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6.4Organisms Respond to Changes in their Environments (A Level only)

Homeostasis (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

15 min read 21 parts Lesson 32 of 44 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Principles of Homeostasis, Negative Feedback and Insulin — AQA A LEVEL BIOLOGY 7402 notes: Explain why internal conditions are controlled, describe negative and positive feedback, and describe how insulin lowers blood glucose.Homeostasis is the maintenance of the internal environ…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Glycogenesis
  • Coordinator
  • Confusing the effects of insulin and glucagon

Section 1 of 21Homeostasis

Homeostasis is the maintenance of the internal environment within restricted limits.

1
Physiological control systems ensure that internal conditions do not stray beyond the narrow limits needed for survival,
  • giving optimal conditions for enzyme action and cell function.
2
Cells rely on many enzyme-controlled reactions, so even a small change in enzyme activity can significantly affect them.
3
At extremes of temperature or pH the bonds holding the enzyme's tertiary structure break,
  • so the active site changes shape, the enzyme denatures and enzyme-substrate complexes can no longer form.
4
Blood glucose concentration must also be controlled: too high lowers water potential of blood (cells lose water), too low reduces respiration.

Section 2 of 21Negative and positive feedback

Most homeostatic control uses negative feedback: a system in which any change is restored to the original level.

Negative and positive feedback
Use A-level language: instead of 'sensors detect change', say receptors detect a stimulus; instead of 'messages are sent to the brain', say the brain acts as a coordinator,
  • then effectors produce a response.
Internal changes can occur in either direction,
  • so negative feedback systems have more than one corrective mechanism, one for a rise and one for a fall.

Negative feedback reduces the effect of the original stimulus to restore the original levels. Positive feedback enhances the effect of the original stimulus and causes conditions to deviate further from the normal range. Worked example: bone repair by osteoblasts and osteoclasts would be identified from the diagram by checking whether the response increases or reduces the original stimulus, and explaining accordingly.

Section 3 of 21Blood glucose and insulin

Blood glucose varies with food consumption (carbohydrates), exercise, and changes in insulin, glucagon and adrenaline. Do not confuse the effects of insulin and glucagon. A rise in blood glucose above normal is detected by cells in the pancreas, and in response the β cells in the islets of Langerhans secrete insulin.

Insulin binds to receptors on target cells (liver, muscle), which causes the addition of glucose transporter proteins to the cell surface membranes, so more glucose enters by facilitated diffusion, and activates enzymes that convert glucose to glycogen (glycogenesis) and increase respiration of glucose.

Blood glucose and insulin
In an exam include all of the steps: the binding of insulin to receptors, the increase in glucose transporter proteins and the activation of enzymes.
Blood glucose then falls and negative feedback reduces insulin secretion.

Section 4 of 21Worked examples

  1. Explain why it is important to control blood glucose concentration. [3]3 marks

  2. Describe the role of negative feedback in homeostasis. [3]3 marks

  3. Describe how insulin reduces blood glucose concentration. [4]4 marks

Section 5 of 21Glucagon

A decrease in blood glucose below the normal range is detected by cells in the pancreas, and in response α cells secrete glucagon. Glucagon causes an increase in blood glucose by activating enzymes that convert glycogen to glucose (glycogenolysis) and that make glucose from amino acids and glycerol (gluconeogenesis); it also reduces glucose uptake by the liver.

Section 6 of 21The second messenger model

Glucagon triggers a cascade involving cAMP:

Glucagon binds to a specific receptor on the surface of a liver cell.
Adenylate cyclase
This activates the enzyme adenylate cyclase in the membrane.
Cyclic AMP (cAMP)
Adenylate cyclase converts ATP to cyclic AMP (cAMP), the second messenger.
Protein kinase A
cAMP activates protein kinase A.
Protein kinase A activates enzymes that convert glycogen to glucose.
Glucose diffuses out of the cell into the blood.

Learn the roles of adenylate cyclase, cAMP and protein kinase.

Section 7 of 21Adrenaline

Adrenaline can also increase blood glucose

  • it binds to receptors on the cell surface membranes of target cells and activates (through adenylate cyclase and cAMP) enzymes that convert glycogen to glucose.
  • The specification covers the second messenger model in the context of glucagon and adrenaline, but be ready to apply it to unfamiliar contexts.

Section 8 of 21The liver in glucose regulation

The liver plays a vital role in regulating blood glucose. Both insulin and glucagon have specific receptors on the membranes of liver cells.

ProcessMeaningHormone
GlycogenesisGlucose to glycogenInsulin
GlycogenolysisGlycogen to glucoseGlucagon, adrenaline
GluconeogenesisFormation of glucose from non-carbohydrates (amino acids, glycerol)Glucagon

Section 9 of 21Linking hormones to negative feedback

Blood glucose control is a negative feedback loop with two corrective pathways.

