Homeostasis (A Level Only)

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Principles of Homeostasis

Homeostasis is the maintenance of the internal environment within restricted limits.
Physiological control systems keep internal conditions within narrow limits to ensure optimal conditions for enzyme action and cell function.
Receptor cells detect changes inside or outside the body and send information to a coordination system, which communicates with effectors to restore conditions to normal.
The nervous system communicates via nerve impulses in neurones; the endocrine system communicates via hormones carried in the blood.
Conditions controlled by homeostasis in mammals include core body temperature, blood pH, blood glucose concentration and water potential of the blood.
Temperature and pH must be maintained because they affect enzyme activity; even small changes in enzyme activity can significantly impact cells.
At low temperatures, molecules have limited kinetic energy so fewer enzyme-substrate complexes form; at high temperatures or extremes of pH, bonds holding the active site break and enzymes denature.

Negative Feedback

Negative feedback is a system in which any change is restored to original levels.
A negative feedback loop involves: a receptor detects a stimulus (a change from normal levels); signals are sent to a coordination system (nervous or hormonal); signals are sent to an effector to carry out a response that reverses the change.
Negative feedback systems have two corrective mechanisms: one for when a factor becomes too low and one for when it becomes too high, increasing the level of control.
Negative feedback reduces the effect of the original stimulus to restore original levels; positive feedback enhances the effect of the original stimulus and causes conditions to deviate further from normal.
In exams, use A-level language: say receptors detect a stimulus and the brain acts as a coordinator.

Glucose Concentration & Insulin

Blood glucose concentration varies due to factors such as consumption of foods containing carbohydrates, exercise, and secretion of hormones that affect blood glucose.
Hormones affecting blood glucose: insulin reduces blood glucose; glucagon increases blood glucose; adrenaline increases blood glucose.
An increase in blood glucose above normal is detected by cells in the pancreas; β cells secrete insulin.
Insulin binds to specific receptors on the membranes of target cells (e.g. liver and muscle cells).
Insulin causes more glucose transporter proteins to be added to the cell surface membranes by fusion of vesicles, increasing permeability to glucose.
Glucose moves into target cells by facilitated diffusion.
Insulin activates enzymes that convert glucose into glycogen; this is glycogenesis.

Glucose Regulation: Glucagon and Adrenaline

A decrease in blood glucose below normal is detected by cells in the pancreas; α cells secrete glucagon.
Glucagon activates enzymes that carry out glycogenolysis (conversion of glycogen to glucose) and gluconeogenesis (conversion of glycerol and amino acids into glucose).
Glucagon does not directly convert molecules into glucose; it activates enzymes that carry out these processes.
The second messenger model of glucagon action: glucagon binds to receptors on target cell membranes (e.g. liver cells); this activates a G protein which activates adenylate cyclase.
Active adenylate cyclase catalyses the conversion of ATP to cyclic AMP (cAMP); cAMP is the second messenger.
cAMP activates protein kinase A, which initiates a cascade of reactions resulting in activation of enzymes that break down glycogen to glucose.
Adrenaline increases blood glucose by binding to receptors on cell surface membranes and activating enzymes that convert glycogen to glucose, via the same second messenger model as glucagon.

Glucose Regulation: the Liver

The liver plays a vital role in regulating blood glucose concentration.
Both insulin and glucagon have specific receptors on the membranes of liver cells.
Insulin binding activates glycogenesis: the synthesis of glycogen from glucose; this lowers glucose concentration in liver cells and allows more glucose to be removed from the blood by diffusion.
Glucagon binding activates glycogenolysis: the breakdown of glycogen to produce glucose, which diffuses into the blood.
Glucagon binding also activates gluconeogenesis: the synthesis of glucose from non-carbohydrate molecules such as fatty acids and amino acids, resulting in release of glucose into the blood.
Remember: genesis = creation; lysis = splitting. Do not confuse these with glycolysis, the first stage of respiration.

