Topic 6: Homeostasis

Question 1

What is homeostasis?

Answer 1

The maintenance of a constant internal environment within restricted limits.

There are constant fluctuations in conditions but these occur around an optimum point. Homeostasis is the ability to return to the optimum and maintain an equilibrium.

Question 2

What is the importance of homeostasis?

Answer 2

• Enzymes and other proteins are sensitive to changes in pH and temperature. Any change reduces rate and could denature them. Maintaining conditions ensures reactions occur at the right rate.
• Changes to water potential of blood and tissue fluid may cause cells to shrivel or burst as a result of osmosis. This is also affected by blood glucose concentration. Constant glucose levels also ensures a reliable source of respiratory substrate.
• The ability to maintain a constant internal environment makes organisms more independent of environmental changes and so are suited to many habitats

Question 3

What is the generic sequence of a homeostatic control mechanism?

Answer 3

• Change in condition
• Detected by receptor
• Coordinator
• Effector
• Response returns condition to optimum

Question 4

Describe how negative feedback works

Answer 4

The change produced by the control system leads to a change in the stimulus, and the receptor turns the system off once the stimulus condition has returned to the optimum.

Question 5

What is the benefit of having separate negative feedback mechanisms in opposite directions?

Answer 5

Having separate mechanisms that control departures from the norm in either direction increases homeostatic control.

It is much more quick having positive actions in both directions than passively returning to the norm in one direction.

Question 6

Describe how positive feedback works

Answer 6

When a deviation from the optimum causes an even greater deviation from the normal

Question 7

Give an example of negative and positive feedback

Answer 7

• Negative: control of blood glucose
• Positive: the production of an action potential in neurones

Question 8

What is the liver and one of its roles?

Answer 8

A large organ made from cells called hepatocytes. It has many roles, one of which is helping to regulate blood glucose concentration.

Question 9

What are the main processes for blood glucose control that occur in the liver?

Answer 9

• Glycogenesis - the conversion of glucose into glycogen. Lowes blood glucose and stores glycogen for use when glucose concentration decreases
• Glycogenolysis - the hydrolysis of glycogen to glucose. Glucose diffuses back into the blood, increasing concentration
• Gluconeogenesis - the production of glucose from sources other than carbohydrate, e.g amino acids and glycerol. Used when the glycogen supply is exhausted

Question 10

What are the main roles of the pancreas?

Answer 10

• Exocrine - produces digestive enzymes, e.g protease, amylase, lipase
• Endocrine - hormones are produced in the islets of Langerhans. Glucagon in alpha cells, insulin in beta cells
• Endocrine - detects changes in blood glucose concentration

Question 11

Describe the structure of the pancreas

Answer 11

Largely made from cells producing its digestive enzymes, but scattered throughout are the hormone-producing cells called the islets of Langerhans.

These have 2 types of cells:
- Alpha cells - larger and produce glucagon
- Beta cells - smaller and produce insulin

These are then secreted into the blood

Question 12

Why does blood glucose concentration need to be controlled?

Answer 12

If blood glucose is too low, respiration rate falls and cells don’t have enough energy, so they die. If blood glucose is too high, blood water potential decreases, causing problems with osmotic pressure.

Question 13

What are some factors affecting blood glucose concentration?

Answer 13

• Carbohydrates (starch, maltose, lactose, sucrose) are hydrolysed into glucose, which is absorbed directly from the diet
• Glycogenolysis - the hydrolysis of glycogen (stored in liver and muscle cells) into glucose in the small intestine
  -Gluconeogenesis - the production of glucose from sources other than carbohydrate
• Glucose is used up gradually by respiring cells

Question 14

Why does blood glucose concentration fluctuate?

Answer 14

Organisms cannot eat continuously and diet varies. Activity also varies, so rate of respiration varies as well.

Question 15

What are some common characteristics of hormones?

Answer 15

• Produced by endocrine glands and secreted directly into the blood
• Carried in blood plasma to target cells
• Bind to receptors on target cells that have a complementary shape
• Effective in very low concentrations but often have widespread and long-lasting effects

Question 16

How is blood glucose increased when a decrease in concentration is detected?

