Topic 14: Homeostasis
 

14.1 Homeostasis in mammals

 

​Students should be able to:

1) explain what is meant by homeostasis and the importance of homeostasis in mammals

2) 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

3) state that urea is produced in the liver from the deamination of excess amino acids

4) describe the structure of the human kidney, limited to:

• fibrous capsule

• cortex

• medulla

• renal pelvis

• ureter

• branches of the renal artery and renal vein

5) 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

6) 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

7) relate the detailed structure of the Bowman’s capsule and proximal convoluted tubule to their functions in the formation of urine

8) describe the roles of the hypothalamus, posterior pituitary gland, antidiuretic hormone (ADH), aquaporins and collecting ducts in osmoregulation

9) 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

10) 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

11) 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

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1) Explain what is meant by homeostasis and the importance of homeostasis in mammals

Homeostasis is the process by which multicellular animals (such as mammals) maintain their internal environment within narrow, optimal limits despite fluctuations in external or internal conditions. In mammals, this means keeping variables such as core body temperature, blood glucose concentration, blood water-potential, pH, and the composition of tissue fluid relatively stable. 

The importance of this regulation cannot be overstated: enzymes and biochemical pathways in mammalian cells operate optimally only within certain ranges of temperature, pH and ionic environment. If these internal conditions stray too far from optimum, enzyme activity declines, structural integrity of proteins may be compromised, and cells may malfunction or die. 
Thus, homeostasis ensures that tissue fluid remains a favourable medium for cells, that metabolic reactions proceed efficiently, and that organs and systems in the mammal function together reliably. Without homeostasis, mammals would be unable to cope with changing environments or internal demands.

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2) Explain the principles of homeostasis in terms of internal and external stimuli, receptors, coordination systems (nervous and endocrine) and effectors (muscles and glands)

The principles of homeostasis involve a control system made up of several components working together: stimuli, receptors, an integrating/coordination centre, effectors, and feedback mechanisms.

• Stimuli: changes (either external or internal) that disturb the steady state. For example, a drop in ambient temperature (external) or a rise in blood glucose after a meal (internal).

• Receptors: cells or sensors that detect the deviation. For instance, thermoreceptors in the skin or hypothalamus detect temperature change; chemoreceptors in the pancreas detect blood-glucose levels.

• Coordination systems: Once receptors detect change, information is sent to a coordination centre such as part of the nervous system (e.g., hypothalamus, brainstem) or an endocrine gland. The nervous system provides rapid, targeted responses; the endocrine system provides slower, longer-lasting regulation via hormones.

• Effectors: these are muscles or glands that carry out responses to restore balance. For example, sweat glands increase output to cool the body; the pancreas (as a gland) secretes insulin to lower blood glucose.

• Feedback (typically negative feedback): The system has the property that when the variable returns towards the set point, the response is reduced or ceased, thus preventing over-correction.

By detecting changes (stimuli), coordinating a response (via nervous or endocrine systems), and actuating effectors (muscles/glands), mammals maintain internal variables within optimum ranges.

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3) Outline the roles of the nervous system and endocrine system in coordinating homeostatic mechanisms, including thermoregulation, osmoregulation and the control of blood glucose concentration

The nervous system and endocrine system are the two main coordination systems for homeostasis, each with distinct but complementary roles.
The nervous system uses rapid electrical impulses and synaptic transmission to effect quick responses to change. For example, if the body temperature falls rapidly, the skin vasoconstricts and skeletal muscles may shiver — both under nervous control.
The endocrine system uses hormones secreted into the blood, which act more slowly but tend to create longer-lasting effects. For example, in osmoregulation, when blood water potential drops, the hypothalamus triggers release of antidiuretic hormone (ADH) from the posterior pituitary; this hormone acts on the kidney to increase water reabsorption.
In blood-glucose regulation, endocrine control is central: the pancreas secretes insulin or glucagon in response to blood-glucose changes. However, nervous inputs (via the autonomic system) modulate hormone release in response to exercise or stress.
Thus, thermoregulation often uses rapid nervous responses, osmoregulation is largely endocrine-driven (although nervous sensors are involved), and blood-glucose control uses both endocrine (primary) and nervous (secondary) regulation. The coordination of both systems is vital for effective homeostasis.