Diabetes

Diabetes is a condition in which the homeostatic control of blood glucose has failed or deteriorated, resulting in high blood glucose and symptoms such as glucose in the urine, dehydration and fatigue.
Type I diabetes: the pancreas fails to produce insulin; onset often in childhood, caused by an autoimmune attack on the β cells.
Type I diabetes is normally treated with insulin injections calculated on the basis of carbohydrate intake and exercise.
Type II diabetes: insulin receptors no longer respond to insulin; development usually occurs in adults and is linked to risk factors such as obesity, a high carbohydrate diet, age and family history.
Treatments for type II diabetes include a low carbohydrate diet, exercise, and medications that help cells take up glucose from the blood.
Individuals with type II diabetes still produce insulin but their insulin receptors are unable to sense insulin.
Public health approaches to type II diabetes include promoting healthy eating and physical activity, reducing intake of processed foods, saturated fats and sugary drinks, and improving nutrition labelling; food industry approaches include reformulating products and continuing to produce and advertise unhealthy foods.

Required Practical: Determining the Concentration of Glucose in Urine

Colorimetry can determine the concentration of glucose in a urine sample of unknown concentration.
The test uses quantitative Benedict's reagent containing potassium thiocyanate, so it does not produce a red precipitate with glucose.
The presence of glucose is measured by the loss of blue colour and formation of a white precipitate, which can be filtered out before analysis.
A positive test is indicated along a spectrum from blue = low concentration to colourless = high concentration.
A calibration curve is set up by preparing a dilution series of glucose solutions, adding a fixed volume of quantitative Benedict's solution, heating in a water bath at at least 70 °C for 5 minutes, filtering, and adding to labelled cuvettes.
The colorimeter is set to red (complementary to blue) and calibrated using a cuvette containing only distilled water (100% transmission).
Plot a graph of glucose concentration against % transmission to create the calibration curve; treat the unknown sample in the same way and use its % transmission to read the glucose concentration from the curve.

Nephron Structure and Function

The kidneys are responsible for osmoregulation (regulating the water content of the blood) and excretion (removal of metabolic waste and excess substances from the blood).
The internal structure of the kidney includes the outer renal cortex, inner renal medulla, renal pelvis where urine is funnelled into the ureter, and thousands of tiny tubes called nephrons.
Nephrons contain the glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
Urine formation occurs in two stages: ultrafiltration (small molecules filtered from blood into Bowman's capsule forming glomerular filtrate) and selective reabsorption (useful molecules taken back from the filtrate into the blood).
In ultrafiltration, the afferent arteriole is wider than the efferent arteriole, creating high blood pressure in the glomerulus that forces small molecules into the Bowman's capsule.
The glomerular filtrate contains amino acids, water, glucose, urea and inorganic ions (Na⁺, K⁺, Cl⁻); blood cells and large proteins remain in the blood as they are too large to pass out.
Features aiding ultrafiltration: capillary endothelium (gaps between cells), basement membrane (mesh of collagen and glycoproteins), and Bowman's capsule epithelium with podocytes (finger-like projections with gaps).

Selective Reabsorption and the Loop of Henle

Selective reabsorption occurs as filtrate passes along the nephron; reabsorbed substances include water, salts, glucose and amino acids, with most reabsorption in the proximal convoluted tubule (PCT).
PCT epithelial cells are adapted with microvilli (increase surface area), co-transporter proteins (each transports a specific solute, e.g. glucose or an amino acid), and many mitochondria (provide energy for sodium-potassium pumps in basal membranes).
Sodium ions are transported from the PCT into surrounding tissues by active transport; chloride ions follow by diffusion due to the electrical gradient; water follows by osmosis; urea moves out by diffusion.
Sugars and amino acids are transported by co-transporter proteins that also transport sodium ions: sodium-potassium pumps actively transport Na⁺ out of epithelial cells into the blood; Na⁺ in the filtrate diffuses into epithelial cells down its concentration gradient; Na⁺ moves via co-transporter proteins, transporting another solute at the same time; solutes then diffuse into the blood.
In the loop of Henle, sodium and chloride ions are pumped out of the ascending limb (impermeable to water) into the medulla, reducing the water potential of the medulla.
The neighbouring descending limb is permeable to water, so water moves out by osmosis into neighbouring capillaries; ions cannot move out of the descending limb due to its low permeability to ions.
The loop of Henle generates a low water potential in the renal medulla, causing reabsorption of water from the distal convoluted tubule and collecting duct by osmosis, producing concentrated urine.

