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AQA A Level Biology Paper 2/3 Exam, Exams of Biology

AQA A Level Biology Paper 2/3 Exam Study Guide Containing 249 Questions with Definitive Solutions 2024-2025.

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2023/2024

Available from 04/07/2024

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Download AQA A Level Biology Paper 2/3 Exam and more Exams Biology in PDF only on Docsity! AQA A Level Biology Paper 2/3 AQA A Level Biology Paper 2/3 Exam Study Guide Containing 249 Questions with Definitive Solutions 2024-2025. Ecosystem - Answer: is the community and the abiotic components of the environment Abiotic factors in an ecosystem - Answer: Temperature Water availability 1 AQA A Level Biology Paper 2/3 Humidity Light intensity Oxygen and carbon dioxide concentration Wind speed Salinity pH of soil Biotic factors in an ecosystem - Answer: Predators Competition Parasites Symbiosis Food availability Disease Community - Answer: All the populations of different species in a habitat Population - Answer: All the individuals of one species in a habitat Habitat - Answer: Place in which an organism lives Ecological niche - Answer: Where an organism lives and it's role within that habitat e.g. feeding rate,which is governed by adaptations to both biotic and abiotic conditions. Measuring frequency of plants or slow moving animals - Answer: Grid the area 2 AQA A Level Biology Paper 2/3 Use data to look for any trends or patterns in distribution Line transect - Answer: A measuring tape is stretched across the habitat and plants touching the line at regular intervals are identified and recorded. This is quick and provides non-quantitative data. Mark release capture (measuring abundance in mobile animals ) - Answer: Large sample of organisms needs to be collected. Use random sampling to identify where to place nets or traps. Sampling method must be standardised e.g. same size trap and same length of time The animals are marked (M) counted and released back into the population and allowed time to remix with the rest of the population The marking method must not harm the animal or affect their survival chances. Then capture a large sample again using the same sampling technique and count the total number of animals you catch (C) and then number of which are marked (R) The population size can be estimated by (MxC) over R What assumptions do we make during capture recapture ? - Answer: No immigration or emigration No births or deaths Marking doesn't effect the likelihood of being recaptured Marked individuals have a chance to mix fully back into population 5 AQA A Level Biology Paper 2/3 Carrying capacity - Answer: The maximum stable population size that an ecosystem can support. The population size varies as a result of abiotic factors and interactions between organisms e.g. intraspecific and interspecific competition and predation. interspecific competition - Answer: Competition between different species. No two species can occupy the exact same niche as there would be too much interspecific competition. The more similar the niche the greater competition and some species can disappear from an area due to being competitively excluded by stronger competitors. intraspecific competition - Answer: Competition between individuals of the same species for the same resources. Population fluctuates around the carrying capacity. Predation - Answer: Is where an organism hunts and kills and eats another organism. The population sizes of prey and predators are linked and fluctuate within narrow limits: As prey increases there's more food for predators so predator population increases aswell. This means more prey is eaten so prey populations begin to fall , which leaves less food for predators so their population decreases. Energy flow - Answer: Energy and nutrients pass through ecosystems from autotrophs (primary producers like plants and algae) to heterotrophs which can be primary (herbivores) secondary or tertiary (carnivores ). Each of these feeding 6 AQA A Level Biology Paper 2/3 roles is a trophic level and energy flows only once through the ecosystem from one trophic level to another to be then lost as heat. A series of trophic levels make up a food chain. Food chains can be interconnected to produce a food web. Which group of organisms do not fit into a single trophic level ? - Answer: Saprobionts which secrete enzymes for extra cellular digestion and then absorb the products. Energy transfer to producers - Answer: Producers produce their own food from atmospheric or aquatic carbon dioxide through photosynthesis using sun light as their main source of energy. However not all light energy is used in photosynthesis as: Some light may not strike chlorophyll Some light is not an appropriate wave length Some of the light is reflected or transmitted through the leaf Energy is lost in respiration and inefficient reactions in photosynthesis so so little energy is converted to glucose so photosynthesis is a very inefficient process( less than 5% energy converted). Most of the sugars produced are used in respiration and the rest is used to make other biological molecules such as cellulose which form the plants biomass. Biomass - Answer: Mass of organic material in an organism and can be measured as mass of carbon or dry mass of tissue per unit area per time. To estimate the biomass in an area , dry the sample at a low temperature as they would burn if temp too high which releases CO2 so would change biomass. Weigh at regular 7 AQA A Level Biology Paper 2/3 Succession - Answer: Is the way in which different species of organisms which make up a community change over a period of time. Where a habitat is newly formed or disturbed changes occur in the biotic community over time. There are two types of succession: Primary - on land that's newly formed or exposed and there is no organic material to start with Secondary - land that's cleared of all plants but where soil remains so succession happens at later stage Harsh environment is colonised by pioneer species as they are specially adapted to cope with harsh conditions - deep long roots, efficient seed dispersal and ability to fix nitrogen. The habitat is changed by pioneer species This changes environment to become more suitable for new species and less suitable for previous species as increase nutrients and water capacity in soil This establishes a new plant species increasing species diversity and the early colonisers are replaced by new species as they are outcompeted Changes in abiotic factors result in a less hostile environment and an increase in biodiversity A climax community develops which depends on the abiotic factors so different ecosystems have different climax communities Conservation - Answer: To prevent climax community from developing in certain areas so there is a variety of habitats. This is maintained by sheep grazing and controlled burning. 10 AQA A Level Biology Paper 2/3 Nitrogen cycle - Answer: The nitrogen cycle is important as provides nitrates to plants which is a source of nitrogen to produce amino acids, nucleic acids and proteins for growth. Nitrogen fixing bacteria are found in root nodules of leguminous plants or found freely in the soil. They convert atmospheric nitrogen into ammonium ions which are used by the plant to make proteins and the bacteria receive sugar and water from the plant to use in respiration- mutualistic relationship. Saprobionts decompose dead matter/organic material containing nitrogen by extracellular digestion where they secrete enzymes so the nitrogen containing compounds are digested externally releasing ammonium ions by ammonification. Nitrifying bacteria oxidise ammonium ions into nitrites and then oxidise nitrites into nitrates. The nitrates are absorbed by the plant via active transport from the soil and can be used to synthesis proteins,etc. Denitrifying bacteria convert nitrates in the soil into nitrogen gas under anaerobic conditions e.g. water logged soil. Phosphorus cycle - Answer: Saprobionts decompose dead organic material using extracellular digestion where they secrete enzymes and then absorb the products of digestion releasing phosphate ions into soils, lakes, rivers and ocean. Some of these phosphate ions dissolved in oceans can by sedimentation form rocks containing phosphate ions and can be returned back to the ocean by erosion. Some of the phosphate ions in lakes are absorbed by plants and algae and used to form organic compounds( ATP , etc) used for growth. Phosphate ions are passed 11 AQA A Level Biology Paper 2/3 onto animals that feed on the plants and algae. Phosphate ions are then lost from the animal in waster products or during death and completes the cycle. Mycorrhizae - Answer: Mutualistic relationship between fungi and the roots of plants. They grow and produce a large system of long thin strands called hyphae. This provides the fungus with sugars such as glucose and sucrose whilst then hyphae increase surface area of root system do can absorb more water and minerals from the soil. Why do we need fertilisers? - Answer: Food production causes mineral ions to be continuously lost from the soil as crops are