Topic 7: Run for your Life — Edexcel A-Level Biology A (Salters-Nuffield)

Movement results from interaction between muscles, tendons, skeleton and ligaments. Muscles contract to generate force; tendons attach muscle to bone and transmit force; bones act as levers; ligaments connect bone to bone and stabilise joints. Antagonistic pairs work opposite each other: as one contracts (agonist), the other relaxes (antagonist), enabling controlled movement. Extensors increase joint angle (e.g., triceps brachii at elbow), flexors decrease joint angle (e.g., biceps brachii). Example: elbow flexion involves biceps contracting and triceps relaxing; extension reverses this.

Skeletal muscle contraction is explained by the sliding filament theory: myosin heads bind to actin, forming cross-bridges, and pivot, pulling actin filaments towards the sarcomere centre so the sarcomere shortens. At rest, tropomyosin blocks myosin-binding sites on actin; troponin holds tropomyosin in place. When a nerve impulse arrives, Ca²⁺ released from the sarcoplasmic reticulum binds troponin, causing tropomyosin to move and expose the binding sites. Myosin heads, energised by ATP hydrolysis catalysed by ATPase, attach, pivot (power stroke), then detach when fresh ATP binds. Repeated cycles slide the filaments, shortening the sarcomere and the muscle fibre. ATP is needed both for the power stroke and for detachment and re-energising of the myosin head.

Aerobic respiration is the controlled release of energy from respiratory substrates such as glucose. Overall, the substrate is split, carbon dioxide is released as a waste product, and hydrogen atoms are reunited with atmospheric oxygen to form water, releasing a large amount of energy. This summary can be represented as C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. The process is not a single explosive reaction but a many-stepped sequence, each step controlled and catalysed by a specific intracellular enzyme. This allows energy to be released in small, usable amounts, often captured as ATP, rather than as heat. The steps occur in the cytoplasm and mitochondria, and the enzymes are specific to each reaction, so the overall rate can be regulated.

Glycolysis splits one hexose, such as glucose, into two three-carbon pyruvate molecules in the cytoplasm, and it runs in both aerobic and anaerobic respiration. First the hexose is phosphorylated using phosphate from ATP, which raises the molecule's reactivity and traps it in the cell; this investment is later repaid. Oxidation then removes hydrogen, forming reduced coenzyme, and substrate-level phosphorylation produces ATP directly. In aerobic respiration pyruvate enters a mitochondrion for the link reaction and Krebs cycle. In anaerobic respiration pyruvate is reduced to lactate, regenerating the oxidised coenzyme so glycolysis can continue when oxygen is absent.

After glycolysis, pyruvate enters a mitochondrion. In the link reaction pyruvate is decarboxylated, releasing CO2, and dehydrogenated, forming reduced NAD; the remaining two-carbon unit enters the Krebs cycle. The Krebs cycle completes the oxidation of glucose, releasing more CO2 and producing ATP, reduced NAD and reduced FAD. These steps occur in the mitochondria because the enzymes for the link reaction and Krebs cycle are located there, whereas glycolysis enzymes are in the cytoplasm. The reduced coenzymes carry hydrogen to the electron transport chain, where oxidative phosphorylation generates most ATP.

Oxidative phosphorylation occurs on the inner mitochondrial membrane. Reduced NAD and reduced FAD deliver electrons to the electron transport chain (ETC). As electrons pass along carriers, energy pumps protons (H⁺) from the matrix into the intermembrane space, creating a proton gradient. Protons then flow back through ATP synthase, which uses the energy of this flow to phosphorylate ADP to ATP. This process is chemiosmosis. The ETC also regenerates NAD and FAD so glycolysis and the Krebs cycle can continue. Oxygen is the final electron acceptor, forming water. The proton gradient stores potential energy; ATP synthase converts it to chemical energy in ATP. For example, one reduced NAD can pump enough protons to produce about 2.5 ATP, though the exact yield varies. Students should be able to describe the sequence and explain the roles of the ETC, chemiosmosis and ATP synthase.