Control of Blood Water Potential

Osmoregulation is the control of the water potential of body fluids; it is an example of homeostasis.
Osmoreceptors are specialised sensory neurones located in the hypothalamus of the brain that monitor the water potential of the blood.
When blood water content decreases: osmoreceptors detect a decrease in water potential; water moves out of osmoreceptor cells causing them to shrink; nerve impulses are sent to the posterior pituitary gland; ADH is released into the blood.
ADH increases the water permeability of the collecting ducts, so the kidneys reabsorb more water; water moves from high water potential in the collecting duct to low water potential in the medulla; a small volume of concentrated urine is produced.
ADH action: ADH molecules bind to receptors on cells lining the collecting duct; vesicles containing aquaporins fuse with the membranes; the number of aquaporins increases, increasing the membrane's permeability to water.
When blood water content increases: osmoreceptors detect an increase in water potential; nerve impulses cause the posterior pituitary gland to release less ADH; water permeability of collecting ducts decreases; the kidneys reabsorb less water; a large volume of dilute urine is produced.

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1.Which of the following is the best definition of homeostasis?
Easy
- AMaintaining the internal environment within restricted limits
- BKeeping the internal environment completely constant at all times
- CAllowing the internal environment to fluctuate freely with external conditions
- DRemoving all waste products from the body
2.The endocrine system communicates within the body using which type of signal?
Easy
- AHormones carried in the blood
- BNerve impulses along neurones
- CElectrical synapses between adjacent cells
- DMechanical vibrations through tissues
3.Which pair of hormones both act to increase blood glucose concentration?
Medium
- AGlucagon and adrenaline
- BInsulin and glucagon
- CInsulin and adrenaline
- DInsulin and thyroxine
4.In the second messenger model of glucagon action, which molecule acts as the second messenger?
Medium
- ACyclic AMP (cAMP)
- BAdenylate cyclase
- CProtein kinase A
- DATP
5.Which of the following correctly describes the difference between glycogenesis and glycogenolysis?
Medium
- AGlycogenesis is the synthesis of glycogen from glucose, while glycogenolysis is the breakdown of glycogen to glucose
- BGlycogenesis is the breakdown of glycogen to glucose, while glycogenolysis is the synthesis of glycogen from glucose
- CGlycogenesis is the synthesis of glucose from non-carbohydrates, while glycogenolysis is the breakdown of glycogen
- DGlycogenesis is the breakdown of glucose during respiration, while glycogenolysis is the synthesis of glycogen
6.Which of the following is a feature of type I diabetes but NOT type II diabetes?
Medium
- AThe pancreas fails to produce insulin
- BInsulin receptors no longer respond to insulin
- COnset is usually linked to obesity in adulthood
- DIt can be managed with a low carbohydrate diet alone
7.In the required practical using quantitative Benedict's reagent and a colorimeter, why is the colorimeter set to red light?
Medium
- ARed is the complementary colour to blue, so a blue solution absorbs red light
- BRed light passes through the solution without being absorbed, giving a baseline reading
- CRed light reacts with the white precipitate to produce a colour change
- DRed light denatures the glucose so it can be measured
8.Which structure of the nephron is the site of ultrafiltration?
Medium
- AGlomerulus and Bowman's capsule
- BProximal convoluted tubule
- CLoop of Henle
- DCollecting duct

