In ecology, describe primary succession from bare rock (pioneer species) to a climax community and why conservation often arrests succession.

1) Pioneer species (e.g., lichens/mosses adapted to extreme abiotic conditions) colonise bare rock; 2) When they die and decompose, saprobionts form a thin layer of humus/soil that retains water and minerals; 3) This makes abiotic conditions less hostile, allowing herbaceous plants, shrubs, and eventually dominant woodland trees (climax community) to outcompete earlier species. Conservation (e.g., grazing, mowing, controlled burning) causes plagioclimax to preserve habitats for rare early-successional species.

How do auxins (IAA) and gibberellins regulate cell elongation and seed germination in plants?

Auxin (IAA): binds to receptors on shoot cell membranes, activating proton pumps that actively transport H⁺ ions into the cellulose cell wall; the lower pH (~4.5) activates expansin enzymes that break hydrogen bonds between cellulose microfibrils, loosening the cell wall so water enters by osmosis and elongates the cell (acid growth hypothesis). Gibberellin (GA): absorbed water stimulates the plant embryo to secrete GA, which diffuses to the aleurone layer and activates transcription of the amylase gene to hydrolyse endosperm starch into glucose.

How does RNA interference (RNAi) via siRNA (small interfering RNA) inhibit translation of target mRNA?

Double-stranded RNA in the cytoplasm is cut by the enzyme Dicer into short double-stranded siRNA fragments. One siRNA guide strand binds to an intracellular enzyme complex (RISC) and pairs via complementary base pairing to a specific target mRNA molecule. The enzyme complex then cleaves (hydrolyses) the mRNA into fragments, preventing translation at the ribosome.

Describe the Light-Dependent Reaction (non-cyclic photophosphorylation) of photosynthesis across the thylakoid membrane.

1) Photoionisation: Light excites electrons in chlorophyll at Photosystem II (PSII) and PSI; 2) Photolysis of water at PSII (2H2O → 4H⁺ + 4e⁻ + O2) replaces lost electrons; 3) Excited electrons pass along an electron transport chain (ETC), releasing energy to pump H⁺ protons from the stroma into the thylakoid lumen; 4) Chemiosmosis: H⁺ diffuses back into the stroma through ATP synthase, synthesising ATP from ADP + Pi; 5) Electrons from PSI + H⁺ reduce NADP to NADPH (catalysed by NADP reductase).

How do Oestrogen (a lipid-soluble steroid hormone) and Epigenetic modifications (DNA methylation and histone acetylation) regulate gene transcription?

Oestrogen diffuses across the phospholipid bilayer into the cytoplasm/nucleus, binds to a complementary receptor site on an inactive transcription factor, causing a conformational change that exposes its DNA-binding site so it binds to a gene's promoter region and activates RNA polymerase. Epigenetics: Increased DNA methylation (adding -CH3 groups to cytosine bases) and decreased histone acetylation (condensing chromatin tightly into heterochromatin) INHIBIT transcription Factor binding and switch genes OFF.

How do rod cells and cone cells in the mammalian retina differ in visual acuity, sensitivity to low light, and retinal convergence?

Rod cells (contain rhodopsin): high sensitivity to low light intensity because multiple rod cells synapse with a single bipolar neurone (retinal convergence / spatial summation), which gives low visual acuity and monochromatic vision. Cone cells (3 types of iodopsin: red, green, blue): packed at the fovea, low sensitivity to light (require bright light), and each cone cell synapses with its own single bipolar neurone (no convergence), giving high visual acuity (resolution) and colour vision.

Trace the four stages of aerobic cellular respiration, stating the exact cellular location and net products of Glycolysis and the Link Reaction.

1) Glycolysis (cytoplasm, anaerobic): Glucose (6C) is phosphorylated by 2 ATP to hexose bisphosphate, splits into 2× triose phosphate (TP), and is oxidised to 2× Pyruvate (3C), yielding a net gain of 2 ATP (substrate-level phosphorylation) and 2 reduced NAD (NADH). 2) Link Reaction (mitochondrial matrix): Pyruvate (3C) is decarboxylated (−CO2) and oxidised (forming NADH) and combines with Coenzyme A to form Acetyl Coenzyme A (2C).

How is a resting potential (−70 mV) maintained across a neurone axon membrane, and how is an action potential (+40 mV) generated and repolarised?

Resting potential (−70 mV): Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in, while the membrane is much more permeable to K⁺ diffusing out than Na⁺ diffusing in. Depolarisation (+40 mV): Stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in down its electrochemical gradient (positive feedback once threshold −55 mV is reached). Repolarisation & Hyperpolarisation: At +40 mV, Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out; K⁺ channels close slowly, causing temporary hyperpolarisation (−80 mV) and the refractory period.

How does the Loop of Henle act as a counter-current multiplier to concentrate urine in the kidney medulla?

