AQA A-level Biology paper 1 Mark scheme, Exams of Applied Biology

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AQA A-level Biology paper 1 Mark scheme

large molecules often contain carbon. why? - Correct answer they can readily form bonds with other carbon atoms. this forms a 'backbone'. other atoms can then attach. describe benedicts test for reducing sugars - Correct answer add equal volumes of the sugar sample and benedicts reagent. heat the mixture in a water bath for 5 minutes. brick red precipitate will form. what is a reducing sugar? - Correct answer a sugar that can reduce (give electrons to) another chemical. describe the benedicts test for non reducing sugars - Correct answer with food sample, add an equal volume of dilute hydrochloric acid in a test tube. place test tube in water bath for 5 mins. add sodium hydrogencarbonate solution. heat resulting solution with an equal volume of benedicts. solution for 5 minutes. turns orange/brown. what does dilute hydrochloric acid do in the benedicts test for non reducing sugars? - Correct answer it hydrolyses any disaccharide into monosaccharides, thus also forming reducing sugars. describe the structure of starch - Correct answer chains of alpha glucose some chains are branched= amylopetcin some chains are coiled= amylose

what purpose does the structure of amylose serve? - Correct answer makes the molecule compact so it is stored more easily. what purpose does the structure of amylopectin serve? - Correct answer more surface area for enzymes to work on, thus glucose molecules can be released at a much quicker rate. how is the structure of starch suited to its function? - Correct answer 1. starch is insolube so it doesnt affect water potential

2. it is large enough that it doesnt diffuse out of cells
3. compact enough so that a lot of it can be stored in a small space
4. branched= each end can simultaneously be acted on by enzymes, so glucose is quickly released how is the structure of glycogen suited to its function? - Correct answer 1. insoluble, so no effect on water potential of cell
5. insoluble, so doesnt diffuse out of cells
6. compact, a lot of it can be stored in a small space
7. very highly branched= each end can simultaneously be acted on by enzymes, so glucose is quickly released why do animals need highly branched glycogen? - Correct answer because animals metabolic rate is faster than plants and so needs glucose to be released quicker. describe the structure of cellulose - Correct answer made up of straight chains of beta glucose the chains are parallel to each other, so H bonds form cross-linkages all the H bonds together strengthens the cellulose forms microfibrils

draw alpha and beta glucose - Correct answer how does the structure of triglycerides relate to their properties? - Correct answer 1. triglycerides have a high ratio of energy-storing carbon-hydrogen bonds to carbon atoms, so its a good source of energy.

2. low mass to energy ratio, so theyre good energy storage molecules.
3. large and non-polar, therefore insoluble, therefore don't affect osmosis.
4. high ratio of hydrogen to oxygen atoms, so can release water when oxidised, hence provide a source of water what are the roles of lipids? - Correct answer 1. source of energy
5. waterproofing
6. insulation
7. protection how does the structure of phospholipids relate to their properties? - Correct answer 1. polar, therefore will position itself to form a bilayer
8. the structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. these are important in cell recognition. draw the structure of an amino acid - Correct answer draw how a dipeptide forms - Correct answer what is meant by 'primary sequence' of a protein - Correct answer The unique sequence of amino acids that make up a protein or polypeptide chain what is meant by 'secondary sequence' of a protein - Correct answer The way in which the primary structure of a polypeptide chain folds e.g. alpha helix or beta pleated sheets. shape is held by H bonds.

what is meant by 'tertiary sequence' of a protein - Correct answer This is the overall 3-D structure of the protein. The shape of the protein is held together by H bonds and ionic bonds and disulfide bridges. what is meant by 'quaternary sequence' of a protein - Correct answer If a protein is made up of several polypeptide chains that are linked in various ways, the way they are arranged is called the quaternary structure. describe the test for proteins - Correct answer add equal volumes of sample and sodium hydroxide to a test tube. add a few drops of dilute copper (II) sulfate and mix. if solution turns purple, protein is present. what are fiberous proteins made up of? - Correct answer made of long molecules arranged to form fibres (e.g. in keratin). Several helices may be wound around each other to form very strong fibres. what are globular proteins made up of? - Correct answer made of chains folded into a compact structure. One of the most important classes are the enzymes. Although these folds are less regular than in a helix, they are highly specific and a particular protein will always be folded in the same way. what kind of protein is collagen? - Correct answer fiberous what kind of protein is haemoglobin? - Correct answer globular what is haemoglobin made up of? - Correct answer two α polypeptide chains two β polypeptide chains an inorganic prosthetic haem group (Fe2+)

