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Flashcards · 9700 · A Level Topic 14
Homeostasis (A Level) flashcards
13 cards on A Level Topic 14, subtopics 14.1 to 14.2. Pick a subtopic or study them all.
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14.1 Homeostasis in mammals
- Homeostasis and negative feedback
- Homeostasis: keeping the internal environment constant, within narrow limits, despite changes. Negative feedback: a change triggers a response that reverses it, returning the variable towards the set point.
- Deamination
- Removal of the amino group from excess amino acids in the liver, producing urea.
- How does ultrafiltration happen in the glomerulus?
- The afferent arteriole is wider than the efferent, giving high hydrostatic pressure. Fluid is forced through endothelial pores, the basement membrane (the filter) and podocyte slits. Blood cells and most plasma proteins stay in the blood.
- Selective reabsorption of glucose in the PCT
- Na⁺–K⁺ pumps on the basal membrane move Na⁺ out of the cell (ATP). Na⁺ enters from the filtrate by co-transport with glucose. Glucose leaves the basal side by facilitated diffusion into the blood. Water follows by osmosis.
- How does ADH respond to a fall in blood water potential?
- Osmoreceptors in the hypothalamus are stimulated; more ADH is released from the posterior pituitary. Vesicles with aquaporins fuse with collecting duct membranes, so more water is reabsorbed by osmosis: a small volume of concentrated urine.
- How does glucagon cause glucose release from a liver cell?
- Glucagon binds its receptor → G-protein activated → adenylyl cyclase makes cAMP (second messenger) from ATP → protein kinase A starts an enzyme cascade (amplification) → glycogen phosphorylase breaks glycogen to glucose.
- Insulin vs glucagon
- Insulin: β cells; released when blood glucose rises; acts on muscle and liver; more GLUT4 in muscle membranes, glycogenesis. Glucagon: α cells; released when glucose falls; acts on liver only; glycogenolysis and gluconeogenesis.
- Glycogenesis vs glycogenolysis vs gluconeogenesis
- Glycogenesis: making glycogen from glucose. Glycogenolysis: breaking glycogen down to glucose. Gluconeogenesis: making new glucose from non-carbohydrates.
- Glucose test strip vs biosensor
- Test strip: enzymes and a chromogen on a pad; colour compared with a chart; semi-quantitative. Biosensor: glucose oxidase immobilised on an electrode; produces a current and a digital reading of the present blood concentration.
14.2 Homeostasis in plants
- The stomatal trade-off
- Open stomata allow CO₂ uptake by diffusion for photosynthesis; closed stomata reduce water loss by transpiration. They open in light and low leaf CO₂ and close in the dark and under water stress.
- How do stomata open?
- Proton pumps (ATP) pump H⁺ out of guard cells; K⁺ channels open and K⁺ diffuses in. Water potential falls and water enters by osmosis. The turgid guard cells bend, because of their uneven walls, and the stoma opens.
- How does abscisic acid (ABA) close stomata?
- Under water stress ABA binds receptors on guard cell membranes. Ca²⁺ enters as a second messenger and proton pumps are inhibited. K⁺ and anions leave, water potential rises, water leaves by osmosis, and the flaccid guard cells close the stoma.
- Features of guard cells
- Thick inner wall next to the pore and thin outer wall; radial cellulose microfibrils; proton pumps, K⁺ channels, chloroplasts and mitochondria.