harvested and not returned to the soil so cannot be decomposed by saprobionts so the minerals are not recycled. Natural fertiliser - Answer: Consist of organic plant or animal matter containing complex organic compounds which release mineral ions as it decays. Advantage: Slow release of nutrients Nutrients not readily leached Improves structure and drainage Useful and cheap means of disposing fame waste Disadvantages: May be difficult to spread Minerals only released slowly Unknown concentration of nutrients 12 AQA A Level Biology Paper 2/3 Light dependent reaction - Answer: Chlorophyll absorbs light energy from the sun which causes electrons to become excited and leave the chlorophyll ( photoionisation) and enter the electron transport chain ( located in thylakoid membrane ) by being up taken by an electron carrier protein. Also a molecule of water is split into protons and electrons and oxygen in a process called photolysis. H2O -> 2H* + 2e' + 1/2 O2. The electrons formed replace the electrons lost by the chlorophyll allowing light energy to constantly be absorbed. Electrons are passed down the electron transport chain in a series of redox reactions and the electrons lose energy as they pass down the chain. This energy actively pumps protons formed by photolysis of water through the thylakoid membrane from the stroma into the lumen of the thylakoid. The protons accumulate in the lumen of thylakoid so a steep concentration gradient causing protons to diffuse back into the stroma through an ATP synthase protein. This produces energy to synthesis ATP from an inorganic phosphate and ADP. This process is called photophosphorylation and the use of proton concentration gradient to synthesis ATP is known as chemiosmosis. The protons produced by photolysis react with coenzyme NADP and electrons released at the end of the electron transport chain to form reduced NADP. The oxygen produced is a waste product and is either used in respiration or diffused our if the leaf. The ATP and the NADP formed by LDR provide the energy and hydrogen needed to form glucose in the LIR Light Independent Reaction (Calvin Cycle) - Answer: Occurs in the stroma of the chloroplast. Ribulose bisphosphate (5C) combines with one molecule of carbon 15 AQA A Level Biology Paper 2/3 dioxide (1C) to produce two molecules of glycerate- 3- phosphate (3C) by being catalysed by the enzyme RuBisCo. These two molecules of GP are reduced into two triose phosphate molecules(3C) using energy from ATP and oxidises reduced NADP into NADP which provides the hydrogen for the reduction reaction and goes back to the LDR to be reduced again. Some of the two triose phosphate molecules are used to regenerate RuBP (5C) using ATP produced from LDR to supply a molecule of phosphate and energy by the hydrolysis of ATP. The rest of the triose phosphate is converted into organic substances such as glucose(1C). Factors limiting the rate of photosynthesis - Answer: CO2 Light intensity Temperature Water availability Benefits of producing crops in a glasshouse - Answer: Achieve higher yields Allows crop to be grown out of season and provides more income Plants can be grown in regions where they would not naturally grow Limiting factors of photosynthesis in a glasshouse - Answer: CO2 concentration - pump CO2 in, use ventilation and paraffin heaters. Light intensity - artificial lighting specific wavelengths can be used Temperature- glass stops heat from escaping, heaters and cooling systems to keep constant optimum temperature and ventilation mechanisms Water availability- use humidifiers to increase amount of water vapour in air to decrease water potential between leaves and air 16 AQA A Level Biology Paper 2/3 Role of mitochondria in respiration - Answer: Inner mitochondrial membrane contains electron transport chain and cristae increase the surface area to produce more ATP Matrix contains the enzymes required for catalysis of the link reaction and Krebs cycle Outer membrane controls the movement of materials in and out of the mitochondria Respiration equation and steps - Answer: C6H12O6+ 6O2 -> 6CO2+ 6H2O + 38 ATP Aerobic respiration has three steps: Glycolysis - cytoplasm The link reaction an Krebs cycle - matrix of mitochondria Oxidative phosphorylation- cristae of mitochondria Glycolysis - Answer: Glucose is phosphorylated using a molecule of phosphate from 2 ATP molecules making glucose phosphate which is more reactive . This phosphorylated glucose splits into two triose phosphate molecules (3C). TP is oxidised forming tie molecules of pyruvate (3C) and also reduces NAD to form two molecules of reduced NAD and produces 4 ATP molecules by substrate level phosphorylation . This leaves a net gain of 2ATP and 2NADH. Pyruvate enters the mitochondrial matrix by active transport for the link reaction. The link reaction - Answer: Pyruvate(3C) is oxidised to acetate(2C) and loses one molecule of CO2 so is decarboxylated and loses a hydrogen reduced NAD to 17 AQA A Level Biology Paper 2/3 A number greater than one implies a shortage of oxygen and anaerobic respiration is occurring aswell. Genotype - Answer: Genetic constitution of and organism determined by the combination of alleles that an organism inherits from the parents. Phenotype - Answer: The expression of the genotype and it's interaction with the environment. Gene - Answer: Sequence of bases of DNA that code for one polypeptide Allele - Answer: Different form of a gene Locus - Answer: Fixed positron of a gene on a chromosome, alleles if a gene are found at the same locus on each chromosome Homozygous - Answer: Both alleles of a certain gene are identical Heterozygous - Answer: Both alleles of a certain gene are different Dominant - Answer: Is always expressed in the phenotype if present in genotype Codominant - Answer: Both alleles are expressed in phenotype 20 AQA A Level Biology Paper 2/3 Recessive - Answer: Is only expressed in phenotype if two copies are present or dominant allele is absent For the investigation of specific genes what factors in an orgasm are important? - Answer: Must show discontinuous variation Reproduce sexually Pure breeding - alleles of patent are known Short life cycle and produce large number of offspring And is small and cheap Monohybrid cross - Answer: Inheritance of a characteristic controlled by a single gene. A homozygous dominant parent is crossed with a homozygous recessive parent to produce four heterozygous offspring. If two of these offspring are crossed there is a 3 to 1 ratio as one is homozygous dominant, two are heterozygous and one is homozygous recessive. Backcross - Answer: Way to determine unknown genotype by crossing with a homozygous recessive phenotype. If any resulting offspring are homozygous recessive the unknown must be heterozygous and if all offspring have the dominant phenotype then the unknown must be homozygous dominant. Codominance cross - Answer: Both alleles are expressed in phenotype so a third phenotype arrises. Example - in snap dragons dominant allele for white is Cw and dominant allele for red is Cr. If both are crossed all the offspring have both alleles 21 AQA A Level Biology Paper 2/3 so phenotype is pink. Is these are crossed there is one red two pink and one white snap dragon. So a 1:2:1 ratio. Multiple alleles - Answer: Genes that have more than two allelic forms for example - blood group system there are three alleles for blood type and give rise to four different phenotypes. Dihybrid cross - Answer: Cross to look at two different genes that are inherited at the same time from a homozygous recessive and a homozygous dominant parent. The F1 offspring are all heterozygous and F2 offspring have ratio 9:3:3:1. Sex determination - Answer: Is determined by combination of chromosomes in a zygote as females have two X chromosomes and makes have an X and a Y. There is always a 50/50 chance of producing a male and female offspring. Sex linkage - Answer: Are genes carried on one of the sex chromosomes. Most genes are carried on the X chromosome as it is larger than Y. This means males are more likely to show recessive phenotypes for genes that are sex linked. Females can be carriers of a sex linked disease as they have two X chromosomes. Look for greater number of affected males than females for evidence of sex linked and look for if an affected mum has a normal son with a recessive phenotype as this is not possible for sex linked. Epistasis - Answer: Where one gene influences the expression of another gene. For example in mice albino genotype Is bb and for them to be melanin they need 22 AQA A Level Biology Paper 2/3 Causes of variation - Answer: Environment Genetic - independent segregation, crossing over, mutations and random fusion of gametes Intraspecific variation - Answer: Variation within a species Interspecific variation - Answer: Variation between different species Random fusion of gametes - Answer: Produces new combination of alleles so the offspring are genetically different from their parents. Independent segregation - Answer: Any one chromosome from any one homologous pairs of chromosomes can enter a gamete with any other one chromosome from another homologous pair. This leads to a different combination of