Answer 16

• The alpha cells in the islets of Langerhans detect the decrease and secrete the hormone glucagon into blood plasma.
• Glucagon binds to transmembrane protein receptor within the cell-surface membrane of a liver cell
• The protein changes shape on the inside of the membrane, activating the enzyme adenylate cyclase.
• This enzyme converts ATP to cyclic AMP (cAMP)
• cAMP acts as a second messenger that binds to the protein kinase enzyme, activating it
• Active protein kinase catalyses the conversion of glycogen to glucose, which moves out the liver cell into blood by facilitated diffusion
• When glycogen stores are depleted, enzymes involved in gluconeogenesis are activated.

This is known as the second messenger model

Question 17

How is blood glucose increased during periods of strenuous exercise or stress?

Answer 17

• Adrenaline is released into the blood to ultimately lead to more ATP production by respiration
• Adrenaline binds to transmembrane protein receptor within the cell-surface membrane of a liver cell
• The protein changes shape on the inside of the membrane, activating the enzyme adenylate cyclase.
• This enzyme converts ATP to cyclic AMP (cAMP)
• cAMP acts as a second messenger that binds to the protein kinase enzyme, activating it
• Active protein kinase catalyses the conversion of glycogen to glucose, which moves out the liver cell into blood by facilitated diffusion
• When glycogen stores are depleted, enzymes involved in gluconeogenesis are activated.

This is known as the second messenger model

Question 18

How does the body respond when blood glucose concentration increases?

Answer 18

• Beta cells in the islets of Langerhans in the pancreas have receptors that detect increases in blood glucose. They then secrete insulin into the blood.
• Almost all body cells apart from red blood cells have glycoprotein insulin receptors, upon binding insulin causes:
• A change in the tertiary structure of glucose transport carrier proteins, causing them to open, allowing more glucose into cells via facilitated diffusion
• Vesicles containing glucose transport proteins fuse with the cell-surface membrane, increasing the density of carrier proteins, so more glucose moves into cells via facilitated diffusion
• Activation of enzymes involved in glycogenesis and the conversion of glucose into fat
• Increasing the rate of respiration of cells to increase their glucose uptake

Question 19

What is diabetes and what are the different types?

Answer 19

Diabetes is a disease in which the patient cannot metabolise carbohydrate, especially glucose.

Type I diabetes - lack of insulin
Type II diabetes - lack of responsiveness to insulin

Question 20

What is type I diabetes?

Answer 20

The body is unable to produce insulin, usually beginning in childhood. It is sometimes an autoimmune response where the beta cells in the islets of Langerhans are attacked by the body.

Question 21

How is type I diabetes controlled?

Answer 21

Controlled by insulin injections. Cannot be taken orally as the insulin protein would be digested. Insulin must be matched exactly with the glucose intake

Question 22

What is type II diabetes?

Answer 22

Normally due to a loss of glycoprotein insulin receptors on cells, or receptors losing their responsiveness to insulin. Usually occurs in people aged 40 years +. Major risk factors include obesity, poor diet and lack of exercise

Question 23

How is type II diabetes controlled?

Answer 23

Usually controlled by regulating carbohydrate intake and matching it to the amount of exercise. In some cases, insulin injections or drugs limiting glucose absorption in the small intestine are used.

Question 24

What is osmoregulation? What is the main part of the body responsible for it?

Answer 24

Osmoregulation is the homeostatic control of blood water potential.

The nephron is the functional unit of the kidney that regulates this.

Question 25

Describe the structure of the kidney

Answer 25

• Fibrous capsule - an outer membrane that protects the kidney
• Cortex - outer region made from renal capsules, convoluted tubules and blood vessels
• Medulla - inner region made from loops of Henle, collecting ducts and blood vessels
• Renal pelvis - funnel-shaped cavity that collects urine into ureter.
• Ureter - a tube that carries urine to the bladder
• Renal artery - supplies kidney with blood
• Renal vein - returns blood to heart

Kidneys are made from millions of tubular structures called nephrons

Question 26

What are the different structures found in the nephron?

Answer 26

• Collecting duct
• Loop of Henle
• Efferent arteriole
• Afferent arteriole
• Renal capsule
• Distal convoluted tubule
• Glomerulus
• Proximal convoluted tubule
• Blood capillaries

Question 27

What is the collecting duct?

Answer 27

A tube into which distal convoluted tubules from many nephrons empty. Lined with epithelial cells and becomes wider as it empties into the kidney pelvis

Question 28

What is the loop of Henle?

Answer 28

A long, hairpin loop that extends from the cortex into the medulla and back again, surrounded by blood capillaries.

Question 29

What are the efferent and afferent arterioles?