1) In the ascending limb (impermeable to water), Na⁺ and Cl⁻ ions are actively pumped out into the medulla interstitial fluid, lowering the water potential (Ψ) of the medulla; 2) In the descending limb (permeable to water, impermeable to ions), water moves out by osmosis into the hypertonic medulla and is carried away by the vasa recta capillaries, so filtrate reaches maximum concentration at the hairpin apex; 3) As filtrate flows past the hypertonic medulla down the collecting duct, water is reabsorbed by osmosis through aquaporins controlled by ADH.

Compare Allopatric Speciation and Sympatric Speciation.

Allopatric speciation: A population is split by a physical/geographical barrier (e.g., ocean, mountain range), preventing gene flow; the two sub-populations experience different environmental selection pressures, accumulate different mutations and allele frequencies via natural selection and genetic drift, until they become reproductively isolated. Sympatric speciation: Occurs within the same geographical area without physical separation via behavioural, temporal, ecological, or polyploid reproductive isolation.

Explain the Second Messenger Model of adrenaline and glucagon action on liver cells (hepatocytes).

1) Adrenaline or glucagon (first messenger, peptide/catecholamine hormone unable to cross the phospholipid bilayer) binds to a specific complementary transmembrane receptor on the liver cell surface; 2) This activates a G-protein, which activates the membrane-bound enzyme adenylyl cyclase; 3) Adenylyl cyclase converts ATP into cyclic AMP (cAMP, the second messenger); 4) cAMP activates protein kinase A enzymes via an intracellular cascade that catalyses glycogenolysis (glycogen → glucose).

How do you apply and interpret the Chi-Squared (χ²) statistical test in genetics and ecology?

χ² = Σ [(O − E)² / E], where O = observed numbers (raw counts, never percentages) and E = expected numbers according to the null hypothesis (there is no significant difference between observed and expected ratios). Degrees of freedom (df) = number of classes − 1. If calculated χ² > critical value at p = 0.05, reject the null hypothesis: there IS a statistically significant difference (probability difference is due to chance is < 5%).

How are totipotent, pluripotent, multipotent, and induced pluripotent stem cells (iPSCs) defined and produced?

Totipotent (zygote and early morula): can divide and differentiate into ANY cell type, including extra-embryonic placental tissue. Pluripotent (embryonic blastocyst inner cell mass): can form almost any body cell type, but not placenta. Multipotent (adult bone marrow) and unipotent (cardiomyocytes): differentiate into a limited range of cell types. iPSCs are created in the lab by reprogramming adult somatic cells using specific transcription factors to regain pluripotency (avoiding ethical destruction of embryos).

Describe the three stages of the Light-Independent Reaction (Calvin Cycle) in the chloroplast stroma.

1) Carbon Fixation: CO2 combines with 5-carbon ribulose bisphosphate (RuBP), catalysed by the enzyme Rubisco, to form two molecules of 3-carbon glycerate 3-phosphate (GP); 2) Reduction: GP is reduced into two molecules of triose phosphate (TP / GALP) using energy from ATP hydrolysis and H atoms from reduced NADP (NADPH); 3) Regeneration: 5 out of every 6 TP molecules are used to regenerate RuBP (using ATP), while 1 TP is used to synthesise hexose sugars, glycerol, fatty acids, or amino acids.

Explain how the medulla oblongata, carotid/aortic baroreceptors, and chemoreceptors control heart rate via the autonomic nervous system.

If blood CO2 rises (lowering blood pH), chemoreceptors in the carotid arteries and aorta send higher-frequency impulses to the cardioacceleratory centre in the medulla oblongata, which sends impulses down the sympathetic nervous system (accelerator nerve releasing noradrenaline) to the SAN to increase heart rate. If blood pressure is too high, baroreceptors signal the cardioinhibitory centre in the medulla, which sends impulses down the parasympathetic vagus nerve (releasing acetylcholine) to the SAN to decrease heart rate.

Trace the stages of the Nitrogen Cycle, naming the specific bacterial processes involved.

1) Nitrogen fixation: Nitrogen-fixing bacteria (free-living Azotobacter or mutualistic Rhizobium in leguminous root nodules) reduce atmospheric N2 gas into ammonium ions (NH4⁺); 2) Ammonification: Saprobionts (decomposers) secrete extracellular enzymes to hydrolyse proteins/urea/DNA in dead organic matter into NH4⁺; 3) Nitrification (aerobic): Nitrifying bacteria oxidise NH4⁺ → Nitrites (NO2⁻) → Nitrates (NO3⁻); 4) Denitrification (anaerobic waterlogged soil): Denitrifying bacteria convert NO3⁻ back into N2 gas.

How does a respirometer measure the rate of oxygen uptake by respiring invertebrates or germinating seeds?

Organisms are placed in a sealed boiling tube with soda lime or potassium hydroxide (KOH) pellets (which absorb all CO2 produced). As the organisms respire aerobically, they consume O2 while the CO2 they exhale is absorbed by KOH, decreasing the gas volume/pressure in the tube and pulling coloured manometer fluid towards the organism. The distance moved by the fluid bubble in a capillary tube of radius r gives volume V = πr²h.

