Edexcel A-level biology 2023 with complete solutions questions and answers

Describe the events of fertilisation that occur after the acrosome reaction. -Membrane of sperm and egg cell fuse together -cortical granules release contents into zona pellucida -Zona pellucida thickens -Haploid nuclei fuse Explain how phospholipids form a cell surface membrane -Hydrophilic parts associate with water -Hydrophobic parts repel water -a bilayer forms with hydrophobic parts pointing in towards the centre of the bilayer Describe the role of the CFTR protein in ensuring that the mucus produced in the lungs has the right consistency -Chloride ions move out of the cells through the CFTR protein -Sodium ions then also move out of the cells -The concentration of the solute increases in the mucus -Water moves out of the cell by osmosis Describe how DNA is organised in a bacterial cell -Circular DNA that is not associated with histones -Plasmids -located in the cytoplasm Describe the light-dependent reactions of photosynthesis -Light energy excites a pair of electrons in PSII -Electrons used in the electron transport chain -Photolysis of water to produce H+, O2 and electrons -Generation of ATP (phosphorylation) -Reduction of NADP Suggest how useful body temperature is in determining time of death -Drop in body temperature is linked to time after death -Factors affect it (eg. environmental temperature, body size etc) -Time of death can be calculated if ambient temperature is known. -However it's only useful for a short amount of time after death. Suggest how useful state of decomposition is in determining time of death -Body decomposes in a specific sequence -Factors affect it (eg. environmental temperature, wounds) -Not useful if body has fully decomposed Why can human DNA polymerase not be used in PCR Human enzymes will not work at high temperatures as they will be denatured Why species of plants cannot be identified from xylem material using PCR and DNA profiling It is made from dead material, there is no DNA present Describe how the production and action of interferon differs from lysozyme Interferon produced by infected cells to inhibit the replication of viruses, whereas lysozyme present in macrophages to destroy bacteria. Suggest why the protein structure of lysozyme is important to the way in which it acts against pathogens -Enzyme, that has an active site of a specific shape -Acts on the cell wall of bacteria Explain why an insect bite, which breaks the surface of the skin, may lead to inflammation -Histamine released from mast cells as a result of damaged tissue -Arterioles dilate causing increased blood flow and capillaries become more permeable -Resulting in an oedema Suggest why reducing inflammation could help reduce a patient's immune response -There will not be such an increase in blood flow to the wound. -Will not be such an increase in macrophages to the area -Fewer macrophages to present antigen to T-helper cells -Without T-helper cell activation B cells cannot be activated etc Explain how changes in the fibrinogen concentration might be a risk factor for CVD -Increased concentrations of fibrinogen leads to increased fibrin -Increases the risk of blood clotting -Which blocks the lumen of arteries Describe the structure of collagen - Primary structure is a repeating amino acid sequence -3 Polypeptide chains -Chains coiled around each other -Cross-links between chains State what is meant by the term sex-linkage -The allele responsible for a trait is located on a sex chromosome -The expression of the trait is linked to gender Explain how studying pollen grains in peat bogs can provide evidence of climate change -Pollen is preserved in peat bogs -A plant species can be identified by its pollen -Climate affects the type of plant growing -Depth of peat correlates with period of time since pollen was produced -Changes in pollen over time indicate changes in climate Describe the role of mRNA in protein synthesis -mRNA is a copy of the DNA -To take information out of the nucleus for translation -Post transcriptional modification to enable more than 1 gene to code for a peptide Explain why it is important that a tRNA molecule can only attach to one amino acid -tRNA is specific for an amino acid -To ensure that the primary structure of the peptide is correct -Otherwise folding will be incorrect Explain why some amino acids can attach to more than 1 type of tRNA molecule -Genetic code is degenerative -Some amino acids have more than one code -There will be a different tRNA for each codon on the mRNA Explain why there are fewer possible anticodons than codons