Homeostasis

Question 1

What is homeostasis?

Answer 1

The maintenance of a constant internal environment.

Question 2

What is negative feedback?

Answer 2

A control mechanism where the body corrects deviations from a set point and turning off the corrective measures as the system gets closer to its normal range.

Question 3

What are some examples of negative feedback loops?

Answer 3

Body temperature, blood pressure, metabolism, regulation of blood sugar, production of red blood cells

Question 4

Why is homeostasis important?

Answer 4

If the temperature is too high or the pH is not optimal, hydrogen/ionic bonds in enzymes may break, changing the tertiary structure of the enzyme. This will change the shape of the active site, so the substrate will no longer fit into the active site (no longer complementary). No enzyme-substrate complexes will form and metabolism stops, so the organism cannot survive. This is only the effect of pH and temperature, lots of other variables also need to be controlled.

Question 5

How is temperature controlled in the body when it is too cold?

Answer 5

If body temperature decreases below 37 degrees C (optimum), the decrease in temperature of the blood is detected by thermoreceptors in the hypothalamus, which sends signals to effectors to cause shivering of muscles and vasoconstriction of blood vessels near the surface of the skin. This causes body temperature to raise. The hypothalamus detects when normal body temperature is restored and stops the shivering and vasodilation to prevent overheating.

Question 6

How is temperature controlled in the body when it is too hot?

Answer 6

The thermoreceptors in the hypothalamus detect when the blood is too hot (over 37 degrees C). The hypothalamus sends signals to stimulate the sweat glands, causing sweating. There is also vasodilation of the blood vessels near the surface of the skin, allowing for heat loss. When the body retains normal temperature, the hypothalamus detects this and reduces the activity of sweat glands, stopping the body from cooling too much.

Question 7

What is positive feedback?

Answer 7

A mechanism whereby feedback after a deviation from a set point causes the corrective measures to remain on. This makes the system deviate even more from its original level.

Question 8

What are some examples of positive feedback?

Answer 8

Influx of Na+ into a cell during the build-up of an action potential (sodium entry increases the permeability of the neurone, triggering entry of more ions). Release of oxytocin during pregnancy (baby pushes against cervix, causing it to stretch, causing nerve impulses to be sent to the brain, which stimulates pituitary to release more oxytocin, which causes smooth muscle lining the uterus to contract, causing the baby to push more).

Question 9

What happens when blood glucose is too high (over 5 mmol dm-3 blood)?

Answer 9

Detected by beta cells in the islets of langerhans region of the pancreas. Insulin is secreted into the blood plasma (it binds to glycoprotein receptors found on nearly all cells except red blood cells). This triggers increased cellular respiration to metabolise glucose. Enzymes activated causing conversion of glucose to fat and glucose to glycogen in liver an muscle cells (glycogenesis). When insulin binds to its receptor proteins on the cell-surface membrane, vesicles in the cell with embedded glucose carrier proteins fuse to the cell-surface membrane (inserting the proteins into the membrane). The carrier proteins now allow glucose to enter the cell via facilitated diffusion. Blood glucose concentration falls to normal, so negative feedback reduces insulin secretion.

Question 10

What happens when blood glucose is too low (below 5 mmol dm-3)?

Answer 10

Detected by alpha cells of the pancreas in the islets of langerhans. Glucagon is secreted into the blood plasma. The receptors are only found on hepatocytes (liver cells). Enzymes become activated to help convert glycogen to glucose (glycogenolysis) and to convert amino acids and other non-carbohydrates to glucose (gluconeogenesis). Glucose entry from the intestines is triggered (any unabsorbed glucose gets absorbed into the blood). Blood glucose rises to normal and negative feedback reduces glucagon secretion.

Question 11

What are some of the physiological effects of adrenaline?

Answer 11

Pupils dilate in the eyes to let more light into the eye/retina to see danger more easily. Heart rate increases to deliver more oxygen and glucose to muscles for increased respiration required for fight or flight. Breathing rate increases to deliver more blood to the lungs. Small arteries in the intestine narrow to divert more blood to the led muscles to run. Glycogen gets converted back to glucose in the liver to provide more glucose for a greater rate of respiration to produce more ATP so you can run faster/muscles contract more.

Question 12

Describe the secondary messenger model of adrenaline and glucagon action.

Answer 12

Adrenaline/glucagon binds to the complementary receptors on a transmembrane protein bound to inactive adenylyl (adenylate) cyclase. This changes the tertiary structure of the transmembrane protein, which activates the adenylyl (adenylate) cyclase by changing the shape of the active site. This activated adenylyl (adenylate) cyclase converts ATP to cyclic AMP, which acts as a secondary messenger. The cAMP changes the shape of (and activates) protein kinase. The active protein kinase catalyses the conversion of glycogen to glucose in a series of steps.

Question 13

What are the adrenal glands?

Answer 13

Endocrine glands found on top of the kidneys which secrete adrenaline into the blood when stressed to raise glucose levels. Stimulates glycogenolysis and inhibits glycogenesis (so works antagonistically to insulin).