maternal and paternal chromosomes in the gamete. How mutation can lead to non-functioning protein - Answer: If the sequence of bases in DNA is altered due to mutation this changes the sequence of amino acids in a polypeptide This changes tertiary structure as bonds form in different places This would lead to loss or reduction in function of the protein Mutagenic agents - Answer: Increase rate of mutation by: 25 AQA A Level Biology Paper 2/3 Acting as a base - chemicals can substitute for a base during DNA replication so change the sequence of amino acids Altering bases- some chemicals can delete or alter bases Changing structure of DNA - UV radiation changes structure of DNA Genetic drift - Answer: Allele frequency's change over time due to chance rather than environmental factors where individuals survive breed and pass on their alleles to the next generation. This can lead to reproductive isolation or speciation and had a great effect in smaller populations as chance is a greater influence. It may occur because of the founder effect where a few organisms from a population become isolated from the rest of the population or genetic bottle neck where the population is reduced to a very small number and when the population size increases there is a reduction in genetic diversity Gene pool - Answer: All the alleles in a population at a particular time. Hardy Weinberg equation - Answer: Predicts the frequency of alleles in a population. p+q=1 where P= frequency of dominant allele Q= frequency of recessive allele p2 + 2pq + q2 = 1 where p2 = frequency of homozygous dominant genotype q2 = frequency of homozygous recessive genotype 2pq = frequency of the heterozygous genotype 26 AQA A Level Biology Paper 2/3 Conditions for Hardy Weinberg principle to work - Answer: Large population Mating between individuals must be random No mutations All genotypes must be equally likely to reproduce There must be no immigration or emigration in the population (no gene flow) Directional selection - Answer: Selection for one extreme and against another extreme in a range of phenotypes causing standard deviations to be smaller in a response to environmental change. Example is antibiotic resistance in bacteria. Stabilising selection - Answer: Selection against both extremes in a range of phenotypes so selects and favours the middle phenotype and acts to prevent change when the environment isn't changing so reduces range of phenotypes. Example is human birth weights as medium sized babies are the most favourable size. Disruptive selection - Answer: Selection for both extremes in a range of phenotypes as they have a selective advantage over the middle intermediate phenotypes which is selected against. Occurs when environment favours more than one phenotype. Example Pacific coho salmon as large males better at fighter and small males can reproduce without being seen. Natural selection - Answer: There will be variation in the population .e.g. due to random mutations 27 AQA A Level Biology Paper 2/3 Specific transcriptional factors can also inhibit transcription by preventing RNA polymerase from binding so stopping transcription. Initiation of transcription by oestrogen - Answer: Oestrogen is a lipid soluble steroid hormone that can bind to specific transcription factors that contain receptor sites for oestrogen. Oestrogen can switch on/ activate a gene and. Therefore initiate transcription by binding with receptor site on transcriptional factor. This removes an inhibitor molecule allowing transcriptional factor to bind to the DNA promoter region and activate transcription. Inhibition of translation of mRNA by RNA interference (RNAi) - Answer: RNAi is where small double stranded RNA molecules prevent mRNA from target genes being translated. RNAi is made by special regulatory genes and moves into the cytoplasm where it becomes single stranded. It then bonds to specific mRNA molecules with a complementary base sequence by complementary base pairing and cuts the mRNA in two small pieces so it can be no longer translated during protein synthesis. Applications of RNAi in scientific research - Answer: Could be used to identify the role of genes in biological pathways where we can determine what the role of a blocked gene is. May be used to block genes that cause disease therefore prevent the disease. Differentiated cells - Answer: Following the formation of a zygote all the cells are identical however specific genes are expressed in different cells leading for them 30 AQA A Level Biology Paper 2/3 to become differentiated and carry out a specific function. They differ from each other as they produce different proteins. All of the specialised cells originally came from stem cells. What are the different types of stem cells ? - Answer: Stem cells are undifferentiated cells and can differentiate into other types of cell and become specialised. They can replace them cells by dividing to form more stem cells. There are four types: Totipotent - can divide and produce any type of body cell and are found in embryos as zygotes for a limited time, during development they translate only part of their DNA resulting in specialisation. Pluripotent - can divide in unlimited numbers and can differentiate into every type of specialised cell apart from the placenta so can be used in treating human disorders. They are also found in embryos as a inner cells in a blastocyst. Multipotent - are found in nature mammals and can divide to form only a limited number of different cell types depending on which tissue they are obtained from. Unipotent - are found in mature mammals and can only differentiate into one type of cell or tissue. Plant stem cells - Answer: Mature plants have many totipotent cells and can develop in vitro (culturing cells or tissues out side of living organism) into whole plants it into plant organs when given the correct conditions. So can clone genetically identical plants this way. 31 AQA A Level Biology Paper 2/3 Cardiomyocytes - Answer: Are heart muscle cells that make up the tissue in our hearts and can be made from unipotent stem cells. Use of stem cells in treating human disorders - Answer: Stem cells are useful because they can replace damaged cells or tissues in patients. They could save lives , could be a replacement for the long donor waiting list and improve quality of life. Adult stem cells - Answer: Multipotent or unipotent obtained from the body tissues of an adult e.g. in bone marrow and can be removed in a simple operation with little risk. However they can only specialise into a limited range of body cells. Embryonic stem cells - Answer: Totipotent or pluripotent obtained from embryos at the early stage of development. They are created in a lab by in vitro fertilisation where stem cells are removed and embryo is destroyed. This is a benefit as they can divide into an unlimited number of all types of body cells, however there are many ethical issues around this. Induced Pluripotent stem cells (ips) - Answer: These are produced from specialised adult body cells (somatic) and are reprogrammed to become pluripotent in a laboratory. Specific protein transcription factors reprograms somatic cells causing them to express genes so they can differentiate into any body cell, therefore they are pluripotent and can self renew so scientists can create a limitless supply. So could replace embryonic stem cells overcoming the 32 AQA A Level Biology Paper 2/3 It also attracts proteins that decrease acetylation of histones and therefore condense the chromatin so the genes become inaccessible to transcription factors. These both inhibit transcription. Decreased methylation allows transcriptional factors to bind to DNA by not being added to a cytosine base. This doesn't attract proteins that decrease acetylation so the gene is switched on and transcription occurs. Benign tumour - Answer: Mass of cells that are non cancerous and do not spread into neighbouring tissue. However they can develop into malignant tumours which are cancerous. Malignant tumour - Answer: Uncontrollably rapidly dividing cells that may spread to other parts of the body and destroy surround tissues. Genes controlling cell division - Answer: Porto-oncogenes stimulates cell division by produced proteins that make cells divide. Tumour suppressor genes slow cell division when it is no longer necessary or if the DNA is damaged by producing proteins that stop cells dividing or self destruct faulty cells. They repair errors in DNA also. Oncogenes - Answer: When a proto-oncogene becomes mutated it becomes a oncogene where it is permanently activated causing normal cells to divide rapidly without need for growth factors and become cancerous. 