AQA A - LEVEL BIOLOGY PAPER 3 TOPIC 1 - 8 EXAM GUIDE QNS & ANS JUNE 2024, Exams of Biology

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A - Level

BIOLOGY

Paper 3 June 2024

TOPIC 1 – 8 EXAM GUIDE

QNS & ANS

1. Describe the structure of a water molecule and explain how its properties are conducive to its role in biological organisms. Answer: A water molecule has a bent shape with one oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom is more electronegative, creating a polar molecule with a partial negative charge near the oxygen and a partial positive charge near the hydrogens. This polarity allows water to form hydrogen bonds, making it an excellent solvent for ionic and other polar molecules. It also contributes to water's high specific heat, surface tension, and cohesion, all of which are essential for life.
2. Explain the fluid mosaic model of cell membranes and its significance in the transport of substances. Answer: The fluid mosaic model describes the cell membrane as a fluid, dynamic structure with various proteins embedded in or attached to a bilayer of phospholipids. The phospholipids have hydrophilic heads and hydrophobic tails, creating a semi- permeable barrier. Proteins serve functions such as transport, signal transduction, and cell recognition, allowing cells to exchange substances with their environment selectively.
3. Discuss how the structure of DNA relates to its function of storing genetic information. Answer: DNA's structure is a double helix with two strands running in opposite directions, composed of nucleotides

containing a phosphate group, a sugar (deoxyribose), and a nitrogenous base. The sequence of bases (adenine, thymine, cytosine, and guanine) encodes genetic information. Base pairing (A with T, C with G) through hydrogen bonds allows for replication and transcription, ensuring genetic information is accurately passed on.

4. Compare and contrast aerobic and anaerobic respiration in terms of energy transfer within cells. Answer: Aerobic respiration occurs in the presence of oxygen and involves glycolysis, the Krebs cycle, and the electron transport chain, resulting in the production of up to 36 ATP molecules per glucose molecule. Anaerobic respiration, in the absence of oxygen, results in less efficient energy transfer, producing only 2 ATP molecules per glucose molecule through glycolysis, followed by fermentation to regenerate NAD+.
5. Analyze the impact of gene mutations on protein synthesis and resulting phenotypic effects. Answer: Gene mutations can alter the sequence of nucleotides in DNA, potentially leading to changes in the amino acid sequence of proteins. This can affect protein structure and function, resulting in various phenotypic effects. For example, a missense mutation may result in a non-functional enzyme, while a nonsense mutation can lead to a truncated protein, both of which can have significant impacts on an organism's traits.
6. Evaluate the role of homeostasis in maintaining optimal internal conditions for enzyme activity. Answer: Homeostasis involves regulatory mechanisms that maintain a stable internal environment, crucial for optimal enzyme activity. Enzymes have specific temperature and pH ranges where they function best. Homeostatic processes, such as thermoregulation and pH balance, ensure that these conditions are met, allowing enzymes to catalyze reactions efficiently and maintain metabolic processes.
7. Discuss the significance of meiosis in generating genetic diversity through independent assortment and crossing over. Answer: Meiosis is a type of cell division that reduces the chromosome number by half, resulting in genetically unique gametes. Independent assortment and crossing over during

meiosis contribute to genetic diversity. Independent assortment refers to the random distribution of maternal and paternal chromosomes to gametes, while crossing over involves the exchange of genetic material between homologous chromosomes, creating new allele combinations.

8. Explain how natural selection can lead to evolution, providing an example of a trait that has evolved in a population. Answer: Natural selection is a mechanism of evolution where individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation. Over time, this can lead to changes in the population's genetic makeup. For example, the peppered moth's coloration evolved from light to dark in polluted areas to better camouflage against predators, demonstrating natural selection.
9. Describe the process of transcription and its role in gene expression. Answer: Transcription is the process by which a segment of DNA is copied into RNA by the enzyme RNA polymerase. During transcription, the DNA double helix unwinds, and one strand serves as a template for synthesizing a complementary RNA strand. This RNA, particularly mRNA, carries the genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm, where it directs protein synthesis.
10. Analyze the effects of a change in an ecosystem on the energy flow between organisms. Answer: Changes in an ecosystem, such as the introduction or removal of a species, can significantly impact energy flow. For instance, removing a top predator can lead to an increase in herbivore populations, which may overconsume vegetation, reducing the energy available to other organisms. Conversely, introducing a new species can disrupt existing food chains, altering the distribution and quantity of energy transfer.
11. Discuss the role of ATP in cellular processes and why it is considered the energy currency of the cell. Answer: ATP (adenosine triphosphate) is a molecule that stores and transfers energy within cells. It releases energy when its terminal phosphate bond is broken, which is then used to power various cellular processes such as muscle contraction, active

transport, and chemical synthesis. Its ability to quickly release energy makes it an efficient energy currency for the cell.

