GCE Advanced Level (A-Levels) Biology Syllabus

State Fick's law and what each term means for diffusion rate.

$$\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$$ Rate increases with greater surface area and steeper concentration gradient, and decreases with longer diffusion distance.

Describe how the alveoli are adapted for efficient gas exchange.

Large total surface area, walls one flattened epithelial cell thick (short diffusion distance), surrounded by a dense capillary network and ventilation maintaining steep concentration gradients, and moist surface for gases to dissolve.

Explain how a counter-current system maximises oxygen uptake in fish gills.

Water flows over the gill lamellae in the opposite direction to blood flow, so a diffusion gradient for oxygen is maintained along the entire length of the lamella, allowing absorption of up to ~80% of the oxygen in the water.

Describe the structure and function of xylem and phloem.

Xylem: dead hollow vessels with lignified walls, transport water and mineral ions upward (transpiration stream). Phloem: living sieve tube elements with companion cells, transport organic solutes (sucrose) by translocation in any direction.

Explain the cohesion-tension theory of water transport in plants.

Water evaporates from leaf mesophyll (transpiration), lowering water potential and pulling water up the xylem; cohesion (hydrogen bonding) between water molecules forms a continuous column, and adhesion to xylem walls maintains tension under negative pressure.

Describe the structure of haemoglobin and oxygen loading/unloading.

Haemoglobin is a quaternary protein with four polypeptide chains each containing a haem (iron) group binding one $\ce{O2}$. Loading occurs at high $p\ce{O2}$ (lungs); unloading at low $p\ce{O2}$ (tissues). Binding is cooperative, giving an S-shaped dissociation curve.

What is the Bohr effect?

At higher carbon dioxide (and lower pH), the oxygen dissociation curve shifts to the right, so haemoglobin's affinity for oxygen decreases and it releases more oxygen to respiring tissues that need it most.

Describe the cardiac cycle in three stages.

Atrial systole: atria contract, pushing blood into ventricles. Ventricular systole: ventricles contract, atrioventricular valves close (first heart sound), semilunar valves open, blood forced into arteries. Diastole: chambers relax, semilunar valves close (second sound), atria fill.

Distinguish tissue fluid formation from reabsorption at a capillary.

At the arteriole end, high hydrostatic pressure exceeds oncotic (osmotic) pressure, forcing plasma out to form tissue fluid. At the venule end, hydrostatic pressure has dropped, so the lower water potential (oncotic pressure) draws water back in; excess is drained by the lymphatic system.

Distinguish the non-specific from the specific immune response.

Non-specific (innate): immediate and same for all pathogens, e.g. physical barriers, phagocytosis and inflammation. Specific (adaptive): slower, antigen-specific, involves lymphocytes and immunological memory (cell-mediated and humoral responses).

Outline the process of phagocytosis.

A phagocyte recognises and engulfs a pathogen, enclosing it in a phagosome. Lysosomes fuse with the phagosome and release lysozymes that hydrolyse the pathogen. Antigens are then displayed on the cell surface (antigen presentation).

Describe the roles of helper T cells, B cells and plasma cells in the humoral response.

Helper T cells bind presented antigens and release cytokines that stimulate the correct B cell. The B cell undergoes clonal selection and divides; some become plasma cells secreting antibodies, others become memory cells for a faster secondary response.

Describe the structure of an antibody and how it works.

An antibody is a Y-shaped quaternary protein with two heavy and two light chains, a constant region and two variable regions forming antigen-binding sites. It is specific to one antigen and causes agglutination, neutralisation, or marks the pathogen for phagocytosis.

Distinguish active from passive, and natural from artificial, immunity.

Active: body makes its own antibodies/memory cells (long-lasting) — natural (infection) or artificial (vaccination). Passive: antibodies received from another source (short-lived, no memory) — natural (placenta/breast milk) or artificial (antibody injection).

Compare DNA and RNA.

DNA: deoxyribose sugar, double-stranded helix, bases A,T,C,G, stable long-term genetic store. RNA: ribose sugar, single-stranded, bases A,U,C,G, shorter, involved in protein synthesis (mRNA, tRNA, rRNA).

