Homeostasis: Biology 9700 (Cambridge International AS & A Level)

A person exercises vigorously in a warm room, and their core body temperature rises above its normal set point of about 37°C. Within a few minutes, thermoreceptors detect this change, and a response is triggered that begins to lower the core body temperature back towards the set point.

Which statement correctly explains why this response is an example of negative feedback, and correctly identifies the receptors and effectors involved?

The first two stages of urine formation in a mammalian kidney are ultrafiltration in the renal (Malpighian) corpuscle and selective reabsorption in the proximal convoluted tubule (PCT).

(a) Explain how the arrangement of blood vessels supplying and draining the glomerulus produces a high hydrostatic pressure, and describe how this pressure results in the formation of glomerular filtrate within the Bowman's capsule. [4]

(b) Blood plasma is the liquid portion of the blood, containing water and dissolved solutes (including large plasma proteins); blood cells are carried within this plasma but are not part of it. Explain why both blood cells and plasma proteins are absent from the glomerular filtrate, even though most other plasma solutes (such as glucose, urea and mineral ions) do appear in it. [2]

(c) Describe how the cells lining the proximal convoluted tubule are adapted for the selective reabsorption of glucose and amino acids, including the role of active transport. [4]

A hiker becomes dehydrated after a long walk in hot conditions, causing the water potential of their blood plasma to fall (become more negative) below its normal value.

(a) Explain, with reference to osmoreceptors and the hypothalamus, how this fall in blood water potential leads to an increased release of antidiuretic hormone (ADH) into the blood. [3]

(b) Explain how an increase in the concentration of ADH in the blood affects the permeability of the collecting duct wall to water, and hence how water is reabsorbed from the fluid in the collecting duct. [4]

(c) Describe the resulting effect of this increased ADH concentration on the volume and concentration of the urine produced, and state what would happen to ADH secretion and to urine output if the hiker instead drank a large volume of water so that blood water potential rose above normal. [3]

After a meal, a person's blood glucose concentration temporarily rises above its normal set point; several hours later, after fasting overnight, it falls again. A little while after the meal, the person also uses a home test strip, containing the enzymes glucose oxidase and peroxidase together with a colourless dye, to check whether any glucose is present in a urine sample.

(a) Name the cells in the pancreas that secrete insulin, and describe two ways in which insulin lowers blood glucose concentration when it is raised. [4]

(b) Glucagon binds to a receptor on the cell-surface membrane of a liver cell and triggers the production of a second messenger inside the cell. Describe the role of this second messenger in bringing about an increase in blood glucose concentration. [4]

(c) Describe how glucose oxidase and peroxidase act together in the test strip to produce a colour change in the presence of glucose, and explain why the appearance of glucose in urine can indicate that blood glucose concentration has remained persistently too high. [2]

During a period of drought, a plant's leaves show a reduced stomatal aperture (the pore of each stoma is much less open) compared with the same species growing in well-watered soil.

Which statement correctly describes the role of guard cells and abscisic acid (ABA) in bringing about this closure of the stomata?

Water is reabsorbed from the fluid in the collecting duct because the tissue fluid of the renal medulla has a very low (very negative) water potential. This gradient is generated by the loop of Henle before ADH ever acts on the collecting duct.

(a) Describe the gross (macroscopic) structure of the kidney, and state where the glomerulus and Bowman's capsule of a nephron are located within this structure. [3]

(b) Describe the general path taken by fluid through a nephron, naming the regions of the nephron in order and stating which of these regions lie in the medulla. [3]

(c) Explain how the loop of Henle establishes a region of very low water potential in the tissue fluid of the medulla, and explain why this gradient is essential for the reabsorption of water from the collecting duct. [4]

Diabetes mellitus is a condition in which blood glucose concentration is not properly controlled, and it exists as two distinct forms, type 1 and type 2.

(a) State what is meant by diabetes mellitus, and explain the underlying cause of type 1 diabetes mellitus. [3]

(b) Explain the underlying cause of type 2 diabetes mellitus, and state one risk factor associated with its development. [3]

(c) State one way in which the usual treatment of type 1 diabetes mellitus differs from the usual treatment of type 2 diabetes mellitus, and explain why this difference follows from their different underlying causes. [2]

As a well-watered plant is exposed to daylight in the morning, its stomata open, allowing gas exchange for photosynthesis to increase.

(a) Describe the arrangement and structure of a pair of guard cells around a stoma, and explain how an increase in guard cell turgor causes the stomatal pore to open. [3]

(b) Explain, with reference to hydrogen ions, potassium ions and water potential, how light stimulates guard cells to open the stomatal pore. [4]

(c) Suggest one advantage to the plant of opening its stomata during daylight hours and closing them again at night. [2]

A person walks outside into very cold conditions without warm clothing, and their core body temperature begins to fall below its normal set point of about 37°C.

(a) Name the type of receptor that detects this fall in temperature, and state the location of the coordination centre that processes this information. [2]

(b) Describe two involuntary physiological responses (not behavioural responses, such as putting on more clothes) that occur in the skin and muscles to reduce heat loss and/or generate heat, explaining how each response helps to return core temperature towards the set point. [4]

(c) Explain how negative feedback prevents core temperature from continuing to rise once it has returned to the set point, so that these responses do not overcorrect. [2]

The islets of Langerhans in the pancreas contain two main types of hormone-secreting cell. Each type directly detects blood glucose concentration itself and responds by changing its rate of hormone secretion, forming part of a negative feedback loop that controls blood glucose concentration.

Which statement correctly identifies these cell types, the hormone each secretes, and the change in blood glucose concentration that increases secretion of that hormone?