Plus de questions

9.Which feature of the proximal convoluted tubule epithelial cells provides energy for the sodium-potassium pumps in their basal membranes?
Medium
- AMany mitochondria
- BMany microvilli
- CMany co-transporter proteins
- DA single layer of cells
10.Which of the following correctly describes the role of the loop of Henle?
Hard
- AIt generates a low water potential in the renal medulla, allowing water to be reabsorbed from the collecting duct by osmosis
- BIt produces concentrated urine directly within the loop itself
- CIt reabsorbs glucose from the filtrate by active transport
- DIt filters blood cells and large proteins out of the blood
11.Which of the following substances are normally present in the glomerular filtrate? (select all that apply)
Medium
- AGlucose
- BAmino acids
- CUrea
- DBlood cells
- ELarge proteins
12.Which of the following are effects of insulin? (select all that apply)
Medium
- AMore glucose transporter proteins are added to the cell surface membranes of target cells
- BGlucose enters target cells by facilitated diffusion
- CEnzymes are activated that convert glucose into glycogen
- DEnzymes are activated that break glycogen down to glucose
- EBlood glucose concentration is raised
13.Which of the following are controlled by homeostasis in mammals? (select all that apply)
Medium
- ACore body temperature
- BBlood pH
- CBlood glucose concentration
- DWater potential of the blood
- EAtmospheric oxygen concentration
14.Negative feedback enhances the effect of the original stimulus, causing conditions to deviate further from the normal range.
Easy
True or false?
15.Insulin is secreted by the β cells of the pancreas when blood glucose concentration rises above the normal range.
Easy
True or false?
16.Individuals with type II diabetes are unable to produce any insulin.
Medium
True or false?
17.Water is reabsorbed from both the ascending and descending limbs of the loop of Henle.
Medium
True or false?
18.ADH increases the number of aquaporins in the membranes of collecting duct cells, increasing their permeability to water.
Medium
True or false?
19.Match each homeostatic process to its correct description.
Medium
- Glycogenesis
- Glycogenolysis
- Gluconeogenesis
- Osmoregulation
- Synthesis of glycogen from glucose
- Breakdown of glycogen to glucose
- Synthesis of glucose from non-carbohydrate molecules
- Regulation of the water content of the blood
20.Match each part of the nephron with its main function.
Medium
- Glomerulus
- Proximal convoluted tubule
- Loop of Henle
- Collecting duct
- Ultrafiltration under high pressure
- Selective reabsorption of glucose and amino acids
- Generates a low water potential in the medulla
- Water reabsorption controlled by ADH
21.Match each hormone or molecule with its role in blood glucose regulation.
Medium
- Insulin
- Glucagon
- Adrenaline
- Cyclic AMP
- Reduces blood glucose by promoting glycogenesis
- Increases blood glucose by promoting glycogenolysis
- Increases blood glucose as part of the 'fight or flight' response
- Acts as a second messenger to activate protein kinase A
22.Place the events of the second messenger model of glucagon action in the correct order.
Medium
- Glucagon binds to receptors on the cell surface membrane of a liver cell
- A G protein is activated, which activates adenylate cyclase
- Adenylate cyclase catalyses the conversion of ATP to cyclic AMP
- Cyclic AMP activates protein kinase A
- Protein kinase A initiates a cascade of reactions that activates enzymes to break down glycogen
23.Place the steps of ultrafiltration in the correct sequence.
Medium
- Blood enters the glomerulus via the afferent arteriole
- High blood pressure builds up in the glomerulus
- Small molecules are forced out of the capillaries
- Small molecules pass through the basement membrane and podocyte gaps
- Glomerular filtrate collects in the Bowman's capsule
24.Place the events that follow a decrease in blood water potential in the correct order.
Medium
- Osmoreceptors in the hypothalamus detect a decrease in blood water potential
- Nerve impulses travel to the posterior pituitary gland
