Homeostasis (A Level only) (AQA A Level Biology): Flashcards
Exam code: 7402
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Define homeostasis.
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Define homeostasis.
Maintaining the internal environment within restricted limits.
Why is homeostasis important?
It ensures optimal conditions for enzyme action and cell function.
What detects changing conditions inside or outside the body?
Receptor cells, which then send information to a coordination system.
Name the two coordination systems and state how each communicates.
The nervous system communicates via nerve impulses in neurones.
The endocrine system communicates via hormones; chemical signals in the blood.
List four conditions controlled by homeostasis in mammals.
Core body temperature.
Blood pH.
Blood glucose concentration.
Water potential of the blood.
Why must temperature and pH be maintained within narrow limits?
Because they affect enzyme activity.
Explain what happens to enzymes at high temperatures.
The bonds holding the active site together break.
The enzyme denatures, so enzyme-substrate complexes can no longer form.
Why do fewer enzyme-substrate complexes form at low temperatures?
Molecules have limited kinetic energy and move slowly, so collisions are infrequent.
At extremes of pH, the bonds holding the active site together , so the enzyme .
At extremes of pH, the bonds holding the active site together break, so the enzyme denatures.
Give two reasons why blood glucose concentration must be kept within narrow limits.
Glucose is an important respiratory substrate.
Glucose can affect the water potential of the blood.
Explain the effect of an increase in dissolved blood glucose on surrounding cells.
It lowers the water potential of the blood.
Water moves out of the surrounding cells by osmosis.
True or False?
A lack of glucose may slow respiration, resulting in a lack of ATP to fuel cellular processes.
True.
Glucose is a key respiratory substrate, so a shortage can reduce ATP production.
Define negative feedback.
A system in which any change is restored to original levels.
What is the role of a receptor in a negative feedback loop?
It detects a stimulus, i.e. a change from normal levels.
Outline the three stages of a negative feedback loop.
A receptor detects a stimulus (a change from normal levels).
Signals are sent to a coordination system.
Signals are sent to an effector, which carries out a response that reverses the change.
Name the two coordination systems involved in negative feedback.
The nervous system.
The hormone (endocrine) system.
In a negative feedback loop, a detects a stimulus and an carries out a response that reverses the change.
In a negative feedback loop, a receptor detects a stimulus and an effector carries out a response that reverses the change.
Why do negative feedback systems have more than one corrective mechanism?
Because changes can occur in either direction.
One mechanism corrects a factor that becomes too low.
Another corrects a factor that becomes too high.
What is the advantage of having two separate corrective mechanisms?
It increases the level of control that can be achieved by negative feedback.
True or False?
Having separate mechanisms that control departures in opposite directions gives greater control.
True.
Separate mechanisms for too high and too low allow changes to be reversed in either direction.
Define positive feedback.
A system that enhances the effect of the original stimulus, causing conditions to deviate further from a normal range.
How does positive feedback differ from negative feedback?
Negative feedback reduces the effect of the original stimulus to restore original levels.
Positive feedback enhances the effect of the original stimulus, causing conditions to deviate further from normal.
feedback reduces the effect of the original stimulus, whereas feedback amplifies it.
Negative feedback reduces the effect of the original stimulus, whereas positive feedback amplifies it.
List three factors that cause blood glucose concentration to vary.
Consumption of foods containing carbohydrates.
Exercise.
Secretion of hormones that affect blood glucose.
Name three hormones that affect blood glucose and state their effect.
Insulin reduces blood glucose.
Glucagon increases blood glucose.
Adrenaline increases blood glucose.
Insulin blood glucose, while glucagon and adrenaline blood glucose.
Insulin reduces blood glucose, while glucagon and adrenaline increase blood glucose.
What type of feedback loop maintains blood glucose within narrow limits?
Negative feedback.
Which cells secrete insulin, and in response to what stimulus?
β (beta) cells in the pancreas.
They respond to an increase in blood glucose above the normal range.
Describe how insulin reduces blood glucose concentration.
Insulin binds to specific receptors on the membranes of target cells.
More glucose transporter proteins are added to the cell surface membranes.
Glucose moves into target cells by facilitated diffusion.
Enzymes are activated that convert glucose into glycogen.
How are additional glucose transporter proteins added to the cell surface membrane?
Vesicles containing glucose transporters fuse with the cell surface membrane.
Name two target cells of insulin.