35 AQA A Level Biology Paper 2/3 Tumour suppressor genes - Answer: If mutation they become inactivated allowing cells to divide uncontrollably as they don't produce a protein to slow down mitosis. Explain how increased methylation of DNA lead to tumour suppressor genes being inactivated ? - Answer: When tumour suppressor genes are hypermethylated transcription is inhibited as chromatin becomes more condensed and transcriptional factors are unable to bind. This causes tumour suppressor genes to not be expressed so the proteins they produce to slow down cell division aren't produced. Explain how decreased methylation of proto-oncogenes could lead to cancer? - Answer: Hypomethylation of proto-oncogenes causes transcription to be stimulated as DNA is less tightly wrapped around histone proteins and transcription factors can bind. The gene is permanently activated so produces proteins that cause cells to divide rapidly causing cancer and is now called an oncogene. Increased oestrogen effect on breast cancer - Answer: Oestrogen can bind to oestrogen receptors and cause stimulation of cell to divide and can cause tumour to grow more rapidly. Oestrogen can activate transcriptional factors causing genes that cause cell division to be switched on. 36 AQA A Level Biology Paper 2/3 Acquired mutations - Answer: Are caused by mutagenic agents and environmental factors such as smoking an radiation after fertilisation. Diagnosis would happen after symptoms appear and treatment depends on the type of cancer as if the cancer is fast growing radiation of chemotherapy can be used , where as if a specific gene is located to be mutated a tumour suppressor gene can make the treatment more targeted. Hereditary mutations - Answer: Are caused by a family history of a certain type of cancer if a certain allele is inherited. If specific cancer causation mutation is known it can be screened in the persons DNA and knowing this means a person can take prevention steps , such as a mastectomy to reduced the risk of breast cancer as early diagnosis increases chance of recovery. Treatment is different for different specific mutations and can develop drugs to effectively target them. Where as some cancer requires more aggressive treatment such as radio and chemotherapy. Gene therapy can treat some mutations by inactivated tumour suppressor genes becoming active again. Genome projects - Answer: DNA sequencing can determine the order of nucleotides and their bases in DNA and sequencing methods work on fragments of DNA so DNA needs to be fragmented first and then resequenced to go back in order. Sequencing projects have read the genomes if a wide range of organisms and methods are continuously updated and have become automated. Genome of simpler organisms - Answer: Determining genome of simpler organisms allows the sequence of proteins that derive from the genetic code of 37 AQA A Level Biology Paper 2/3 Sticky end - Answer: Single stranded sections of DNA that over hang at the end of a double stranded molecule these single stranded bases are exposed. Using a gene machine to produce fragments of DNA - Answer: Fragments of DNA can now be synthesised in a lab from scratch without needing a preexisting DNA template. Sequence of nucleotide bases of a gene are determined by the desired protein and the mRNA codons are looked up and complementary DNA triplets are worked out and errors are rejected so there are no introns or non coding DNA. However gene machine and reverse transcriptase both don't produce fragments with sticky ends. How sticky ends can be produced from double stranded fragments - Answer: Heat DNA to break hydrogen bonds and then once cooled add primers which stop them complementary base pairing with DNA polymerase and nucleotides. In Vivo cloning steps - Answer: Is where DNA fragments are amplified/cloned in living cells. Firstly, the DNA must be prepared for insertion by adding a promoter region at the start and a terminator sequence at the end. The promoter region is a DNA sequence that attaches RNA polymerase and transcriptional factors to start transcription and the production of mRNA. The terminator region is a DNA sequence that releases RNA polymerase and ends transcription. The DNA fragment is then inserted into a vector , which is a carrier used to transport DNA into a host cell so that the foreign DNA can be replicated and expressed. Viruses or liposomes( phospholipid vesicle) can be used as vectors but a plasmid is often used to transfer DNA into bacterial cells and can be used as a 40 AQA A Level Biology Paper 2/3 genetic marker. A plasmid is small circular DNA found in cytoplasm of bacteria cells and can replicate independently of cell replication. The vector DNA is cut (in the middle of one of the marker genes for plasmids) using the same restriction endonuclease enzyme that was used to isolate the DNA fragment containing the target gene. This is so sticky ends of the vector are complementary to the sticky ends of the DNA fragment and the sticky ends join by DNA ligase by joining the sugar phosphate backbone of the two strands of DNA into one strand. This forms a recombinant plasmid/ DNA but may also form the original plasmid and circular DNA, which may be difficult to separate but there marker genes can be used to identify the correct recombinant plasmid. The vector with recombinant DNA is used to transfer the gene into host cells, if a plasmid vector is used host cells need to be persuaded to take in the plasmid vector and it's DNA and is said to be transformed. For bacterial host cells this requires mixing the plasmids and bacteria in a solution containing calcium ions to make the bacterial c In Vitro cloning steps - Answer: Is where copies of DNA fragments are made outside of a living organism using the polymerase chain reaction( PCR). This can be used to make millions of DNA fragment copies. Firstly, the DNA fragment is added to a machine called thermocycler with DNA polymerase, free nucleotides and primers. Primers are short sequences of single stranded nucleotides with a specific base sequence that is complementary to the start of the DNA fragments and allow DNA polymerase to bind and start DNA synthesis, whilst also preventing single stranded DNA to join back together. DNA polymerase is an enzyme that creates new double stranded DNA strands. The mixture is heated to 95 degrees to 41 AQA A Level Biology Paper 2/3 break the hydrogen bonds and separate the doubly strands of DNA. The temperature is then cooled to 40/50/60 degrees and causes primers to attach to complementary base sequences at the ends of the single strands. The temperature is raised to 72 degrees which is optimum for DNA polymerase to work and free nucleotides attach to the template strand by complementary base pairing forming new strands of DNA. Two copies of the fragment DNA are formed in one cycle and PCR is complete then the process is repeated and the DNA is doubled in quantity each time producing millions of copies. Function of primers - Answer: Allow DNA polymerase to bind and start DNA synthesis, whilst preventing single stranded DNA joining back to each other. How may plants be genetically modified ? - Answer: Adding genes for resistance to herbicides, insects, pests and viral diseases into various plant species Adding genes to develop tolerance to extreme environmental conditions like temperature Adding genes to improve the nutrient content of foods How may bacteria be genetically modified ? - Answer: By in vivo cloning as foreign DNA can be inserted to produce lots of a useful protein such as insulin which is the extracted an purified. How may animals be genetically modified ? - Answer: The desired gene is inserted into a fertilised egg so that all body cells will carry a copy of this gene. A growth hormone gene would allow animals to grow larger and at a faster rate. Adding a 42 AQA A Level Biology Paper 2/3 However, there are many ethical issues with how far gene therapy should be taken. Gel electrophoresis - Answer: Is the process of separating DNA fragments on the basis of their length for analysis to produce a genetic fingerprint or so the defective gene can be located using a gene probe. DNA samples are amplified using PCR ( making numerous copies of areas of DNA that contain variable number tandem repeats) and cut into fragments by using restriction enzymes. The fragments are marked either with a fluorescent tag so the fragments can be viewed under UV light as bands or are radioactively labeled and an autoradiograph identity's the position of fragment after separation. The fragments are placed into wells at one end of a thin slab of gel and covered in a buffer solution that conducts electricity. An electric current is passed through the gel and the DNA is attracted to the positive electrode at the far end of the gel as DNA carries a slight negative charge. The molecules have to diffuse through the gel and smaller lengths of DNA move faster and travel further so the fragments separate according to size. Genetic fingerprinting steps - Answer: DNA extraction- First DNA is extracted from a sample of tissues and PCR will increase the quantity of DNA. Digestion - Then DNA is cut into fragment using restriction endonucleases to produce fragments containing the variable number tandem repeats. Separation- Gel electrophoresis separates the DNA fragments and they become single stranded by immersing in alkali. 