State the two Hardy–Weinberg equations and the 5 conditions required for a population to remain in Hardy–Weinberg equilibrium.

Allele frequencies: p + q = 1. Genotype frequencies: p² + 2pq + q² = 1 (where p² = homozygous dominant, 2pq = heterozygous carriers, q² = homozygous recessive). Always start calculations by finding q = √(q²). Five conditions: 1) No mutations; 2) Random mating; 3) Large population size (no genetic drift); 4) Isolated population (no immigration/emigration gene flow); 5) No natural selection (all genotypes equally fertile/viable).

Why is saltatory conduction along a myelinated neurone much faster than conduction along an unmyelinated neurone, and what are the roles of the refractory period?

The lipid myelin sheath (formed by Schwann cells) acts as an electrical insulator preventing ion movement across the membrane except at the Nodes of Ranvier. Local currents therefore jump from node to node (saltatory conduction), vastly increasing conduction velocity. The refractory period ensures: 1) Action potentials are propagated unidirectionally; 2) Action potentials remain discrete (do not merge); 3) It limits the maximum frequency of impulses.

What is the role of Reverse Transcriptase and cDNA when isolating a human gene (such as the human insulin gene) from pancreatic β-cells?

Pancreatic β-cells contain high concentrations of mature insulin mRNA (which has already had its non-coding introns spliced out). Reverse transcriptase (isolated from retroviruses) synthesises a single-stranded complementary DNA (cDNA) strand from the mRNA template, and DNA polymerase then builds the second strand to form double-stranded intron-free cDNA that prokaryotes can directly transcribe and translate.

Describe the sequence of transmission across a cholinergic (acetylcholine, ACh) synapse and distinguish spatial vs temporal summation.

1) Action potential arrives at the synaptic knob, opening voltage-gated Ca²⁺ channels so Ca²⁺ ions diffuse in; 2) Ca²⁺ influx causes synaptic vesicles containing ACh to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis; 3) ACh diffuses across the cleft and binds to nicotinic Na⁺ receptor channels on the postsynaptic membrane, opening them so Na⁺ enters (generating an EPSP); 4) Acetylcholinesterase (AChE) hydrolyses ACh into choline + ethanoic acid (recycled using ATP). Spatial summation = multiple presynaptic neurones; Temporal summation = high-frequency impulses from one neurone.

Contrast benign and malignant tumours, and explain how mutations in proto-oncogenes and tumour suppressor genes lead to cancer.

Malignant tumours grow rapidly, are unencapsulated, invade surrounding tissues, and metastasise via blood/lymph; benign tumours grow slowly and remain encapsulated. Proto-oncogenes stimulate normal cell division; a gain-of-function mutation or hypomethylation turns them into oncogenes that permanently activate receptor/growth factor signalling. Tumour suppressor genes (e.g., p53, BRCA1) slow cell division, repair DNA, or trigger apoptosis; loss-of-function mutation or hypermethylation of BOTH alleles removes the brake on mitosis.

Compare Student's t-test, Spearman's Rank Correlation Coefficient (rs), and Chi-Squared (χ²) when choosing a statistical test in A-Level Biology.

1) Student's t-test (unpaired or paired): tests whether there is a significant difference between the MEANS of two continuous, normally distributed data sets. 2) Spearman's Rank Correlation: tests whether there is a significant CORRELATION (association) between two paired continuous/ranked variables. 3) Chi-Squared (χ²): tests for a significant difference/association between CATEGORIC frequency count data.

Describe the Krebs Cycle (in the mitochondrial matrix) and Oxidative Phosphorylation (across the inner mitochondrial cristae).

Krebs Cycle: Acetyl CoA (2C) combines with a 4C acceptor (oxaloacetate) to form a 6C molecule (citrate), which undergoes serial decarboxylation and dehydrogenation to regenerate the 4C acceptor; per cycle it yields 2 CO2, 1 ATP (substrate-level), 3 reduced NAD, and 1 reduced FAD. Oxidative Phosphorylation: NADH and FADH2 donate H atoms (splitting into H⁺ + e⁻); electrons pass along the cristae ETC, pumping H⁺ into the intermembrane space; H⁺ flows back through ATP synthase (chemiosmosis) to make ATP, and O2 acts as the final electron acceptor (½O2 + 2H⁺ + 2e⁻ → H2O).

State the expected F2 phenotypic ratios for dihybrid inheritance (9:3:3:1), autosomal linkage, codominance, and recessive/dominant epistasis.

Unlinked dihybrid cross (AaBb × AaBb): 9 : 3 : 3 : 1. Autosomal linkage (without crossing over): 3 : 1 ratio (same as monohybrid), with small numbers of recombinant offspring generated by crossing over. Codominance (monohybrid heterozygote cross): 1 : 2 : 1. Recessive epistasis (homozygous recessive at one locus masks another): 9 : 3 : 4. Dominant epistasis: 12 : 3 : 1. Complementary gene action: 9 : 7.

