Everything from the 5 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Homeostasis is the maintenance of a constant internal environment: core temperature, blood glucose concentration and blood water potential are all kept within narrow limits around a set point. This matters because enzymes work best within a narrow range of temperature and pH, and cells shrink or swell if the water potential of the tissue fluid changes. A change away from the set point is a stimulus, which may be internal (more heat from respiration during exercise) or external (a cold day). Receptors detect it; a coordination system, either the nervous system (fast, short-lived, impulses along neurones) or the endocrine system (slower, longer-lasting, hormones in the blood), sends instructions; effectors, which are muscles or glands, bring about a response. In negative feedback the response reverses the original change, so the value returns towards the set point.
Negative feedback does not mean that something bad happens. It means the response is in the opposite direction to the change: a rise causes responses that lower the value, a fall causes responses that raise it.
Arterioles do not move up and down in the skin. Vasodilation and vasoconstriction change the diameter of arterioles, which changes how much blood flows through the surface capillaries.
Excess amino acids cannot be stored, so in the liver they are deaminated: the amine group is removed as ammonia and converted into urea, which the kidneys excrete. Each kidney has a fibrous capsule, an outer cortex, an inner medulla and a renal pelvis that drains into the ureter; it is supplied by branches of the renal artery and drained by the renal vein. Its functional units are about a million nephrons: glomerulus and Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. In ultrafiltration, high hydrostatic pressure in the glomerulus (the afferent arteriole is wider than the efferent) forces water and small solutes through the endothelium, the basement membrane and the gaps between podocytes; cells and large proteins stay in the blood. In selective reabsorption in the PCT, Na+/K+ pumps and cotransporters, with microvilli and many mitochondria, return all the glucose and amino acids, most ions and much water to the blood.
Urea is made in the liver, not in the kidney. The kidney only removes urea that the blood brings to it.
Ultrafiltration is not selective: everything small enough passes through, including glucose and amino acids. The selecting is done afterwards, by reabsorption in the PCT.
Osmoregulation is the control of the water potential of the blood. Osmoreceptors in the hypothalamus detect it. When the water potential falls (after sweating, or eating salty food), they lose water by osmosis and shrink, stimulating neurosecretory cells; these made ADH (antidiuretic hormone) in the hypothalamus and it is released from their endings in the posterior pituitary gland into the blood. ADH binds to receptors on the cells of the collecting ducts, triggering vesicles containing aquaporins (water channel proteins) to fuse with the membrane facing the lumen. The duct wall becomes more permeable, more water moves by osmosis into the concentrated tissue fluid of the medulla and then the blood, and a small volume of concentrated urine is made. When the water potential rises, less ADH is released, aquaporins are taken back into vesicles and a large volume of dilute urine results. This is negative feedback.
ADH does not pump water. It only changes the permeability of the collecting duct to water; the water moves passively by osmosis into the concentrated medulla.
Blood glucose is kept near 5 mmol dm−3 by negative feedback. The α and β cells of the islets of Langerhans in the pancreas are both receptors and the source of the hormones. When glucose is high, β cells secrete insulin: in muscle cells vesicles with GLUT4 transporters fuse with the cell surface membrane, so more glucose enters by facilitated diffusion; in liver cells (and muscle) enzymes for glycogenesis are activated, and glucose is respired faster. When glucose is low, α cells secrete glucagon, which acts on liver cells: it binds to a cell surface receptor, causing a conformational change that activates a G protein, which stimulates adenylyl cyclase to make cyclic AMP (the second messenger). cAMP activates protein kinase A, starting an enzyme cascade of phosphorylations that amplifies the signal until glycogen phosphorylase breaks down glycogen to glucose. Test strips and biosensors measure glucose using glucose oxidase (glucose to gluconic acid and hydrogen peroxide) and, on strips, peroxidase with a colourless chromogen that becomes coloured.
The hormone is the first messenger; it never enters the cell. cAMP is the second messenger that carries the signal through the cytoplasm.
Glycogen, glucagon and glycogenesis are easy to confuse. Glucagon is the hormone; glycogen is the storage polysaccharide; glycogenesis makes glycogen; glycogenolysis breaks it down.
Stomata open and close in response to environmental conditions. Opening lets carbon dioxide diffuse in for photosynthesis; closing reduces water loss by transpiration, so the size of the aperture is a balance between the two. Stomata usually open in light and close in darkness, and they also follow a daily (circadian) rhythm that continues even in constant conditions. Each stoma is bordered by two guard cells with chloroplasts and unevenly thickened walls: the inner wall next to the pore is thicker and cellulose microfibrils run around the cell. To open, ATP-driven proton pumps move H+ out, K+ ions flow in, the water potential of the guard cells falls, water enters by osmosis and the turgid cells bow apart. To close, K+ leaves, water follows and the cells become flaccid. In water stress the plant makes abscisic acid (ABA), which binds to guard cell receptors and causes Ca2+ ions to enter the cytoplasm as a second messenger; this opens channels so K+ and anions leave and the stomata close.
Stomata do not open because the plant needs to transpire. They open to let carbon dioxide in; water loss is the unavoidable cost, which is why they close when water is short.
When K+ enters, the guard cell's water potential becomes lower (more negative), so water moves in. Do not say the water potential rises as ions enter.