Homeostasis in Mammals (Cambridge (CIE) A Level Biology): Flashcards
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Define homeostasis.
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Define homeostasis.
Homeostasis is the maintenance of a constant internal environment, keeping conditions within narrow limits despite changes in internal and external factors.
Why is homeostasis important in mammals?
Cells work most efficiently in near-optimum conditions.
Maintaining a stable internal environment (e.g. core temperature, blood glucose and blood water potential) ensures enzymes and cells function properly.
Outline the general sequence of a homeostatic control pathway.
Stimulus (change in internal or external factor)
→ receptor detects the change
→ coordination system (nervous or endocrine)
→ effector (muscle or gland)
→ response that restores the norm.
Define negative feedback.
Negative feedback is a control mechanism in which a change in a factor triggers a response that reverses the change, returning the factor towards its set point.
Which two coordination systems are used in homeostasis, and give an example effector of each.
Nervous system and endocrine system.
Effectors are muscles and glands.
Structures that detect a change in a factor are called .
Structures that detect a change in a factor are called receptors.
True or False?
In negative feedback, the response reinforces the original change.
False.
In negative feedback the response reverses the original change, returning the factor towards its set point.
Define deamination.
Deamination is the removal of the amino group (–NH2) from an amino acid, carried out in the liver.
Where and from what is urea produced?
Urea is produced in the liver from the deamination of excess amino acids.
Why must excess amino acids be broken down?
Amino acids cannot be stored by the body.
Excess amino acids are deaminated in the liver so their carbon skeletons can be used, and the amino group is converted to urea for excretion.
Urea is produced in the liver from the of excess amino acids.
Urea is produced in the liver from the deamination of excess amino acids.
True or False?
Urea is produced in the kidneys.
False.
Urea is produced in the liver. The kidneys then excrete it in the urine.
Name the main regions of the human kidney from the outside inwards.
Cortex (outer region)
→ medulla (inner region)
→ renal pelvis (central chamber leading to the ureter).
What is the fibrous capsule of the kidney?
The fibrous capsule is the tough outer layer of connective tissue that surrounds and protects the kidney.
The outermost region of the kidney tissue is the .
The outermost region of the kidney tissue is the cortex.
What is the renal pelvis?
The renal pelvis is the central chamber of the kidney that collects urine and channels it into the ureter.
What is the function of the ureter?
The ureter carries urine from the renal pelvis of the kidney to the bladder.
Compare the renal artery and the renal vein.
The renal artery brings oxygenated blood, high in urea, into the kidney.
The renal vein carries deoxygenated blood, low in urea, away from the kidney.
List the parts of a nephron in the order that filtrate passes through them.
Bowman's capsule (surrounding the glomerulus)
→ proximal convoluted tubule
→ loop of Henle
→ distal convoluted tubule
→ collecting duct.
What is the glomerulus?
The glomerulus is a knot of capillaries sitting inside the Bowman's capsule, where ultrafiltration of the blood occurs.
What is the Bowman's capsule?
The Bowman's capsule is the cup-shaped start of the nephron that surrounds the glomerulus and collects the glomerular filtrate.
What is the loop of Henle?
The loop of Henle is the hairpin-shaped section of the nephron that dips into the medulla and is involved in establishing a water potential gradient.
The first coiled tubule after the Bowman's capsule is the convoluted tubule.
The first coiled tubule after the Bowman's capsule is the proximal convoluted tubule.
Compare the afferent and efferent arterioles of the glomerulus.
The afferent arteriole carries blood into the glomerulus.
The efferent arteriole carries blood out of the glomerulus and is narrower, helping to create high hydrostatic pressure.
What do the distal convoluted tubule and collecting duct connect to?
The distal convoluted tubule drains into the collecting duct.
Several collecting ducts carry urine to the renal pelvis.
Define ultrafiltration.
Ultrafiltration is the filtration of blood under high hydrostatic pressure, forcing small molecules out of the glomerulus and into the Bowman's capsule to form glomerular filtrate.
What produces the high hydrostatic pressure needed for ultrafiltration?
The efferent arteriole is narrower than the afferent arteriole.
This creates high hydrostatic pressure in the glomerulus, forcing fluid into the Bowman's capsule.
Which components of blood are held back during ultrafiltration, and which pass into the filtrate?
Held back: blood cells and large plasma proteins (too large to pass the basement membrane).
Pass into filtrate: water, glucose, amino acids, salts and urea.
How are podocytes adapted for their role in the Bowman's capsule?
Podocytes are cells with finger-like projections that wrap around the capillaries.
The gaps between them let filtrate pass easily into the Bowman's capsule.
Define selective reabsorption.
Selective reabsorption is the process of taking useful substances back from the filtrate into the blood, occurring mainly in the proximal convoluted tubule (PCT).
What is reabsorbed from the filtrate in the proximal convoluted tubule?
All of the glucose and amino acids.