Explain why DNA replication is described as semi-conservative.

DNA helicase unwinds and separates the two strands; each original strand acts as a template, and DNA polymerase joins complementary free nucleotides. Each new molecule contains one original (conserved) strand and one newly synthesised strand.

Define gene, allele, genome and proteome.

Gene: a sequence of DNA bases coding for a polypeptide (or functional RNA). Allele: an alternative version of a gene. Genome: the complete set of genetic material in an organism. Proteome: the full range of proteins a cell/organism can produce.

Describe transcription.

In the nucleus, RNA polymerase binds and unwinds the gene; one strand (template) is used to assemble complementary RNA nucleotides forming pre-mRNA. In eukaryotes, introns are spliced out to give mature mRNA, which leaves through a nuclear pore.

Describe translation.

mRNA binds to a ribosome; tRNA molecules with complementary anticodons bring specific amino acids to each codon. Peptide bonds form between adjacent amino acids, building a polypeptide until a stop codon is reached.

State the key features of the genetic code.

It is a triplet code (three bases = one codon = one amino acid), degenerate (most amino acids have several codons), non-overlapping, and universal (the same in nearly all organisms).

Define genotype, phenotype, homozygous, heterozygous, dominant and recessive.

Genotype: the alleles present. Phenotype: observable characteristics. Homozygous: two identical alleles. Heterozygous: two different alleles. Dominant: allele expressed in the heterozygote. Recessive: allele only expressed when homozygous.

In a monohybrid cross between two heterozygotes ($Aa \times Aa$), state the expected genotypic and phenotypic ratios.

Genotypic ratio $1\,AA : 2\,Aa : 1\,aa$; phenotypic ratio $3$ dominant $: 1$ recessive.

What is the chi-squared test used for in genetics, and give the formula.

To test whether observed offspring ratios differ significantly from expected ratios (goodness of fit). $$\chi^{2} = \sum \frac{(O - E)^{2}}{E}$$ If $\chi^{2}$ exceeds the critical value at $p = 0.05$, the difference is significant and the null hypothesis is rejected.

Distinguish continuous from discontinuous variation and give a cause of each.

Continuous variation: a range of values controlled by many genes (polygenic) plus environment, e.g. height. Discontinuous variation: distinct categories controlled by one or few genes, little environmental effect, e.g. blood group.

State the Hardy-Weinberg equations and what they predict.

$$p + q = 1 \quad\text{and}\quad p^{2} + 2pq + q^{2} = 1$$ where $p$ and $q$ are allele frequencies; $p^{2}$, $2pq$, $q^{2}$ are the homozygous dominant, heterozygous and homozygous recessive genotype frequencies. It predicts constant allele frequencies in a non-evolving population.

Distinguish directional, stabilising and disruptive selection.

Directional: favours one extreme, shifting the mean (e.g. antibiotic resistance). Stabilising: favours the mean, reducing variation (e.g. human birth weight). Disruptive: favours both extremes against the mean, can lead to two phenotypes.

Define a species and explain allopatric vs sympatric speciation.

A species is a group of organisms that can interbreed to produce fertile offspring. Allopatric speciation: populations separated by a geographical barrier diverge. Sympatric speciation: reproductive isolation arises within the same area (e.g. behavioural or temporal isolation).

State the taxonomic hierarchy and the three domains.

Hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. The three domains are Bacteria, Archaea and Eukarya.

Give the index of diversity formula and what it measures.

$$d = \frac{N(N-1)}{\sum n(n-1)}$$ where $N$ is the total number of organisms of all species and $n$ is the number of each species. A higher value indicates greater species diversity and usually a more stable ecosystem.

Write the overall equation for photosynthesis and the two main stages.

$$\ce{6CO2 + 6H2O ->[\text{light}] C6H12O6 + 6O2}$$ Stage 1: light-dependent reactions in the thylakoid membranes (produce ATP, reduced NADP, oxygen). Stage 2: light-independent reactions (Calvin cycle) in the stroma (fix $\ce{CO2}$ into sugars).

Describe the light-dependent reactions.