Answer 29

• Efferent: tiny vessel leaving the renal capsule with a smaller diameter than the afferent arteriole, increasing blood pressure in the glomerulus
• Afferent: tiny vessel that arises from renal artery and supplies nephron with blood

Question 30

What is the renal capsule?

Answer 30

Closed end at the start of the nephron. Cup-shaped and surrounds a mass of blood capillaries called the glomerulus. Has an inner layer made from specialised cells called podocytes.

Question 31

What is the distal convoluted tubule?

Answer 31

A series of loops surrounded by capillaries. Walls made from epithelial cells but surrounded by fewer capillaries than the proximal convoluted tubule

Question 32

What is the glomerulus?

Answer 32

A many-branched knot of capillaries from which fluid is forced out the blood

Question 33

What is the proximal convoluted tubule?

Answer 33

A series of loops surrounded by blood capillaries. Has walls made from epithelial cells with microvilli

Question 34

How are the blood capillaries structured in the nephron?

Answer 34

A concentrated network of capillaries surrounds the proximal convoluted tubule, loop of Henle and distal convoluted tubule, from where they reabsorb mineral salts, glucose and water. These then form the renal vein to return blood to the heart.

Question 35

What is one of the most important functions of the kidney ?

Answer 35

Maintaining the water potential of blood plasma, and therefore the tissue fluid too (osmoregulation).

Question 36

What are the main stages of osmoregulation in the kidney?

Answer 36

• Formation of glomerular filtrate by ultrafiltration
• Reabsorption of glucose and water by proximal convoluted tubule
• Maintenance of a sodium ion gradient in the medulla by the loop of Henle
• Reabsorption of water by the distal convoluted tubule and the collecting ducts

Question 37

What passage does blood take through the nephron?

Answer 37

Blood enters the kidney through the renal artery, which branches into many afferent arterioles, each of which enters a renal capsule of a nephron. The afferent arteriole divides to give a complex called the glomerulus. The glomerular capillaries merge to form the efferent arteriole, which sub-divides into capillaries around the nephron tubules, then feeding into the renal vein.

Question 38

How is the glomerular filtrate formed in the renal capsule?

Answer 38

By ultrafiltration in the glomerulus.

The glomerular capillary walls are made from endothelial cells with pores between them. The diameter of the afferent arteriole is greater than the efferent arteriole, so hydrostatic pressure builds up in the glomerulus. Water, glucose and mineral ions are squeezed out the capillary, forming the glomerular filtrate. Blood cells and large proteins are too large to pass into the renal capsule

Question 39

What resists the movement of filtrate out the glomerulus during ultrafiltration?

Answer 39

• Capillary endothelial cells
• Connective tissue (basement membrane)
• Podocytes (inner layer of renal capsule)
• Hydrostatic pressure of fluid in renal capsule space
• Low water potential of blood in the glomerulus

Question 40

What are some adaptations that help filtrate move out the glomerulus during ultrafiltration?

Answer 40

• Podocytes have spaces between them - filtrate can pass through gaps between them and their branches
• Endothelium of glomerular capillaries has spaces between cells

Question 41

Where does most reabsorption of water and glucose occur in the nephron?

Answer 41

Proximal convoluted tubule

Question 42

How are proximal convoluted tubule epithelial cells adapted for reabsorption of glucose and water?

Answer 42

• Have microvilli for a large surface area to reabsorb substances from filtrate in the lumen
• Have infoldings at their bases to give a large surface area to transfer reabsorbed substances into blood capillaries
• Have a high density of mitochondria for ATP for active transport

Question 43

Describe the process of reabsorption of glucose and other substances in the proximal convoluted tubule

Answer 43

• 3 Na+ actively transported out epithelial cells of proximal convoluted tubule into interstitial (tissue) fluid, then into the blood
• Lowers Na+ concentration inside cell compared to that of glomerular filtrate in lumen of proximal convoluted tubule
• Na+ diffuse through co-transport proteins into epithelial cells
• Each type of co-transport protein brings glucose, amino acids, Cl- or vitamins in with the Na+
• Diffusion of soluble substances into epithelial cells lowers water potential, so water moves into epithelial cell, interstitial fluid, then capillary via osmosis
• Lipid-soluble substances move across epithelial cell-surface membranes by diffusion
• Glucose, amino acids, Cl- and vitamins diffuse down their concentration gradients by facilitated diffusion from epithelial cells into capillaries

Question 44

What is the structure of the loop of Henle and what is it responsible for?

