6.4 - Homeostasis is the maintenance of a stable internal environment

What is homeostasis?

Internal environment is maintained within

set limits around an optimum.

Why is it important that core temperature

remains stable?

Maintain stable rate of enzyme-controlled

reactions & prevent damage to membranes.

Temperature too low = enzyme & substrate

molecules have insufficient kinetic energy.

Temperature too high = enzymes denature.

Why is is important that blood pH

remains stable?

Maintain stable rate of enzyme-controlled

reactions (& optimum conditions for other

proteins).

Acidic pH = H + ions interact with H-bonds & ionic

bonds in tertiary structure of enzymes → shape

of active site changes so no ES complexes form.

Why is it important that blood glucose

concentration remains stable?

● Maintain constant blood water potential:

prevent osmotic lysis / crenation of cells.

● Maintain constant concentration of respiratory

substrate: organism maintains constant level of

activity regardless of environmental conditions.

Define negative and positive feedback.

Negative feedback: self-regulatory mechanisms

return internal environment to optimum when

there is a fluctuation.

Positive feedback: a fluctuation triggers

changes that result in an even greater deviation

from the normal level.

Outline the general stages involved in

negative feedback

Receptors detect deviation →

coordinator → corrective mechanism by

effector → receptors detect that

conditions have returned to normal.

Suggest why separate negative

feedback mechanisms control

fluctuations in different directions.

Provides more control, especially in case

of ‘overcorrection’, which would lead to a

deviation in the opposite direction from

the original one.

Suggest why coordinators analyse inputs

from several receptors before sending an

impulse to effectors.

● Receptors may send conflicting

information.

● Optimum response may require

multiple types of effector.

Why is there a time lag between

hormone production and response by an

effector?

It takes time to:

● produce hormone

● transport hormone in the blood

● cause required change to the target

protein

Name the factors that affect blood

glucose concentration.

● Amount of carbohydrate digested from

diet.

● Rate of glycogenolysis.

● Rate of gluconeogenesis.

Define glycogenesis, glycogenolysis and

gluconeogenesis

Glycogenesis: liver converts glucose into the storage

polymer glycogen.

Glycogenolysis: liver hydrolyses glycogen into glucose

which can diffuse into blood.

Gluconeogenesis: liver converts glycerol & amino acids into

glucose.

Outline the role of glucagon when blood

glucose concentration decreases.

1. 𝞪 cells in Islets of Langerhans in pancreas detect

decrease & secrete glucagon into bloodstream.

2. Glucagon binds to surface receptors on liver cells &

activates enzymes for glycogenolysis &

gluconeogenesis.

3. Glucose diffuses from liver into bloodstream.

Outline the role of adrenaline when blood

glucose concentration decreases.

1. Adrenal glands produce adrenaline. It

binds to surface receptors on liver cells &

activates enzymes for glycogenolysis.

2. Glucose diffuses from liver into

bloodstream.

Outline what happens when blood

glucose concentration increases.

1. 𝝱 cells in Islets of Langerhans in pancreas detect

increase & secrete insulin into bloodstream.

2. Insulin binds to surface receptors on target cells to:

a) increase cellular glucose uptake

b) activate enzymes for glycogenesis (liver & muscles)

c) stimulate adipose tissue to synthesise fat

Describe how insulin leads to a decrease

in blood glucose concentration

● Increases permeability of cells to glucose.

● Increases glucose concentration gradient.

● Triggers inhibition of enzymes for

glycogenolysis.

How does insulin increase permeability

of cells to glucose?

● Increases number of glucose carrier

proteins.

● Triggers conformational change which

opens glucose carrier proteins.

How does insulin increase the glucose

concentration gradient?

● Activates enzymes for glycogenesis in

liver & muscles.

● Stimulates fat synthesis in adipose

tissue.

Use the secondary messenger model to

explain how glucagon and adrenaline

work.

1. Hormone-receptor complex forms.

2. Conformational change to receptor activates G-protein.

3. Activates adenylate cyclase, which converts ATP to

cyclic AMP (cAMP).

4. cAMP activates protein kinase A pathway.

5. Results in glycogenolysis.

Explain the causes of Type 1 diabetes

and how it can be controlled.

Body cannot produce insulin e.g. due to

autoimmune response which attacks 𝝱

cells of Islets of Langerhans.

Treat by injecting insulin.

Explain the causes of Type 2 diabetes

and how it can be controlled.

Glycoprotein receptors are damaged or become

less responsive to insulin.

Strong positive correlation with poor diet / obesity.

Treat by controlling diet and exercise regime.

Name some signs and symptoms of

diabetes.