Question 14

What are the common symptoms of diabetes?

Answer 14

• hyperglycaemia (too much blood sugar) can lead to dehydration
• extreme thirst
• weight loss
• long term can damage blood vessels in retina
• glucose in the urine
• can damage kidneys and nerves
• hypoglycaemia (not enough blood sugar) caused by missing meals or injecting too much insulin, can cause sweating and unconsciousness

Question 15

What is the cause of type 1 diabetes?

Answer 15

Mainly found in young people, caused by an autoimmune attack on the pancreas meaning insulin cannot be produced, so glucose can’t be taken into cells.

Question 16

What is the cause of type 2 diabetes?

Answer 16

Mainly found in adults, caused when glycoprotein receptors become less responsive to insulin, so less glucose is taken into cells.

Question 17

What are the treatments for type 1 diabetes?

Answer 17

• regular insulin injections (can’t be taken orally as would be digested)
• regular monitoring of blood sugar levels by biosensors to ensure the correct dose is administered

Question 18

What are the treatments for type 2 diabetes?

Answer 18

Healthy diet (control sugar intake) and regular exercise. There are some drugs which reduce the rate at which glucose is absorbed by the body, which can help control levels.

Question 19

What are the treatments for type 2 diabetes?

Answer 19

Healthy diet (control sugar intake) and regular exercise. There are some drugs which reduce the rate at which glucose is absorbed by the body, which can help control levels.

Question 20

Describe the structure of the excretory system.

Answer 20

Blood containing wastes enters the renal artery from the aorta, entering the kidneys. The blood is filtered and certain substances are selectively reabsorbed at the nephrons in the kidneys. Blood without wastes leaves the kidneys via the renal vein, leading to the vena cava. Urine is produced in the kidneys, which travels down the ureters to be stored in the bladder then excreted through the urethra.

Question 21

Describe the structure of the kidney.

Answer 21

The renal artery carries blood with wastes to the nephrons, while the renal vein carries blood without wastes away from the nephrons. The capsule is a fibrous outer membrane for protection. The cortex is the outer region made of renal (Bowman’s) capsules, convoluted tubules and blood vessels. The medulla (pyramids) is the inner region made of loops of Henle, collecting ducts and blood vessels. Nephrons are tubular structures which form the basic structural and functional units of the kidneys. Collecting ducts lead to a minor calyx, which leads to a major calyx, which leads to the renal pelvis, which collects the urine to go down the ureter.

Question 22

Describe the process of ultrafiltration.

Answer 22

Blood enters the glomerulus (a bundle of capillaries looped inside the Bowman’s capsule) from the afferent arteriole under high hydrostatic pressure. A filtrate, containing small molecules such as water, glucose, amino acids and urea, is forced through small pores in the capillary endothelium by ultrafiltration. The filtrate then passes through the basement membrane (a porous protein mesh which acts as a filter). Larger proteins / blood cells and platelets are too large to fit through the pores in the basement membrane, so remain in the blood. The filtrate then passes through the Bowman’s capsule epithelium by moving between podocyte branches called foot processes (podocytes are specialised cells that line the Bowman’s capsule).

Question 23

Why is the efferent arteriole smaller in diameter than the afferent arteriole?

Answer 23

When the blood is filtered at the glomerulus, it significantly decreases in volume (as it loses lots of water and other small molecules), so the efferent arteriole has a smaller diameter to maintain high blood pressure in blood leaving the glomerulus.

Question 24

Describe the selective reabsorption of substances in the proximal convoluted tubule (PCT)?

Answer 24

All the glucose in the filtrate is reabsorbed in the PCT, along with some Na+ and K+, some water and all amino acids.

Question 25

What are some adaptations of the proximal convoluted tubule?

Answer 25

• the epithelium of the PCT is folded and has many microvilli to increase surface area for reabsorption of substances such as glucose, amino acids and ions.
• epithelial cells have many carrier/channel proteins to increase the rate of facilitated diffusion
• epithelial cells have many mitochondria which release ATP from respiration for the active transport/cotransport of glucose to ensure it all returns to the blood.

Question 26

How is glucose removed and reabsorbed from the filtrate into the blood in the PCT?

Answer 26

Sodium ions and glucose are absorbed together by cotransport via a carrier protein. Sodium ions are removed from the epithelial cell by active transport into the blood (using Na+/K+ pump). This maintains a low concentration of sodium ions in the epithelial cell, allowing sodium ions to enter by facilitated diffusion taking glucose with them. Glucose moves by facilitated diffusion into the blood through a channel protein.

Question 27

Is all the glucose in the filtrate always reabsorbed?

Answer 27

If circulating glucose levels are high enough, all the glucose transporters in the proximal convoluted tubule become saturated, meaning their capacity to move glucose is exceeded and the excess cannot be reabsorbed, so some glucose may appear in the urine.

Question 28

How is the concentration of urine produced related to the loop of Henle?

Answer 28

The concentration of urine is directly proportional to the length of the loop of Henle (animals with a longer loop of Henle produce more concentrated urine as more water can be reabsorbed into the blood). Animals which live in dry environments tend to have longer loops of Henle as a result.

