Cambridge Pre-U Biology (Principal Subject) Syllabus

Define osmosis precisely.

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane, down a water-potential gradient.

What is water potential ($\Psi$), its components, and the water potential of pure water?

$$\Psi = \Psi_{s} + \Psi_{p}$$ where $\Psi_{s}$ is solute potential and $\Psi_{p}$ is pressure potential. Pure water at atmospheric pressure has $\Psi = 0\ \text{kPa}$; adding solute makes $\Psi$ negative.

Why is a large surface-area-to-volume ratio important for gas exchange, and how does it change with size?

A large SA:V ratio shortens diffusion distance and increases exchange area, allowing sufficient gas exchange. As an organism gets larger, volume increases faster than surface area, so SA:V falls, requiring specialised exchange surfaces.

List the features of an efficient gas-exchange surface (alveolus).

Large surface area, thin (one-cell-thick) walls for a short diffusion path, moist surface, good blood supply and ventilation to maintain a steep concentration gradient, and permeability to gases.

Write Fick's law of diffusion as it applies to gas exchange.

$$\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$$

Compare the structure and function of arteries, veins, and capillaries.

Arteries: thick muscular/elastic walls, narrow lumen, carry blood at high pressure away from heart. Veins: thinner walls, wide lumen, valves, return blood at low pressure. Capillaries: one-cell-thick endothelium for exchange with tissues.

Describe the role of haemoglobin and the shape of the oxygen dissociation curve.

Haemoglobin binds oxygen reversibly to form oxyhaemoglobin: $\ce{Hb + 4O2 <=> Hb(O2)4}$. The dissociation curve is sigmoid (S-shaped) due to cooperative binding, loading $\ce{O2}$ in the lungs and unloading it in respiring tissues.

Explain the Bohr effect.

The Bohr effect is the rightward shift of the oxygen dissociation curve when $\ce{CO2}$ concentration (and acidity) rises; haemoglobin's affinity for oxygen decreases, so more $\ce{O2}$ is released to actively respiring tissues.

Define homeostasis and explain negative feedback.

Homeostasis is the maintenance of a constant internal environment within narrow limits. Negative feedback: a deviation from the set point is detected and triggers a corrective response that reverses the change, restoring the norm.

Outline how blood glucose is lowered by insulin.

Rising blood glucose is detected by $\beta$-cells of the pancreatic islets, which secrete insulin. Insulin increases glucose uptake by cells and stimulates glycogenesis (glucose to glycogen) in the liver and muscle, lowering blood glucose.

Describe the role of the kidney nephron in ultrafiltration.

At the glomerulus, high hydrostatic (blood) pressure forces water, glucose, ions and urea through the basement membrane into the Bowman's capsule, forming glomerular filtrate; large proteins and blood cells are retained in the blood.

State the resting potential value of a neurone and the ion gradient that maintains it.

The resting potential is about $-70\ mV$ (inside negative), maintained by the sodium-potassium pump exporting $3\ \ce{Na+}$ for every $2\ \ce{K+}$ imported, plus membrane permeability to $\ce{K+}$.

Describe the events of an action potential.

A stimulus opens voltage-gated $\ce{Na+}$ channels causing depolarisation (to about $+40\ mV$); $\ce{Na+}$ channels close and $\ce{K+}$ channels open causing repolarisation; brief hyperpolarisation follows before the resting potential is restored. It is all-or-nothing.

How is an impulse transmitted across a cholinergic synapse?

Depolarisation opens $\ce{Ca^2+}$ channels; $\ce{Ca^2+}$ influx triggers vesicles to release acetylcholine by exocytosis; it diffuses across the cleft and binds receptors on the postsynaptic membrane, opening $\ce{Na+}$ channels to depolarise it; acetylcholinesterase then breaks it down.

What is the role of antagonistic muscles, using the elbow as an example?

Antagonistic muscles work in opposing pairs across a joint: at the elbow the biceps (flexor) contracts to bend the arm while the triceps (extensor) relaxes, and vice versa to straighten it.

