Chapter 16 - Homestasis

Question 1

What is homeostasis?

Answer 1

The maintenance of a constant internal
environment within restricted limits within an
organism.

Question 2

What is the importance of homeostasis?

Answer 2

pH(affects ionic and hydrogen bonds) and temperature affect enzymes;
may reduce efficiency of enzymes/denature enzymes;
need reactions to occur at a constant, predictable rate;
Changes to water potential causes water to enter or leave cells;
so cells shrink and expand;
so blood glucose must be maintained

Question 3

What is negative feedback?

Answer 3

Where a change triggers a response which reduces the effect of a change

Question 4

What is positive feedback?

Answer 4

Positive feedback occurs when a deviation from an optimum causes a greater deviation.

Question 5

What cells are part of the Islet of Langerhans and what hormones do they secrete?

Answer 5

Alpha cells- larger pancreatic cells that secrete glucagon
Beta cells-smaller pancreatic cells that secrete insulin

Question 6

What is glycogenesis, glycogenolysis and gluconeogenesis?

Answer 6

Glycogenesis is the conversion of glucose to glycogen
Glycogenolysis is the breakdown of glycogen to glucose
Gluconeogenesis is the production of glucose from non-carbohydrate sources, such as amino acids and glycerol

Question 7

How does insulin play a role in the control of blood glucose concentration?

Answer 7

Causes a change in the tertiary structure of the glucose carrier protein causing them to change shape
Allows more glucose to enter cells via facilitated diffusion
Also activates enzymes that convert glucose to glycogen and fat
Increases the number of glucose transport channel

Question 8

How does glucagon play a role in the control of blood glucose concentration?

Answer 8

Binds to specific receptor;
On muscle / liver cell;
Activation of enzymes (in liver);
Hydrolysis of glycogen; (Facilitated) diffusion
of glucose out of (liver cells) cells;
Increases blood glucose levels;

Question 9

How does adrenaline play a role in the control of blood glucose concetration?

Answer 9

Raises blood glucose concentration by
Attaches to protein receptors on cell-surface membrane of target cells
Activates enzymes that cause the breakdown of glycogen to glucose

Question 10

How does the second messenger model work in relation to glucagon and adrealine?

Answer 10

Adrenaline binds to a transmembrane protein receptor on cell-surface membrane of liver cell
Binding of adrenaline causes a conformational change of protein inside the membrane
This activates the enzyme adenyl cyclase converting ATP to cyclic AMP
The cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and activating it
Active enzyme protein kinase converts glycogen to glucose, which moves out of the liver cell via facilitated diffusion into blood

Question 11

What are the differences between Type 1 and Type 2 diabetes?

Answer 11

Type 1- due to a lack of insulin being produced (begins in childhood)
Type 2- due to the loss of glycoprotein receptors on body cells/ lack of response to insulin by body cells

Question 12

How are Type 1 and Type 2 diabetes controlled?

Answer 12

Type 1 is controlled by insulin injections(which is monitored by biosensors to ensure the correct dosage of insulin is given), skin patches(to allow insulin to be absorbed through the skin), controlling carbohydrate intake
Type 2 is controlled by regulating the carbohydrate intake and matching it to amount of exercise taken(can be supplemented with insulin injections)

Question 13

What is the structure of the kidney?

Answer 13

• Fibrous capsule - outer membrane protects kidney
• Cortex - made up of renal capsules, convoluted tubules and blood vessels
• Medulla - contains nephrons
• Renal pelvis - collects urine into ureter
• Ureter - carries urine
• Renal artery - supplies kidney with blood via aorta
• Renal vein - returns blood to heart via vena cava

Question 14

What is the structure of the nephron?

Answer 14

Bowman’s Capsule- closed end at the start of nephron that surrounds the glomerulus
Proximal convuluted tube-Loops surrounded by capillaries (contain microvilli) where selective reabsorption takes place
Loop of Henle-Long hairpin loop that extends from cortex into medulla where sodium ion gradient is maintained
Distal convuluted tube-loops surrounded by fewer capillaries (contain microvilli) where reabsorption of water takes place
Collecting tube- where the distal convoluted tubules from nephrons empty

Question 15

What are the structures of the blood vessels surrounding the nephron

Answer 15

Afferent arteriole- Where blood enters the nephron
Efferent arteriole - Where blood leaves nephron
Glomerulus- branched knot of capillaries where fluid is forced out of the blood
Capillaries

Question 16

How are glucose and water reabsorbed by the proximal convoluting tubule?

Answer 16

Sodium ions actively transported out of epithelial cells into blood capillaries(lowering concentration)
Sodium ions diffuse down a concentration gradient from lumen of epithelial cell to lining via facilitated diffusion
through co-transport carrier proteins(along with glucose e.g)
Molecules reabsorbed into blood

Question 16

How is glomerular filtrate formed?

Answer 16

Higher hydrostatic pressure in renal capsule means that smaller molecules(e.g glucose) can pass through the basement membrane
Proteins are too large so they cannot pass through the membrane
Presence of podocytes allow filtrate to pass between them.

Question 17

Structure of loop of Henle?

Answer 17

Has descending and ascending limb
Ascending limb- thicker walls and impermeable to water
Descending limb- narrow walls and permeable to water

Question 18

How is water reabsorbed into the blood?

Answer 18

Glucose in capillaries lowers the water potential
Water moves out into interstitial space via osmosis down water potential gradient

Question 19

How is the sodium ion gradient maintained by the loop of Henle?

Answer 19

In the ascending limb,
Na+ ions diffuse out via active transport
Water remains because the walls of the ascending limb are impermeable
This increases the concentration of Na+ ions in medulla, decreasing water potential

In descending limb-
Water moves out by osmosis and some sodium ions stay in as the walls have low permeability
Concentration of filtrate increases.

Question 20

What is the role of the hypothalamus and posterior pituituary gland?

Answer 20

Hypothalamus contains osmoreceptors that detect changes in water potential levels and produce ADH
Posterior pituitary gland- secretes ADH via signal from hupothalamus

Question 21

How is water reabsorbed by the distal convoluted tubule?

Answer 21

Water moves out by osmosis down water potential gradient
Controlled by ADH (more ADH if you’re dehydrated, less if you’re hydrated) which increases their permeability

Question 22

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

Answer 22

Detected by osmoreceptors in hypothalamus
Hypothalamus secretes more ADH
ADH attaches to receptors on collecting tube/distal convoluted tubule
This increases the permeability of cells to water by causing aquaporins to join to cell surface membrane of epithelial cell membrane
More water is reabsorbed by osmosis into DCT/collecting duct as they are more aquaporins within the cell membrane
This produces a smaller volume of concentrated urine

Question 23

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

Answer 23

Detected by osmoreceptors in hypothalamus→ less ADH secreted → decreased permeability of cells to water(less aquaporins embed in cell membrane)→ less water reabsorbed by osmosis →larger volume of less concentrated urine