Liver cells.
Muscle cells.
Define glycogenesis.
The conversion of glucose into glycogen.
Insulin is secreted by the cells of the pancreas and causes glucose to enter cells by diffusion.
Insulin is secreted by the β (beta) cells of the pancreas and causes glucose to enter cells by facilitated diffusion.
True or False?
Insulin increases the permeability of target cells to glucose.
True.
More glucose transporter proteins are added to the membranes, increasing permeability to glucose.
Which cells secrete glucagon, and in response to what stimulus?
α (alpha) cells in the pancreas.
They respond to a decrease in blood glucose below the normal range.
How does glucagon increase blood glucose concentration?
It activates enzymes that convert glycogen to glucose in glycogenolysis.
It activates enzymes that convert glycerol and amino acids into glucose.
Define glycogenolysis.
The conversion of glycogen to glucose.
Describe the second messenger model of glucagon action.
Glucagon binds to receptors on the cell surface membranes of target cells, e.g. liver cells.
This activates a G protein, which activates the enzyme adenylate cyclase.
Adenylate cyclase catalyses the conversion of ATP to cyclic AMP (cAMP).
cAMP activates protein kinase A, which initiates a cascade that activates enzymes that break down glycogen to glucose.
In the second messenger model, what acts as the second messenger?
Cyclic AMP (cAMP).
What is the role of adenylate cyclase in glucagon action?
It catalyses the conversion of ATP to cyclic AMP (cAMP).
Glucagon activates a G protein, which activates , converting ATP into .
Glucagon activates a G protein, which activates adenylate cyclase, converting ATP into cyclic AMP (cAMP).
What does protein kinase A do once activated by cAMP?
It initiates a cascade of reactions that activate enzymes which break down glycogen to glucose.
Besides glycogenolysis, which other process does glucagon stimulate to raise blood glucose?
The conversion of glycerol and amino acids into glucose.
True or False?
Glucagon directly converts glycogen into glucose.
False.
Glucagon does not directly convert molecules; it activates enzymes that carry out these processes.
What effect does adrenaline have on blood glucose concentration?
It increases blood glucose concentration.
How does adrenaline raise blood glucose concentration?
It binds to receptors on the cell surface membranes of target cells.
It activates enzymes that convert glycogen to glucose.
Describe the second messenger model of adrenaline action.
Adrenaline binds to receptors on the membrane of liver cells.
Adenylate cyclase is activated.
Adenylate cyclase converts ATP to the second messenger cyclic AMP (cAMP).
cAMP activates protein kinase A, which initiates a cascade that activates enzymes that break down glycogen to glucose.
Which molecule acts as the second messenger in adrenaline's action?
Cyclic AMP (cAMP); it activates protein kinase A inside the cell.
Adrenaline binds to a receptor and activates , which converts ATP into the second messenger .
Adrenaline binds to a receptor and activates adenylate cyclase, which converts ATP into the second messenger cyclic AMP (cAMP).
What is the role of protein kinase A in adrenaline action?
It initiates a cascade of reactions that activate enzymes which break down glycogen to glucose.
Which enzyme catalyses the conversion of ATP to cAMP?
Adenylate cyclase.
Adrenaline increases blood glucose by activating enzymes that convert to .
Adrenaline increases blood glucose by activating enzymes that convert glycogen to glucose.
True or False?
Adrenaline uses the same second messenger mechanism as glucagon.
True.
Both act via adenylate cyclase and the second messenger cAMP.
On which cells does adrenaline act to raise blood glucose?
Target cells such as liver cells, where it binds to receptors on the cell surface membrane.
What is the role of the liver in blood glucose regulation?
It plays a vital role in regulating blood glucose concentration.
Both insulin and glucagon have specific receptors on the membranes of liver cells.
Define glycogenesis.
The synthesis of glycogen from glucose.
Define glycogenolysis.
The breakdown of glycogen to produce glucose.
Define gluconeogenesis.
The synthesis of glucose from non-carbohydrate molecules, e.g. glycerol and amino acids.
Which hormone activates glycogenesis in liver cells?
Insulin.
Which hormone activates glycogenolysis and gluconeogenesis?
Glucagon.
How does glycogenesis lower blood glucose?
It lowers the glucose concentration inside liver cells.
This allows more glucose to be removed from the blood by diffusion.
is the synthesis of glycogen from glucose, whereas is the breakdown of glycogen into glucose.