How do you calculate NPP (Net Primary Production), Net Production of Consumers (N), and Respiratory Quotient (RQ)?

NPP = GPP (Gross Primary Production) − R (respiratory heat losses by plants). Consumer Net Production: N = I − (F + R), where I = chemical energy in ingested food, F = energy lost in faeces/urine, and R = respiratory energy loss. Respiratory Quotient: RQ = Volume of CO2 produced ÷ Volume of O2 consumed (Carbohydrates = 1.0, Proteins ≈ 0.9, Lipids ≈ 0.7; RQ > 1.0 indicates anaerobic respiration is occurring).

How do you use replica plating and antibiotic resistance marker genes (e.g., ampicillin and tetracycline resistance in plasmid pBR322) to identify transformed bacteria?

A plasmid has two antibiotic resistance genes (Amp^R and Tet^R). The foreign gene is inserted into the middle of the Tet^R gene (insertional inactivation). Bacteria are first grown on agar containing ampicillin (killing non-transformed bacteria that didn't take up a plasmid). Colonies are then transferred via a velvet pad (replica plating) onto agar containing tetracycline; colonies that grow on ampicillin but DIE on tetracycline contain the recombinant plasmid.

Explain the 3 thermal stages of the Polymerase Chain Reaction (PCR) for in vitro DNA amplification.

Reaction mixture contains target DNA, free dNTP nucleotides, forward/reverse DNA primers, and thermostable Taq DNA polymerase: 1) Denaturation (95°C): breaks hydrogen bonds to separate the DNA double helix into two single strands; 2) Annealing (55°C): cools so short single-stranded DNA primers bind (anneal) to complementary 3' ends of each template strand; 3) Extension/Synthesis (72°C): optimum temperature for Taq polymerase to synthesise complementary DNA strands (doubling DNA each cycle: 2ⁿ).

What happens to the lengths of the Sarcomere, I-band, A-band, and H-zone when a myofibril contracts?

Remember the mnemonic 'HIA' (only A stays constant): As actin and myosin filaments slide past each other without shortening themselves, the Z-lines move closer together (sarcomere shortens), the light I-band (actin only) becomes NARROWER, the H-zone (myosin only) becomes NARROWER, while the dark A-band (full length of myosin filaments) stays CONSTANT in width.

Explain the Sliding Filament Theory of skeletal muscle contraction, specifying the roles of Ca²⁺, Tropomyosin, Troponin, and ATP.

1) Action potential travels down T-tubules, causing the sarcoplasmic reticulum (SR) to release Ca²⁺ ions into the sarcoplasm; 2) Ca²⁺ binds to troponin, causing it to change shape and pull tropomyosin away from the actin-myosin binding sites on the actin filament; 3) Myosin heads (with ADP+Pi attached) bind to actin to form actomyosin cross-bridges, nod forward in a 'power stroke' (releasing ADP+Pi), and pull actin filaments inwards; 4) A new ATP molecule binds to the myosin head to break the cross-bridge, and ATPase hydrolyses ATP → ADP + Pi to recock the myosin head.

Explain how ultrafiltration in the Renal Bowman's Capsule and selective reabsorption in the Proximal Convoluted Tubule (PCT) work.

Ultrafiltration: The afferent arteriole is wider than the efferent arteriole, creating high hydrostatic pressure in the glomerulus that forces water, glucose, amino acids, urea, and ions through the capillary endothelium, basement membrane (the main molecular sieve stopping RBCs and large plasma proteins), and podocyte foot processes into the Bowman's capsule. Selective reabsorption in PCT: 100% of glucose and amino acids and ~85% of water/Na⁺ are reabsorbed into peritubular capillaries via Na⁺ co-transport (driven by basolateral Na⁺/K⁺ ATPase pumps and microvilli brush border).

How are DNA probes, genetic fingerprinting (VNTRs), and gel electrophoresis used in medical diagnostics and forensics?

Variable Number Tandem Repeats (VNTRs / minisatellites) are non-coding DNA regions whose repeat lengths vary uniquely between individuals (except identical twins). DNA is amplified by PCR, cut at specific palindromic recognition sites by restriction endonucleases, and loaded onto an agarose gel; negatively charged DNA fragments (due to PO4³⁻ groups) migrate towards the positive anode, with shorter fragments moving faster/further. Radioactive or fluorescently labelled single-stranded DNA probes hybridise to target alleles after Southern blotting.

A-Level Biology II: Photosynthesis, Cellular Respiration, Nervous/Endocrine Coordination, Homeostasis, Genetics & Gene Expression

A2 / Year 13 A-Level Biology mastery: Light-Dependent & Calvin Cycle reactions, Glycolysis/Krebs/Oxidative Phosphorylation, action potentials & cholinergic synapses, sliding filament muscle contraction, nephron osmoregulation, Hardy-Weinberg, epistasis, epigenetics, and PCR/CRISPR.

