Homeostasis

• Stimulus, receptor, coordinator, effector, feedback — and inside the cell, a first messenger outside, a second messenger inside, and a cascade that multiplies the signal.
• Pump basal, co-transporter luminal. Made in the hypothalamus, released from the posterior pituitary. Receptor, G-protein, adenylyl cyclase, cAMP, protein kinase A, cascade, glycogen phosphorylase. H+ out, K+ in, water in, open.

• 14.1.1 I can explain — explain what is meant by homeostasis and the importance of homeostasis in mammals
• 14.1.2 I can explain — explain the principles of homeostasis in terms of internal and external stimuli, receptors, coordination systems (nervous system and endocrine system), effectors (muscles and glands) and negative feedback
• 14.1.3 I can state — state that urea is produced in the liver from the deamination of excess amino acids
• 14.1.4 I can describe — describe the structure of the human kidney, limited to: • fibrous capsule • cortex • medulla • renal pelvis • ureter • branches of the renal artery and renal vein
• 14.1.5 I can identify — identify, in diagrams, photomicrographs and electron micrographs, the parts of a nephron and its associated blood vessels and structures, limited to: • glomerulus • Bowman's capsule • proximal convoluted tubule • loop of Henle • distal convoluted tubule • collecting duct
• 14.1.6 I can describe — describe and explain the formation of urine in the nephron, limited to: • the formation of glomerular filtrate by ultrafiltration in the Bowman's capsule • selective reabsorption in the proximal convoluted tubule
• 14.1.7 I can relate — relate the detailed structure of the Bowman's capsule and proximal convoluted tubule to their functions in the formation of urine
• 14.1.8 I can describe — describe the roles of the hypothalamus, posterior pituitary gland, antidiuretic hormone (ADH), aquaporins and collecting ducts in osmoregulation
• 14.1.9 I can describe — describe the principles of cell signalling using the example of the control of blood glucose concentration by glucagon, limited to: • binding of hormone to cell surface receptor causing conformational change • activation of G-protein leading to stimulation of adenylyl cyclase • formation of the second messenger, cyclic AMP (cAMP) • activation of protein kinase A by cAMP leading to initiation of an enzyme cascade • amplification of the signal through the enzyme cascade as a result of activation of more and more enzymes by phosphorylation • cellular response in which the final enzyme in the pathway is activated, catalysing the breakdown of glycogen
• 14.1.10 I can explain — explain how negative feedback control mechanisms regulate blood glucose concentration, with reference to the effects of insulin on muscle cells and liver cells and the effect of glucagon on liver cells
• 14.1.11 I can explain — explain the principles of operation of test strips and biosensors for measuring the concentration of glucose in blood and urine, with reference to glucose oxidase and peroxidase enzymes
• 14.2.1 I can explain — explain that stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration
• 14.2.2 I can explain — explain that stomata have daily rhythms of opening and closing
• 14.2.3 I can describe — describe the structure and function of guard cells and explain the mechanism by which they open and close stomata
• 14.2.4 I can describe — describe the role of abscisic acid in the closure of stomata during times of water stress, including the role of calcium ions as a second messenger

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured, not killed; ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as variables, method, recording, graphs and evaluation rather than as theory.