what is collagen made up of? describe its structure? - Correct answer three polypeptide chains wound around each other each of the three chains is a coil itself Hydrogen bonds form between these coils Collagen molecules form further chains with other collagen molecules and form Covalent Cross Links with each other, which are staggered along the molecules to further increase stability. Collagen molecules wrapped around each other form Collagen Fibrils which themselves form Collagen Fibres. what are the functions of collagen? - Correct answer 1. Form the structure of bones

2. Makes up cartilage and connective tissue
3. Prevents blood that is being pumped at high pressure from bursting the walls of arteries
4. Is the main component of tendons, which connect skeletal muscles to bones what type of bond forms between the phosphate group and deoxyribose sugar in DNA? - Correct answer phosphodiester bond why is DNA stable? - Correct answer 1. the phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
5. many hydrogen bonds link the base pairs together, so it adds strength overall. how is DNA adapted to carrying out its function? - Correct answer 1. it is very stable, thus rarely mutates
6. the H bonds can easily be separated during DNA replication and protein synthesis
7. extremely large, thus carries a lot of genetic information
8. base pairing allows DNA to replicate and transfer info as mRNA.

describe semi conservative replication - Correct answer - DNA helicase unwinds separates the two strands of DNA into single strands by breaking the H bonds between the bases

• this therefore forms 2 base templates (the bases on each strand are exposed)
• free complimentary nucleotides align with the bases according to complimentary base pairing
• they are joined via condensation reaction with DNA polymerase
• the new DNA molecule is formed what is meant by resolution - Correct answer the minimum distance apart that two objects can be in order for them to appear as separate items. before cell fractionation, why is the tissue placed in a cold, isotonic buffer? - Correct answer cold: reduce enzyme activity that may break down the organelles isotonic: same water potential as tissue, this prevents organelles shrinking/bursting due to osmotic loss/gain of water buffered: so the pH doesn't fluctuate, so that nothing alters the organelle structures or affects enzyme functioning what are the advantages of an electron microscope? - Correct answer electron beam has a short wavelength and the microscope can therefore has high resolution. the negatively charged beam can be focused using electromagnets. what are the limitations of using a transmission electron microscope? - Correct answer the whole system must be in vacuum, so living specimens cant be studied. complex staining process the image may contain artefacts

what is the function of SER? - Correct answer synthesises, stores and transports lipids and carbohydrates what is the function of RER? - Correct answer provides a large surface area to synthesise proteins and glycoproteins provides a transport pathway for materials throughout the cell what is the function of golgi apparatus? - Correct answer add carbs to proteins to form glycoproteins produce secretory enzymes secrete carbohydrates transport, modify and store lipids form lysosomes what is meant by 'organ'? - Correct answer a combination of tissues that are coordinated to perform a variety of functions. describe binary fission - Correct answer the circular DNA molecule replicates and both copies attach to the cell membrane. plasmids also replicate cell membrane grows between the two DNA molecules, dividing the cytoplasm into two. a new cell wall forms between the two DNA molecules. two daughter cells are produced, each with a single copy of circular DNA and a variable number of plasmids. how do viruses replicate? - Correct answer they attach to their host cell with their attachment proteins. nucleic acids are injected into host cell. the genetic info contains "instructions" for the host cells metabolic processes to start producing the viral components, which are assembled into new viruses.

describe cell mediated immunity - Correct answer 1. phagocytes injest pathogens.