After a period of anaerobic respiration in animals, lactate accumulates in muscle tissue and diffuses into the blood. It is transported to the liver, where it is oxidised back to pyruvate. This oxidation reaction requires oxidised NAD, which is converted into reduced NAD. The resulting pyruvate has two main fates: it can enter the link reaction and Krebs cycle to be metabolised aerobically, producing ATP, or it can be converted into glucose via gluconeogenesis. The extra oxygen consumed after exercise (oxygen debt) acts as the final electron acceptor in the electron transport chain. This allows the reoxidation of reduced NAD, sustaining ATP production for recovery processes.

Cardiac muscle is myogenic, meaning it initiates its own contractions without external nervous stimulation. The heartbeat is coordinated by an electrical pathway starting at the sinoatrial node (SAN), which acts as a pacemaker by generating waves of depolarisation across the atria, causing atrial systole. A layer of non-conducting tissue prevents these waves from reaching the ventricles directly. Instead, the electrical activity reaches the atrioventricular node (AVN), which introduces a slight delay to allow the ventricles to fill. The impulse then travels down the bundle of His and into the Purkyne fibres, triggering ventricular systole from the apex upwards. Electrocardiograms (ECGs) record this electrical activity; abnormal traces help diagnose cardiovascular diseases like arrhythmias or myocardial infarctions.

Cardiac output is the volume of blood pumped by one ventricle per minute, calculated as cardiac output = heart rate × stroke volume. If heart rate is 70 beats min⁻¹ and stroke volume is 70 cm³, cardiac output = 70 × 70 = 4900 cm³ min⁻¹, which is 4.9 dm³ min⁻¹. During exercise, increased cardiac output and ventilation deliver oxygen more rapidly to respiring muscles and remove carbon dioxide faster. Chemoreceptors detect rising carbon dioxide concentration in the blood; the cardiovascular control centre in the medulla oblongata alters sympathetic and parasympathetic output to the sinoatrial node, raising heart rate, while the ventilation centre in the medulla oblongata stimulates intercostal muscles and the diaphragm to increase ventilation rate and depth.

A muscle fibre is a multinucleate cell with a sarcolemma, sarcoplasm and many myofibrils. Each myofibril repeats sarcomeres of actin and myosin, giving striations; T-tubules and sarcoplasmic reticulum spread excitation. Slow twitch fibres have many mitochondria, myoglobin and capillaries, so they resist fatigue for endurance. Fast twitch fibres have fewer mitochondria and less myoglobin, but more glycogen and glycolytic enzymes, so they contract powerfully for short bursts. Some fast fibres are oxidative-glycolytic, combining moderate fatigue resistance with speed. Structure links to physiology: capillary density and myoglobin support aerobic respiration, while myosin ATPase activity and sarcoplasmic reticulum calcium release affect contraction speed.

Negative feedback reverses a change: a receptor detects a deviation from the set point, a coordination centre processes it, and an effector responds to restore the original level. This keeps systems within narrow limits, as in thermoregulation, blood glucose and blood pH. Positive feedback amplifies a change, moving the system further from the set point; examples include oxytocin during childbirth and platelet activation in clotting. The principle is that negative feedback opposes the initial stimulus, so fluctuations are corrected and homeostasis is maintained. Positive feedback is less common and usually has a defined end point, such as delivery of a baby or formation of a clot.

Homeostasis maintains a stable internal environment despite external change. During exercise, muscle respiration releases heat, so core temperature rises. Dynamic equilibrium means conditions fluctuate around a set point rather than staying fixed. The hypothalamus contains thermoreceptors that detect blood temperature and coordinates responses. When too hot, vasodilation of skin arterioles and increased sweating increase heat loss; when too cold, vasoconstriction and reduced sweating conserve heat, while shivering generates heat through increased muscle respiration. Negative feedback restores the set point. For example, during a 10 km run, core temperature may rise by 1–2 °C; sweating and skin blood flow increase, then return to normal after exercise. This prevents enzyme denaturation and maintains optimum conditions for metabolism.