Linking hormones to negative feedback
After a meal glucose rises, β cells release insulin and glucose falls; between meals or in exercise glucose falls, α cells release glucagon (and the adrenal glands release adrenaline) and glucose rises.
As glucose returns to the normal range the stimulus for hormone secretion is removed, so the response switches off.
Remember that hormones are carried in the blood and act only on cells with complementary receptors,
  • which is why they can have effects on specific target cells such as hepatocytes.

Section 10 of 21Worked examples

  1. Describe the role of cAMP when glucagon binds to a liver cell. [3]3 marks

  2. Define gluconeogenesis and glycogenolysis. [2]2 marks

  3. Explain why glucagon and insulin are both needed. [2]2 marks

Section 11 of 21Diabetes

Diabetes
Diabetes is a condition in which homeostatic control of blood glucose has failed or deteriorated,
  • giving symptoms such as glucose in the urine, dehydration and fatigue.
  • There are two typestype I (the body does not produce enough insulin,
  • because β cells are destroyed by an autoimmune response; treated with insulin injections) and type II.
    • (cells lose responsiveness to insulin, linked to obesity and diet;
      • managed with diet, exercise and sometimes medication)
Health advisers may promote public health campaigns encouraging healthy eating and regular physical activity, recommend lower intake of processed foods, saturated fats and sugary drinks,
  • and ask governments to improve nutrition-labelling laws.
Interpreting intervention data: Step 1, describe the trend; compare with the region without intervention (in Region B,
  • where no intervention took place, incidence rose more steeply from 5.0 to 8.6).
Diabetes
Step 3, interpret the meaning: the difference suggests public health campaigns and food reformulation may have helped slow the rise in type II diabetes in Region A.
Step 4, acknowledge limitations or alternative explanations (other differences between regions, other changes over time).

Section 12 of 21Required practical: colorimetry and urine glucose

Colorimetry can determine the glucose concentration of a urine sample of unknown concentration, using quantitative Benedict's reagent (containing potassium thiocyanate).

1
Prepare a dilution series of glucose solutions, using pipettes to add different volumes of stock solution and distilled water to labelled tubes.
2
Add a fixed volume of quantitative Benedict's solution to each tube.
3
Heat in the same water bath for 5 minutes, then filter.
4
Place the cuvette in the colorimeter and obtain the % transmission of light through each solution.
5
Plot a calibration curve (concentration against % transmission) on graph paper.
6
Treat the unknown sample in the same way and read its concentration from the curve.

Section 13 of 21Kidney and nephron structure

The kidneys carry out osmoregulation (regulating the water content of the blood) and excretion (removal of metabolic waste and excess substances). The internal structure includes the cortex, medulla and renal pelvis. Kidneys contain many tiny tubes, nephrons, the functional units that form urine.

Section 14 of 21Ultrafiltration

Urine formation has two stagesultrafiltration, then selective reabsorption.

1
In ultrafiltration, small molecules are filtered out of the blood into the Bowman's capsule under high pressure, forming glomerular filtrate.
2
The pressure arises because the afferent arteriole is wider than the efferent arteriole.
3
The blood in the glomerular capillaries is separated from the lumen of the capsule by two cell layers and a basement membrane: gaps between the endothelial cells of the capillary allow small molecules through; the basement membrane, a mesh of collagen and glycoproteins, allows small molecules to pass but not large proteins; the epithelial cells of the capsule have finger-like projections called podocytes with gaps between them.
4
Many of the substances in the filtrate are needed by the body and are reabsorbed into the blood as it passes along the nephron.

Section 15 of 21Worked examples

  1. Describe how you would use a colorimeter to find the concentration of glucose in a urine sample. [5]5 marks

  2. Explain why glucose appears in the urine of a person with diabetes. [2]2 marks

  3. Describe how the structures of the glomerular capillary and Bowman's capsule allow ultrafiltration. [3]3 marks

  4. Region B had no intervention and incidence of type II diabetes rose from 5.0 to 8.6. Evaluate what this suggests. [2]2 marks

Section 16 of 21Selective reabsorption in the proximal convoluted tubule

Many substances in the glomerular filtrate are needed by the body, including glucose, amino acids, vitamins, mineral ions and water. They are reabsorbed into the blood as the filtrate passes along the nephron, and most selective reabsorption takes place in the proximal convoluted tubule (PCT).

Adaptation of PCT epithelial cellHow it aids reabsorption
Many microvilli on the luminal membraneIncrease the surface area for reabsorption
Many co-transporter proteins in the luminal membraneEach type transports a specific solute (e.g. glucose or a particular amino acid) across the membrane together with sodium ions
Many mitochondriaProvide ATP for the sodium-potassium pumps
1
Sodium ions are actively transported out of the epithelial cells into the blood (by sodium-potassium pumps), lowering sodium concentration inside the cells.
2
Sodium ions diffuse from the filtrate into the cells down a concentration gradient.
3
These sodium ions move via co-transporter proteins in the membrane,
  • and as they move the proteins transport another solute at the same time, e.g. glucose or an amino acid.
4
Once inside the epithelial cells the solutes diffuse down their concentration gradients into the blood.