- ADH is released into the blood
- Aquaporins are added to the collecting duct cell membranes
- More water is reabsorbed and a small volume of concentrated urine is produced
25.Is the patient likely to have type 1 or type 2 diabetes?
Medium
- AType 1 diabetes
- BType 2 diabetes
- CNeither type of diabetes
- DBoth types equally likely
26.Which features of the patient's notes support a diagnosis of type 1 rather than type 2 diabetes? (select all that apply)
Hard
- APatient age 9
- BHigh cytotoxic T cell activity
- CTreatment with insulin injections by pump
- DSymptoms of increased thirst and urine production
- EWeight loss
27.Which of the following is a valid reason why her blood glucose concentration decreased after 60 minutes even though her blood contained no insulin?
Hard
- AGlucose was excreted in her urine
- BGlucose was used in respiration by her cells
- CGlucagon converted glucose into glycogen
- DAdrenaline increased the uptake of glucose by cells
28.The same experiment was repeated on a non-diabetic woman whose blood glucose was 75 mg per 100 cm³ before drinking the sugary solution. Compared with the diabetic woman, which change would you predict in the non-diabetic woman's blood glucose curve?
Hard
- AA smaller rise in blood glucose that returns to normal more quickly
- BA larger rise in blood glucose that stays high for longer
- CNo change in blood glucose at any point
- DA rise followed by a permanent decrease below the starting value
29.The loop of Henle itself produces concentrated urine.
Medium
True or false?
30.Which part of the loop of Henle is impermeable to water?
Medium
- AAscending limb
- BDescending limb
- CBoth limbs
- DNeither limb
31.What indicates a positive result when quantitative Benedict's reagent reacts with glucose?
Medium
- AA loss of blue colour with formation of a white precipitate
- BA colour change from blue to brick red
- CA colour change from colourless to blue
- DFormation of a red precipitate
32.Why is the white precipitate filtered out before the solution is placed in the colorimeter?
Medium
- AIt would scatter light and give an inaccurate transmission reading
- BIt absorbs red light and would raise the reading
- CIt reacts with the cuvette to change its colour
- DIt increases the concentration of glucose in the sample
33.What is the main process that takes place in the proximal convoluted tubule?
Medium
- ASelective reabsorption
- BUltrafiltration
- COsmoregulation
- DExcretion of urea
34.How do the microvilli on PCT epithelial cells aid reabsorption?
Medium
- AThey increase the surface area for reabsorption
- BThey provide energy for the sodium-potassium pumps
- CThey transport specific solutes across the membrane
- DThey lower the water potential of the surrounding tissues
35.What is the role of the co-transporter proteins in the PCT epithelial cell membranes?
Hard
- AThey transport a specific solute such as glucose into the cell alongside sodium ions
- BThey pump sodium ions out of the cell into the blood
- CThey provide ATP for active transport
- DThey allow water to move out of the PCT by osmosis
36.Which type of feedback mechanism is shown by this process?
Hard
- APositive feedback
- BNegative feedback
- COsmoregulation
- DHomeostatic control of pH
37.Which of the following are examples of conditions controlled by homeostasis in mammals? (select all that apply)
Medium
- ACore body temperature
- BBlood pH
- CBlood glucose concentration
- DWater potential of the blood
- EBlood cell diameter
38.Which of the following best describes the role of a coordination system in a homeostatic feedback loop?
Medium
- AIt detects a change from normal levels.
- BIt carries out a response that reverses the change.
- CIt receives signals from receptors and communicates with effectors.
- DIt enhances the original stimulus to cause further deviation.
39.Match each hormone with its effect on blood glucose concentration.
Medium
- Insulin
- Glucagon
- Adrenaline
- Reduces blood glucose concentration
- Increases blood glucose concentration by activating enzymes that convert glycogen to glucose