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4) Describe the deamination of amino acids and outline the formation of urea in the urea cycle (biochemical detail of the urea cycle is not required)

When amino acids are in excess (for example after abundant protein intake), the amino groups cannot be stored and must be removed. In the liver, amino acids undergo deamination: the amino (-NH₂) group is removed, producing ammonia (NH₃) (or ammonium ions NH₄⁺). The remaining carbon skeletons can be oxidised for energy, converted to fats or carbohydrates, or used in other metabolic pathways.
As ammonia is toxic, mammals convert it into a less-toxic compound, urea, via the urea cycle. The liver produces urea, which is then transported in blood to the kidneys, where it is excreted in urine.
This process helps maintain nitrogen balance and prevents accumulation of toxic ammonia. It is thus part of homeostasis in mammals: removal of nitrogenous waste and maintaining safe internal chemical conditions.

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5) Describe the gross structure of the kidney and the detailed structure of the nephron with its associated blood vessels (using photomicrographs and electron micrographs)

The kidneys are paired organs that sit in the abdominal cavity and are responsible for filtering blood, removing waste products, and regulating the water and ion content of body fluids. Each kidney has an outer cortex and an inner medulla region. The cortex contains many renal corpuscles and proximal/distal tubules; the medulla contains loops of Henle and collecting ducts which descend into the renal pelvis. The renal artery supplies blood; the renal vein drains blood; the ureter carries away urine.
Within the kidney, the functional unit is the nephron, which is associated with its own vascular supply. Each nephron includes:

• A glomerulus (a tuft of capillaries) supplied by an afferent arteriole and drained by an efferent arteriole.

• Bowman's capsule surrounding the glomerulus, where ultrafiltration occurs.

• The proximal convoluted tubule (PCT) in the cortex, where most reabsorption occurs.

• The loop of Henle (descending and ascending limbs) extending into the medulla, helping create a concentration gradient.

• The distal convoluted tubule (DCT) in the cortex, involved in regulated ion and water reabsorption.

• The collecting duct, which receives filtrate from several nephrons and passes into the medulla and pelvis.
  Surrounding the tubules are peritubular capillaries (and the vasa recta in juxtamedullary nephrons) which pick up reabsorbed substances and return them to the circulation. This detailed structure enables filtration, selective reabsorption and excretion, allowing the kidney to maintain stable internal conditions.

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6) Describe how the processes of ultrafiltration and selective reabsorption are involved in the formation of urine in the nephron

In the nephron, urine formation involves two main stages: ultrafiltration and selective reabsorption. During ultrafiltration, blood enters the glomerulus under relatively high pressure (due to the afferent versus efferent arteriole diameter difference). Fluid and small solutes (water, salts, glucose, amino acids, urea) are forced through the capillary walls, basement membrane and Bowman's capsule into the nephron tubule. Large proteins and blood cells are retained in the capillary.
Following filtration, the filtrate passes through the PCT, loop of Henle, DCT and collecting duct. During selective reabsorption, useful substances are reabsorbed back into the blood via the peritubular capillaries and vasa recta: e.g., nearly all glucose and amino acids, variable amounts of water, many ions. The descending limb of the loop of Henle is permeable to water, while the ascending limb actively pumps out ions to generate a gradient, aiding water reabsorption in the medulla. The result is urine that contains excess water, wastes (urea), and any substances in surplus, thereby adjusting the volume and composition of body fluids to maintain homeostasis.

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7) Describe the roles of the hypothalamus, posterior pituitary gland, antidiuretic hormone (ADH), aquaporins and collecting ducts in osmoregulation

Osmoregulation is the control of the water potential and hence water content of body fluids. The hypothalamus contains osmoreceptors that detect changes in the water potential (or osmolarity) of the blood. When the blood becomes too concentrated (water potential falls), the hypothalamus signals the posterior pituitary gland to release antidiuretic hormone (ADH) into the circulation. ADH travels to the kidneys and acts on the cells of the collecting ducts (and to an extent the late DCT). In response, aquaporin channels are inserted into the luminal membranes of the collecting-duct cells, increasing their permeability to water. As a result, more water is reabsorbed from the urine filtrate into the medullary interstitium and back into the blood, thus producing a smaller volume of more concentrated urine. If water potential is too high (blood too dilute), ADH secretion falls, aquaporins are removed, collecting ducts become less permeable, and a larger volume of dilute urine is produced. This negative-feedback loop helps maintain stable internal water balance in mammals.