Glycogenesis is the synthesis of glycogen from glucose, whereas glycogenolysis is the breakdown of glycogen into glucose.
is the synthesis of glucose from non-carbohydrate molecules such as glycerol and amino acids.
Gluconeogenesis is the synthesis of glucose from non-carbohydrate molecules such as glycerol and amino acids.
True or False?
Glycogenolysis results in the diffusion of glucose into the blood.
True.
The breakdown of glycogen releases glucose, which then diffuses into the blood.
True or False?
Glycogenolysis is the same process as glycolysis.
False.
Glycogenolysis is the breakdown of glycogen to glucose, whereas glycolysis is the first stage of respiration.
What is diabetes?
A condition in which the homeostatic control of blood glucose has failed or deteriorated, which can result in high blood glucose.
Give three symptoms associated with diabetes.
Glucose in the urine.
Dehydration.
Fatigue.
What causes type I diabetes?
The pancreas fails to produce insulin.
This is caused by an autoimmune attack on the β cells.
How is type I diabetes treated?
With insulin injections, calculated on the basis of carbohydrate intake and exercise.
What is the underlying cause of type II diabetes?
Insulin receptors no longer respond to insulin.
List risk factors for type II diabetes.
Obesity.
A high carbohydrate diet.
Age.
Family history.
Describe three treatments for type II diabetes.
A low carbohydrate diet.
Exercise.
Medications that help cells take up glucose from the blood.
True or False?
Individuals with type II diabetes are unable to produce any insulin.
False.
They still have insulin-producing cells and can produce insulin, but their receptors are unable to sense insulin.
In type I diabetes the pancreas fails to produce , whereas in type II diabetes the insulin no longer respond.
In type I diabetes the pancreas fails to produce insulin, whereas in type II diabetes the insulin receptors no longer respond.
Give two approaches health advisers use to reduce type II diabetes.
Promoting public health campaigns that encourage healthy eating and regular physical activity.
Recommending reduced intake of processed foods, saturated fats and sugary drinks.
Give one way the food industry can help reduce type II diabetes risk.
Reformulating products to reduce sugar, salt and fat content.
When evaluating data that links diet with diabetes, what factors should you consider?
Whether the sample is representative (e.g. sample size, human subjects).
Whether the data suggest causation or just correlation.
Whether statistical analysis has been carried out.
Whether the research could be biased, e.g. funded by the food industry.
What technique is used to determine the concentration of glucose in a urine sample?
Colorimetry, with the result compared against a calibration curve.
What is special about quantitative Benedict's reagent?
It contains potassium thiocyanate, so it does not produce a red precipitate when it contacts glucose.
How is the presence of glucose indicated with quantitative Benedict's reagent?
By the loss of blue colour and the formation of a white precipitate.
Along what colour spectrum is a positive test indicated?
Blue = low concentration.
Colourless = high concentration.
Why must the solutions be filtered before analysis?
To remove the white precipitate.
Describe how to produce the calibration curve.
Prepare a dilution series of glucose solutions of known concentration.
Add a fixed volume of quantitative Benedict's solution and heat in a water bath at least 70 °C for 5 minutes.
Filter each solution and measure the % transmission in a colorimeter.
Plot glucose concentration against % transmission.
Why is the colorimeter wavelength set to red?
Red is the complementary colour to blue, so a blue solution absorbs red light, allowing a reading to be taken.
Why is the colorimeter calibrated with distilled water?
It gives 100% transmission of light, providing a point of comparison for all other readings.
How is the glucose concentration of the unknown urine sample determined?
Treat the sample in the same way as the glucose solutions and measure its % transmission.
Read its concentration from the calibration curve.
The colorimeter is set to a wavelength and measures the percentage of light through each solution.
The colorimeter is set to a red wavelength and measures the percentage transmission of light through each solution.
True or False?
Quantitative Benedict's reagent produces a red precipitate with glucose.
False.
It contains potassium thiocyanate, so glucose is indicated by a loss of blue colour and a white precipitate.
What are the two main functions of the kidneys?
Osmoregulation: regulating the water content of the blood.
Excretion: the removal of metabolic waste and excess substances from the blood.
Define osmoregulation.
Regulating the water content of the blood.
Define excretion.
The removal of metabolic waste and excess substances from the blood.
Name the three main regions of the internal structure of the kidney.
The outer renal cortex.