45 AQA A Level Biology Paper 2/3 Hybridisation- radioactive or fluorescent probes are used to bind with non coding DNA. The probes have complementary base sequence to the variable number tandem repeats which differ in lengths between individuals. Development- an X Ray film is placed over sample and the film is exposed to radiation from the radioactive probes which are attracted to the DNA fragments. As a result a series of bands are seen which is unique to each individual or fluorescent probes which are exposed to UV light. DNA from crime scenes can be compared to suspect DNA if the banding patterns are identical. Variable number tandem repeats - Answer: Are non coding base sequences that repeat over and over and in each individual there is a unique pattern of these repeats as they differ from each person. This means the probability of two individuals have the same genetic fingerprint is very low as the chance of having same number of VNTRs is low at each place found in DNA , this makes non coding regions as a whole that are used in genetic fingerprinting. DNA inherited from same parents within a small family group will have similar DNA. Uses of DNA fingerprinting - Answer: In forensic science - to compare samples of DNA collected from crime scenes to possible suspects. However the DNA could be contaminated or be a innocent bystanders DNA In genetic - as individuals inherit half genetic material from each parent there band on DNA fingerprint should correspond to parents DNA. In medical diagnosis- can help diagnose genetic disorders or cancer which might usually not have symptoms till later life. 46 AQA A Level Biology Paper 2/3 In plant and animal breeding - can be used to prevent inbreeding which decreases the gene pool and can identify how closely related individual animals are. DNA probes - Answer: Following gel electrophoresis DNA probes can identify whether a gene or allele is present within the DNA fragments. A DNA/gene probe is a single stranded, short sequence of bases which are complementary to a particular base sequence for example the target gene being screened for. A DNA probe will bind( hybridise) to the target gene/allele if it is present in a sample. To identify whether the DNA probe has attached the probe has a label attached so it can be detected. This is either a radioactive label detected using X- Ray film or a fluorescent label detected using UV light. The probe can be made complementary to a specific allele that causes a particular disease or complementary to a variable number tandem repeat for use in genetic fingerprinting. Genetic screening - Answer: Involves using DNA probes produced by PCR using the mutant gene as a template to screen patients for important genes/ alleles. This means the DNA probe will bind(hybridise)to the target mutant gene of its present in a sample. Unlike genetic fingerprinting the coding DNA is tested and one probe is used that is complementary to a base sequence on mutant gene so the DNA fragments carrying gene can be removed and purified. There are three main uses of screening: Help identify inherited conditions as people with family history of disease may choose to be screened for the mutant allele To see how patient will respond to certain drugs 47 AQA A Level Biology Paper 2/3 enough glucose for respiration in cells to produce ATP so cells cannot carry out normal functions. Negative feedback - Answer: Is a change from a set point or norm which is detected by receptors and stimulates corrective physiological mechanisms by effectors to restore the system to the norm or set point. The possession of multiple separate negative feedback mechanisms controls departures in different directions from the original state giving a greater degree of control. Positive feedback - Answer: Is not a homeostatic mechanism as positive feedback mechanisms amplifies the change as the effectors respond to further the increase from the set or normal level. An example is when body temperature drops below 37 degrees positive feedback causes temperature to fall further. Three sources glucose in bloods stream comes from - Answer: From glucose absorbed following hydrolysis of carbohydrates from a person diet From the hydrolysis of glycogen stored in muscle and liver knowing as glycogenolysis From gluconeogenesis where glucose is produced from amino acids or glycerol in the liver Hormones - Answer: Hormones are produced by endocrine glands and are transported to target cells which have specific receptor proteins in the cell membrane or cytoplasm which the hormone binds to. 50 AQA A Level Biology Paper 2/3 Role of adrenaline with blood glucose concentration - Answer: Adrenaline is secreted from the adrenal glands when blood glucose concentration is low, during stress and exercise. This increases blood glucose concentration by binding to receptors in target cell membrane of liver cells activating an enzyme which stimulates the conversion of glycogen into glucose ( glycogenolysis) and inactivates an enzyme which stimulates the conversion of glycogen from glucose ( glycogenesis). It also activates glucagon secretion and inhibits insulin secretion. Second messenger model - Answer: Both adrenaline and glucagon activate glycogenolysis inside a cell via a second messenger model. The hormone approaches receptor site on liver cell membrane which is a complimentary shape to the hormone so hormone attaches to receptor site and activates an enzyme called adenylate cyclase inside the membrane. Adenylate cyclase converts ATP into cyclic AMP which acts as a second messenger that activates the enzyme protein kinase A. This then activates a chain of reactions that breaks down glycogen into glucose. Negative feedback of increased blood glucose from the norm - Answer: When blood glucose concentration rises above the normal blood glucose concentration, this is detected by the Islets of Langerhans in the pancreas which secretes B cells which secrete insulin into the blood and travels to target cells in the liver and muscle. Insulin then binds to these specific receptors on the target cells membrane. This increases the permeability of the cells as extra specific channel proteins are inserted into the membrane increasing the uptake of glucose by the cells by facilitated diffusion. Insulin activates enzymes which stimulate the 51 AQA A Level Biology Paper 2/3 conversion of glucose to glycogen in the liver and muscle cells which can be stored and is known as glycogenesis. Insulin activates enzymes which stimulate the conversion of glucose to fats for storage in adipose tissue. Insulin also increases respiration rates of glucose in cells. This results in a lower concentration of glucose in the blood and the concentration is restored back to normal. Negative feedback of decreased blood glucose from the norm - Answer: When blood glucose concentration falls below the normal blood glucose concentration, this is detected by the Islets of Langerhans in the pancreas which secretes alpha cells which secrete glucagon into the blood and travels to target cells in the liver. Glucagon then binds to these specific receptors on the target cells membrane. Glucagon activates enzymes which catalyse the conversion of glycogen to glucose in the liver and is known as glycogenolysis. Glucagon activates enzymes that stimulate the conversion of non carbohydrates such as amino acids and glycerol to glucose which is known as gluconeogenesis. Glucagon activates enzymes which stimulate fatty acid release in adipose tissue. Glucagon decreases the rate of respiration of glucose in cells. This results in a higher concentration of glucose in the blood and the concentration is restored back to normal. Diabetes - Answer: Diabetes mellitus is a condition where the body is no longer able to control blood glucose concentrations effectively and there are two main types: Type 1 - is caused at childhood by the inability to produce insulin due to the body's immune system attacking beta cells. This causes blood glucose 52 AQA A Level Biology Paper 2/3 the filtrate by osmosis. The filtrate thay remains is urine which is made up of water and dissolved salts, idea and hormones. Describe two ways which the cells of the PCT are adapted to their function ? - Answer: Microvilli increase surface area for reabsorption of glucose from PCT. Highly infolded plasma membrane at base of PCT cells maximises surface area for reabsorption of glucose into capillary. Many mitochondria provide ATP needed for active transport of glucose as it's the site of aerobic respiration. Maintenance of sodium ion gradient in the medulla by the loop of Henle - Answer: Filtrate enters loop of Henle from PCT and drops down into the medulla and rises back up to the cortex. The ascending limb is permeable to ions and at the start of the ascending limb sodium ions and chloride ions are actively transported out of the ascending limb cells into the tissue fluid against a concentration gradient using ATP. The ascending limb is impermeable to water so the water potential of the tissue fluid in the medulla becomes lower as there is a greater concentration of sodium ions. Near the bottom of the ascending limb the ions move out by facilitated diffusion as the filtrate as a high concentration of ions. The descending limb is impermeable to sodium and chloride ions but is highly permeable to water so water moves out of the descending limb cells and enters the tissue fluid of the medulla by osmosis. This makes the filtrate in the descending limb more concentrated so reduces