Level: intermediate
Languages: en → en
Cards: 32
Markdown: A-Level Biology II: Photosynthesis, Cellular Respiration, Nervous/Endocrine Coordination, Homeostasis, Genetics & Gene Expression (.md)

Flashcards (32)

#	Front (Term / Question)	Back (Definition / Answer)
1	In ecology, describe primary succession from bare rock (pioneer species) to a climax community and why conservation often arrests succession.	1) Pioneer species (e.g., lichens/mosses adapted to extreme abiotic conditions) colonise bare rock; 2) When they die and decompose, saprobionts form a thin layer of humus/soil that retains water and minerals; 3) This makes abiotic conditions less hostile, allowing herbaceous plants, shrubs, and eventually dominant woodland trees (climax community) to outcompete earlier species. Conservation (e.g., grazing, mowing, controlled burning) causes plagioclimax to preserve habitats for rare early-successional species.
2	How do auxins (IAA) and gibberellins regulate cell elongation and seed germination in plants?	Auxin (IAA): binds to receptors on shoot cell membranes, activating proton pumps that actively transport H⁺ ions into the cellulose cell wall; the lower pH (~4.5) activates expansin enzymes that break hydrogen bonds between cellulose microfibrils, loosening the cell wall so water enters by osmosis and elongates the cell (acid growth hypothesis). Gibberellin (GA): absorbed water stimulates the plant embryo to secrete GA, which diffuses to the aleurone layer and activates transcription of the amylase gene to hydrolyse endosperm starch into glucose.
3	How does RNA interference (RNAi) via siRNA (small interfering RNA) inhibit translation of target mRNA?	Double-stranded RNA in the cytoplasm is cut by the enzyme Dicer into short double-stranded siRNA fragments. One siRNA guide strand binds to an intracellular enzyme complex (RISC) and pairs via complementary base pairing to a specific target mRNA molecule. The enzyme complex then cleaves (hydrolyses) the mRNA into fragments, preventing translation at the ribosome.
4	Describe the Light-Dependent Reaction (non-cyclic photophosphorylation) of photosynthesis across the thylakoid membrane.	1) Photoionisation: Light excites electrons in chlorophyll at Photosystem II (PSII) and PSI; 2) Photolysis of water at PSII (2H2O → 4H⁺ + 4e⁻ + O2) replaces lost electrons; 3) Excited electrons pass along an electron transport chain (ETC), releasing energy to pump H⁺ protons from the stroma into the thylakoid lumen; 4) Chemiosmosis: H⁺ diffuses back into the stroma through ATP synthase, synthesising ATP from ADP + Pi; 5) Electrons from PSI + H⁺ reduce NADP to NADPH (catalysed by NADP reductase).
5	How do Oestrogen (a lipid-soluble steroid hormone) and Epigenetic modifications (DNA methylation and histone acetylation) regulate gene transcription?	Oestrogen diffuses across the phospholipid bilayer into the cytoplasm/nucleus, binds to a complementary receptor site on an inactive transcription factor, causing a conformational change that exposes its DNA-binding site so it binds to a gene's promoter region and activates RNA polymerase. Epigenetics: Increased DNA methylation (adding -CH3 groups to cytosine bases) and decreased histone acetylation (condensing chromatin tightly into heterochromatin) INHIBIT transcription Factor binding and switch genes OFF.
6	How do rod cells and cone cells in the mammalian retina differ in visual acuity, sensitivity to low light, and retinal convergence?	Rod cells (contain rhodopsin): high sensitivity to low light intensity because multiple rod cells synapse with a single bipolar neurone (retinal convergence / spatial summation), which gives low visual acuity and monochromatic vision. Cone cells (3 types of iodopsin: red, green, blue): packed at the fovea, low sensitivity to light (require bright light), and each cone cell synapses with its own single bipolar neurone (no convergence), giving high visual acuity (resolution) and colour vision.
7	Trace the four stages of aerobic cellular respiration, stating the exact cellular location and net products of Glycolysis and the Link Reaction.	1) Glycolysis (cytoplasm, anaerobic): Glucose (6C) is phosphorylated by 2 ATP to hexose bisphosphate, splits into 2× triose phosphate (TP), and is oxidised to 2× Pyruvate (3C), yielding a net gain of 2 ATP (substrate-level phosphorylation) and 2 reduced NAD (NADH). 2) Link Reaction (mitochondrial matrix): Pyruvate (3C) is decarboxylated (−CO2) and oxidised (forming NADH) and combines with Coenzyme A to form Acetyl Coenzyme A (2C).
8	How is a resting potential (−70 mV) maintained across a neurone axon membrane, and how is an action potential (+40 mV) generated and repolarised?	Resting potential (−70 mV): Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in, while the membrane is much more permeable to K⁺ diffusing out than Na⁺ diffusing in. Depolarisation (+40 mV): Stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in down its electrochemical gradient (positive feedback once threshold −55 mV is reached). Repolarisation & Hyperpolarisation: At +40 mV, Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out; K⁺ channels close slowly, causing temporary hyperpolarisation (−80 mV) and the refractory period.