2. the phagocyte places antigens from pathogen on its cell surface membrane, so it becomes an antigen presenting cell.
3. receptors of T-helper cells have a complimentary fit to these antigens.
4. this attachment activates T cells to divide rapidly by mitosis and form a clone of genetically identical cells.
5. the cloned T cells: a. develop into memory cells b. stimulate phagocytosis c. stimulate B cells to divide and secret their antibody d. activate cytotoxic T cells. describe humoral immunity - Correct answer 1. the surface antigen of a pathogen is taken up by B cell
6. the B cell processes it and presents the antigen on its surface
7. T-helper cells attach to the processed antigens, thus activating the B cell
8. B cells divide by mitosis to produce a clone of plasma cells
9. these secrete the specific antibody that can bind to the pathogens antigen
10. the antigens destroy the pathogen
11. some B cells develop into memory cells what is passive immunity and how is it produced? - Correct answer produced by the introduction of antibodies into individuals from an outside source. immunity is acquired immediately. no contact with pathogen/s needed. short term immunity: antibodies aren't replaced when broken down, memory cells not formed. e.g. anti-venom

what is active immunity and how is it produced? - Correct answer produced by stimulating the production of antibodies by the individuals own immune system. contact with pathogen/s needed. immunity takes time to develop natural active immunity - Correct answer results from an individual becoming infected with a disease under normal circumstances. long term body produces its own antibodies artificial active immunity - Correct answer involves inducing an immune response in an individual WITHOUT them suffering the symptoms of the disease. e.g. vaccinations memory cells produced what are the features of a successful vaccination programme? - Correct answer 1. vaccine must be economically available in sufficient quantities

2. must have few side effects
3. available means of producing, storing and transporting the vaccine
4. must be possible to produce herd immunity why can vaccinations not be effective? - Correct answer 1. immunity may not be induced in people with defective immune systems
5. individuals may develop disease after the vaccination but before their immunity levels are high enough to prevent it.
6. pathogen may mutate frequently, so vaccines cant prevent them from harming people
7. certain pathogens conceal themselves from the body's immune system e.g. by hiding within cells

5. individuals may object to vaccinations due to religious, ethical or medical reasons what are antibodies made up of? - Correct answer 4 polypeptide chains:

• 2 are 'heavy chains'
• 2 are 'light chains' variable (binding) region how does HIV replicate? - Correct answer HIV attaches most commonly to T- helper cells the protein capsid fuses with the cell-surface membrane the RNA and enzymes of HIV enter T-helper cell reverse transcriptase converts the viruses RNA to DNA the new DNA enters nucleus is inserted into T-h cells DNA HIV DNA in nucleus creates mRNA using the cells enzymes, which contains the instructions for making new viral proteins. the viral proteins are assembled into new HIV features of specialised gas exchange surfaces - Correct answer - a large surface area to volume ratio
• very thin therefore short diffusion pathway
• selectively permeable
• a transport system how do insects carry out gas exchange? - Correct answer 1. insects contract their muscles, squeezing the trachea, enabling mass movement.

2. a diffusion gradient is established
3. The ends of tracheoles are in contact with individual cells, contain a small amount of fluid in which the gases are dissolved. The fluid is drawn into the muscle tissue during exercise. This increases the surface area of air in contact

with the cells. Gases diffuse in through the spiracles and down the tracheae and tracheoles. describe and explain the countercurrent exchange principle - Correct answer -the blood and the water flow in opposite directions -blood that is loaded with O2 meets water, which already has maximum O -blood with little O2 meets with water with O2 removed -so, diffusion concentration gradient established and maintained across the entire width of the gill lamellae -so, 80% of O2 in water absorbed into fish blood how does the structure of a plant leaf link to gas exchange? - Correct answer -all cells are close to stomata, and therefore surface, therefore a short diffusion pathway -inter-connecting air spaces in the mesophyll so that gases can readily contact mesophyll cells describe and explain why plant stomata links to gas exchange and its benefits - Correct answer stomata are like minute pores, on the underside of leaves. each has guard cells, which control the rate of gas exchange guard cells close when water loss is excessive how do insects limit water loss? - Correct answer -small surface area to volume ratio -waterproof coverings -spiracles which can close how do xerophytes limit water loss? - Correct answer -thick waxy cuticle -rolled up leaves: traps water vapour -hairy leaves: traps moist air -stomata in pits/grooves: traps moist air