its water potential. The water in the tissue fluid of the medulla is reabsorbed init the vasa texts by osmosis. This causes a gradient of sodium ions in the medulla which lowers the water potential 55 AQA A Level Biology Paper 2/3 so that if the water potential of the blood falls water can be reabsorbed from the collecting duct due to the release of ADH. Long loop of Henle means the animal is in a dry environment and needs to conserve water and a short loop of Henle means the animal is in aquatic conditions so doesn't need to conserve as much water. Distal convoluted tubule and collecting ducts - Answer: After leaving the loop of Henle the filtrate moves into the distal convoluted tubule in the cortex and finally to the collecting duct which drips down into the medulla. As the loop of Henle creates a sodium ion concentration gradient in the medulla, the tissue fluid has a lower water potential so water moves out of the filtrate in the collecting ducts by osmosis and be reabsorbed by the vase recta. If the water potential is too low in the blood more water is reabsorbed by osmosis into the blood so the urine is more concentrated and if the water potential is too high in the blood less water is reabsorbed by osmosis into the blood so the urine is more dilute. ADH(antidiuretic) in osmoregulation - Answer: ADH increases permeability of the collecting duct cells by increasing transcription and causes more aquaporins to be inserted into the membrane of the cells lining the collecting duct. Aquaporins are protein water channels that allow water to move down their water potential gradient and out into the medulla then the vase recta increasing the amount of water reabsorbed from the filtrate. When ADH levels fall the aquaporins are removed from the membranes and the collecting duct becomes less permeable. 56 AQA A Level Biology Paper 2/3 Osmoreceptors in hypothalamus detect decrease in the water potential which stimulates pituitary gland to release more ADH into the blood and makes the cells of the collecting duct more permeable to water. This means when filtrate travels down collecting duct the low water potential of the medulla causes water to move by osmosis from the filtrate to the medulla and then into the blood so more water is reabsorbed, so a small volume of highly concentrated urine is produced which helps the body to conserve water. Osmoreceptors in hypothalamus detect an increase in the water potential which stimulates pituitary gland to release less ADH into the blood and makes the cells of the collecting duct less permeable to water. This means when filtrate travels down collecting duct far less water is reabsorbed by osmosis into the blood, so a large volume of dilute urine is produced which helps the body to excrete excess water. Negative feedback of osmoregulation - Answer: If there is an increase in water potential over the normal water potential of the blood this is detected by osmoreceptors in the hypothalamus. This releases less ADH from the posterior pituitary gland so the collecting duct walls are less permeable to water. This causes less water to be reabsorbed into the blood and more urine to be produced. This decreases the water potential in the blood back to the normal. If there is a decrease in water potential below the normal water potential of the blood this is detected by osmoreceptors in the hypothalamus. This releases more ADH from the posterior pituitary gland so the collecting duct walls are more permeable to water. This causes more water to be reabsorbed into the blood and 57 AQA A Level Biology Paper 2/3 moves down the shoot but light from one side causes IAA to move to the shaded side from the light side. The causes a higher concentration of IAA to build up on the shaded side causing more cell elongation than on the light side. The shaded grows at a faster rate causing the shoot to bend and grow towards light. Negative phototropism and IAA - Answer: Negative phototropism happens when the roots grow away from light due to uneven distribution of IAA. IAA is produced by cells in the tip of the shoot and is transported down to the growing region in the end of the roots where growth is stimulated. IAA is transported evenly when it moves down the root but light from one side causes IAA to move to the shaded side from the light side. The causes a higher concentration of IAA to build up on the shaded side and inhibits growth. The shaded grows at a slower rate causing the root to bend and grow away from light. Positive gravitropism and IAA - Answer: Positive gravitropism happens when the roots grow towards gravity due to uneven distribution of IAA. IAA is produced by cells in the tip of the shoot and is transported down to the growing region in the end of the roots where growth is stimulated. IAA is transported evenly when it moves down the roots but gravity causes IAA to move down to the lower side. The causes a higher concentration of IAA to build up on the lower side inhibiting growth . The lower side grows at a slower rate than the upper side causing the root to bend and grow towards gravity. Negative gravitropism and IAA - Answer: Negative gravitropism happens when the shoots grow away from gravity due to uneven distribution IAA. IAA is produced by 60 AQA A Level Biology Paper 2/3 cells in the tip of the shoot and is transported down to the growing region in the tip of the shoot where growth is stimulated. IAA is transported evenly when it moves down the shoot but gravity causes IAA to move to the lower side. The causes a higher concentration of IAA to build up on the lower side causing more cell elongation than on the upper side. The lower side grows at a faster rate causing the shoot to bend and grow upwards away from gravity. What are the simple responses in simple organism ? - Answer: Taxes and Kineses. These are simple innate (does not have to be learned) forms of behaviour that rely on simple reflex actions and allow mobile organisms to respond to environmental changes and maintain them in favourable conditions for survival. They are genetically determined, show a stereotyped pattern in all members of same species and consists of a chain of reflexes. Taxes in simple organisms - Answer: A taxis is a simple directional response to a stimulus and is carried out by the whole organism. Phototaxis is movement towards or away from light. Chemotaxis is movement towards or away from a chemical. Kinesis in simple organisms - Answer: A kineses is a non directional response to a stimulus. The rate of movement or rate of change of direction is related to intensity of the stimulus and kineses results in random movement patterns. In dry air woodlice move faster and turn less often to increase their chance of moving away from drier air. This increases their chance of survival as it reduces water loss as humid air has higher water potential so less water lost by diffusion and keeps 61 AQA A Level Biology Paper 2/3 them concealed. As the air becomes more humid they move slower and turn more often so they are more likely to stay in that area. Structure of the nervous system - Answer: Nervous system is divided into two sections, the central , which is made up of the brain and spinal cord , and the peripheral which is made up of the sensory and motor neurones. This is made up of a complex network of neurones and the main three types are: Sensory neurones carry nerve impulses from receptors toward the central NS Motor neurones carry nerve impulses away from the central NS to effectors Relay neurones carry electrical nerve impulses between sensory and motor neurones. A stimulus is detected by receptor cells ( which can be cells or proteins on cell surface membrane) and sends and electrical impulse to the sensory neurone. When the impulse reaches the end of a neurone , neurotransmitters take the information across to the next neurone which sends and electrical impulse. The central NS (coordinator) processes information and send impulses along motor neurones to an effector ( cells which bring about a response to a stimulus). The peripheral nervous system is split into two systems: The somatic which controls conscious activities The autonomic which controls unconscious activities and has two divisions which have the opposite effect on the body. The sympathetic nervous system gets the body ready for action (fight or flight) and the parasympathetic calms the body down (rest and digest). Neurones - Answer: Pass electrical impulses along their length and stimulate their target cells by secreting chemical neurotransmitters directly onto them resulting 62 AQA A Level Biology Paper 2/3 generator potential reaches the threshold an action potential will be triggered. The greater the pressure , the greater the generator potential. Once the pressure had been registered no further action potentials are sent until the pressure is released. Structure of the eye - Answer: Light enters through the pupil, which is just a gap in the iris, so muscles of the iris control the amount of light that enters through the pupil. Light rays are focused by the lens onto the retina which lines the inside of the eye and contains photoreceptor cells, mainly rod and cone cells , which detect light. The fives of the retina is where there are lots of cone cells but no rod cells. Nerve impulses from rod and cone cells synapse to bipolar neurones which synapse with sensory neurones and bundle together to form the optic nerve which send impulses to the brain. Where the optic nerve leave the eye is called the blind spot and contains no photoreceptors. Rod cells - Answer: Found in the peripheral parts of the retina and are not found in the blind spot or optic nerve. They contain rhodopsin which is very sensitive to low light intensities and breaks down when it absorbs light, causing a chemical change which alters the membranes permeability to sodium ions and a generator potential is created and if it reaches the threshold a nerve impulse is sent to a bipolar neurone and take the impulse to the optic nerve then to the brain. In the absence of further stimulation rhodopsin is reformed and during a process that requires ATP, however in bright light the breakdown happens faster than rhodopsin can reform so rod cells do not function and are bleached. Rod cells show retinal convergence where several rod cells synapse to one bipolar neurone. 