There are stop codons that tRNA do not bind to State the meaning of the term bacterial infection When bacteria are inside cells Describe an investigation, that could be carried out in a laboratory, to determine a suitable combination of antibiotics to use against Mycobacterium tuberculosis -Culture bacteria with agar using aseptic techniques -Different antibiotics used in a number of different combinations -Apply antibiotics using filter paper soaked in antibiotic -Incubate bacteria with antibiotic for a period of time (greater than 12 hours) -Most suitable combination will have the largest zone of inhibition State the meaning of the term species Groups of organisms that can breed together to produce fertile offspring Explain how gel electrophoresis can be used to determine the father of a cub -Collect DNA samples, from blood, skin, etc., from mother, cub and all possible fathers. -Use restriction endonucleases, load onto gel and apply a current. -Visualise through southern blotting, adding fluorescent probes -Match DNA bands Describe the structure of a cell wall in a xylem vessel -cellulose molecules as microfibrils held together by hydrogen bonds -mesh of microfibrils -lignified -Lignin distributed in spirals -Presence of pits -Presence of pectin and hemicelluse in cell wall Explain an ethical issue relating to the use of prenatal screening -risk of false positive and a healthy foetus could be aborted -may result in an abortion and it is unethical to cause death of a foetus Explain how the properties of water make it an ideal transport medium -solvent -Hydrogen bonds form between water molecules and solute molecules -cohesion between molecules State what is meant by the term allele -different form of the same gene -found at the same locus on a chromosome Explain how amino acids would be incorporated into a surfactant protein -mRNA attaches to a ribosome -pairing between codons on mRNA and anticodons on tRNA -Amino acids carried by tRNA -Amino acids joined by peptide bonds -Formation of secondary structure Explain the effect that a diet high in salt could have on a person's risk of developing CVD -Increases risk of CVD -causes higher blood pressure -increases risk of damage to endothelium of artery -inflammatory response -atheroma/plaque formation -narrowing of arteries increases risk of blood clots explain why thicker mucus is produced if the functioning of the CFTR protein is impaired -Chloride ions cannot leave the cell -Sodium ions do not move out of the cell -Water moves into the cells by osmosis Describe the roles of the Golgi apparatus and vesicles in the formation and transport of extracellular enzymes -proteins enter the Golgi apparatus -modification inside -enzyme packaged into vesicles -vesicles fuse with cell membrane Explain how a group of cells can produce the same enzyme -genes can be activated or deactivated -these cells receive the same signal -all have the gene for the enzyme activated -resulting in production of mRNA for the enzyme explain the function of the plasma desmata -Allows transport and communication between cells -cytoplasmic connection compare and contrast the structure of cellulose and amylopectin -Both are polysaccharides -Both contain 1,4 glycosidic bonds -cellulose contains B-glucose whereas amylopectin contains A-glucose -Cellulose is not branched Explain why a second sperm cannot fertilise an egg cell -Cortical reaction -Results in the thickening of zona pellucida -So other sperm cannot reach the egg cell membrane Explain how an electron micrograph image shows the Golgi apparatus -Membrane bound sacs -Sacs in stacks of different sizes -Cisternae are flattened -No ribosomes Describe the role of the Golgi apparatus. -Modifies proteins e.g. Addition of carbohydrate chain to form glycoprotein -Packages proteins into vesicles for export by exoctosis Describe the end result of mitosis (NOT CYTOKINESIS) -Chromosomes decondensed (chromatids separated) -Nucleus divided Describe the process that takes place inside a cell during specialisation -Genes are activated -Transcribed to mRNA -Translation of mRNA to proteins -Proteins determine function of cell Give the differences between organs and tissues -Tissuese are made out of cells and organs made out of tissues -Tissues made out of similar types of cells and organs made out of different tissues -organs have more functions than tissues Explain what is meant by the term sustainable -A resource that can be renewed -Available to future generations Explain what is meant by the zone of inhibition The area where there is no bacteria Explain what is meant by the term niche the