-reduced surface area to volume ratio by trapping air, water potential gradient is reduced describe the process of inspiration - Correct answer external intercostals contract internal intercostals relax ribs move up and outwards, increasing thorax volume diaphragm contracts, further increasing thorax volume air pressure in lungs becomes lower than atmospheric pressure, so air is forced in describe the process of expiration - Correct answer internal intercostals contract external intercostals relax ribs movie in and downwards diaphragm relaxes this decreases thorax volume air pressure in lungs becomes greater than atmospheric pressure, so air is forced out structure of haemoglobin - Correct answer 4 polypeptide chains

• 2 alpha
• 2 beta each chain has a haem group which contains a ferrous Fe2+ ion. what is the vena cava connected to? - Correct answer right atrium what is aorta connected to? - Correct answer left ventricle what is pulmonary artery connected to? - Correct answer right ventricle

what is pulmonary vein connected to? - Correct answer left atrium how is tissue fluid formed - Correct answer high hydrostatic pressure at arterial end due to contractions of ventricles this causes tissue fluid to be forced out of the blood plasma this is ultrafiltration describe the process of lipid digestion - Correct answer in small intestine, bile salts combine with fat droplets and break them down smaller (emulsification). lipase enzymes from pancreas hydrolyse lipids into fatty acids, glycerol and monoglycerides. short chain fatty acids: diffuse directly into blood via epithelial cell longer chain fatty acids: combine with bile salts to form micelles, which break down into fatty acids and monoglycerides which diffuse into epithelial cells. once there, they recombine into triglycerides which can form chylomicrons. artery structure - Correct answer thick muscle layer so that they can dilate and contract to smooth blood flow. thick elastic layer so artery can stretch and recoil with systole and diastole, smoothing blood flow. overall thickness of wall is large so that it doesn't burst under pressure. why do arterioles have thinner elastic layer than arteries? - Correct answer blood pressure is lower. vein structure - Correct answer muscle layer is thinner, because they carry blood away from tissues, and so have less need to dilate. elastic layer is thinner because of lower blood pressure, which doesn't need recoil.

overall thickness of wall is thinner due to low blood pressure. valves at regular intervals to ensure there's no backflow of blood. why do veins have thinner walls than arteries? - Correct answer thinner walls because blood is at lower pressure, and so less stretch and recoil with each contraction of ventricles. capillary structure - Correct answer one cell thick endothelium. allows rapid diffusion. numerous, highly branched, so provides a larger surface area. narrow lumen and diameter. spaces in lining to allow tissue fluid to form. how does water travel into/through plants? - Correct answer the water particles have higher water potential than the root hair cell, and so they enter the cell by osmosis down a water potential gradient. the water potential of the root hair cell is now higher than the cortex cell, and so water enters the cortex cell by osmosis. symplast pathway: water enters root hair cell by osmosis, and travels from the cytoplasm of one cell to another by plasmodesmata apoplast pathway: water enters the root by entering the spaces in cell walls. water moves up xylem due to H bonding in water. water travels until it reaches the casparian strip, where water needs to travel via symplast pathway. what is plasmodesmata? - Correct answer strands of cytoplasm linking one cell to another that passes through pores in cellulose cell walls

explain cohesion-tension theory - Correct answer water is drawn from cell to cell by osmosis. it moves across the root cortex to keep a constant column of water. this creates 'tension' on the column of water in the xylem. water shows cohesion due to its H bonding. this makes the column very strong. so, water is pulled up the xylem to replace the water lost by evaporation. water moves between the cells of a leaf along the water potential gradient. what is the role of companion cells? - Correct answer they load/unload organic solutes into the sieve tube describe the transfer of sucrose into sieve elements - Correct answer sucrose is manufactured from photosynthesising cells. it diffuses by facilitated diffusion from photosynthesising cells to companion cells, down a concentration gradient. H+ actively transported into the spaces between cell walls. H+ diffuses into sieve tube elements. sucrose molecules also transported with them, through co-transport. what is a 'source' cell? - Correct answer cells that photosynthesise and make sugars what is a 'sink' cell? - Correct answer cells that use/store organic solutes explain mass flow hypothesis - Correct answer 1. sugars build up at source, lowering water potential of source cells.