9	How does the Loop of Henle act as a counter-current multiplier to concentrate urine in the kidney medulla?	1) In the ascending limb (impermeable to water), Na⁺ and Cl⁻ ions are actively pumped out into the medulla interstitial fluid, lowering the water potential (Ψ) of the medulla; 2) In the descending limb (permeable to water, impermeable to ions), water moves out by osmosis into the hypertonic medulla and is carried away by the vasa recta capillaries, so filtrate reaches maximum concentration at the hairpin apex; 3) As filtrate flows past the hypertonic medulla down the collecting duct, water is reabsorbed by osmosis through aquaporins controlled by ADH.
10	Compare Allopatric Speciation and Sympatric Speciation.	Allopatric speciation: A population is split by a physical/geographical barrier (e.g., ocean, mountain range), preventing gene flow; the two sub-populations experience different environmental selection pressures, accumulate different mutations and allele frequencies via natural selection and genetic drift, until they become reproductively isolated. Sympatric speciation: Occurs within the same geographical area without physical separation via behavioural, temporal, ecological, or polyploid reproductive isolation.
11	Explain the Second Messenger Model of adrenaline and glucagon action on liver cells (hepatocytes).	1) Adrenaline or glucagon (first messenger, peptide/catecholamine hormone unable to cross the phospholipid bilayer) binds to a specific complementary transmembrane receptor on the liver cell surface; 2) This activates a G-protein, which activates the membrane-bound enzyme adenylyl cyclase; 3) Adenylyl cyclase converts ATP into cyclic AMP (cAMP, the second messenger); 4) cAMP activates protein kinase A enzymes via an intracellular cascade that catalyses glycogenolysis (glycogen → glucose).
12	How do you apply and interpret the Chi-Squared (χ²) statistical test in genetics and ecology?	χ² = Σ [(O − E)² / E], where O = observed numbers (raw counts, never percentages) and E = expected numbers according to the null hypothesis (there is no significant difference between observed and expected ratios). Degrees of freedom (df) = number of classes − 1. If calculated χ² > critical value at p = 0.05, reject the null hypothesis: there IS a statistically significant difference (probability difference is due to chance is < 5%).
13	How are totipotent, pluripotent, multipotent, and induced pluripotent stem cells (iPSCs) defined and produced?	Totipotent (zygote and early morula): can divide and differentiate into ANY cell type, including extra-embryonic placental tissue. Pluripotent (embryonic blastocyst inner cell mass): can form almost any body cell type, but not placenta. Multipotent (adult bone marrow) and unipotent (cardiomyocytes): differentiate into a limited range of cell types. iPSCs are created in the lab by reprogramming adult somatic cells using specific transcription factors to regain pluripotency (avoiding ethical destruction of embryos).
14	Describe the three stages of the Light-Independent Reaction (Calvin Cycle) in the chloroplast stroma.	1) Carbon Fixation: CO2 combines with 5-carbon ribulose bisphosphate (RuBP), catalysed by the enzyme Rubisco, to form two molecules of 3-carbon glycerate 3-phosphate (GP); 2) Reduction: GP is reduced into two molecules of triose phosphate (TP / GALP) using energy from ATP hydrolysis and H atoms from reduced NADP (NADPH); 3) Regeneration: 5 out of every 6 TP molecules are used to regenerate RuBP (using ATP), while 1 TP is used to synthesise hexose sugars, glycerol, fatty acids, or amino acids.
15	Explain how the medulla oblongata, carotid/aortic baroreceptors, and chemoreceptors control heart rate via the autonomic nervous system.	If blood CO2 rises (lowering blood pH), chemoreceptors in the carotid arteries and aorta send higher-frequency impulses to the cardioacceleratory centre in the medulla oblongata, which sends impulses down the sympathetic nervous system (accelerator nerve releasing noradrenaline) to the SAN to increase heart rate. If blood pressure is too high, baroreceptors signal the cardioinhibitory centre in the medulla, which sends impulses down the parasympathetic vagus nerve (releasing acetylcholine) to the SAN to decrease heart rate.
16	Trace the stages of the Nitrogen Cycle, naming the specific bacterial processes involved.	1) Nitrogen fixation: Nitrogen-fixing bacteria (free-living Azotobacter or mutualistic Rhizobium in leguminous root nodules) reduce atmospheric N2 gas into ammonium ions (NH4⁺); 2) Ammonification: Saprobionts (decomposers) secrete extracellular enzymes to hydrolyse proteins/urea/DNA in dead organic matter into NH4⁺; 3) Nitrification (aerobic): Nitrifying bacteria oxidise NH4⁺ → Nitrites (NO2⁻) → Nitrates (NO3⁻); 4) Denitrification (anaerobic waterlogged soil): Denitrifying bacteria convert NO3⁻ back into N2 gas.