role of an organism in an ecosystem Explain what is meant by the term polygenic inheritance -More than one gene for a single characteristic -on more than one locus -Gives rise to continuous variation -Genes can interact with each other Describe how skin flora protect the body from infection -Prevent growth of bacteria -Competition for space -release of chemicals Describe how carbon dioxide is removed from air into oceans -Dissolves in the water -For carbon fixation -By photosynthesis of sea plants Explain why a couple may chose not to have a prenatal test for a genetic disorder -Result may be inaccurate -Increased risk of miscarriage due to procedure -false positive may lead to termination of healthy foetus -Abortion may be against values of the parents Name the prenatal test that can be used to detect genetic disorders at 11 weeks of pregnancy Chorionic villus sampling Explain how a change in the primary structure of factor VIII (enzyme) could cause difficulties in blood clotting -Different sequence of amino acids -Changes in R groups change bonding -changing of active site shape prevents subrstrate from binding -Reducing production of fibrin Describe the functions of carrier proteins in a cell surface membrane -Used in movement of large, polar molecules -Facilitated diffusion, movement from high to low concentration -Active transport which requires ATP to move against concentration gradient Compare and contrast the molecular structure of globular and fibrous proteins -both are chains of amino acids held in place by peptide bonds -both contain hydrogen bonds holding molecule in 3d shape -globular proteins have hydrophilic amino acids on the outside whereas fibrous proteins have hydrophobic groups on the outside -globular have tertiary or quaternary structure whereas fibrous have little tertiary structure -globular are folded into compact shapes whereas fibrous have long chains Describe and explain the sequence of events that occurs at a synapse after a neurotransmitter is released. -neurotransmitter diffuses across the gap -binds to receptors on post-synaptic membrane -voltage gated channels open -causing a depolarisation -if sufficient then action potential set up in adjacent cell -neurotransmitter is broken down so there is not a prolonged action potential in post-synaptic membrane Why too little exercise may not be good for the health of a person -Increased risk of obesity -Coronary heart disease -High blood pressure -Diabetes -Osteoporosis Why too much exercise may not be good for the health of a person -Wear and tear on joints -Suppression of immune system Brain activity was recorded whilst carrying out face identification, suggest why fMRI was used. -Active areas have more oxygenated blood -Active areas involved in face identification identified. -Improves validity of study -Shows brain in real time -High resolution -Safety, e.g. does not use x rays Suggest why fast twitch muscles are less red in colour than slow twitch -Less myoglobin present -Less haemoglobin present (less blood) -Fewer capillaries -Respiration is mainly anaerobic State the homeostatic control mechanism that returns the pH of the blood to its original level Negative feedback Explain how the pH of the blood of a sprinter is returned to its original level after a race -Low pH due to acid (lactate) in the blood -Lactate is taken to the liver -Converted into glycogen -Oxygen debt -Required to convert lactate back to pyruvate -Which is taken to the matrix of mitochondria and oxidised to CO2 and H2O through Krebs -Chemoreceptors detect a change in pH -Causes breathing rate to increase -CO2 from blood diffuses into alveoli and is breathed out. State the location of rhodopsin within a rod cell Outer segment Explain what is meant by the term oxidative phosphorylation -electrons passed through electron transport chain -lose energy -used to add phosphate to ADP -chemiosmosis by protons down ATP synthase -Oxygen is the final electron acceptor How can the active form of phytochrome be converted back to the inactive form -leave it in the dark -exposing it to far red light Describe the changes in the heart that will bring about an increase in cardiac output -heart rate increases -Stroke volume increases -SAN activity increases -AVN time delay decreases -more blood returning to heart causes muscle to stretch -ventricles contract with greater force Explain why genes found on the sex chromosome pair have a different pattern of inheritance -X chromosome carries genes not on y -Therefore males only have 1 copy of some genes -If only one inherited then it will be expressed Describe how low