2. water therefore enters cells by osmosis.
3. sugars are then actively transported into sieve tubes by transfer cells
4. this lowers water potential in sieve tube, so water enters by osmosis.
5. this causes high pressure potential at source at sink, sugars are unloaded by companion cells
6. this raises water potential

how can you investigate mass flow using aphids? - Correct answer 1. cut across their mouthparts. the mouthparts act as a sampling tube that they collect the phloem contents.

2. If the phloem is punctured with a hollow tube then the sap oozes out, showing that there is high pressure (compression) inside the phloem (this is how maple syrup is tapped).
3. If the xylem is punctured then air is sucked in, showing that there is low pressure (tension) inside the xylem. how can you investigate transport using radioactive tracers? - Correct answer a plant is supplied with 14C, which becomes incorporated into organic substances in the plant. The plant is then frozen in liquid nitrogen to kill and fix it quickly, and placed onto photographic film in the dark. The resulting autoradiograph shows the location of compounds containing 14C. this shows that organic compounds (presumably sugars) are transported downwards from the leaf to the roots. define genome - Correct answer the complete set of genes in a cell define proteome - Correct answer the full range of proteins that a cell is able to produce describe the process of transcription - Correct answer DNA unwinds and separates (H bond between bases broken) template strands form due to exposed bases free RNA nucleotides align with template strand through complimentary base pairing RNA polymerase bonds the nucleotides together, forming a molecule of mRNA complimentary to the DNA

mRNA leaves nucleus via nuclear pore splicing occurs- introns removed describe the process of translation - Correct answer 1. mRNA associates with a ribosome in the cytoplasm

2. tRNA molecules with complimentary anticodons align with the first 2 mRNA codons
3. peptide bond forms between the amino acid molecules
4. the tRNA that bound to the first codon is free to leave the ribosome
5. the ribosome moves along the mRNA by one codon and now a new tRNA with a complimentary anticodon enters the ribosome
6. this continues until the ribosome reaches a stop codon the 2 mechanisms that mean that meiosis will result in genetic variation - Correct answer independent segregation crossing over what happens during independent segregation? - Correct answer during meiosis 1 each chromosome aligns with its homologous partner at the equator of the cell the combination of maternal/paternal chromosomes aligns randomly one chromosome of each pair goes to daughter cell- by chance what happens during crossing over? - Correct answer when the chromosomes align, the chromatids of each pair become twisted around one another tension is built and portions of chromatids are broken off, which rejoin with the chromatid of the homologous partner so, new genetic combinations of maternal and paternal alleles are produced what is ATP made up of? - Correct answer adenine ribose

3 phosphates how does ATP store energy? - Correct answer has 3 phosphates, the bonds between these are unstable and so have a low activation energy, hence easily broke. when they break, they release energy. what does ATP synthase catalyse? - Correct answer the formation of ATP from ADP in what scenarios is ATP reformed? - Correct answer during photosynthesis, in chlorophyll containing plants during respiration (oxidative phosphorylation) substrate level phosphorylation why is ATP a good immediate energy source in a cell - Correct answer the instability of the phosphate bonds. each ATP molecule releases less energy than glucose molecule, and so is smaller and more manageable the hydrolysis of ATP to ADP is a single reaction that releases immediate energy what energy requiring processes need ATP? - Correct answer metabolic processes movement active transport secretion activation of molecules which has a longer wavelength: light or a beam of electrons? - Correct answer light