Light excites electrons in chlorophyll (photosystems); electrons pass down the electron transport chain making ATP by chemiosmosis (photophosphorylation). Photolysis splits water ($\ce{2H2O -> 4H+ + 4e- + O2}$); NADP is reduced to form reduced NADP.

Outline the three stages of the Calvin cycle.

(1) $\ce{CO2}$ combines with ribulose bisphosphate (RuBP, 5C) catalysed by rubisco to give two glycerate-3-phosphate (GP, 3C). (2) GP is reduced to triose phosphate (TP) using ATP and reduced NADP. (3) TP is used to regenerate RuBP and to form glucose.

Write the equation for aerobic respiration and name its four stages.

$$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}$$ Stages: glycolysis (cytoplasm), the link reaction and Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane).

Summarise glycolysis: location, inputs and net products.

Occurs in the cytoplasm. Glucose (6C) is phosphorylated using 2 ATP, then split and oxidised to 2 pyruvate (3C). Net yield per glucose: 2 ATP and 2 reduced NAD.

Describe anaerobic respiration in animals and in yeast/plants.

Animals: pyruvate is reduced by reduced NAD to lactate ($\ce{C3H6O3}$), regenerating NAD for glycolysis. Yeast/plants: pyruvate is converted to ethanol and $\ce{CO2}$, also regenerating NAD. Both yield only the 2 ATP from glycolysis.

Define gross primary production (GPP) and net primary production (NPP) and give the relationship.

GPP is the total chemical energy fixed by producers; NPP is the energy left for growth after respiration. $$\text{NPP} = \text{GPP} - R$$ where $R$ is respiratory loss. NPP is available to the next trophic level.

Give the equation for efficiency of energy transfer between trophic levels.

$$\%\,\text{efficiency} = \frac{\text{energy in higher trophic level}}{\text{energy in lower trophic level}} \times 100$$ Transfer is typically only ~10% because energy is lost as heat (respiration), in excretion and in non-consumed parts.

Outline the nitrogen cycle's four key processes.

Nitrogen fixation: $\ce{N2}$ to ammonia by nitrogen-fixing bacteria. Ammonification: decomposers convert organic N to ammonium. Nitrification: nitrifying bacteria oxidise ammonium to nitrite then nitrate. Denitrification: denitrifying bacteria convert nitrate back to $\ce{N2}$ (anaerobic).

Define primary succession and explain the role of pioneer species.

Primary succession is the colonisation of bare, lifeless ground. Pioneer species (e.g. lichens) tolerate harsh conditions and, on dying, form humus/soil, changing the environment so new species can colonise, leading through seral stages to a stable climax community.

Describe the structure and function of a reflex arc.

Stimulus $\to$ receptor $\to$ sensory neurone $\to$ relay neurone (in CNS) $\to$ motor neurone $\to$ effector $\to$ response. It is rapid, automatic and protective because it does not require conscious brain processing.

Explain how a resting potential is established in a neurone.

The sodium-potassium pump actively transports 3 $\ce{Na+}$ out for every 2 $\ce{K+}$ in; the membrane is more permeable to $\ce{K+}$ which diffuses out. This makes the inside negative relative to the outside, giving a resting potential of about $-70\,\text{mV}$.

Describe how an action potential is generated and propagated.

A stimulus opens voltage-gated $\ce{Na+}$ channels causing depolarisation (to about $+40\,\text{mV}$); $\ce{Na+}$ channels close and $\ce{K+}$ channels open causing repolarisation, then hyperpolarisation. Local currents depolarise the next region; in myelinated axons it jumps node to node (saltatory conduction).

Explain the all-or-nothing principle and how stimulus strength is coded.

An action potential only fires if the threshold is reached, and is always the same size regardless of stimulus strength (all-or-nothing). A stronger stimulus is coded by a higher frequency of action potentials and by more neurones firing.

Describe synaptic transmission at a cholinergic synapse.

An action potential opens voltage-gated $\ce{Ca^2+}$ channels; calcium influx causes vesicles to fuse and release acetylcholine into the cleft. ACh binds receptors on the postsynaptic membrane, opening $\ce{Na+}$ channels to depolarise it. Acetylcholinesterase then breaks down ACh.

Define homeostasis and explain negative feedback.