Question 29

Describe how water is reabsorbed in the loop of Henle.

Answer 29

Sodium ions are actively transported out of the ascending loop of Henle, using ATP provided by the many mitochondria in the cells of its wall. This creates a low water potential in the interstitial region (part of the medulla between the two limbs of the loop of Henle). The walls of the ascending limbs are very thick and practically impermeable to water, so barely any escapes by osmosis. Walls of the descending limb are very permeable to water, so lots of water passes out of the filtrate in the descending limb by osmosis due to the water potential gradient. Water then moves from the interstitial space to the capillaries to be moved away. As the filtrate moves down the descending limb, its water potential decreases and it reaches its lowest point at the base of the loop. Na+ diffuses out of the filtrate at the base of the ascending limb and Na+ is actively pumped out along its length, increasing the water potential of the filtrate. There is an interstitial region between the collecting duct and the ascending limb, in which there is a water potential gradient (highest in the cortex and lowest in the medulla). The collecting duct is permeable to water as as the filtrate moves down it, water passes out by osmosis and is passed into capillaries. The loop of Henle acts as a counter-current multiplier, ensuring there is always a water potential gradient to draw water out of the collecting duct. When the filtrate leaves the collecting duct it is now urine and is more concentrated than the blood.

Question 30

What is the role of the distal convoluted tubule?

Answer 30

The cells lining the distal convoluted tubule are lined with microvilli and mitochondria, allowing them to reabsorb material from the filtrate by active transport. Under the influence of various hormones, its permeability is altered to make final adjustments to the water and salt level to control blood pH.

Question 31

What is osmoregulation?

Answer 31

Osmoregulation refers to the balancing of salt and water levels in the blood/cells.

Question 32

How is water taken into and out of the body?

Answer 32

In:
- via the diet
- produced in aerobic respiration
Out:
- excretion (urination)
- breathing out (exhalation)
- sweating

Question 33

Why is osmoregulation important?

Answer 33

If there is too high a water potential in the blood, water will enter cells by osmosis, causing them to swell and they may eventually burst (lysis). If there is too low a water potential in the blood, water will leave the cells in the body via osmosis. This has a dehydrating effect and can lead to cell death.

Question 34

What is ADH (antidiuretic hormone)?

Answer 34

Osmoregulation is controlled in part by ADH, which makes the distal convoluted tubule and collecting duct more permeable to water, so more gets absorbed back into the bloodstream.

Question 35

What happens if your water potential in the blood becomes too high?

Answer 35

High blood water potential may be caused by water intake increasing or salts being used in metabolism/ being excreted not being replaced. The increase in water potential is detected and osmoreceptors of the hypothalamus are stimulated. Efforts are made to reduce water level (thirst centre not stimulated). The hypothalamus increases the frequency of nerve impulses to the posterior pituitary gland to reduce the release of ADH. With little ADH in the blood, the permeability of the collecting duct to water and urea decreases. Less water is reabsorbed into the blood from the collecting duct, so a large volume of dilute urine is produced. The fall in water potential is detected by osmoreceptors in the hypothalamus. The pituitary gland raises its ADH release back to normal levels by negative feedback and correct water potential is restored.

Question 36

What happens if your blood water potential becomes too low?

Answer 36

Low blood water potential may be caused by reduced water intake, increased sweating or increased salt intake. The change is detected and osmoreceptors (which have shrunk due to water loss) of the hypothalamus are stimulated. The hypothalamus secretes ADH which passes to the posterior pituitary gland to be secreted into the capillaries. ADH binds to specific protein receptors on cells lining the distal convoluted tubule and collecting duct - this activates phosphorylase enzymes within cells, causing vesicles to move to and fuse with the cell-surface membrane. The vesicles contain aquaporins (water channel proteins), which become added to the cell-surface membrane, and water permeability increases, so more water is reabsorbed by osmosis. ADH causes the collecting duct to become more permeable to urea, which moves into the interstitial region, lowering its water potential. This encourages more water loss from the collecting duct. A small amount of concentrated urine is produced. The rise in water potential is detected by osmoreceptors and the pituitary gland reduces ADH release, so the permeability of the collecting duct returns to normal and correct water potential is restored.

Question 37

What is the method for core practical 11 (glucose dilution series)?

Answer 37

• use distilled water and a stock solution of glucose to produce a dilution series (use c1v1=c2v2 to calculate the required volumes of glucose solution and make up the rest of the required volume with distilled water)
• add Benedict’s solution to each solution and heat in a water bath
• ensure the solutions are fully mixed, then pour into cuvettes and measure the absorbance of the solution using a colorimeter with a red filter. Between each reading the colorimeter should be zeroed with distilled water.
• make a calibration curve of the concentration of the glucose solution against absorbance (plot absorbance on the y-axis and concentration of glucose on the x-axis). Draw a line of best fit
• add Benedict’s reagent and heat the urine samples (use the same volume as was used for the glucose solutions), then read off the glucose concentration associated with absorbance value obtained