- Increases blood glucose concentration by binding to receptors on liver cell membranes
40.Place the steps of insulin action in the correct order.
Medium
- Insulin binds to specific receptors on the membranes of target cells.
- Vesicles containing glucose transporters fuse with the cell surface membrane.
- Glucose moves into target cells by facilitated diffusion.
- Enzymes are activated that convert glucose into glycogen.
41.Complete the paragraph describing events when blood water content decreases.
Medium
Osmoreceptors are located in the ____ of the brain. If the water potential of the blood decreases, water moves ____ the osmoreceptor cells, causing them to shrink. This stimulates the ____ gland to release ADH.
42.Name the processes that occur in the liver in response to insulin and glucagon.
Medium
Insulin binding activates enzymes that carry out ____, the synthesis of glycogen from glucose. Glucagon binding activates enzymes that carry out ____, the breakdown of glycogen to glucose, and ____, the synthesis of glucose from non-carbohydrate molecules.
43.Name the structures of the nephron in the order filtrate passes through them.
Easy
Filtrate forms in the Bowman's capsule, then passes through the ____, the ____, the distal convoluted tubule and finally the ____.
44.In a study of diabetes, the mean insulin sensitivity of type II diabetics was 0.40 (± 0.24) before surgery and 1.10 (± 0.87) one month after surgery. What is the increase in mean insulin sensitivity?
Medium
45.In the required practical, a water bath must be heated to at least what temperature when using quantitative Benedict's reagent? Give your answer in °C.
Medium
46.Name the process by which glucose is synthesised from non-carbohydrate molecules such as amino acids and fatty acids in the liver.
Medium
47.Name the specialised sensory neurones that monitor the water potential of the blood.
Easy
48.Name the hormone released by the posterior pituitary gland that increases the water permeability of the collecting ducts.
Medium
49.Name the membrane protein that ADH causes to be added to collecting duct cell membranes, increasing their permeability to water.
Medium
50.Complete the description of ion and water movement in the loop of Henle.
Medium
Sodium and chloride ions are pumped out of the ____ limb into the surrounding medulla, which lowers the water potential of the medulla. Water then moves out of the ____ limb into neighbouring capillaries by osmosis.
51.Complete the table explaining how features of the PCT aid its function.
Medium
Many microvilli increase the ____ for reabsorption. Many mitochondria provide ____ for the sodium-potassium pumps. Co-transporter proteins transport a specific ____ across the membrane.
52.In a study of diabetes, the mean insulin sensitivity of type II diabetics was 0.40 (± 0.24) before surgery and 1.10 (± 0.87) one month after surgery. Calculate the increase in mean insulin sensitivity. Give your answer to 2 decimal places.
Medium
53.Define the term homeostasis.
Medium
54.Explain how the endocrine system functions to bring about responses in the body.
Hard
55.Explain why the regulation of body temperature is important in mammals.
Hard
56.Explain how the maintenance of a constant body temperature in humans allows metabolic reactions within cells to proceed at maximum efficiency.
Hard
57.Describe the action of insulin in lowering blood glucose concentration.
Hard
58.In a study of diabetes, the mean insulin sensitivity of type II diabetics was 0.40 (± 0.24) before surgery and 1.10 (± 0.87) one month after surgery. What is the increase in mean insulin sensitivity?
Medium
- A-0.3
- B1.7
- C2.7
- D0.7
59.In the required practical, a water bath must be heated to at least what temperature when using quantitative Benedict's reagent? Give your answer in °C.
Medium
- A63 °C
- B70 °C
- C84 °C
- D77 °C
60.In a study of diabetes, the mean insulin sensitivity of type II diabetics was 0.40 (± 0.24) before surgery and 1.10 (± 0.87) one month after surgery. Calculate the increase in mean insulin sensitivity. Give your answer to 2 decimal places.
Medium
- A0.7
- B1.7
- C-0.3
- D2.7