High blood glucose concentration

● Glucose in urine

● Polyuria

● Polyphagia

● Polydipsia

● Blurred vision

● Sudden weight loss

● Blurred vision

Suggest how a student could produce a

desired concentration of glucose solution

from a stock solution.

Volume of stock solution = required concentration x

final volume needed / concentration of stock solution.

Volume of distilled water = final volume needed -

volume of stock solution.

Outline how colorimetry could be used to

identify the glucose concentration in a

sample.

1. Benedict’s test on solutions of known glucose

concentration. Use colorimeter to record absorbance.

2. Plot calibration curve: absorbance (y-axis), glucose

concentration (x-axis).

3. Benedict’s test on unknown sample. Use calibration curve

to read glucose concentration at its absorbance value.

Define osmoregulation

Control of blood water potential via

homeostatic mechanisms.

Describe the gross structure of a

mammalian kidney.

Fibrous capsule: protects kidney.

Cortex: outer region consists of Bowman’s capsules, convoluted

tubules, blood vessels.

Medulla: inner region consists of collecting ducts, loops of Henle,

blood vessels.

Renal pelvis: cavity collects urine into ureter.

Ureter: tube carries urine to bladder.

Renal artery: supplies kidney with oxygenated blood.

Renal vein: returns deoxygenated blood from kidney to heart.

Describe the structure of a nephron.

Bowman’s capsule at start of nephron: cup-shaped, surrounds

glomerulus, inner layer of podocytes.

Proximal convoluted tubule (PCT): series of loops surrounded

by capillaries, walls made of epithelial cells with microvilli.

Loop of Henle: hairpin loop extends from cortex into medulla.

Distal convoluted tubule : similar to PCT but fewer capillaries.

Collecting duct: DCT from several nephrons empty into

collecting duct, which leads into pelvis of kidney.

Describe the blood vessels associated

with a nephron

Wide afferent arteriole from renal artery enters renal

capsule & forms glomerulus: branched knot of

capillaries which combine to form narrow efferent

arteriole.

Efferent arteriole branches to form capillary network

that surrounds tubules.

Explain how glomerular filtrate is formed.

Ultrafiltration in Bowman’s capsule.

High hydrostatic pressure in glomerulus forces small

molecules (urea, water, glucose, mineral ions) out of

capillary fenestrations AGAINST osmotic gradient.

Basement membrane acts as filter. Blood cells & large

molecules e.g. proteins remain in capillary.

How are cells of the Bowman’s capsule

adapted for ultrafiltration?

● Fenestrations between epithelial cells

of capillaries.

● Fluid can pass between & under

folded membrane of podocytes.

State what happens during selective

reabsorption and where it occurs.

Useful molecules from glomerular filtrate

e.g. glucose are reabsorbed into the

blood.

Occurs in proximal convoluted tubule.

Outline the transport processes involved

in selective reabsorption.

How are cells in the proximal convoluted

tubule adapted for selective

reabsorption?

● Microvilli: large surface area for co-transporter

proteins.

● Many mitochondria: ATP for active transport

of glucose into intercellular spaces.

● folded basal membrane: large surface area.

What happens in the loop of Henle?

1. Active transport of Na + & Cl - out of ascending limb.

2. Water potential of interstitial fluid decreases.

3. Osmosis of water out of descending limb

(ascending limb is impermeable to water).

4. Water potential of filtrate decreases going down

descending limb: lowest in medullary region,

highest at top of ascending limb.

Explain the role of the distal convoluted

tubule.

Reabsorption:

a) of water via osmosis

b) of ions via active transport

permeability of walls is determined by

action of hormones.

Explain the role of the collecting duct.

Reabsorption of water from filtrate into

interstitial fluid via osmosis through

aquaporins.

Explain why it is important to maintain an

Na + gradient.

Countercurrent multiplier: filtrate in collecting

ducts is always beside an area of interstitial fluid

that has a lower water potential.

Maintains water potential gradient for maximum

reabsorption of water.

What might cause blood water potential

to change?

● level of water intake

● level of ion intake in diet

● level of ions used in metabolic

processes or excreted

● sweating

Explain the role of the hypothalamus in

osmoregulation.

1. Osmosis of water out of osmoreceptors

in hypothalamus causes them to shrink.

2. This triggers hypothalamus to produce

more antidiuretic hormone (ADH).

Explain the role of the posterior pituitary

gland in osmoregulation.

Stores and secretes the ADH produced

by the hypothalamus.

Explain the role of ADH in

osmoregulation.

1. Makes cells lining collecting duct more permeable to water:

● Binds to receptor → activates phosphorylase → vesicles

with aquaporins on membrane fuse with cell-surface

membrane.

2. Makes cells lining collecting duct more permeable to urea:

● water potential in interstitial fluid decreases.

● more water reabsorbed = more concentrated urine.