All the substances leaving the PCT for the surrounding tissues eventually enter nearby capillaries down their concentration gradients; water follows by osmosis. Remember that reabsorption of glucose and amino acids by co-transport is an active process (indirect active transport, driven by the sodium gradient).

Section 17 of 21The loop of Henle

While most water and salts are reabsorbed in the PCT, the loop of Henle and collecting duct are also involved.

The loop of Henle
In the ascending limb, sodium and chloride ions are actively pumped out into the medulla and the wall is impermeable to water; in the descending limb water leaves by osmosis because the tissue fluid of the medulla has a low water potential.
The loop of Henle generates a steep water potential gradient across the medulla, maximising the reabsorption of water.
Concentrated urine is not produced in the loop itself: its role is to lower the water potential of the medulla so that water can be reabsorbed by osmosis from the distal convoluted tubule and collecting duct, concentrating the urine.

Section 18 of 21Osmoregulation and ADH

Control of the water potential of body fluids is osmoregulation, an example of homeostasis. Specialised sensory neurones called osmoreceptors in the hypothalamus monitor the water potential of the blood.

When blood water potential decreases (less water): osmoreceptors detect the decrease; nerve impulses stimulate release of ADH from the posterior pituitary gland; ADH travels in the blood to the kidneys; ADH molecules bind to receptors on the surface of cells lining the collecting duct; this increases the water permeability of these cells (vesicles containing aquaporins fuse with the membrane); more water is reabsorbed by osmosis from the collecting duct; a small volume of concentrated urine is produced.

When blood water content increases (e.g. high fluid intake): (1) osmoreceptors detect an increase in water potential; (2) nerve impulses along sensory neurones go to the posterior pituitary; (3) it releases less ADH; (4) the water permeability of the collecting ducts decreases; (5) less water is reabsorbed; (6) a large volume of dilute urine is produced. This is negative feedback.

Section 19 of 21Worked examples

  1. Explain how the cells of the proximal convoluted tubule are adapted for reabsorption. [4]4 marks

  2. Describe how glucose is reabsorbed from the filtrate into the blood. [4]4 marks

  3. Explain the role of the loop of Henle in producing concentrated urine. [3]3 marks

  4. Describe how a person's kidneys respond to dehydration. [5]5 marks

Section 20 of 21Key terms

TermMeaning
HomeostasisMaintenance of a stable internal environment.
Negative feedbackA change is reversed to restore the original level.
Positive feedbackA change is amplified so conditions move further from normal.
InsulinHormone from β cells of the pancreas that lowers blood glucose.
GlycogenesisConversion of glucose to glycogen.
CoordinatorPart of the control system (e.g. brain) that processes information from receptors.
TermMeaning
GlucagonHormone from α cells that raises blood glucose.
AdrenalineHormone that raises blood glucose via glycogenolysis.
Second messengerIntracellular molecule (cAMP) that relays the hormone signal.
Adenylate cyclaseEnzyme that converts ATP to cAMP.
Protein kinaseEnzyme activated by cAMP that activates other enzymes.
GluconeogenesisProduction of glucose from non-carbohydrate molecules.
GlycogenolysisBreakdown of glycogen to glucose.
TermMeaning
DiabetesCondition in which blood glucose control has failed or deteriorated.
ColorimeterInstrument measuring the absorbance or transmission of light through a solution.
Calibration curveGraph of known concentrations used to find an unknown.
NephronFunctional unit of the kidney that forms urine.
UltrafiltrationFiltration of small molecules from blood into the Bowman's capsule under pressure.
PodocyteEpithelial cell of the Bowman's capsule with finger-like projections.
OsmoregulationRegulation of the water content of the blood.
TermMeaning
Proximal convoluted tubuleThe part of the nephron where most selective reabsorption occurs.
Co-transporter proteinProtein that moves two solutes across a membrane together.
Loop of HenlePart of the nephron creating a low water potential in the medulla.
OsmoreceptorSensory neurone monitoring the water potential of the blood.
ADHHormone from the posterior pituitary that increases the water permeability of the collecting duct.
Collecting ductTube in which water is reabsorbed under ADH control.

Section 21 of 21Exam technique

1
Writing 'sensors' and 'messages' instead of receptors and coordinators.
2
Confusing the effects of insulin and glucagon.
3
Missing the stepsbinding to receptors, more transporters, enzyme activation.
4
Describing negative feedback as 'making the change bigger'.
1
Mixing up glycogenesis and glycogenolysis.
2
Missing adenylate cyclase from the cascade.
3
Saying glucagon is made by β cells.
4
Thinking hormones enter the cellthey bind to surface receptors.
1
Saying type II diabetes is caused by lack of insulin production only.
2
Omitting the calibration curve in colorimetry.
3
Saying large proteins are filtered out of the blood into the filtrate.
4
Concluding causation from data without noting limitations.
1
Saying the loop of Henle produces concentrated urine.
2
Saying ADH is produced by the kidney.
3
Describing reabsorption of glucose as simple diffusion.
4
Forgetting that osmoreceptors detect the water potential of the blood.

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