Homeostasis is the maintenance of a stable internal environment within narrow limits. Negative feedback: a deviation from the set point is detected by receptors and triggers responses by effectors that reverse the change, returning conditions to normal.

Describe the control of blood glucose by insulin and glucagon.

High glucose: pancreatic $\beta$-cells release insulin, increasing glucose uptake and glycogenesis (glucose to glycogen), lowering blood glucose. Low glucose: $\alpha$-cells release glucagon, stimulating glycogenolysis and gluconeogenesis, raising blood glucose.

Explain the second messenger model of adrenaline/glucagon action.

The hormone (first messenger) binds a membrane receptor, activating adenylate cyclase, which converts ATP to cyclic AMP (cAMP, the second messenger). cAMP activates protein kinases that catalyse glycogenolysis, releasing glucose.

Distinguish type 1 from type 2 diabetes.

Type 1: $\beta$-cells are destroyed (often autoimmune), so little/no insulin is produced; treated with insulin injections. Type 2: cells become insensitive/lose responsiveness to insulin (receptor down-regulation), usually later onset; managed by diet, exercise and drugs.

Describe ultrafiltration in the kidney nephron.

Blood enters the glomerulus under high hydrostatic pressure (afferent arteriole wider than efferent). Water, glucose, ions and urea are forced through the basement membrane (a molecular filter) into the Bowman's capsule, while blood cells and proteins remain in the blood.

Explain how the loop of Henle and ADH control water reabsorption.

The loop of Henle sets up a high salt concentration in the medulla (counter-current multiplier), lowering water potential. When the body is dehydrated, ADH is released and makes the collecting duct walls more permeable, so more water is reabsorbed by osmosis, producing concentrated urine.

Explain how transcription factors and oestrogen control gene expression.

Transcription factors bind to specific DNA sequences (promoters) to switch transcription on or off. Oestrogen is lipid-soluble and binds an intracellular receptor; the complex acts as a transcription factor, entering the nucleus to bind DNA and stimulate transcription of target genes.

Explain how epigenetics regulates gene expression without changing the base sequence.

Through reversible chemical modifications to DNA and histones: increased DNA methylation and histone deacetylation make chromatin condense, switching genes off; decreased methylation/increased acetylation allow transcription, switching genes on. These marks can be influenced by the environment.

Explain how RNA interference (RNAi) regulates gene expression.

Small interfering RNA (siRNA) or microRNA binds to a complementary mRNA molecule. This leads to the mRNA being broken down or blocked from translation, preventing the production of the corresponding protein (post-transcriptional gene silencing).

Describe how a tumour suppressor gene and an oncogene can cause cancer.

Tumour suppressor genes (e.g. p53) normally slow division or trigger apoptosis; if mutated/hypermethylated and silenced, control is lost. Proto-oncogenes become oncogenes when a mutation makes them permanently active, driving uncontrolled cell division and tumour formation.

Outline the polymerase chain reaction (PCR).

Cycles of: denaturation at ~$95\,^{\circ}\text{C}$ (separate strands), annealing at ~$55\,^{\circ}\text{C}$ (primers bind), and extension at ~$72\,^{\circ}\text{C}$ (Taq polymerase builds new strands). DNA amount doubles each cycle, giving exponential in vitro amplification.

Describe how recombinant DNA is made and inserted into a host using restriction enzymes and ligase.

A restriction enzyme cuts both the desired gene and a plasmid (vector) at the same recognition site, leaving complementary sticky ends. The gene and plasmid are joined by DNA ligase to form recombinant DNA, which is taken up by a host cell (e.g. bacterium) to be expressed.

What are marker genes used for in genetic engineering?

Marker genes (e.g. for antibiotic resistance or fluorescence) are inserted alongside the gene of interest so transformed host cells that took up the recombinant plasmid can be identified and selected from those that did not.

Describe how gel electrophoresis separates DNA fragments and its use in genetic fingerprinting.

DNA fragments are loaded into wells in agarose gel; an electric field draws negatively charged DNA toward the anode. Smaller fragments move faster and travel further, separating by size. Comparing the resulting band patterns of variable repeats (VNTRs) gives a genetic fingerprint unique to an individual.