serotonin levels can affect transmission of impulses in the brain -Less neurotransmitter -Less depolarisation -Less chance of action potential occurring in post synaptic membrane Describe how MDMA could affect the transmission of impulses in the brain -Binds to receptors on post synaptic membrane -Increased serotonin released -More action potentials generated Explain how the use of MDMA could result in symptoms of depression -Depletion of serotonin -post synaptic membrane becomes less responsive to serotonin -Less serotonin associated with depression Describe the reaction that joins two a-glucose molecules together to form a disaccharide -condensation reaction -water is formed Explain the electrical changes in an axon that allow nerve impulses to be detected -potential difference in the axon changes -due to voltage gated sodium channels opening -Na+ move into the axon, causing depolarisation -followed by K channels opening -K+ move out of the axon, causing repolarisation Describe three phase drug testing -Phase 1: test drug on a small number of healthy volunteers -Phase 2: Test drug on a small number of patients with the disease -Phase 3: Test drug on a large number of patients with disease -Use of double blind trial with placebo in either 2 or 3 structure of centrioles -Pair of hollow tubes -At right angles -Arranged in groups of 9 microtubules Structure of Golgi Apparatus -Stacks of flattened sacs (cisternae) -Cisternae are curved --Vesicles Explain why xylem is described as a tissue -Group of similar cells -With a similar function Explain the importance of meiosis in plants -Produces genetic variation within the species -which would increases the survival chances of the species -Produces haploid cells for reproduction Explain the advantages of using bioplastics compared to plastics derived from oil -bioplastics are sustainable as more plants can be grown -Less pollution as bioplastics are biodegradable Describe an advantage and disadvantage of using ropes from plant fibres rather than oil-based plastics -They are sustainable -They are not as strong State what is meant by tensile strength Force the fibre can withstand before breaking Describe the positions of lignified tissues in the stem -Sclerenchyma on the outside of the vascular bundle -Xylem is on the inner side of the vascular bundle Describe the process that occurs at a synapse that leads to habituation -Repeated stimulus causes calcium channels on the pre-synaptic membrane not to open -Fewer calcium ions diffuse into the membrane -Fewer vesicles fuse with pre-synaptic membrane -Less neurotransmitter diffuses across the gap -Action potential less likely to occur in post-synaptic membrane Describe the events that occur during prophase -Nuclear envelope breaks down -Spindle fibres form -Chromosomes condense and become visible -Centrioles migrate to opposite poles state the characteristic features of antibodies -Glycoprotien -Light and heavy chains with disulfide bridges between peptides -Variable region with antigen binding site -Constant region -Produced by plasma cells -Involved in opsonisation, agglutination, lysis etc Describe the immune response of the body to viral infections -T killer cells cause lysis of infected cells due to release of enzymes -Macrophages engulf virus particles -Plasma cells produce antibodies -Which cause opsonisation/agglutination etc -Memory cells made to protect from future infections Describe how vaccines produce artificial immunity -Antigen injected -Antigen engulfed by macrophage -Macrophage is an APC to T helper cells -T helper cells activate B cells/ T killer cells -B and T memory cells made Describe the role of microorganisms in recycling organic matter in a dead animal -microorganisms secrete enzymes -Which breaks down proteins into amino acids etc -Products of decomposition are taken up by microorganisms -Glucose used in respiration -Releases CO2 into atmosphere -other breakdown products return to soil What is a trophic level An energy level in a food chain Describe how rigor Mortis occurs -Muscle cells deprived of oxygen -Respiration becomes anaerobic, producing lactic acid -pH falls -Enzymes inhibited, inhibiting respiration -ATP no longer produced -Bonds between muscle proteins become fixed Compare succession in a dead body with that in plant communities -As organisms feed the environment changes -Allowing conditions suitable for growth of other species (eg microbes or plants) -In plant succession early species replaced over time -Most early insects remain as other colonise the body -Climax community of plants formed -Longer timescale in plants