Respiration, muscles and the internal environment
Homeostasis and the kidney
Homeostasis keeps internal conditions within narrow limits using negative feedback: a receptor detects a change, a coordinator (e.g. hypothalamus) triggers effectors that reverse it. Thermoregulation: vasodilation, sweating (too hot); vasoconstriction, shivering, raised metabolism (too cold). The liver deaminates excess amino acids, making urea. In the nephron: ultrafiltration in the glomerulus (high hydrostatic pressure; basement membrane and podocytes filter out cells and proteins); selective reabsorption in the proximal convoluted tubule (all glucose, most water and ions; microvilli, many mitochondria, co-transport); the loop of Henle creates a high solute concentration in the medulla (ascending limb pumps out Na⁺/Cl⁻, impermeable to water); the collecting duct reabsorbs water by osmosis. ADH from the posterior pituitary makes collecting-duct cells insert aquaporins, so more water is reabsorbed when blood ψ is low.
≈ 55 minATP, glycolysis and anaerobic respiration
ATP (adenine + ribose + 3 phosphates) is the immediate energy source: hydrolysis to ADP + Pᵢ by ATP hydrolase releases a small, usable amount of energy; it is resynthesised by ATP synthase. Glycolysis (cytoplasm): glucose is phosphorylated (2 ATP used), split into 2 triose phosphate, then oxidised to 2 pyruvate, producing 4 ATP (net 2) and 2 reduced NAD. Without oxygen, NAD must be regenerated: in mammals pyruvate → lactate (lactate dehydrogenase); in yeast/plants pyruvate → ethanol + CO₂. Anaerobic respiration yields only 2 ATP per glucose. Respiratory quotient RQ = CO₂ produced ÷ O₂ used: carbohydrate 1.0, protein ~0.9, lipid ~0.7. Lipids release the most energy per gram (more H atoms).
≈ 50 minBlood glucose control
Blood glucose is kept near 5 mmol dm⁻³ by the islets of Langerhans in the pancreas. High glucose: β cells release insulin, which binds receptors on liver, muscle and fat cells; more GLUT4 transporters move to membranes; glucose is converted to glycogen (glycogenesis) and fat, and respiration increases. Low glucose: α cells release glucagon, which binds liver receptors; via the second messenger cyclic AMP and an enzyme cascade (protein kinase), glycogen is broken down (glycogenolysis) and glucose is made from amino acids and glycerol (gluconeogenesis). Adrenaline also raises glucose via cAMP. Type 1 diabetes: no insulin (autoimmune), treated with insulin injections. Type 2: cells less responsive to insulin, linked to obesity; treated with diet, exercise and drugs. Glucose is measured with test strips/biosensors using glucose oxidase.
≈ 45 minMuscle contraction and plant responses
Skeletal muscle fibres contain myofibrils made of repeating sarcomeres (Z line to Z line): thin actin and thick myosin filaments overlap. At the neuromuscular junction, ACh depolarises the sarcolemma; the impulse travels down T-tubules; the sarcoplasmic reticulum releases Ca²⁺. Ca²⁺ binds troponin, moving tropomyosin off the binding sites on actin. Myosin heads bind (cross-bridges), perform the power stroke (releasing ADP + Pᵢ), then ATP binds to detach them and is hydrolysed to recock the head. Actin slides over myosin: the I band and H zone shorten; the A band stays the same. Phosphocreatine regenerates ATP rapidly. Plants: auxin (IAA) causes cell elongation (acid growth) and phototropism; gibberellin causes stem elongation and stimulates amylase in seed germination; abscisic acid closes stomata in drought; the Venus fly trap closes by rapid changes in turgor after two hair touches.
≈ 55 minKrebs cycle and oxidative phosphorylation
Link reaction (matrix): pyruvate (3C) is decarboxylated (CO₂) and dehydrogenated (reduced NAD), forming acetyl (2C) which joins coenzyme A. Krebs cycle (matrix): acetyl CoA + oxaloacetate (4C) → citrate (6C); citrate is decarboxylated twice and dehydrogenated four times back to oxaloacetate. Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP (substrate-level). Two turns per glucose. Oxidative phosphorylation (inner membrane, cristae): reduced NAD/FAD release electrons to the electron transport chain; energy pumps H⁺ into the intermembrane space; H⁺ flow back through ATP synthase (chemiosmosis) makes ATP. Oxygen is the final electron acceptor, forming water. Total ≈ 32 ATP per glucose. Redox indicators (DCPIP, methylene blue) accept electrons and lose colour.
≈ 55 min