65 AQA A Level Biology Paper 2/3 Rod cells are very sensitive to light ( action potentials are triggered when rods are stimulated at low light intensity) and if many rod cells are stimulated at the same time, because of retinal convergence, the generator potentials combine and reach the threshold to trigger and action potential (spacial summation). Rod cells are unable to distinguish colours so only give monochromatic vision. Rod cells have a low visual acuity (ability to tell apart points that are close together in detail) as many rod cells share the same neurone( retinal convergence) so cant tell light from two points close together apart. Cone cells - Answer: Found at a high concentration in the fovea, a low concentration at the periphery of retina and none at the blind spot or optic nerve. They contain iodopsin which does not break down in dim light(not sensitive to dim light) but is very sensitive to high light intensities and breaks down when it absorbs light, causing a chemical change which alters the membranes permeability to sodium ions and a generator potential is created and if it reaches the threshold a nerve impulse is sent to a bipolar neurone and take the impulse to the optic nerve then to the brain. There are three different types of cone each containing a different iodopsin pigment: red sensitive , blue sensitive and green sensitive and give information in colour because of the trichromatic theory. Each cone cell synapses to its own bipolar neurone. Cone cells are less sensitive to light ( action potentials are triggered when cones are stimulated at high light intensities) as more light is needed for come cell to trigger an action potential to its own bipolar neurone. Cone cells have high visual acuity (ability to tell apart points that are close together in detail) because each cone synapse with its own 66 AQA A Level Biology Paper 2/3 bipolar neurone which generates impulses from two cone cells close together so the brain can distinguish between two close points and see them in detail. Colour blindness - Answer: Sex linked disease where in absence of red sensitive cones, red light will be detected by green sensitive comes and the brain cannot distinguish between red and green. How is heart rate controlled? - Answer: Cardiac muscle is myogenic as contractions arise from within the cardiac muscle it's self and controls the regular heart beat. Sinoatrial node within the right atrium sends and electrical impulse across the atrial walls causing both atria to contract. Non conducting collagen tissue prevents the impulse being passed to the ventricles. Impulses reach atrioventricular node which send an electrical impulse down the bundle of His. However there is a slight delay before AVN sends an impulse to allow atria to fully empty before ventricles contract. Impulses pass down into purkyne fibres which carries impulse to the ventricular walls causing both ventricles to contracts simultaneously. Autonomic nervous system - Answer: Is made up fo two antagonistic systems the sympathetic which involves the neurotransmitter noradrenaline and has an excitatory effect during stress, danger or exercise and the parasympathetic which involves acetylcholine and has an inhibitory effect during rest. They are antagonistic as the actions oppose each other because the sympathetic prepares for activity and the parasympathetic slows down and conserves energy. The sinoatrial node generates electrical impulses that contacts cardiac muscles but the 67 AQA A Level Biology Paper 2/3 which secretes noradrenaline and binds to receptors on SAN, increasing the heart rate causing blood pressure to rise to normal levels. Structure of a myelinated motor neurone - Answer: A neurone( nerve cell) is adapted to carrying impulses by containing a cell body which contains a large amount of RER for the production of proteins and neurotransmitters. The cell body elongates into the axon which carries impulses away from the cell. Also contains dendrimer which are extensions of cell membrane and cytoplasm and further divide into dendrites and carry impulses to the cell body. Some neurones contain Schwann cells which wrap around the axon and provide protection and insulation building up layers of their membranes that contain myelin( lipid) and make up the myelin sheath. These neurones are called myelinated and there are gaps in between Schwann cells containing no myelin sheath called the nodes of Ranvier. Myelinated neurones are able to transmit nerve impulses faster than non myelinated neurones. Maintenance of the resting potential - Answer: The resting potential is when the outside of the axon is more positively charged than the inside as there is more positive ions out side the axon. This means the axon is polarised as there is a difference in charge called potential difference and is usually -65 to -70 mV. The resting potential is maintained by a balance between sodium ions and potassium ions inside and outside axon controlled by: phospholipid bilayer being impermeable to charged particles so sodium and potassium ions cannot diffuse straight across so specific intrinsic proteins which are either gated or leak channels allow ions to diffuse through by facilitated diffusion. Sodium potassium pumps are 70 AQA A Level Biology Paper 2/3 proteins that actively transport sodium ions out of the axon and potassium ions into the axon. 3 Na+ are actively transported out of the axon and 2 K+ are actively transported into the axon via the Na+/K+ pump. The membrane is more permeable to potassium ions than sodium ions so sodium ions cannot diffuse back into the axon but potassium ions rapidly diffuse out of the axon through potassium channels down a concentration gradient. This creates an electrochemical gradient as the inside is more negative than the outside of the axon. This means K+ diffuses out of the axon much faster than Na+ diffuses in causing the inside the membrane to be more negative than the outside of the membrane at rest. Generation of Action potential - Answer: When a stimulus is received by a receptor or nerve ending this causes depolarisation and if the axon is depolarised enough an action potential can be generated. The stages of an action potential are: When the axon is stimulated this causes gated sodium ion channels to open making the membrane more permeable to sodium ions , so sodium ions rapidly diffuse into the axon causing depolarisation. If the potential difference reaches the threshold this causes more gated sodium ion channels to open so more sodium ions rapidly diffuse into the axon until the membrane potential becomes positive (+40mV) and this is the action potential. When the potential difference is +40 mV all the gated sodium ion channels close and gated potassium channels open increasing the membranes permeability to potassium ions so potassium ions rapidly diffuse out of the axon (repolarisation). So many potassium ions diffuse out of the axon that they cause a more negative potential difference than the 71 AQA A Level Biology Paper 2/3 resting potential. This is known as hyperpolarisation and causes the relative refractory period. This causes gated potassium ion channels to close and the sodium-potassium pump restores the the concentration gradient for sodium and potassium ions and returns the resting potential to normal. All or nothing principle - Answer: If the threshold is reached then action potentials will be generated at a constant size and speed down as the all of nothing principle. A larger stimulus will increase the frequency of action potentials instead of making them larger. Passage of action potential in non-myelinated neurones - Answer: Action potentials are spread along the neurone as depolarisation causes local electrical currents to be set up which causes gated sodium ion channels to open in adjacent regions. This causes sodium ions to enter adjacent regions which then become depolarised. Passage of action potential in myelinated neurones - Answer: Because of the Schwann cells making up the myelin sheath providing electrical insulation and preventing the movement of ions across the membrane, depolarisation can only occur at the nodes of Ranvier in the gaps between Schwann cells. This means that the impulse jumps from node to node and is known as saltatory conduction and greatly increases the speed of impulses travelling across the axon. This uses less energy as less ATP is used by active transport of the Na+/K+ pump during the refractory period. 