• “The loop of Henle creates a concentration gradient by a countercurrent mechanism, so…” Correct Identify the loop of Henle; never explain it. Outcome 14.1.6 is limited to ultrafiltration and selective reabsorption in the PCT. You need to find the loop in a diagram or micrograph (it is in the medulla) and to know that the medulla's tissue fluid has a low water potential. How that arises is not in the syllabus.
• “ADH is made in the pituitary gland.” Correct ADH is made in the hypothalamus and released from the posterior pituitary. The two roles belong to two structures, and a question on 14.1.8 asks for both.
• “Glucose is actively pumped into the PCT cells.” Correct The pump moves Na+ out across the basal membrane. Glucose enters across the luminal membrane by co-transport with Na+, and leaves across the basal membrane by facilitated diffusion. Name the membrane every time.
• “Glucagon enters the liver cell and breaks down glycogen.” Correct Glucagon never enters the cell. It binds a cell surface receptor; a G-protein, adenylyl cyclase, cAMP and protein kinase A relay the signal; glycogen phosphorylase breaks down glycogen.
• “Insulin and glucagon both act on muscle and liver.” Correct Insulin acts on muscle and liver cells; glucagon acts on liver cells only, because muscle cells have no glucagon receptors.
• “A biosensor uses glucose oxidase and peroxidase to make a colour.” Correct Peroxidase and the colourless chromogen belong to the test strip. A biosensor has glucose oxidase on an electrode and gives a current.
• “Guard cells open the stoma when they lose water.” Correct They open when they gain water and become turgid: H+ out, K+ in, water potential down, water in by osmosis, and the thicker inner wall makes each cell bow away from the pore.
• “Homeostasis keeps conditions in the body exactly the same.” Repair It keeps the internal environment relatively constant: each factor fluctuates within narrow limits around a set point.
• “Negative feedback means the response has a bad effect.” Repair “Negative” means the response reverses the change that caused it.
• “Urea is made in the kidneys.” Repair Urea is made in the liver by the deamination of excess amino acids; the kidneys excrete it.
• “The Bowman's capsule filters the blood by active transport.” Repair Ultrafiltration is driven by hydrostatic pressure, which is high because the afferent arteriole is wider than the efferent. No ATP is used to filter.
• “The podocytes are the filter.” Repair The basement membrane is the filter. The podocytes' slits let filtrate through into the capsule space.
• “Glucose is not filtered, so there is none in urine.” Repair Glucose is in the filtrate at the same concentration as in plasma; there is none in urine because it is all reabsorbed in the PCT.
• “The Na+/K+ pump on the microvilli pumps Na+ into the cell.” Repair The pump is on the basal membrane and moves Na+ out of the cell. Na+ enters across the luminal membrane by diffusion through co-transporters, carrying glucose with it.
• “ADH makes the collecting duct pump water out.” Repair ADH causes aquaporins to be inserted into the luminal membrane; water leaves by osmosis into the medulla, whose tissue fluid has a lower water potential.
• “When you drink a lot, more ADH is released to get rid of the water.” Repair Less ADH is released; fewer aquaporins remain, less water is reabsorbed, and a large volume of dilute urine forms.
• “cAMP breaks down glycogen.” Repair cAMP activates protein kinase A, which starts the cascade; glycogen phosphorylase, the last enzyme, breaks down glycogen.
• “Amplification happens because glucagon is made in large amounts.” Repair Amplification happens inside the cell: at each step of the cascade one active enzyme activates many molecules of the next, so a few hormone molecules give a very large response.
• “Glucagon turns glucose into glycogen.” Repair Insulin promotes glycogenesis; glucagon promotes glycogen breakdown. Watch the spelling: glycogen is the store, glucagon the hormone.
• “A biosensor changes colour when glucose is present.” Repair A test strip changes colour; a biosensor produces a current proportional to glucose concentration.
• “K+ is actively pumped into guard cells.” Repair H+ is actively pumped out; K+ then diffuses in through K+ channels down its electrochemical gradient.
• “ABA closes stomata by making guard cells turgid.” Repair ABA, through Ca2+, makes K+ and negative ions leave, so water leaves by osmosis and the guard cells become flaccid.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• Interleave with the chapters that use this one. Topic 15 uses ADH, glucagon and insulin as its examples of the endocrine system (15.1.1, 15.1.2): when you reach it, re-answer “compare nervous and endocrine coordination using ADH as the hormone”. Topic 12 (respiration) supplies the ATP for every pump in this chapter: re-answer “why do PCT cells have many mitochondria”. Topic 13 (photosynthesis) is the other side of the stomatal balance: re-answer “why does stomatal closure lower the rate of photosynthesis”. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 14 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• There is no multiple-choice paper on A Level content, so Topic 14 is written out in full. A structured question asks you to state where urea is made, describe the kidney, describe and explain ultrafiltration and PCT reabsorption, describe the roles of ADH and aquaporins, set out the glucagon cascade in order, explain glucose control by negative feedback, or explain how guard cells open and close. Order and compartments matter: which membrane the pump sits in, which cells glucagon reaches, where ADH is made and where it is released.
• Kidney photomicrographs and electron micrographs to identify and measure (recall magnification and scale-bar arithmetic); tables of plasma, filtrate and urine concentrations to turn into percentages reabsorbed; blood glucose, insulin and glucagon traces to describe with figures and explain; amplification factors to multiply through a cascade; biosensor or colorimeter calibration lines to read an unknown from; stomatal aperture against time of day.
• No investigation is named in Topic 14, but it supplies two natural Paper 5 contexts. A glucose oxidase–peroxidase assay: glucose concentration (made by proportional dilution) against absorbance, a boiled-reagent control, timing of the reading as the main source of error. Stomatal impressions: pore width measured with a calibrated eyepiece graticule, compared between treatments with a t-test whose formula and critical values the question provides.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.