12. Explain how the nervous system and endocrine system interact to respond to external stimuli. Answer: The nervous system and endocrine system work together to maintain homeostasis and respond to external stimuli. The nervous system detects changes and sends rapid electrical signals to effectors, while the endocrine system releases hormones into the bloodstream, causing slower but longer-lasting effects. For example, in the fight-or-flight response, the nervous system triggers immediate reactions, and the endocrine system sustains the response by releasing adrenaline.
13. Evaluate the importance of genetic variation in a population for survival and evolution. Answer: Genetic variation is crucial for a population's survival and evolution as it provides the raw material for natural selection. Populations with greater genetic diversity have a better chance of having individuals with traits that can adapt to changing environments, resist diseases, and survive predators. This variation increases the likelihood that some individuals will survive and reproduce, ensuring the population's long-term viability.
14. Describe the process of osmosis and its significance in the movement of water across cell membranes. Answer: Osmosis is the passive movement of water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. This process is vital for maintaining cell turgor, regulating cell volume, and transporting nutrients and waste products. It also plays a role in the absorption of water in plant roots and the kidneys' reabsorption of water.
15. Explain the concept of an ecological niche and how it relates to species competition and coexistence. Answer: An ecological niche is the role and position a species has in its environment, including its habitat, resources, and relationships with other organisms. It encompasses all the factors necessary for a species' survival and reproduction.

Species with different niches can coexist without direct competition, while those with overlapping niches may compete for resources, potentially leading to competitive exclusion or niche differentiation.

1. Describe the structure and function of DNA and RNA. How do they differ from each other? Answer: DNA is a double-stranded helix that stores genetic information, while RNA is single-stranded and plays a role in protein synthesis.
2. Explain the process of transcription and translation in gene expression. Answer: Transcription is the synthesis of RNA from a DNA template, while translation is the process of synthesizing proteins using the mRNA template.
3. What is the significance of genetic variation in evolution and adaptation? Answer: Genetic variation allows for diversity within a population, which is essential for adaptation to changing environments and evolution.
4. Discuss the role of enzymes in biological reactions, including their structure and function. Answer: Enzymes are proteins that act as catalysts, speeding up chemical reactions by lowering the activation energy required.
5. How do cells exchange substances with their environment? Discuss the processes of diffusion, osmosis, and active transport. Answer: Cells exchange substances through diffusion (movement of molecules from high to low concentration), osmosis (movement of water across a selectively permeable membrane), and active transport (movement of molecules against a concentration gradient with the use of energy).
6. Describe the structure and function of organelles within a eukaryotic cell, such as the nucleus, mitochondria, and endoplasmic reticulum. Answer: The nucleus contains the cell's genetic material,

mitochondria produce ATP through cellular respiration, and the endoplasmic reticulum is involved in protein synthesis and lipid metabolism.

7. Explain the role of ATP in cellular energy transfer and metabolism. Answer: ATP is the primary energy currency in cells and is used to power cellular processes through the hydrolysis of its phosphate bonds.
8. How do organisms respond to changes in their external environment? Provide examples of both external and internal stimuli. Answer: Organisms respond to changes in their environment through behaviors (e.g., seeking shelter during a storm) and physiological responses (e.g., sweating to cool down).
9. Discuss the role of mitosis and meiosis in cell division and genetic variation. Answer: Mitosis is the process of cell division for growth and repair, while meiosis is involved in the formation of gametes and generates genetic diversity through recombination.
10. How do hormones regulate gene expression and coordinate physiological processes within an organism? Answer: Hormones are chemical messengers that bind to specific receptors on target cells to regulate gene expression and coordinate functions such as growth, metabolism, and reproduction.
11. Describe the process of photosynthesis in plants, including the role of chloroplasts, light, and carbon dioxide. Answer: Photosynthesis is the process by which plants convert light energy into chemical energy (glucose) using chloroplasts, light, and carbon dioxide.
12. How do organisms obtain and transfer energy through food chains and food webs? Provide examples of producers, consumers, and decomposers. Answer: Energy flows through ecosystems via food chains and webs, with producers (plants) converting sunlight into energy, consumers (animals) eating producers, and decomposers

breaking down organic matter.