what are the two types of electron microscope? - Correct answer transmission electron microscope scanning electron microscope how does a transmission electron microscope work? - Correct answer it has an electron gun which produces a beam of electrons this is focused onto a specimen by condenser electromagnets the beam passes through the specimen some parts of the specimen absorb the electrons and appear dark and vice versa an image is produced why cant the highest resolution images not be produced by TEM? - Correct answer there are difficulties preparing the specimen, which can limit resolution a higher energy electron beam may destroy the specimen how does a scanning electron microscope work? - Correct answer it directs an electron beam onto a specimen surface rather than penetrating it. the beam is passed back and forth across a specimen in a regular pattern, depending on its contours a computer is used to build up a 3D image how to calibrate an eyepiece graticule? - Correct answer use a stage micrometer and align it with the eyepiece graticule. then, calculate the length of each subdivision define ecology - Correct answer the study of the inter-relationships between organisms and their environment, including abiotic and biotic factors. define ecosystem - Correct answer dynamic systems made up of a community and all the non-living factors of its environment. It includes all the living

organisms that interact with one another and also with the physical and non- physical factors present. biotic - Correct answer living abiotic - Correct answer non-living define population - Correct answer a group of individuals that occupy the same habitat at the same time and are potentially able to interbreed. define community - Correct answer all the populations of different species living and interacting in a particular place at the same time. define habitat - Correct answer the place where an organism normally lives define ecological niche - Correct answer The position occupied by an organism in a particular ecosystem, dependent upon the resources it uses. The more resources that are taken into account then the more carefully defined the organism's niche will be, the organism will become more specialised. interspecific competition - Correct answer occurs when individuals of DIFFERENT species compete for resources such as food, light, water etc. intraspecific competition - Correct answer occurs when individuals of the SAME species compete for resources such as foo, light, water etc. predation - Correct answer one organism is consumed by another. explain predation - Correct answer predators eat their prey, reducing prey population with fewer prey left, there is greater competition between predators for food

some cant get the food and die, thereby reducing predator population the prey population therefore increases so, the predator population also increases two ways in which pathogens can cause disease - Correct answer releases toxins damages cell tissue A vaccine can be used to produce immunity. Describe how memory cells are important in this process. (3) - Correct answer important because when individual is in contact with virus again, antibodies can be produced at a faster rate in a secondary immune response, and so the pathogen can be destroyed faster. Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf (3) - Correct answer it enters via stomata, which are opened as guard cells open. if diffuses down a concentration gradient through the air spaces. There are large numbers of mitochondria in the epithelial cell. Explain how these organelles help the cell to absorb the products of digestion. (2) - Correct answer mitochondria provide the ATP this is needed for active transport of substances into the cell red blood cells are biconcave discs. Explain one advantage of a biconcave disc over a spherical cell of the same volume in transporting oxygen. - Correct answer Large (surface) area; For diffusion; Describe how oxygen in air in the alveoli enters the blood in capillaries. - Correct answer diffusion across alveoli epithelium Attaching lactase to the beads is a more efficient use of lactase than adding the lactase

directly to cow's milk. Suggest three reasons why it is more efficient to attach lactase to the beads. - Correct answer lactase bead can be reused continuous process lactase not present in milk what is a vaccine? - Correct answer Contains antigen / proteins / dead / weakened microorganism / pathogen / virus / bacteria; Stimulates production of antibodies / plasma cells / memory cells where is amylase produced? - Correct answer salivary glands, pancreas, small intestine where is maltase produced? - Correct answer epithelium of small intestine where is lactose produced? - Correct answer pancreas, small intestine where are proteases produced? - Correct answer stomach, pancreas, small intestine describe the involvement of RER in producing chylomicrons - Correct answer RER synthesises proteins in the ribosomes, which are needed to form chylomicrons. suggest how chylomicrons leave the epithelial cell. - Correct answer by exocytosis because theyre too large to leave by other methods whats the difference between species richness and diversity index? - Correct answer Species richness measures only number of (different) species / does not measure number of individuals.

directional selection - Correct answer selection favours individuals in one direction from the mean of the population. e.g. when environment changes, those with advantageous allele outside of the mean will be more likely to survive and breed. stabilising selection - Correct answer if environmental conditions are the same, those with alleles closer to the mean are favored. they are more likely to survive and reproduce e.g. birth weight. courtship behaviour results in reproductive success by... - Correct answer 1. allowing recognition of own species

2. identifying those capable of breeding
3. forming a pair bond
4. synchronising mating