Engineering and Science Admissions Test (ESAT) Biology Module Syllabus

Write the reversible reaction by which haemoglobin transports oxygen.

$$\ce{Hb + 4O2 <=> Hb(O2)4}$$ Haemoglobin binds oxygen in the lungs (high $\ce{O2}$) to form oxyhaemoglobin and releases it in respiring tissues (low $\ce{O2}$).

Describe the double circulation of the human heart and why it is advantageous.

Blood passes through the heart twice per circuit: the pulmonary circuit (heart $\rightarrow$ lungs $\rightarrow$ heart) and the systemic circuit (heart $\rightarrow$ body $\rightarrow$ heart). This maintains high blood pressure to the body, allowing efficient, rapid delivery of oxygen.

Compare arteries, veins, and capillaries.

Arteries: thick muscular/elastic walls, narrow lumen, carry blood at high pressure away from the heart, no valves. Veins: thinner walls, wide lumen, low pressure, valves to prevent backflow, carry blood to the heart. Capillaries: one-cell-thick walls for exchange of substances between blood and tissues.

Describe how the alveoli are adapted for efficient gas exchange.

Large total surface area (millions of alveoli), thin (one-cell-thick) walls for a short diffusion distance, moist lining for gases to dissolve, and a dense capillary network that maintains steep concentration gradients for $\ce{O2}$ and $\ce{CO2}$.

Write the word and balanced symbol equations for aerobic respiration.

Glucose + oxygen $\rightarrow$ carbon dioxide + water (+ energy). $$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}$$

Write the equations for anaerobic respiration in human muscle and in yeast.

Human muscle: glucose $\rightarrow$ lactic acid. $$\ce{C6H12O6 -> 2C3H6O3}$$ Yeast (fermentation): glucose $\rightarrow$ ethanol + carbon dioxide. $$\ce{C6H12O6 -> 2C2H5OH + 2CO2}$$

Why does aerobic respiration release much more energy than anaerobic respiration?

Aerobic respiration completely oxidises glucose to $\ce{CO2}$ and $\ce{H2O}$, releasing about $2880\,\text{kJ\,mol}^{-1}$. Anaerobic respiration only partially breaks down glucose, leaving energy locked in lactic acid or ethanol, so it yields far less ATP per glucose molecule.

What is oxygen debt, and how is it repaid?

Oxygen debt is the extra oxygen needed after exercise to break down the lactic acid that accumulated during anaerobic respiration. It is repaid by continued deep, fast breathing, oxidising lactic acid to $\ce{CO2}$ and water (or converting it back to glucose in the liver).

Name the parts of a reflex arc in the correct order.

Stimulus $\rightarrow$ receptor $\rightarrow$ sensory neurone $\rightarrow$ relay neurone (in CNS/spinal cord) $\rightarrow$ motor neurone $\rightarrow$ effector (muscle or gland) $\rightarrow$ response. Reflexes are rapid and automatic.

How is a nerve impulse transmitted across a synapse?

The impulse triggers release of neurotransmitter molecules from the pre-synaptic neurone into the synaptic cleft. These diffuse across and bind to receptors on the post-synaptic membrane, generating a new impulse in the next neurone.

Compare nervous and hormonal (endocrine) communication.

Nervous: electrical impulses along neurones, very fast, short-lived, very precise/localised target. Hormonal: chemical messengers (hormones) carried in the blood, slower, longer-lasting, and often act on multiple target organs.

State the source and main effect of insulin and glucagon.

Both are made by the pancreas. Insulin (when blood glucose is high) stimulates cells to take up glucose and the liver to store it as glycogen, lowering blood glucose. Glucagon (when blood glucose is low) stimulates the liver to convert glycogen to glucose, raising blood glucose.

What does adrenaline do and when is it released?

Adrenaline is released from the adrenal glands in response to fear or stress (the 'fight or flight' response). It raises heart rate, breathing rate, and blood glucose concentration, and diverts blood to the muscles to prepare the body for action.

Define homeostasis and explain negative feedback.

Homeostasis is the maintenance of a constant internal environment despite external changes. Negative feedback is the control mechanism: when a factor deviates from its set point, a response is triggered that reverses the change and restores the set point.

Describe how the body responds to a rise in core body temperature.

Thermoregulatory centre detects the rise; sweat glands secrete more sweat (evaporation cools the skin), vasodilation occurs (more blood flows near the skin surface to lose heat by radiation), and hairs lie flat (erector muscles relax). These reduce body temperature back toward $37\,^{\circ}\text{C}$.

Outline the role of the kidney and ADH in controlling water balance (osmoregulation).

Kidneys filter the blood and reabsorb water/useful substances, excreting excess as urine. When blood is too concentrated, the pituitary releases more ADH (antidiuretic hormone), making the kidney tubules more permeable so more water is reabsorbed, producing less, more concentrated urine.

What is the function of the kidney nephron's main stages (filtration and reabsorption)?