Notes de révision

## Principles of Homeostasis - **Homeostasis** = maintaining the internal environment within restricted limits. - Ensures optimal conditions for enzyme action and cell function. - **Receptor cells** detect changes; send info to a **coordination system**; effectors restore conditions. - **Nervous system** communicates via nerve impulses in neurones; **endocrine system** via hormones (chemical signals in blood). - Controlled conditions include: core body temperature, blood pH, blood glucose concentration, water potential of the blood. ### Temperature & pH - Maintained within narrow limits because they affect enzyme activity. - Low temp: molecules have limited kinetic energy, fewer collisions, fewer enzyme-substrate complexes. - Body temp: more kinetic energy, frequent collisions, many complexes. - High temp: bonds holding active site break → enzyme denatures; no complexes form. - Extremes of pH: same denaturation effect. ### Blood Glucose Concentration - Needs narrow limits because: - glucose is an important respiratory substrate; lack slows respiration → less ATP. - glucose affects water potential of blood: increase lowers water potential → water moves out of cells by osmosis; decrease raises water potential → water moves into cells. ## Negative Feedback - **Negative feedback** = a system in which any change is restored to original levels. - Steps: 1) receptor detects stimulus (change from normal); 2) signals sent to coordination system (nervous or hormone); 3) signals sent to effector → response reverses change. - Systems have two corrective mechanisms: one for too low, one for too high. - **Positive feedback** enhances the original stimulus, causing deviation further from normal range. ## Glucose Concentration & Insulin - Blood glucose varies due to: consumption of carbohydrates, exercise, secretion of hormones. - Hormones: **insulin** (reduces blood glucose), **glucagon** (increases), **adrenaline** (increases). - **Insulin action** (β cells in pancreas secrete when glucose high): 1. Insulin binds to specific receptors on target cell membranes (liver, muscle). 2. More glucose transporter proteins added to cell surface membranes (vesicles fuse), increasing permeability to glucose. 3. Glucose moves into target cells by facilitated diffusion. 4. Enzymes activated that convert glucose into glycogen → **glycogenesis**. ## Glucose Regulation: Glucagon - **Glucagon** secreted by α cells in pancreas when blood glucose low. - Activates enzymes that: convert glycogen to glucose (**glycogenolysis**); convert glycerol and amino acids into glucose. - **Second messenger model**: 1. Glucagon binds to receptors on target cell membranes (liver). 2. Activates G protein → activates **adenylate cyclase**. 3. Adenylate cyclase catalyses ATP → **cyclic AMP (cAMP)** (second messenger). 4. cAMP activates **protein kinase A** → cascade → activates enzymes that break down glycogen to glucose. ## Glucose Regulation: Adrenaline - Adrenaline binds to receptors on liver cell membranes. - Same second messenger model: activates adenylate cyclase → cAMP → protein kinase A → cascade → enzymes break down glycogen to glucose. ## Glucose Regulation: the Liver - Liver regulates blood glucose; insulin and glucagon have specific receptors on liver cell membranes. - Insulin binding activates: - **glycogenesis**: synthesis of glycogen from glucose; lowers glucose in liver cells, allows more glucose from blood by diffusion. - Glucagon binding activates: - **glycogenolysis**: breakdown of glycogen to glucose; glucose diffuses into blood. - **gluconeogenesis**: synthesis of glucose from non-carbohydrate molecules (e.g. fatty acids, amino acids); glucose released into blood. ## Diabetes - Diabetes = homeostatic control of blood glucose failed/deteriorated; can result in high blood glucose, glucose in urine, dehydration, fatigue. - **Type I**: pancreas fails to produce insulin; onset often childhood; autoimmune attack on β cells; treated with insulin injections (calculated on carbohydrate intake and exercise). - **Type II**: insulin receptors no longer respond to insulin; usually adults; linked to obesity, high carbohydrate diet, age, family history; treatments: low carbohydrate diet, exercise, medications helping cells take up glucose. - Individuals with type II still produce insulin; receptors unable to sense it. - Evaluating approaches: public health campaigns, food reformulation; consider sample representativeness, causation vs correlation, statistical analysis, bias. ## Required Practical: Determining the Concentration of Glucose in Urine - Uses **quantitative Benedict's reagent** containing potassium thiocyanate; no red precipitate; presence of glucose measured by loss of blue colour and formation of white precipitate. - Positive test spectrum: blue = low concentration → colourless = high concentration. - **Calibration curve method**: 1. Prepare dilution series of glucose solutions. 2. Add fixed volume of quantitative Benedict's solution to each tube. 3. Heat in water bath at least 70 °C for 5 minutes. 4. Filter contents; add fixed volume to labelled cuvettes (removes white precipitate). 5. Set colorimeter to red (complementary to blue). 6. Calibrate with distilled water (100% transmission). 7. Measure % transmission for each. 8. Plot glucose concentration vs % transmission → calibration curve. - Unknown sample treated same way; use % transmission and calibration curve to find concentration. ## Nephron Structure - Kidneys: **osmoregulation** (regulating water content of blood) and **excretion** (removal of metabolic waste and excess substances). - Internal structure: outer **renal cortex**, inner **renal medulla**, **renal pelvis** (funnels urine into ureter), thousands of nephrons. - Nephron structures: glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct. ## Nephron Function - Urine formation: **ultrafiltration** (small molecules filtered into Bowman's capsule → glomerular filtrate) and **selective reabsorption** (useful molecules returned to blood). - **Ultrafiltration**: afferent arteriole wider than efferent → high blood pressure in glomerulus; forces small molecules into Bowman's capsule. Filtrate contains amino acids, water, glucose, urea, inorganic ions (Na+, K+, Cl-). Blood cells and large proteins remain. - Features aiding ultrafiltration: capillary endothelium (gaps), basement membrane (mesh of collagen and glycoproteins), Bowman's capsule epithelium (podocytes with gaps). - **Selective reabsorption** mostly in proximal convoluted tubule (PCT). Adaptations: microvilli (increase surface area), co-transporter proteins (transport specific solutes), many mitochondria (provide energy for sodium-potassium pumps). - Mechanisms: Na+ actively transported; Cl- follows by diffusion; water by osmosis; urea by diffusion; sugars and amino acids by co-transporter proteins with Na+. - **Loop of Henle**: Na+ and Cl- pumped out of ascending limb (impermeable to water) into medulla → lowers water potential of medulla; descending limb permeable to water → water moves out by osmosis. Generates low water potential in medulla; water reabsorbed from distal convoluted tubule and collecting duct by osmosis → concentrated urine. ## Control of Blood Water Potential - **Osmoregulation** = control of water potential of body fluids. - **Osmoreceptors** in hypothalamus monitor blood water potential. - **Low blood water content**: osmoreceptors detect decrease → water moves out of cells, they shrink → nerve impulses to posterior pituitary → releases **ADH** → increases water permeability of collecting ducts → kidneys reabsorb more water → small volume of concentrated urine. - **ADH action**: binds to receptors on collecting duct cells → vesicles with **aquaporins** fuse with membrane → more aquaporins → increased permeability to water. - **High blood water content**: osmoreceptors detect increase → less ADH released → decreased water permeability → less water reabsorbed → large volume of dilute urine.