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8) Explain how the blood glucose concentration is regulated by negative feedback control mechanisms, with reference to the effects of insulin on muscle cells and liver cells and the effect of glucagon on liver cells

Blood glucose regulation is a classic example of homeostasis via negative feedback. After a carbohydrate-rich meal, blood glucose concentration rises. Pancreatic β-cells detect this and secrete insulin. Insulin acts on liver cells to promote uptake of glucose, conversion to glycogen (glycogenesis) and inhibition of gluconeogenesis; it acts on muscle cells to enhance glucose uptake and storage as glycogen and increase glucose use. As glucose levels fall toward normal, insulin secretion decreases. Conversely, if blood glucose drops (during fasting or exercise), α-cells in the pancreas release glucagon. Glucagon acts on liver cells to promote glycogen breakdown (glycogenolysis), and stimulate gluconeogenesis, thereby raising blood glucose. As levels return to normal, glucagon secretion decreases. Through these opposing hormone actions, the body maintains blood glucose within narrow limits, ensuring energy supply to tissues (particularly the brain) and preventing harmful extremes.

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9) 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
  Cell signalling is the means by which hormones exert their effect on target cells via specific molecular pathways. In the case of glucagon controlling blood glucose: glucagon binds to a specific receptor on the surface of a liver cell (hepatocyte). Binding causes a conformational change in the receptor, which then activates an associated G-protein inside the membrane. The G-protein stimulates the enzyme adenylyl cyclase, which converts ATP into the second-messenger cyclic AMP (cAMP). cAMP then activates protein kinase A (PKA). PKA triggers an enzyme cascade through phosphorylation: each activated enzyme phosphorylates other enzymes, creating an amplification of the signal (one glucagon molecule triggers many downstream responses). Eventually the cascade activates an enzyme (for example, glycogen phosphorylase) that catalyses glycogen breakdown to glucose, which is released into the blood. This mechanism illustrates how hormonal signals produce major functional changes through a few initial signals, illustrating speed, specificity and amplification in signalling.

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10) explain how negative feedback control mechanisms regulate blood glucose concentration

When blood glucose concentration deviates from its normal range in mammals, the body uses a negative feedback loop to restore it. In negative feedback the system detects a change (stimulus) and triggers responses (effectors) that move the variable back toward its set-point.

When blood glucose rises (after a meal)

• The stimulus: absorption of carbohydrates raises blood glucose concentration.

• Receptors: β-cells in the pancreatic islets detect the high glucose.

• The pancreas secretes insulin, a hormone that lowers blood glucose.

• Effects of insulin on liver cells and muscle cells:

  • Liver cells: Insulin stimulates glucose uptake and conversion of glucose to glycogen (glycogenesis), and inhibits glycogen breakdown (glycogenolysis) and gluconeogenesis. Consequently the liver removes glucose from the blood and stores it, lowering blood glucose.

  • Muscle cells: Insulin promotes uptake of glucose into muscle cells (and fat cells), increasing glucose usage and storage (as glycogen) in muscle. This reduces the concentration of glucose in the blood. 

• As blood glucose falls toward the set-point, insulin secretion decreases (shutdown of the effector), completing the negative feedback loop.

When blood glucose falls (between meals, fasting, exercise)

• Stimulus: blood glucose concentration falls below the normal level.

• Receptors: α-cells in the pancreatic islets detect low glucose.

• The pancreas secretes glucagon, a hormone that raises blood glucose.

• Effect of glucagon on liver cells:

  • Glucagon triggers the liver to convert stored glycogen into glucose (glycogenolysis) and release it into the blood. It also stimulates gluconeogenesis (making glucose from amino acids and other non-carbohydrate sources) and inhibits glycogen synthesis. This increases blood glucose concentration.

