Organisms exchange substances with their environment
Blood vessels and haemoglobin
Arteries: thick muscle and elastic tissue, narrow lumen, high pressure. Veins: wide lumen, thin walls, valves, low pressure. Capillaries: one-cell-thick endothelium, short diffusion distance. Tissue fluid forms at the arterial end of capillaries where hydrostatic pressure exceeds the water potential effect of plasma proteins; it returns at the venous end and via lymph. Haemoglobin binds O₂ cooperatively, giving an S-shaped (sigmoid) dissociation curve: high saturation in lungs, unloading in respiring tissues. Bohr shift: more CO₂ lowers affinity (curve shifts right), so more O₂ is released where it's needed. Fetal haemoglobin has higher affinity (curve left). CO₂ is carried mostly as HCO₃⁻ (carbonic anhydrase in red cells), with the chloride shift balancing charge.
≈ 50 minGas exchange surfaces
Efficient exchange surfaces follow Fick's law: rate ∝ (surface area × concentration difference) ÷ thickness. Alveoli: huge surface area, one-cell-thick squamous epithelium next to capillary endothelium, steep gradient kept by ventilation and blood flow, elastic fibres for recoil. Airways: cartilage (trachea, bronchi) keeps them open; goblet cells make mucus; cilia sweep it away; smooth muscle controls diameter. Ventilation: inspiration (diaphragm and external intercostals contract, volume ↑, pressure ↓); expiration largely passive. Pulmonary ventilation = tidal volume × breathing rate. Fish gills use countercurrent flow to keep a gradient along the whole lamella; insects use tracheae and spiracles; leaves use stomata and air spaces.
≈ 45 minXylem, phloem and transpiration
Xylem vessels are dead, hollow tubes with no end walls and lignified walls (strength, waterproof) with pits. Water evaporates from mesophyll cells and diffuses out through stomata (transpiration), creating tension; cohesion between water molecules (hydrogen bonds) and adhesion to vessel walls pull a continuous column up: cohesion–tension. Water crosses the root via the apoplast (cell walls) and symplast (cytoplasm), but the Casparian strip forces it into the symplast at the endodermis. Transpiration increases with light, temperature and wind and decreases with humidity; measured with a potometer (water uptake). Xerophytes reduce water loss (sunken stomata, hairs, rolled leaves, thick cuticle). Phloem sieve tubes with companion cells move sucrose by mass flow from source to sink: sucrose loaded by co-transport lowers ψ, water enters, hydrostatic pressure pushes sap.
≈ 50 minDigestion and absorption
Digestion hydrolyses large insoluble molecules into small soluble ones. Carbohydrates: amylase (salivary glands, pancreas) hydrolyses starch to maltose; membrane-bound disaccharidases on the ileum epithelium (maltase, sucrase, lactase) produce monosaccharides. Proteins: endopeptidases cut within chains (more ends); exopeptidases remove terminal amino acids; membrane-bound dipeptidases split dipeptides. Lipids: bile salts emulsify fats into small droplets (larger surface area) and form micelles; lipase hydrolyses ester bonds to monoglycerides and fatty acids. Absorption in the ileum: glucose and amino acids by co-transport with Na⁺ (gradient maintained by the Na⁺/K⁺ pump); micelles carry fatty acids to epithelial cells, which diffuse in, are rebuilt into triglycerides in the SER, packaged with proteins into chylomicrons, exocytosed and enter lacteals (lymph). Villi and microvilli give a huge surface area; thin epithelium; good blood supply.
≈ 45 minThe heart and cardiac cycle
Mammals have a double circulation: pulmonary (right ventricle → lungs) and systemic (left ventricle → body). The left ventricle wall is thickest to pump blood at high pressure. Cardiac cycle: atrial systole (atria contract, AV valves open), ventricular systole (ventricles contract; pressure rises; AV valves close when ventricular pressure exceeds atrial; semilunar valves open when it exceeds arterial), diastole (relaxation; semilunar valves close; atria fill). Valves open and close only due to pressure differences. Heartbeat is myogenic: the SAN (pacemaker) sends a wave of excitation over the atria; the AVN delays it, then passes it down the bundle of His and Purkyne fibres so ventricles contract from the apex up. Cardiac output = stroke volume × heart rate.
≈ 45 min