Homeostasis: Maintaining the internal environment within restricted limits.
Negative feedback: A system in which any change is restored to original levels.
Positive feedback: Enhances the effect of the original stimulus and causes conditions to deviate further from a normal range.
Insulin: Hormone secreted by β cells in the pancreas that reduces blood glucose.
Glucagon: Hormone secreted by α cells in the pancreas that increases blood glucose.
Adrenaline: Hormone that increases blood glucose by activating enzymes that convert glycogen to glucose.
Glycogenesis: The synthesis of glycogen from glucose.
Glycogenolysis: The breakdown of glycogen to produce glucose.
Gluconeogenesis: The synthesis of glucose molecules from non-carbohydrate molecules, e.g. fatty acids and amino acids.
Second messenger model: Sequence where glucagon/adrenaline bind to receptors, activate adenylate cyclase, convert ATP to cAMP (second messenger), activate protein kinase A, leading to activation of enzymes that break down glycogen.
Type I diabetes: Condition where the pancreas fails to produce insulin; caused by autoimmune attack on β cells.
Type II diabetes: Condition where insulin receptors no longer respond to insulin.
Ultrafiltration: Small molecules are filtered out of the blood and into the Bowman's capsule of the kidney nephron, forming glomerular filtrate.
Selective reabsorption: Useful molecules are taken back from the filtrate and returned to the blood.
Osmoregulation: The control of the water potential of body fluids.
ADH (antidiuretic hormone): Hormone released by posterior pituitary gland that increases water permeability of collecting ducts, causing more water reabsorption.
Osmoreceptors: Specialised sensory neurones that monitor the water potential of the blood, located in the hypothalamus.
Aquaporins: Water channel proteins; vesicles containing them fuse with collecting duct cell membranes in response to ADH, increasing water permeability.

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