13. Explain the process of DNA replication, including the roles of DNA polymerase, primase, helicase, and ligase. Answer: DNA replication is the process of copying a DNA molecule, with DNA polymerase synthesizing new strands, helicase unwinding the DNA helix, primase creating RNA primers, and ligase sealing the gaps.
14. Discuss the impact of mutations on genetic variation and inheritance. Provide examples of different types of mutations. Answer: Mutations are changes in the DNA sequence that can lead to genetic variation and may impact an organism's phenotype. Examples include point mutations (substitution, insertion, deletion), frameshift mutations, and chromosomal abnormalities.
15. How do organisms adapt to changes in their environment through natural selection and evolution? Provide examples of adaptations in different species. Answer: Natural selection favors traits that increase an organism's fitness and survival in a specific environment, leading to adaptations such as camouflage, mimicry, and specialized body structures.
16. Describe the structure and function of carbohydrates, lipids, proteins, and nucleic acids in biological systems. Answer: Carbohydrates provide energy and structural support, lipids serve as energy storage and cell membranes, proteins perform various functions as enzymes, receptors, and antibodies, and nucleic acids store and transmit genetic information.
17. Analyze the process of cellular respiration in eukaryotic cells, including glycolysis, the Krebs cycle, and the electron transport chain. Answer: Cellular respiration is the process of converting glucose into ATP by glycolysis (cytoplasm), the Krebs cycle (mitochondria), and the electron transport chain (mitochondrial membrane).
18. Discuss the differences between sexual and asexual

reproduction in organisms. How do each contribute to genetic variation? Answer: Sexual reproduction involves the fusion of gametes from two parents, leading to genetic variation through recombination, while asexual reproduction produces offspring genetically identical to the parent.

19. How do environmental factors influence gene expression and phenotype in organisms? Provide examples of both internal and external factors. Answer: Environmental factors such as temperature, light, diet, and stress can impact gene expression and phenotypic traits in organisms, influencing growth, development, and behavior.
20. Explain the process of protein synthesis, including transcription, mRNA processing, translation, and post- translational modifications. Answer: Protein synthesis involves transcription (DNA to mRNA), mRNA processing (splicing, capping, polyadenylation), translation (mRNA to protein), and post-translational modifications (folding, cleavage, glycosylation).
21. Describe the structure and function of cell membranes, including the roles of phospholipids, proteins, and carbohydrates in membrane transport and signaling. Answer: Cell membranes consist of a lipid bilayer with embedded proteins and carbohydrates, which regulate the passage of molecules into and out of the cell and mediate cell- cell communication.
22. Analyze the process of DNA profiling and its applications in forensic science, paternity testing, and genetic disease diagnosis. Answer: DNA profiling uses techniques such as PCR and gel electrophoresis to analyze DNA sequences for identification, paternity testing, and diagnosing genetic disorders.
23. Discuss the role of feedback mechanisms in maintaining homeostasis in organisms, such as temperature regulation and blood glucose levels. Answer: Feedback mechanisms involve negative (reversing a change) and positive (amplifying a change) feedback loops to

maintain internal balance and stability (homeostasis) in organisms.

24. Explain the process of meiosis and its significance in sexual reproduction, gamete formation, and genetic diversity. Answer: Meiosis is the cellular division that produces haploid gametes (sperm and eggs) for sexual reproduction, reshuffling genetic material through crossing over and independent assortment.
25. How do changes in the environment, such as pollution, deforestation, and climate change, impact ecosystems and biodiversity? Answer: Environmental changes can disrupt ecosystems, leading to habitat loss, species extinctions, and altered biogeochemical cycles, affecting biodiversity and ecosystem stability.
26. Describe the process of natural selection and adaptation in response to changes in the environment. How does this process drive evolution? Answer: Natural selection acts on heritable traits that confer advantages in specific environments, leading to adaptation and the accumulation of genetic changes over generations (evolution).
27. Discuss the role of mutagens, carcinogens, and DNA repair mechanisms in maintaining genetic stability and preventing mutations in cells. Answer: Mutagens (chemicals, radiation), carcinogens (cancer- causing agents), and DNA repair mechanisms (proofreading, excision repair) play a role in protecting cells from mutations and genomic instability.
28. Analyze the process of gene regulation and protein expression in cells, including transcription factors, enhancers, promoters, and repressors. Answer: Gene regulation involves the control of gene expression through regulatory proteins (transcription factors), enhancers, promoters, and repressors, modulating the synthesis of specific proteins.