Define gene, allele, and genotype.

A gene is a length of DNA coding for a polypeptide. An allele is an alternative version (form) of a gene at a given locus. The genotype is the alleles an organism possesses for a characteristic.

State the base-pairing rules in DNA and the bond that holds the strands.

Adenine pairs with thymine (A-T, two hydrogen bonds) and cytosine pairs with guanine (C-G, three hydrogen bonds); the two antiparallel strands are held by these hydrogen bonds between complementary bases.

Why is DNA replication described as semi-conservative?

Each new DNA molecule consists of one original (parental) strand acting as a template and one newly synthesised strand, so half of each daughter molecule is conserved from the parent.

Outline transcription.

RNA polymerase unwinds DNA and uses the template (antisense) strand to assemble a complementary mRNA strand (with uracil replacing thymine) by forming phosphodiester bonds; the mRNA then leaves the nucleus.

Outline translation.

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

What is the genetic code's degeneracy and why is it significant?

The code is degenerate because most amino acids are specified by more than one of the 64 triplet codons; this reduces the effect of point (substitution) mutations, as a changed base may still code for the same amino acid.

Name the stages of mitosis in order and state its outcome.

Prophase, metaphase, anaphase, telophase (PMAT). Outcome: two genetically identical diploid daughter cells, used for growth, repair and asexual reproduction.

How does meiosis differ from mitosis in outcome and chromosome number?

Meiosis involves two divisions producing four genetically different haploid gametes (chromosome number halved, $2n \to n$), with genetic variation from crossing over and independent assortment; mitosis gives two identical diploid cells.

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

Genotypic ratio $1\,TT : 2\,Tt : 1\,tt$; phenotypic ratio $3$ dominant : $1$ recessive.

What phenotypic ratio is expected in the F2 of a dihybrid cross between two heterozygotes ($RrYy \times RrYy$)?

$9:3:3:1$ (9 both dominant : 3 one dominant : 3 other dominant : 1 both recessive), assuming the genes assort independently.

State the Hardy-Weinberg equations and what the terms represent.

$$p + q = 1 \qquad p^{2} + 2pq + q^{2} = 1$$ where $p$ and $q$ are allele frequencies; $p^{2}$ = homozygous dominant, $2pq$ = heterozygous, $q^{2}$ = homozygous recessive frequencies.

Distinguish natural selection from genetic drift.

Natural selection is the non-random differential survival and reproduction of organisms based on advantageous alleles. Genetic drift is a random change in allele frequencies due to chance, most significant in small populations.

Differentiate directional, stabilising, and disruptive selection.

Directional: favours one extreme, shifting the mean. Stabilising: favours the intermediate phenotype, reducing variation. Disruptive: favours both extremes against the mean, potentially splitting the population.

Define a species and outline allopatric speciation.

A species is a group of organisms that can interbreed to produce fertile offspring. Allopatric speciation: a geographical barrier isolates populations, which then diverge under different selection pressures/drift until they can no longer interbreed.

How is recombinant DNA produced and inserted into a bacterial plasmid?

A restriction endonuclease cuts both the gene and the plasmid, leaving complementary sticky ends; the gene and plasmid are joined by DNA ligase (forming phosphodiester bonds), and the recombinant plasmid is taken up by a host bacterium as a vector.

Describe the three repeating temperature steps of one PCR cycle.

Denaturation at about $95\,^{\circ}\text{C}$ separates the strands; annealing at about $55\,^{\circ}\text{C}$ lets primers bind; extension at about $72\,^{\circ}\text{C}$ lets Taq polymerase synthesise new strands. The cycle roughly doubles DNA each time.

State the overall balanced equation for photosynthesis.

$$\ce{6CO2 + 6H2O ->[\text{light}][\text{chlorophyll}] C6H12O6 + 6O2}$$

Where do the light-dependent and light-independent reactions occur, and what does each produce?