17	How does a respirometer measure the rate of oxygen uptake by respiring invertebrates or germinating seeds?	Organisms are placed in a sealed boiling tube with soda lime or potassium hydroxide (KOH) pellets (which absorb all CO2 produced). As the organisms respire aerobically, they consume O2 while the CO2 they exhale is absorbed by KOH, decreasing the gas volume/pressure in the tube and pulling coloured manometer fluid towards the organism. The distance moved by the fluid bubble in a capillary tube of radius r gives volume V = πr²h.
18	State the two Hardy–Weinberg equations and the 5 conditions required for a population to remain in Hardy–Weinberg equilibrium.	Allele frequencies: p + q = 1. Genotype frequencies: p² + 2pq + q² = 1 (where p² = homozygous dominant, 2pq = heterozygous carriers, q² = homozygous recessive). Always start calculations by finding q = √(q²). Five conditions: 1) No mutations; 2) Random mating; 3) Large population size (no genetic drift); 4) Isolated population (no immigration/emigration gene flow); 5) No natural selection (all genotypes equally fertile/viable).
19	Why is saltatory conduction along a myelinated neurone much faster than conduction along an unmyelinated neurone, and what are the roles of the refractory period?	The lipid myelin sheath (formed by Schwann cells) acts as an electrical insulator preventing ion movement across the membrane except at the Nodes of Ranvier. Local currents therefore jump from node to node (saltatory conduction), vastly increasing conduction velocity. The refractory period ensures: 1) Action potentials are propagated unidirectionally; 2) Action potentials remain discrete (do not merge); 3) It limits the maximum frequency of impulses.
20	What is the role of Reverse Transcriptase and cDNA when isolating a human gene (such as the human insulin gene) from pancreatic β-cells?	Pancreatic β-cells contain high concentrations of mature insulin mRNA (which has already had its non-coding introns spliced out). Reverse transcriptase (isolated from retroviruses) synthesises a single-stranded complementary DNA (cDNA) strand from the mRNA template, and DNA polymerase then builds the second strand to form double-stranded intron-free cDNA that prokaryotes can directly transcribe and translate.
21	Describe the sequence of transmission across a cholinergic (acetylcholine, ACh) synapse and distinguish spatial vs temporal summation.	1) Action potential arrives at the synaptic knob, opening voltage-gated Ca²⁺ channels so Ca²⁺ ions diffuse in; 2) Ca²⁺ influx causes synaptic vesicles containing ACh to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis; 3) ACh diffuses across the cleft and binds to nicotinic Na⁺ receptor channels on the postsynaptic membrane, opening them so Na⁺ enters (generating an EPSP); 4) Acetylcholinesterase (AChE) hydrolyses ACh into choline + ethanoic acid (recycled using ATP). Spatial summation = multiple presynaptic neurones; Temporal summation = high-frequency impulses from one neurone.
22	Contrast benign and malignant tumours, and explain how mutations in proto-oncogenes and tumour suppressor genes lead to cancer.	Malignant tumours grow rapidly, are unencapsulated, invade surrounding tissues, and metastasise via blood/lymph; benign tumours grow slowly and remain encapsulated. Proto-oncogenes stimulate normal cell division; a gain-of-function mutation or hypomethylation turns them into oncogenes that permanently activate receptor/growth factor signalling. Tumour suppressor genes (e.g., p53, BRCA1) slow cell division, repair DNA, or trigger apoptosis; loss-of-function mutation or hypermethylation of BOTH alleles removes the brake on mitosis.
23	Compare Student's t-test, Spearman's Rank Correlation Coefficient (rs), and Chi-Squared (χ²) when choosing a statistical test in A-Level Biology.	1) Student's t-test (unpaired or paired): tests whether there is a significant difference between the MEANS of two continuous, normally distributed data sets. 2) Spearman's Rank Correlation: tests whether there is a significant CORRELATION (association) between two paired continuous/ranked variables. 3) Chi-Squared (χ²): tests for a significant difference/association between CATEGORIC frequency count data.
24	Describe the Krebs Cycle (in the mitochondrial matrix) and Oxidative Phosphorylation (across the inner mitochondrial cristae).	Krebs Cycle: Acetyl CoA (2C) combines with a 4C acceptor (oxaloacetate) to form a 6C molecule (citrate), which undergoes serial decarboxylation and dehydrogenation to regenerate the 4C acceptor; per cycle it yields 2 CO2, 1 ATP (substrate-level), 3 reduced NAD, and 1 reduced FAD. Oxidative Phosphorylation: NADH and FADH2 donate H atoms (splitting into H⁺ + e⁻); electrons pass along the cristae ETC, pumping H⁺ into the intermembrane space; H⁺ flows back through ATP synthase (chemiosmosis) to make ATP, and O2 acts as the final electron acceptor (½O2 + 2H⁺ + 2e⁻ → H2O).