29. How do cells communicate and coordinate physiological processes through cell signaling pathways, such as hormones, neurotransmitters, and growth factors? Answer: Cell signaling involves the transmission of signals (hormones, neurotransmitters) through receptors and intracellular pathways to regulate cell growth, differentiation, and responses to stimuli.
30. Explain the principles of genetic inheritance, including Mendel's laws of segregation and independent assortment. How do these principles apply to human traits? Answer: Mendel's laws describe the inheritance of traits through genes and genetic crosses, with segregation (alleles separate during gamete formation) and independent assortment (genes sort independently) influencing genetic diversity and inheritance patterns in humans.
31. Describe the process of DNA transcription, including the roles of RNA polymerase, promoters, enhancers, and transcription factors. Answer: Transcription is the synthesis of RNA from a DNA template by RNA polymerase, with promoters (regulatory regions), enhancers (DNA elements), and transcription factors (regulatory proteins) determining gene expression.
32. Discuss the role of epigenetics in regulating gene expression and phenotypic variation, including DNA methylation, histone modifications, and non-coding RNAs. Answer: Epigenetics involves heritable changes in gene expression without altering the DNA sequence, influenced by factors such as DNA methylation, histone modifications, and non-coding RNAs.
33. Analyze the process of cell division in eukaryotic cells, including the phases of mitosis and meiosis, chromosome structure, and cell cycle regulation. Answer: Cell division involves mitosis (somatic cells) for growth and repair and meiosis (germ cells) for gamete formation and genetic diversity, regulated by checkpoints and cyclin- dependent kinases.
34. How do biotechnological techniques, such as genetic

engineering, cloning, and CRISPR-Cas9, impact agriculture, medicine, and biotechnology? Answer: Biotechnological techniques allow for the manipulation of genetic material in organisms, leading to advancements in agriculture (GM crops), medicine (gene therapy), and biotechnology (gene editing).

35. Describe the process of protein folding and post- translational modifications, including chaperones, disulfide bonds, glycosylation, and phosphorylation. Answer: Protein folding involves the correct 3D structure formation, assisted by chaperones, disulfide bonds, and post- translational modifications (glycosylation, phosphorylation) that modify protein function and stability.
36. Discuss the impacts of genetic mutations on protein structure and function, including missense, nonsense, and frameshift mutations. Answer: Genetic mutations can alter the amino acid sequence of a protein, resulting in changes to its structure and function, such as missense (substitution), nonsense (premature stop), and frameshift (insertion/deletion) mutations.
37. How do organisms maintain genetic diversity and adaptability through sexual reproduction, genetic recombination, and gene flow? Answer: Genetic diversity is maintained through sexual reproduction, genetic recombination (crossing over), and gene flow (exchange of alleles between populations), enhancing adaptability to changing environments.
38. Explain the process of DNA repair mechanisms, such as base excision repair, nucleotide excision repair, and mismatch repair, in preserving genome integrity. Answer: DNA repair mechanisms remove or correct damaged DNA bases, lesions, or mismatches caused by environmental factors or errors in replication, preserving genome stability and preventing mutations.
39. Compare and contrast prokaryotic and eukaryotic cells in terms of cell structure, organelles, genetic material, and metabolic processes.

Answer: Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have a nucleus, organelles, and linear chromosomes, driving differences in cell structure, function, and metabolic pathways.