Light-dependent reactions occur on the thylakoid membranes, producing ATP, reduced NADP and $\ce{O2}$ (from photolysis of water). The light-independent reactions (Calvin cycle) occur in the stroma, fixing $\ce{CO2}$ into carbohydrate using ATP and reduced NADP.

Outline the Calvin cycle.

$\ce{CO2}$ combines with ribulose bisphosphate (RuBP, 5C) catalysed by rubisco to form two molecules of glycerate-3-phosphate (GP, 3C); GP is reduced to triose phosphate (TP) using ATP and reduced NADP; some TP forms glucose, and the rest regenerates RuBP.

State the overall balanced equation for aerobic respiration.

$$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}$$ releasing energy used to make ATP.

Name the four stages of aerobic respiration and their locations.

Glycolysis (cytoplasm), the link reaction (mitochondrial matrix), the Krebs cycle (matrix), and oxidative phosphorylation/electron transport chain (inner mitochondrial membrane/cristae).

What is the net ATP yield of glycolysis and the final electron acceptor in aerobic respiration?

Glycolysis yields a net of $2$ ATP (and $2$ reduced NAD) per glucose. The final electron acceptor of the electron transport chain is oxygen, which combines with electrons and $\ce{H+}$ to form water.

Compare the products of lactate (animal) and ethanol (yeast) anaerobic respiration.

In animals, pyruvate is reduced to lactate (regenerating NAD): $\ce{C3H4O3 + 2H -> C3H6O3}$. In yeast/plants, pyruvate is decarboxylated and reduced to ethanol and $\ce{CO2}$: $\ce{C6H12O6 -> 2C2H5OH + 2CO2}$.

Define habitat, population, community, and ecosystem.

Habitat: the place an organism lives. Population: all individuals of one species in an area. Community: all populations of different species in an area. Ecosystem: the community plus the abiotic (non-living) environment, interacting as a unit.

Why does biomass and energy decrease along a food chain, and roughly by how much per trophic level?

Energy is lost at each level through respiration (heat), excretion and unineaten/undigested material, so only about $10\%$ of energy is transferred to the next trophic level; this limits chains to about $4$–$5$ links.

Outline the role of microorganisms in the nitrogen cycle.

Nitrogen-fixing bacteria convert $\ce{N2}$ to ammonia; nitrifying bacteria oxidise ammonia to nitrite then nitrate ($\ce{NH3 -> NO2^- -> NO3^-}$); denitrifying bacteria reduce nitrate back to $\ce{N2}$; decomposers (ammonifying bacteria) release ammonia from dead organic matter.

Explain how eutrophication leads to the death of aquatic organisms.

Excess nitrate/phosphate (e.g. from fertiliser run-off) causes an algal bloom; algae block light, killing plants; decomposing dead matter is broken down by aerobic bacteria whose multiplication depletes dissolved oxygen, suffocating fish and other aerobic organisms.

How does the greenhouse effect contribute to global warming?

Greenhouse gases (e.g. $\ce{CO2}$, $\ce{CH4}$) absorb and re-emit long-wave infrared radiation emitted by the Earth, trapping heat in the atmosphere; rising concentrations from burning fossil fuels enhance this, raising mean global temperature.

Distinguish in-situ from ex-situ conservation with examples.

In-situ conservation protects species in their natural habitat (e.g. nature reserves, national parks). Ex-situ conservation protects species outside their habitat (e.g. zoos, botanic gardens, seed banks, captive breeding programmes).

Define biodiversity and name the levels at which it is measured.

Biodiversity is the variety of living organisms in an area. It is measured at three levels: genetic diversity (variety of alleles within a species), species diversity (number and relative abundance of species), and ecosystem/habitat diversity.

State Simpson's Index of Diversity formula and what a higher value indicates.

$$D = 1 - \sum \left( \frac{n}{N} \right)^{2}$$ where $n$ is the number of individuals of each species and $N$ the total; a value closer to $1$ indicates higher biodiversity and a more stable ecosystem.