Ultrafiltration in the glomerulus forces water, glucose, ions, and urea out of the blood under high pressure. Selective reabsorption returns all glucose, needed ions, and most water to the blood. The remaining urea, excess water, and ions form urine.

What are auxins, and how do they cause phototropism and gravitropism in plants?

Auxins are plant growth hormones produced in shoot/root tips. In phototropism, auxin accumulates on the shaded side of a shoot, causing greater elongation there so the shoot bends toward light (positive phototropism). In roots, auxin inhibits elongation, causing roots to bend downward (positive gravitropism).

Name three commercial uses of plant hormones.

Auxins as selective weedkillers and in rooting powders; gibberellins to end seed dormancy, induce flowering, and produce larger fruit; ethene (ethylene) to ripen fruit during transport/storage.

State the structure of DNA, including base pairing rules.

DNA is a double helix of two polynucleotide strands held by complementary base pairing: adenine pairs with thymine ($\ce{A=T}$, 2 hydrogen bonds) and cytosine pairs with guanine ($\ce{C#G}$, 3 hydrogen bonds). Each nucleotide = a sugar (deoxyribose), a phosphate, and a base.

Define gene, allele, genotype, and phenotype.

Gene: a section of DNA coding for a particular protein/trait. Allele: a different version (variant) of a gene. Genotype: the alleles an organism has (e.g. $Bb$). Phenotype: the observable characteristics resulting from the genotype and environment.

In a monohybrid cross between two heterozygous parents ($Bb \times Bb$), state the genotypic and phenotypic ratios.

Genotypic ratio $1\,BB : 2\,Bb : 1\,bb$. Phenotypic ratio $3$ dominant : $1$ recessive. The recessive phenotype has probability $\frac{1}{4}$ (25%).

Distinguish between homozygous and heterozygous, and dominant and recessive alleles.

Homozygous: two identical alleles ($BB$ or $bb$). Heterozygous: two different alleles ($Bb$). A dominant allele is expressed even with one copy; a recessive allele is only expressed when homozygous (two copies).

Explain the difference between continuous and discontinuous variation, giving an example of each.

Continuous variation: a range of values with no distinct categories, controlled by many genes and influenced by the environment (e.g. height, mass). Discontinuous variation: distinct categories with no intermediates, controlled by one or few genes (e.g. blood group, tongue rolling).

Outline Darwin's theory of evolution by natural selection.

Organisms show genetic variation; more offspring are produced than can survive, causing competition. Individuals with advantageous alleles are better adapted, are more likely to survive and reproduce (survival of the fittest), and pass on those alleles. Over many generations, beneficial alleles become more common and species evolve.

How does antibiotic resistance in bacteria provide evidence for natural selection?

Random mutation produces a few bacteria with resistance. When antibiotics are present, non-resistant bacteria die but resistant ones survive and reproduce, passing on the resistance allele. Over time the resistant population dominates — a rapid, observable example of natural selection.

Compare asexual and sexual reproduction.

Asexual: one parent, no gametes/fusion, offspring genetically identical (clones), fast, no variation (e.g. bacteria, bulbs). Sexual: two parents, fusion of haploid gametes (fertilisation), offspring genetically different, creates variation but is slower and needs two organisms.

Define a food chain and food web, and explain why energy decreases along trophic levels.

A food chain shows the flow of energy from one organism to the next (producer $\rightarrow$ consumers). A food web is interconnected food chains. Energy decreases at each trophic level because it is lost as heat (respiration), in movement, and in undigested/excreted material, so only about $10\%$ is passed on.

Describe the main stages of the carbon cycle.

$\ce{CO2}$ is removed from the air by photosynthesis (fixed into glucose). Carbon passes along food chains by feeding. It returns to the air as $\ce{CO2}$ through respiration of organisms, decomposition by microbes, and combustion of fossil fuels and wood.

Outline the role of the nitrogen cycle's key bacteria.

Nitrogen-fixing bacteria convert $\ce{N2}$ to nitrogen compounds/ammonia. Decomposers convert proteins to ammonia (ammonification). Nitrifying bacteria convert ammonia to nitrites then nitrates. Denitrifying bacteria convert nitrates back to $\ce{N2}$, returning nitrogen to the atmosphere.

State two human activities that reduce biodiversity and one consequence of each.

Deforestation: destroys habitats, reducing species numbers and adding $\ce{CO2}$ to the atmosphere. Pollution (e.g. sewage, fertiliser runoff): causes eutrophication and death of aquatic organisms. (Also: global warming from greenhouse gases shifts/destroys habitats.)

Write the word and balanced symbol equations for photosynthesis and state where it occurs.

Carbon dioxide + water $\xrightarrow{\text{light, chlorophyll}}$ glucose + oxygen. $$\ce{6CO2 + 6H2O ->[light][chlorophyll] C6H12O6 + 6O2}$$ It occurs in the chloroplasts of plant cells and is an endothermic reaction (absorbs light energy).