72 AQA A Level Biology Paper 2/3 responses and multiple impulses may be combined to form a single response due to these features: Unidirectionality- synapses can only pass the impulse in one direction as the transmitter is only released by the presynaptic knob and receptors are only doing on postsynaptic knob. Summation - if the stimulus is weak not enough neurotransmitter is released to generate an action potential by reaching the threshold but impulses can be combined by summation to build up enough neurotransmitter to generate and action potential. There are two types: Spatial summation is where a number of different presynaptic neurones release enough neurotransmitter at the same time to cause enough depolarisation to exceed the threshold of the post synaptic neurone and trigger an action potential Temporal summation- a single presynaptic neurone releases small amounts of neurotransmitter many times in a short period which add up and cause enough depolarisation to exceed the threshold and trigger an action potential in the post synaptic neurone. Inhibition - on the postsynaptic membranes on some neurones there are chloride ion channels which can be opened when activated by a certain neurotransmitter. This causes chloride ions to flood into the postsynaptic knob and make it more negative than at rest ( hyperpolarisation). This means the membrane can't depolarise enough to trigger an action potential so they are known as inhibitory synapses. 75 AQA A Level Biology Paper 2/3 Excitatory and inhibitory neurotransmitters - Answer: Excitatory neurotransmitters depolarise the postsynaptic membrane causing an action potential to be triggered if the threshold is reached. Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane preventing an action potential being triggered. Effect of drugs on synapse - Answer: Some drugs have a similar shape as neurotransmitters so they mimic their action at receptors and more receptors are activated. Some drugs block receptors so they can't be activated by neurotransmitters so fewer receptors are activated. Some drugs inhibit the enzyme that breaks down the neurotransmitter so there is more neurotransmitter in the synaptic cleft to bind to receptors. Some drugs stimulate the release of excess neurotransmitter from the presynaptic neurone so more receptors are activates. Some drugs inhibit the release of neurotransmitter from the presynaptic neurone so fewer receptors are activated. 3 types of muscle - Answer: Skeletal muscle - can be voluntarily controlled and contracts rapidly when stimulated by nerves , act as effectors , fatigues rapidly and is attached to bones via inelastic tendons and when the muscle contracts it pulls on the skeleton causing the bone to move. Smooth muscle - cannot he voluntarily controlled and is found in walls of tubular organs such as arteries and the gut. It contacts slowly and doesn't fatigue. Cardiac muscle - doesn't fatigue and is only found in the heart 76 AQA A Level Biology Paper 2/3 Antagonistic pairs of muscles - Answer: Skeletal muscles are attached to bones by tendons and ligaments attach bones to other bones. Pairs of skeletal muscles contact where they shorten and relax where they return to their normal length to move bones at a joint. Muscles that work together to move a bone are called antagonistic pairs and are pressed against an incompressible skeleton so act as levers. The contacting muscle is called an agonist and the relaxing muscle is called an antagonist. Structure of a skeletal muscle - Answer: Skeletal muscle is made up of large bundles of muscle fibres/cells bound together by connective tissue which is continuous with the tendons which connect muscle to bone. The cell membrane of muscle fibres is called the sarcolemma which folds inwards and stick into the sarcoplasm. These folds are called transverse (T) tubules and help spread electrical impulses throughout the sarcoplasm so they reach all parts of muscle fibre. The sarcoplasm contains many mitochondria to provide ATP needed for muscle contraction. Muscle fibres are multinucleate and contains internal membranes called sarcoplasmic reticulum that stores and releases calcium ions needed for muscle contraction. Within each muscle fibre there are numerous myofibrils which are composed of repeated sarcomeres. Structure of a myofibril - Answer: Myofibril consist of bundles of thick and thin filaments that move past each other making the muscles contract. The thick filaments are made up of the protein myosin and the thin filaments are made up of the protein actin. Viewed under a microscope there are repeated dark and light 77 AQA A Level Biology Paper 2/3 molecule of ATP attaches to myosin head and causes cross bridges to break so the myosin head detached and then reattaches with ADP and Pi to a different binding site further along the action filament and a new cross bridge is formed and the cycle is repeated. Many cross bridges form and break very rapidly pulling the actin filament along which shortens the sarcomeres causing muscle contraction. Calcium ions activate the enzyme ATP hydrolase which hydrolyses ATP into ADP and Pi to provide energy needed for myosijbhesd to return to its original position. Muscle relaxation - Answer: When the muscle stops being stimulated calcium ions leave the binding sites and are actively transported back into the sarcoplasmic reticulum which requires ATP. The causes tropomyosin to block the binding sites on actin again so the myosin head can't attach to actin and muscle contraction cannot occur and causes actin filaments to slide back into their relaxed position lengthening the sarcomere. Roles of tropomyosin in myofibril contraction - Answer: When the muscle is relaxed the tropomyosin blocks the myosin binding sites on actin filaments and when they are moved this exposes the binding sites so cross bridges can form. Roles of calcium ions in myofibril contraction - Answer: The opening of calcium ion channels in the sarcoplasmic reticulum allows calcium ions to rapidly diffuse into the sarcoplasm. This causes the tropomyosin molecules that were blocking the myosin binding sites on actin filaments to move away and expose the binding sites for cross bridges to form. Calcium ions also active ATP hydrolase in the myosin head. 80 AQA A Level Biology Paper 2/3 Roles of actin and myosin in myofibril contraction - Answer: Myosin heads bind to binding sites on actin filaments forming a cross bridge myosin heads change their position/angle, pulling the actin filaments along which is known as the power stoke. The myosin head detached and then reattaches further along the action filament which repeats the cycle. Roles of ATP in myofibril contraction - Answer: Binding of ATP to the myosin head allows it to detach from the actin filament. ATP hydrolysis provides the energy for the myosin head to return to its original position, the power stroke ( allows myosin heads to change angle and pull actin) and the formation and breaking of actinomyosin cross bridges. Roles of the sarcoplasmic reticulum in myofibril contraction - Answer: Contains high concentration of calcium ions in relaxed muscle and upon stimulation gated calcium ion channels in the sarcoplasmic reticulum open allowing calcium ions to diffuse rapidly into the sarcoplasm. Roles of ATP and phosphocreatine in muscle contraction - Answer: Energy is needed for muscles to contract and ATP gets used up very quickly so it has to be continually generated in three main ways: Aerobic respiration- most ATP is generated via oxidative phosphorylation in the cells mitochondria and only works when there is oxygen present so is good for long periods of low intensity exercise. 81 AQA A Level Biology Paper 2/3 Anaerobic respiration - ATP is made rapidly by glycolysis but produces less energy than aerobic respiration and producers lactic acid as a waste product and cause muscle fatigue. So this is good for short periods of high intensity exercise. Phosphocreatine system- ATP is resynthesised using the phosphate and the energy released from splitting phosphocreatine into creatine so ADP is phosphorylated. PCr is stored in muscle cells and generates ATP rapidly. PCr runs out after a short period of time during short bursts of vigorous exercise and then can be resynthesised when then muscle is relaxed. PCr + ADP -> Cr + ATP Features of slow twitch fibres - Answer: Long term steady movements e.g endurance events Small diameter of fibres More capillaries More mitochondria Slow contraction speed Slow rate of pumping calcium ions ATP hydrolysed slowly in myosin heads Oxidative phosphorylation through aerobic respiration is the main source of ATP Low glycogen content as glucose arrives via the blood and is used in aerobic respiration. High myoglobin content Slow rate of fatigue 82