• As blood glucose rises toward the set-point, glucagon secretion reduces, completing the negative feedback loop.

Summary of negative‐feedback regulation

• The system keeps blood glucose relatively constant by opposing changes: when glucose is too high → insulin acts to lower it; when glucose is too low → glucagon acts to raise it.

• The effectors (liver, muscle) are the tissues that carry out the corrective actions.

• This regulation ensures that glucose is available for tissues (especially the brain) but avoids the damage caused by extreme hyper- or hypoglycaemia.

• It is an excellent example of homeostasis: detection of change → response → correction → reduction of stimulus.

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11) 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

Modern diagnostic tools for monitoring glucose concentration – especially in diabetic care – use enzyme-based test strips or biosensors. In a typical test strip for urine or blood glucose, the sample is applied to a pad containing the enzyme glucose oxidase, which oxidises glucose to gluconic acid and hydrogen peroxide (H₂O₂). The H₂O₂ then reacts (via the enzyme peroxidase) with a dye in the strip causing a colour change; the intensity of colour correlates with glucose concentration (which can be compared to a chart or read electronically). In biosensors, the same initial enzyme reaction occurs but the subsequent reaction triggers an electrical signal proportional to glucose concentration, enabling quantitative readings. These devices exploit specific enzyme reactions (glucose oxidase → peroxidase) to convert biological information (glucose level) into a measurable signal, allowing rapid monitoring of blood or urine glucose and thus aiding homeostatic management in mammals.

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14.2 Homeostasis in plants

 

​Students should be able to:

1) 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

2) explain that stomata have daily rhythms of opening and closing

3) describe the structure and function of guard cells and explain the mechanism by which they open and close stomata

4) 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

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1. Stomatal response to environmental conditions: balancing CO₂ uptake and water loss

Plants must continuously regulate their internal environment (and the environment of their leaf cells) so that photosynthesis can proceed efficiently while avoiding excessive water loss. One key mechanism for this is the opening and closing of stomata (pores) on the leaf surface.

• When stomata are open, carbon dioxide (CO₂) from the atmosphere can diffuse through the stomatal pore into the leaf internal air spaces, then into mesophyll cells where photosynthesis takes place. This is essential for the plant to fix carbon in the light-independent reactions.

• However, when stomata are open, water vapour inside the leaf (which is at higher humidity than the external air) diffuses out — this is transpiration. Excessive water loss can lead to dehydration, loss of turgor, and damage to cells.

• Therefore, the plant must balance:

  • Maximising or ensuring sufficient CO₂ uptake for efficient photosynthesis,

  • While minimising excessive water loss (especially under dry, hot, or windy conditions).

• The stomata respond to environmental cues such as: light intensity, CO₂ concentration inside leaf, humidity outside, water availability in soil, temperature, and wind.

  • For example: In bright light, photosynthesis demands more CO₂, so stomata often open. But if the air is very dry or water in the soil is scarce, the plant may partially close stomata to conserve water.

  • If internal CO₂ falls (because it is being used up), the plant may open stomata to allow more in. But if the leaf is losing too much water (i.e., transpiration rate high), stomata may close.

• In summary: the stomatal aperture is regulated in response to environmental changes so that the trade-off between CO₂ uptake and water conservation is managed, maintaining homeostasis of the leaf/cell environment (turgor, hydration, gas supply).

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2. Daily rhythms of stomatal opening and closing

Stomata do not simply respond randomly; they also follow circadian (daily) rhythms and predictable patterns of opening and closing throughout the day-night cycle.

• Typically, stomata open after dawn (when light arrives) and continue open during the day, allowing photosynthesis to proceed. At night, when photosynthesis cannot occur (or is much reduced) and when transpiration might still occur with no CO₂ benefit, many stomata close.

• Even in fairly constant conditions (light, temperature), many plants maintain a daily rhythm of stomatal status (opening and closing) driven internally (circadian clock) rather than purely by external cues.

• This rhythm helps the plant anticipate regular changes (dawn, dusk) and thereby reduce water loss when photosynthetic benefit is low (e.g., at night) and maximise gas exchange when the benefit is high (day).