Explain the role of 3 inorganic ions required by plants -Nitrate, used in amino acids to synthesise proteins used for growth -Calcium, strengthens cell walls -Magnesium, production of chlorophyll for photosynthesis Explain the role of enzymes in genetic modification -restriction endonucleases cut gene out of animal DNA -Amplify using DNA polymerase in PCR -Same restriction endonucleases to produce sticky ends -H bonds formed between bases at sticky ends -DNA ligase joins gene to plasmid -Phosphodiester bonds form Describe the structure of the modified plasmid in genetic modification -Circle of DNA -Contains bacterial and animal gene -Marker gene to show if modification is successful (e.g. antibiotic resistance) Suggest why plants rather than bacteria may be used to produce a protein -Easier to manage growth (sterile conditions not needed) -Safer -Edible drugs can be produced -Bacteria may not have correct amino acid to produce the protein Describe the risks associated with the use of GMOs -Gene transfer to the species -which may result in transfer of antibiotic resistance (marker gene) -There may be harmful effects from the genes on humans Compare hormones with nervous coordination -chemical vs nerve impulse -Transported in blood vs carried by neurone -slow vs fast -long lasting vs short lasting -Widespread vs localised Explain what is meant by negative feedback -Control mechanism that returns a change away from the normal back to the normal -Ensures a constant value Suggest why performance enhancing drugs should be banned -unfair advantage -unethical -health risks Explain how calcium ions are involved in synaptic transmission -Impulse reaches pre-synaptic membrane -Calcium channels open -Vesicles fuse with membrane -Exocytosis of neurotransmitter into synaptic cleft Suggest why impulses only travel in one direction across the synapse -Neurotransmitter receptors only found on postsynaptic membrane -Neurotransmitter only found in the presynaptic knob what happens when a rod cell responds to light Cation channels close and rod cell becomes hyperpolarised Explain the role of IAA in phototropism -IAA produced in growing root tips -accumulates on the shaded side of root -Causes cell elongation on that side -Shoot grows towards the light What are the symptoms of Parkinson's -Muscle stiffness -Muscle tremors -Slowness of movement -Poor balance Explain how 'dopamine agonists' might be a useful treatment for Parkinson's disease -Agonists mimic dopamine -Bind to dopamine receptors at synapses -Trigger action potentials Suggest why L-Dopa might be a useful treatment for patients with Parkinson's disease. -L-dopa can reach the brain -L-dopa is converted to dopamine -Dopamine triggers action potentials at synapses -This alleviates symptoms Explain how MDMA could affect levels of serotonin in the brain. -Higher levels of serotonin in the synapse -Inhibits reabsorption of serotonin into the neurone -Can reverse pumps to release more serotonin into the synapse. Describe the ethical issues involved in the use of animals in a drug trial -Idea that overall good should outweigh the harm to animals -Idea that animals should never be used -Idea that as few animals should be used as possible -Welfare of animals should be important Explain why muscles work in antagonistic pairs -Muscles can only work in one direction -So there is a need to create an opposite force -So must have extensors and flexors Explain how the appearance of a myofibril changes when a muscle contracts -Sarcomere shortens -Actin and mysosin molecules remain the same length What is the protein in the thick muscle filament Myosin What is the protein the thin muscle filament Actin Describe the interaction between troponin and tropomyosin when a skeletal muscle fibre contracts -Ca2+ binds to troponin -Troponin changes shape -Causes tropomyosin to move Explain how an imbalance of serotonin may contribute to ill health -Serotonin is a neurotransmitter -Low levels of serotonin is linked to depression Distinguish between nature and nurture -Nature involves genes -Nurture involves environment Describe how MRI may be used to investigate brain tumours -Identification of brain tumour location -Size -Type -Monitoring growth/ treatment What functions are associated with the cerebral hemispheres -Thinking -Feeling emotions -Controlling voluntary movements -Memory -Ability to see Describe the role of electron carriers in the electron transport chain -Electron movement towards ATP synthase -Through redox reactions -Releases energy -Used to pump protons into the intermembrane space of the mitochondrion What is the role