Most of the water and mineral ions are also reabsorbed back into the blood.
How is glucose reabsorbed in the proximal convoluted tubule?
Sodium ions are actively pumped out of the PCT cells into the blood.
Na^+^ then diffuses back in from the filtrate, co-transporting glucose with it; glucose then enters the blood by facilitated diffusion.
How is a proximal convoluted tubule cell adapted for selective reabsorption?
Microvilli on the surface give a large surface area.
Many mitochondria provide ATP for active transport, and co-transporter proteins are present in the membrane.
The filtration of blood in the glomerulus under high pressure is called .
The filtration of blood in the glomerulus under high pressure is called ultrafiltration.
True or False?
Glucose is normally present in the urine of a healthy person.
False.
In a healthy person all glucose is reabsorbed by selective reabsorption in the proximal convoluted tubule, so none appears in the urine.
Define osmoregulation.
Osmoregulation is the control of the water potential of the blood, keeping it within narrow limits.
Which structure monitors the water potential of the blood?
Osmoreceptors in the hypothalamus detect changes in the water potential of the blood.
Where is antidiuretic hormone (ADH) released from?
ADH is made in the hypothalamus and released into the blood from the posterior pituitary gland.
What does ADH do to the collecting duct?
ADH makes the collecting duct walls more permeable to water by inserting more aquaporins into the membranes.
More water is reabsorbed, producing less, more concentrated urine.
What are aquaporins?
Aquaporins are water-channel proteins in cell membranes that allow water to move across by osmosis.
How does the body respond when blood water potential is too low (dehydration)?
Osmoreceptors in the hypothalamus detect the low water potential.
More ADH is released → collecting ducts become more permeable → more water reabsorbed → small volume of concentrated urine.
How does the body respond when blood water potential is too high (overhydration)?
Less ADH is released → collecting ducts become less permeable → less water reabsorbed → large volume of dilute urine.
ADH increases the number of water-channel proteins called in the collecting duct membrane.
ADH increases the number of water-channel proteins called aquaporins in the collecting duct membrane.
Which hormones control blood glucose, and where are they made?
Insulin (from β cells) and glucagon (from α cells).
Both are made in the islets of Langerhans in the pancreas.
What is the effect of insulin on muscle and liver cells?
Insulin increases glucose uptake into muscle cells.
In liver cells it stimulates glycogenesis (conversion of glucose to glycogen) and reduces glucose release, lowering blood glucose.
What is the effect of glucagon on liver cells?
Glucagon stimulates glycogenolysis (breakdown of glycogen to glucose) in the liver.
Glucose is released into the blood, raising blood glucose.
How is blood glucose control an example of negative feedback?
A rise in blood glucose triggers insulin release, which lowers it.
A fall in blood glucose triggers glucagon release, which raises it.
Each response reverses the change, returning glucose to normal.
Define second messenger.
A second messenger is a molecule (e.g. cyclic AMP) produced inside a cell that relays a signal from a hormone bound at the cell surface to the inside of the cell.
In cell signalling, what happens when glucagon binds to its cell surface receptor?
Binding causes a conformational change in the receptor.
This activates a G-protein, which in turn stimulates the enzyme adenylyl cyclase.
What is the role of adenylyl cyclase and cyclic AMP (cAMP) in glucagon signalling?
Adenylyl cyclase catalyses the formation of the second messenger cyclic AMP (cAMP).
cAMP then activates protein kinase A, initiating an enzyme cascade.
How is the glucagon signal amplified inside the cell?
Through an enzyme cascade: each activated enzyme activates many more enzymes by phosphorylation.
This greatly amplifies the original signal.
What is the final cellular response of the glucagon signalling pathway?
The final enzyme in the pathway is activated and catalyses the breakdown of glycogen into glucose, which is released into the blood.
The second messenger formed in the glucagon signalling pathway is cyclic .
The second messenger formed in the glucagon signalling pathway is cyclic AMP (cAMP).
Which two enzymes are used in test strips and biosensors to measure glucose?
Glucose oxidase and peroxidase.
What reaction does glucose oxidase catalyse on a test strip?
Glucose oxidase catalyses the oxidation of glucose to gluconic acid.
This reaction also produces hydrogen peroxide (H2O2).
What is the role of peroxidase on a glucose test strip?
Peroxidase catalyses the reaction of hydrogen peroxide with a colourless chemical.
This produces a coloured product, and the intensity of colour depends on the glucose concentration.
How is glucose concentration read from a test strip?
The colour produced on the strip is compared against a colour chart.
This gives an approximate glucose concentration in blood or urine.
How does a biosensor improve on a simple test strip?
A biosensor converts the chemical reaction into an electrical signal proportional to the glucose concentration.
This gives a precise, digital reading rather than a colour comparison.
The enzyme oxidase catalyses the oxidation of glucose on a test strip.
The enzyme glucose oxidase catalyses the oxidation of glucose on a test strip.
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