• Understanding this point emphasises that stomatal control is not just reactive but also predictive (anticipatory), contributing to the plant’s internal regulation (homeostasis) of gas exchange and water status.

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3. Structure and function of guard cells; mechanism of stomatal opening and closing

A detailed understanding of how stomata are constructed and how guard cells operate is vital.

Structure of guard cells:

• A stoma (plural stomata) consists of a pore in the leaf epidermis, flanked by two specialized guard cells.

• Guard cells are kidney- or bean-shaped in many dicotyledonous plants; in monocots they may be dumb-bell shaped.

• The inner wall of each guard cell (adjacent to the pore) is thicker and less elastic; the outer wall (away from the pore) is thinner and more elastic.

• Each guard cell contains chloroplasts (in many species) and the usual machinery of a plant cell, enabling metabolic regulation.

Function of guard cells + mechanism of opening/closing:

• Opening mechanism (when conditions favourable for CO₂ uptake):

  • Light triggers guard cell photosynthesis (in species where guard cells contain chloroplasts), producing ATP and changing osmotic conditions.

  • Potassium ions (K⁺) are actively pumped into the guard cells from neighbouring cells or the apoplast; this requires ATP.

  • The accumulation of K⁺ lowers the water potential inside guard cells, so water enters by osmosis from neighbouring cells/channels.

  • As water enters, guard cells become turgid; their shape changes (because of the differential thickness of inner/outer walls), which causes the pore to open as the guard cells bulge apart.

  • Water and other ions (e.g., Cl⁻, malate²⁻) may also accumulate to balance charge and ensure osmotic influx.

  • The opened stomatal pore allows gas exchange: CO₂ influx, O₂ outflux, and water vapour outflow (transpiration).

• Closing mechanism (when conditions unfavourable):

  • When water is scarce, or when the plant detects stress (e.g., drought), guard cells lose K⁺ ions (and other osmotic solutes) and water flows out, reducing their turgor.

  • Loss of turgor causes the guard cells to become flaccid and the pore closes (the cells collapse together, narrowing the gap).

  • Additional signals (such as the hormone abscisic acid) trigger ion efflux, reducing osmotic potential and causing closure (see point 4).

• Summary of mechanism: Opening: K⁺ in → water in → turgid guard cells → pore open. Closing: solutes out → water out → guard cells flaccid → pore closed.

• Understanding how structure (wall thickness differences, chloroplasts, cell shape) enables function (controlled pore opening/closing) ties into the idea of homeostasis in plants: regulating internal gas and water status.

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4. Role of abscisic acid (ABA) in stomatal closure during water stress, including the role of calcium ions as a second messenger

When plants experience water stress (e.g., drought, high temperature, low soil water availability), they must rapidly reduce water loss; stomatal closure is a key response. The hormone abscisic acid (ABA) plays a central role.

• Abscisic acid (ABA) is produced (or its concentration increases) in plant tissues when water potential falls or when roots detect low soil moisture; it is transported to leaves (and guard cells).

• In guard cells, ABA triggers a cascade of events:

  • It binds to receptor proteins, initiating a signal transduction pathway.

  • One key part of the pathway is the increase in cytosolic calcium ion concentration (Ca²⁺) in guard cells (calcium acts as a second messenger).

  • Elevated Ca²⁺ causes activation of ion channels (for K⁺ and Cl⁻) in the guard cell membranes → K⁺ and Cl⁻ ions exit guard cells into apoplast.

  • Loss of these osmotic ions causes water to leave guard cells by osmosis, reducing turgor, and therefore stomata close.

  • This reduces transpiration and conserves water; it also reduces CO₂ uptake, temporarily limiting photosynthesis, but the trade-off prioritises water conservation under stress.

• The role of Ca²⁺ as second messenger emphasises that the plant’s response is cellular and signalling-based, not just passive.

• Thus the mechanism: water stress → increased ABA → Ca²⁺ signalling in guard cells → ion efflux → water efflux → guard cells become flaccid → stomatal closure.

• The process shows plant homeostasis: under adverse environmental conditions (water shortage) the plant adjusts stomatal aperture to maintain internal water balance, even if this means reducing photosynthetic uptake of CO₂ temporarily.

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