Answer 44

It is a hairpin-shaped tubule extending into the medulla. It has a descending limb (narrow, thin walls, highly permeable to water) and an ascending limb (wider, thick walls, impermeable to water).

It is responsible for water being reabsorbed by the collecting duct, so urine has a lower water potential than the blood.

Question 45

What does the length of the loop of Henle mean?

Answer 45

The concentration of urine produced is directly related to the length of the loop of Henle. Longer loop of Henle = more concentrated urine

Question 46

What is the significance of the direction of movement of fluid through the two limbs of the loop of Henle?

Answer 46

The fluid in the two limbs move in opposite directions, so the exchange of substances is greater. The filtrate in the collecting duct with a lower water potential meets interstitial fluid with an even lower water potential. There is a water potential gradient along the whole collecting duct, so more water enters interstitial fluid and blood by osmosis.

The loop of Henle acts as a counter-current multiplier

Question 47

Describe the process of reabsorption of water by the collecting duct and loop of Henle

Answer 47

• Na+ ions actively transported out ascending limb of loop of Henle using ATP
• Creates low water potential in interstitial region between 2 limbs - thick wall of ascending limb is impermeable to water so very little escapes
• Walls of descending limb are very permeable to water so it leaves filtrate into interstitial space, then blood
• Filtrate progressively loses water as it moves down descending limb, lowering water potential, lowest at the tip of hairpin
• At base of ascending limb, Na+ diffuses out, up the limb Na+ is actively transported out, so filtrate has progressively higher water potential
• In interstitial space between ascending limb + collecting duct, there is a water potential gradient, highest in cortex, decreasing into medulla
• Collecting duct permeable to water, so as filtrate moves down, moves into blood vessels
• As water passes out filtrate, water potential decreases, but also decreases in interstitial space so water moves out the whole length of collecting duct by osmosis

Question 48

Describe the reabsorption of material in the distal convoluted tubule

Answer 48

The cells in the tubule wall have microvilli and many mitochondria that let them reabsorb material rapidly from filtrate by active transport. The main role is to make final adjustments to the water and salts reabsorbed, and to the pH of blood. The permeability of walls can be affected by hormones.

Question 49

What are some of the key components of the osmoregulation negative feedback loop?

Answer 49

• Hypothalamus - region of the brain which acts as a control centre for the autonomic nervous system and regulates blood water potential
• Osmoreceptors - cells within the hypothalamus that detect changes in blood water potential
• Pituitary gland - endocrine gland situated at the base of the brain. Has an anterior (front) and posterior (back) part
• Antidiuretic hormone ADH - hormone produced by hypothalamus and secreted by the posterior pituitary gland

Question 50

How does water potential of the blood decrease?

Answer 50

• Too little water being consumed
• High sweating
• Large amounts of ions taken in

Question 51

How does the body respond to a fall in water potential?

Answer 51

• Osmoreceptors in hypothalamus detect fall in blood water potential
• Osmoreceptors lose water by osmosis and shrink
• Hypothalamus produces more ADH, secreted into capillaries by posterior pituitary gland
• ADH binds to receptors on distal convoluted tubule and collecting duct
• Activates phosphorylase enzyme within cell
• Causes vesicles with aquaporins to fuse with cell-surface membrane, increasing water permeability
• Increased permeability of collecting duct to urea, which passes out, decreasing water potential of interstitial space
• Combined effect causes more water to leave collecting duct by osmosis
• More water reabsorbed into blood, more concentrated urine produced
• Doesn’t increase water potential of blood, just prevents it from decreasing
• Osmoreceptors detect rise in water potential and send fewer impulses to pituitary gland
• Pituitary gland reduces release of ADH and permeability of collecting ducts reverts to original state

Question 52

What could cause an increase in blood water potential?

Answer 52

• Large volumes of water consumed
• Salts excreted / used in metabolism and not replaced in the diet

Question 53

How does the body respond to an increase in blood water potential?

Answer 53

• Osmoreceptors in hypothalamus detect rise in water potential and increase frequency of nerve impulses to pituitary gland to reduce its release of ADH
• Less ADH in the blood = decrease in permeability of collecting ducts to water and urea
• Less water reabsorbed into the blood from collecting duct
• More dilute urine produced and water potential of blood falls
• When water potential has returned to normal, osmoreceptors in hypothalamus cause pituitary to raise its ADH release back to normal