25	State the expected F2 phenotypic ratios for dihybrid inheritance (9:3:3:1), autosomal linkage, codominance, and recessive/dominant epistasis.	Unlinked dihybrid cross (AaBb × AaBb): 9 : 3 : 3 : 1. Autosomal linkage (without crossing over): 3 : 1 ratio (same as monohybrid), with small numbers of recombinant offspring generated by crossing over. Codominance (monohybrid heterozygote cross): 1 : 2 : 1. Recessive epistasis (homozygous recessive at one locus masks another): 9 : 3 : 4. Dominant epistasis: 12 : 3 : 1. Complementary gene action: 9 : 7.
26	How do you calculate NPP (Net Primary Production), Net Production of Consumers (N), and Respiratory Quotient (RQ)?	NPP = GPP (Gross Primary Production) − R (respiratory heat losses by plants). Consumer Net Production: N = I − (F + R), where I = chemical energy in ingested food, F = energy lost in faeces/urine, and R = respiratory energy loss. Respiratory Quotient: RQ = Volume of CO2 produced ÷ Volume of O2 consumed (Carbohydrates = 1.0, Proteins ≈ 0.9, Lipids ≈ 0.7; RQ > 1.0 indicates anaerobic respiration is occurring).
27	How do you use replica plating and antibiotic resistance marker genes (e.g., ampicillin and tetracycline resistance in plasmid pBR322) to identify transformed bacteria?	A plasmid has two antibiotic resistance genes (Amp^R and Tet^R). The foreign gene is inserted into the middle of the Tet^R gene (insertional inactivation). Bacteria are first grown on agar containing ampicillin (killing non-transformed bacteria that didn't take up a plasmid). Colonies are then transferred via a velvet pad (replica plating) onto agar containing tetracycline; colonies that grow on ampicillin but DIE on tetracycline contain the recombinant plasmid.
28	Explain the 3 thermal stages of the Polymerase Chain Reaction (PCR) for in vitro DNA amplification.	Reaction mixture contains target DNA, free dNTP nucleotides, forward/reverse DNA primers, and thermostable Taq DNA polymerase: 1) Denaturation (95°C): breaks hydrogen bonds to separate the DNA double helix into two single strands; 2) Annealing (55°C): cools so short single-stranded DNA primers bind (anneal) to complementary 3' ends of each template strand; 3) Extension/Synthesis (72°C): optimum temperature for Taq polymerase to synthesise complementary DNA strands (doubling DNA each cycle: 2ⁿ).
29	What happens to the lengths of the Sarcomere, I-band, A-band, and H-zone when a myofibril contracts?	Remember the mnemonic 'HIA' (only A stays constant): As actin and myosin filaments slide past each other without shortening themselves, the Z-lines move closer together (sarcomere shortens), the light I-band (actin only) becomes NARROWER, the H-zone (myosin only) becomes NARROWER, while the dark A-band (full length of myosin filaments) stays CONSTANT in width.
30	Explain the Sliding Filament Theory of skeletal muscle contraction, specifying the roles of Ca²⁺, Tropomyosin, Troponin, and ATP.	1) Action potential travels down T-tubules, causing the sarcoplasmic reticulum (SR) to release Ca²⁺ ions into the sarcoplasm; 2) Ca²⁺ binds to troponin, causing it to change shape and pull tropomyosin away from the actin-myosin binding sites on the actin filament; 3) Myosin heads (with ADP+Pi attached) bind to actin to form actomyosin cross-bridges, nod forward in a 'power stroke' (releasing ADP+Pi), and pull actin filaments inwards; 4) A new ATP molecule binds to the myosin head to break the cross-bridge, and ATPase hydrolyses ATP → ADP + Pi to recock the myosin head.
31	Explain how ultrafiltration in the Renal Bowman's Capsule and selective reabsorption in the Proximal Convoluted Tubule (PCT) work.	Ultrafiltration: The afferent arteriole is wider than the efferent arteriole, creating high hydrostatic pressure in the glomerulus that forces water, glucose, amino acids, urea, and ions through the capillary endothelium, basement membrane (the main molecular sieve stopping RBCs and large plasma proteins), and podocyte foot processes into the Bowman's capsule. Selective reabsorption in PCT: 100% of glucose and amino acids and ~85% of water/Na⁺ are reabsorbed into peritubular capillaries via Na⁺ co-transport (driven by basolateral Na⁺/K⁺ ATPase pumps and microvilli brush border).
32	How are DNA probes, genetic fingerprinting (VNTRs), and gel electrophoresis used in medical diagnostics and forensics?	Variable Number Tandem Repeats (VNTRs / minisatellites) are non-coding DNA regions whose repeat lengths vary uniquely between individuals (except identical twins). DNA is amplified by PCR, cut at specific palindromic recognition sites by restriction endonucleases, and loaded onto an agarose gel; negatively charged DNA fragments (due to PO4³⁻ groups) migrate towards the positive anode, with shorter fragments moving faster/further. Radioactive or fluorescently labelled single-stranded DNA probes hybridise to target alleles after Southern blotting.