40. How do organisms respond to changes in their internal and external environments through feedback mechanisms, homeostasis, and adaptive behaviors? Answer: Organisms use feedback mechanisms, such as hormones and the nervous system, to regulate internal conditions (temperature, pH, glucose) and adapt to external stimuli (stress, predators), maintaining homeostasis and survival in changing environments. Cells Question: Explain the role of the cell membrane in maintaining cell homeostasis. Answer: The cell membrane controls the movement of substances in and out of the cell, maintaining internal balance and facilitating communication with the external environment. Organisms Exchange Substances with Their Environment Question: Compare and contrast the mechanisms of diffusion and osmosis in cells. Answer: Diffusion is the passive movement of molecules from high to low concentration, while osmosis specifically refers to the movement of water molecules across a selectively permeable membrane. Genetic Information, Variation, and Relationships Between Organisms Question: Discuss the significance of meiosis in generating genetic diversity. Answer: Meiosis is a cell division process that produces haploid cells, contributing to genetic variation through independent assortment and crossing over. Energy Transfers in and Between Organisms Question: Illustrate the process of ATP synthesis and its role in cellular energy transfer. Answer: ATP synthesis occurs in the mitochondria and

involves the conversion of ADP and inorganic phosphate into ATP, providing energy for cellular processes. Organisms Respond to Changes in Their Internal and External Environments Question: Explain how the nervous system coordinates responses to stimuli in the human body. Answer: The nervous system detects stimuli, processes information, and transmits signals to effectors to produce responses, ensuring homeostasis. Genetics Question: Define the terms genotype and phenotype and provide an example of each. Answer: Genotype refers to an organism's genetic makeup, while phenotype is the observable characteristics resulting from the genotype, such as eye color or height. The Control of Gene Expression Question: Describe the role of transcription factors in gene regulation. Answer: Transcription factors bind to specific DNA sequences, influencing the initiation of transcription and regulating gene expression in response to internal and external signals. Biological Molecules Question: Investigate the structure and function of lipids in living organisms. Answer: Lipids are diverse molecules that serve as energy storage, structural components of cell membranes, and signaling molecules in cellular processes. Cells Question: Compare and contrast prokaryotic and eukaryotic cells in terms of structure and function. Answer: Prokaryotic cells lack a nucleus and membrane- bound organelles, while eukaryotic cells have a nucleus and various organelles with specialized functions. Organisms Exchange Substances with Their Environment Question: Analyze the role of active transport in maintaining

ion concentrations in nerve cells. Answer: Active transport uses energy to move ions against their concentration gradient, ensuring proper ion balance for nerve cell function. Genetic Information, Variation, and Relationships Between Organisms Question: Discuss the process of DNA replication and its significance in passing genetic information to daughter cells. Answer: DNA replication involves the synthesis of two identical DNA strands from a parent molecule, ensuring genetic continuity and transmission of traits. Energy Transfers in and Between Organisms Question: Evaluate the efficiency of aerobic respiration compared to anaerobic respiration in producing ATP. Answer: Aerobic respiration generates a significantly higher amount of ATP per glucose molecule than anaerobic respiration, due to the complete oxidation of glucose in the presence of oxygen. Organisms Respond to Changes in Their Internal and External Environments Question: Describe the role of hormones in coordinating physiological responses in multicellular organisms. Answer: Hormones are chemical messengers produced by endocrine glands that regulate various physiological processes by binding to target cells and eliciting specific responses. Genetics Question: Explain the principles of Mendelian genetics and provide an example of a monohybrid cross. Answer: Mendelian genetics describes the inheritance of traits based on dominant and recessive alleles, as demonstrated in a monohybrid cross involving a single gene. The Control of Gene Expression Question: Investigate the role of epigenetic modifications in gene regulation. Answer: Epigenetic modifications alter gene expression without changing the underlying DNA sequence, influencing cellular differentiation and responses to environmental cues.

Biological Molecules Question: Explore the structure and function of proteins in living organisms. Answer: Proteins are macromolecules composed of amino acids, serving diverse roles as enzymes, structural components, transporters, and signaling molecules in biological processes. Cells Question: Examine the process of cell division in eukaryotic cells and its significance in growth and repair. Answer: Cell division involves mitosis for growth and tissue repair, ensuring the formation of identical daughter cells with the same genetic information. Organisms Exchange Substances with Their Environment Question: Compare and contrast endocytosis and exocytosis in cells. Answer: Endocytosis involves the uptake of substances into the cell through vesicle formation, while exocytosis releases substances outside the cell by fusing vesicles with the cell membrane.