of ATP synthase in aerobic respiration? -Stalked particle -Protons move through ATP synthase (chemiosmosis) -Due to electrochemical gradient -Sufficient energy released -Which allows ADP + Pi to join, making ATP What is active transport -Movement against a concentration gradient -ATP is needed Suggest what happens in a plant response to light after H-bonds are broken in the cell wall. -Cell absorbs water -By osmosis -Causing it to elongate Describe the role of the myelin sheath -Insulation -depolarisation occurs at nodes -impulse jumps from node to node -Saltatory conduction -Faster conduction Explain how the structure of the axon cell membrane is related to conduction of nerve impulses -Phospholipid bilayer restricts ion movement -Proteins span the membrane -Sodium potassium pump moves ions -Voltage gated channels allow diffusion of ions Use fick's law to explain how the rate of diffusion is affected by asthma -Rate of diffusion decreased -Lower concentration gradient -Surface area also decreased Suggest how drinking coffee causes a change in sweat production -Temperature of blood increases -Thermoreceptors in hypothalamus detect this -Send impulses to sweat glands Explain how the SAN ensures oxygenated blood enters the aorta -SAN initiates the impulse -Causing the atria to contract -Forcing oxygenated blood into the left ventricle -Electrical activity from SAN received by AVN and travels down Bundle of His -Causing left ventricle to contract, forcing blood into the aorta Explain the term myogenic -Stimulation generated within the muscle -Resulting in depolarisation Describe the role of visual stimulation on the development of the visual cortex during the critical period -Neurones synapse with cells in ocular dominance columns -Stimuli along neurones required to strengthen connections -Stimulus must be during critical period to develop connections with visual cortex Describe how the resting potential is maintained in the neurone -Sodium ions move out and potassium ions move in -Through the sodium potassium pump -Which uses ATP -Potassium ions diffuse out -Sodium ions cannot diffuse back into the neurone as voltage gated sodium channels are closed Discuss the ethical issues that might arise from the use of invertebrates in investigations -Undeveloped nervous system so do not feel pain, no ethical issues -Unethical to use any living organism in an experiment -Acceptable under certain circumstances (eg benefit to human health) -May not be acceptable under any circumstance Explain why the atrioventricular valves need to close -Ventricle needs to contract and force blood into the arteries -Prevents the backflow of blood into the atria on contraction Explain the health risk of having a very high BMI -High blood pressure causes damage to the endothelium lining -Atherosclerosis causes narrowing of lumen of arteries -Heart disease as a result of narrowing of coronary arteries -Stroke as a result of reduced blood flow to brain -Joint damage as a result of increased mass causing damage to cartilage -Type 2 diabetes caused by decreased sensitivity of insulin receptors Explain why cigarette smokers have a higher risk of developing lung cancer than non-smokers -Cigarettes contain carcinogens -Increases the number of mutations in cells -Resulting in uncontrolled cell division Explain why data may not be suitable for predicting future climate change -Cannot assume trend will continue (extrapolation is inaccurate) -Not enough data collected (does not go back far enough) -Data fluctuates Describe the role of Calcium ions in muscle fibres -Calcium ions bind to troponin, causing them to change shape -Displaces tropomyosin -Exposing myosin binding sites Describe the appearance of the chromosomes in the cells undergoing the metaphase stage of mitosis. -Condensed and visible -Seen as chromatids -Joined by spindle fibres -In the middle of the cell Explain how the bacterial molecules could trigger a specific immune response -Bacterial molecules engulfed by a macrophage -Becomes a macrophage APC (antigen on MHC) -T helper with complementary CD4 bind to APC -Cytokines released by active T helper cells causing the production of B effector cells -Plasma cells secrete antibodies State the location of chlorophyll in a chloroplast Thylakoid membrane Explain the role of oxygen in human respiration. -Final electron acceptor in the electron transport chain -In oxidative phosphorylation -ATP is synthesises -Reduced NAD are oxidised State one difference between the structure of pyruvate and the structure of the acetyl group in acetyl CoA. Pyruvate has 3 carbons, acetyl has 2 carbons