The inner renal medulla.
The renal pelvis.
What happens at the renal pelvis?
Urine is funnelled into the ureter.
What is a nephron?
The functional unit of the kidney; a tiny tube responsible for the formation of urine.
List the structures found within a nephron, in order.
Glomerulus.
Bowman's capsule.
Proximal convoluted tubule.
Loop of Henle.
Distal convoluted tubule.
Collecting duct.
Nephrons span the and of the kidney and lead into the pelvis.
Nephrons span the cortex and medulla of the kidney and lead into the pelvis.
True or False?
The renal cortex is the outer region of the kidney.
True.
The cortex is the outer region and the medulla is the inner region.
Where is urine funnelled before it enters the ureter?
The renal pelvis, at the centre of the kidney.
Name the two stages of urine formation.
Ultrafiltration: small molecules are filtered out of the blood into the Bowman's capsule.
Selective reabsorption: useful molecules are returned from the filtrate to the blood.
Define ultrafiltration.
The filtration of small molecules out of the blood and into the Bowman's capsule, forming the glomerular filtrate.
Why is there high blood pressure in the glomerulus?
The afferent arteriole is wider in diameter than the efferent arteriole.
Which substances form the glomerular filtrate?
Amino acids, water, glucose, urea and inorganic ions (Na+, K+, Cl−).
Name the three layers that filter small molecules during ultrafiltration.
The capillary endothelium.
The basement membrane.
The Bowman's capsule epithelium (podocytes).
Blood cells and large are too large to leave the capillaries, so they remain in the .
Blood cells and large proteins are too large to leave the capillaries, so they remain in the blood.
Define selective reabsorption.
The reabsorption of useful substances from the glomerular filtrate back into the blood.
Where does most selective reabsorption occur?
The proximal convoluted tubule.
Describe three adaptations of proximal convoluted tubule epithelial cells.
Many microvilli to increase the surface area for reabsorption.
Many co-transporter proteins in the luminal membrane.
Many mitochondria to provide energy for the sodium-potassium pumps.
Describe how glucose is reabsorbed by co-transport in the proximal convoluted tubule.
Sodium-potassium pumps actively transport sodium ions out of the epithelial cells into the blood.
This lowers the sodium concentration inside the cells, so sodium ions in the filtrate diffuse in via co-transporter proteins, carrying glucose with them.
Glucose then diffuses down its concentration gradient into the blood.
True or False?
The ascending limb of the loop of Henle is impermeable to water.
True.
Water cannot follow the ions out directly; instead, water leaves the descending limb by osmosis.
What is the role of the loop of Henle?
To lower the water potential of the medulla.
This allows water to be reabsorbed from the collecting duct, producing concentrated urine.
Define osmoregulation.
The control of the water potential of body fluids.
What are osmoreceptors and where are they located?
Specialised sensory neurones that monitor the water potential of the blood.
They are located in the hypothalamus of the brain.
Describe the response to a decrease in blood water potential.
Osmoreceptors in the hypothalamus detect the decrease in water potential.
Nerve impulses are sent to the posterior pituitary gland, which releases ADH into the blood.
ADH increases the water permeability of the collecting ducts.
More water is reabsorbed, producing a small volume of concentrated urine.
Which gland releases ADH?
The posterior pituitary gland.
How does ADH increase the water permeability of collecting duct cells?
ADH binds to receptors on the cells lining the collecting duct.
Vesicles containing aquaporins fuse with the cell membranes.
The number of aquaporins increases, increasing permeability to water.
What is the role of ADH?
To increase the water permeability of the collecting ducts, increasing water reabsorption.
Describe the response to an increase in blood water content.
Osmoreceptors detect the increase in water potential.
Nerve impulses cause the posterior pituitary to release less ADH.
The water permeability of the collecting ducts decreases, so less water is reabsorbed.
A large volume of dilute urine is produced.
What type of urine is produced when blood water content is low?
A small volume of concentrated urine.
ADH increases the number of in the collecting duct membranes, increasing their permeability to .
ADH increases the number of aquaporins in the collecting duct membranes, increasing their permeability to water.
Where does water move as it is reabsorbed from the collecting duct?
From a high water potential in the collecting duct to a low water potential in the medulla.
True or False?
When blood water potential is high, more ADH is released.
False.
Less ADH is released, so less water is reabsorbed and a large volume of dilute urine is produced.
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