Data on exercise and health must be analysed and interpreted carefully. Too much exercise can cause wear and tear on joints, such as osteoarthritis of the knee in long-distance runners, and can suppress the immune system, increasing susceptibility to infections. Too little exercise increases the risk of obesity, cardiovascular disease (CVD) and diabetes. When interpreting data, distinguish correlation from causation: a correlation shows a relationship between variables, but causation requires evidence that one variable directly causes the other. For example, a negative correlation between exercise level and CVD incidence does not prove exercise prevents CVD, because other factors such as diet or genetics may be involved. Evaluate study design, sample size and confounding variables before drawing conclusions.

Medical technology restores or replaces damaged tissues so that injured or disabled people can take part in sport. Keyhole surgery uses small incisions and an endoscope with a camera, so surgeons repair joints such as the knee or shoulder with less tissue damage, less pain and faster rehabilitation than open surgery. Prostheses replace missing limbs or joint surfaces; materials such as titanium, carbon fibre and polyethylene are chosen for strength, low density and biocompatibility. A running blade stores elastic strain energy and returns it during push-off, while a hip or knee replacement restores a pain-free range of movement. Assessment requires linking each technology to a specific sporting benefit, such as earlier return to training or improved mobility, rather than describing surgery alone.

Performance-enhancing substances include anabolic steroids, erythropoietin (EPO), human growth hormone and stimulants. Ethical positions differ. A consequentialist or utilitarian view judges an action by its outcomes: if doping harms health, misleads spectators or damages fair competition, it is wrong. A deontological view judges the act itself: doping breaks the rules and the athlete's duty to compete honestly, so it is wrong regardless of benefits. A libertarian or autonomy view argues that informed adults should be free to accept risks, provided opponents also consent. A fairness view stresses that doping undermines equal opportunity and the spirit of sport. Discussion requires weighing evidence, considering stakeholders such as athletes, spectators and governing bodies, and reaching a justified conclusion.

Gene expression is controlled at transcription. Transcription factors are proteins that bind specific DNA sequences near a gene's promoter, helping or blocking RNA polymerase. Activators increase transcription; repressors reduce it. Hormones can act as signals: a lipid-soluble hormone such as oestrogen diffuses through the cell membrane, binds its receptor, and the complex enters the nucleus to bind DNA as a transcription factor, switching genes on or off. This allows cells to respond to internal and external signals without changing the genome. For example, in a liver cell, cortisol binds an intracellular receptor, and the complex activates genes for gluconeogenesis. The same gene may be active in one cell type but silent in another because different transcription factors are present.

Investigating the rate of respiration typically involves using a respirometer to measure oxygen consumption in organisms like germinating seeds or maggots. A chemical such as soda lime or potassium hydroxide is placed in the apparatus to absorb the carbon dioxide produced. Consequently, any decrease in gas volume is solely due to oxygen uptake. As oxygen is consumed, the pressure drops, drawing a coloured liquid along a capillary tube. By measuring the distance moved by the fluid over a set time, and knowing the capillary's internal radius, you can calculate the volume of oxygen absorbed (using πr²h). Dividing this volume by the time taken and the mass of the organisms gives the rate of respiration, often expressed in cm³ g⁻¹ min⁻¹.

In this core practical, a spirometer trace is used to measure breathing before and after exercise. Tidal volume is the volume of air moved into or out of the lungs in one normal breath, read as the vertical height of a trace wave converted using the spirometer calibration. Breathing rate is the number of breaths per minute, found by counting waves over a known time. Respiratory minute ventilation is tidal volume multiplied by breathing rate, for example 500 cm³ × 12 min⁻¹ = 6000 cm³ min⁻¹. Oxygen consumption is found from the slope of the trace as the oxygen in the closed spirometer is used up; the carbon dioxide produced is absorbed, so the trace drifts downwards. Exercise increases tidal volume, breathing rate, minute ventilation and oxygen consumption.