Metabolic Physiology

Question 1

How is temperature homeostasis maintained

Answer 1

36.5-37.5 degrees celcius, varying by 0.5 with circadian rhythm.

Heat generation
Basal metabolic rate
Heat from digestion
Muscle activity (increased by exercise)

Heat loss
Distribution of blood to skin
Countercurrent exchange (warmer arterial blood transfers heat to nearby veins, returning heat to the core) - affected by vasoconstriction
Difference between body temperature & environment (60% of body heat lost via IR radiation)
Insulation & clothing (heat lost through conduction & convection)
Evaporation of water (wet skin cools the body much faster)

Question 2

What is basal metabolic rate, and how is it measured?

Answer 2

Rate of energy production by catabolism of food under standard conditions
Fasted for > 12 hours
Mental & physical rest
Corrected for age, sex and surface area
200kJ/m²/Hr is normal for adult male

Measured through calorimetry
Direct:
Person placed in an insulated room, controlled external temp.
Heat produced warms up water in coils in the room, which can be measured to determine heat energy released by the person
Indirect:
Spirometry to record oxygen consumption, and infer metabolic rate from this data

Affected by:
Age, sex, body size, food, exercise, pregnancy, hormones, genetics, environmental temperature

Question 3

Why do we have enzymes?

Answer 3

Most reactions in the body would not happen quickly enough to sustain life, so enzymes catalyse them, by lowering the activation energy required to allow the reaction to proceed - the total change in energy remains the same.

Carbonic anhydrase increases the speed of CO₂ to bicarbonate production by 100x.
Urease increases speed of urea hydrolysis by 1000x

Question 4

What are cofactors, and what is the induced fit hypothesis?

Answer 4

Cofactors are required but not consumed in enzymatic reactions.

Often metallic
Magnesium in hexokinase
Ferrous iron for cytochrome oxidase
Zinc for carbonic anhydrase

Or organic molecules
Co-enzyme A
Co-enzyme B12

Induced fit reflects enzymes being are able to undergo conformational change, with different energy states, promoting interaction with substrates.
Such as hexokinase in glucose metabolism, or Hb binding to oxygen

Question 5

What determines the rate of enzyme function?

IMAGE

Answer 5

Rate constant of the enzyme

Temperature - Increasing kinetic energy speeds up reactions until the enzyme denatures

pH - Ionisation at active site will be affected by pH, and each enzymes has an optimal pH

Question 6

In context of enzyme function

Explain the Michaelis Menton Equation?

IMAGE of graph

Answer 6

V₀ = Vmax [s] / Kₘ + [s]

Describes how the rate of enzyme function changes with substrate concentration, until fully saturated with substrate.

T = 0 and therefore there is plenty of [E] (Enzyme), [S] (Substrate). At this time, V = V₀

E+S <k₋₁--k₁> ES <k₂--k₋₂> E+P

1 V₀ = k₂[ES]
The velocity of the equation is dependent on the concentration of ES

2 Rate of formation of ES
ES = k₁[E][S]

3 Rate of dissociation of ES
ES = k₂[ES]+k₋₁[ES]

At steady state, formation and dissociation will be equal.

k₁[E][S]=k₂[ES]+k₋₁[ES]
k₁[E][S]=(k₂+k₋₁)[ES]
([E][S])/[ES] = (k₂+k₋₁)/k₁

4 Kₘ = (k₂+k₋₁)/k₁

Rearranging for [ES]
5 [ES] = [E][S]/Kₘ

In context of T = 0

6 [S]free is ~ [S]total
7 [E] = [E]total - [ES]

Substituting into equation 5
[ES] = ([E]total - [ES])[S]/Kₘ
[ES] = [E]total[S]/Kₘ + [S]

Then substitute into equation 1
8 V₀ = k₂[E]total[S]/Kₘ + [S]

Vmax occurs when all active sites are occupied

Vmax = k₂[ES]max
Vmax = k₂[E]total
9 [E]total=Vmax/k₂

Substituting equation 8 into 9

V₀ = Vmax([s]/Kₘ + [S])

This is the Michaelis-Menton equation

k are the constants forwards and backwards on a reversible reaction

Km = Michaelis constant

https://youtu.be/NVDxNal06zM

Question 7

What factors affect the respiratory quotient (RQ)/

Answer 7

Metabolic
Food
Carbohydrate molecules contain oxygen, meaning less oxygen is needed for each molecule of CO₂ produced, with an RQ of 1
Fats have far less oxygen, meaning much more is required to produce CO₂, with RQ of 0.7
Proteins are in the middle at 0.8
Exercise & Hyperventilation
Any metabolic process that produces energy or heat will affect the CO₂/O₂ balance, and therefore the RQ
Kussmaul breathing to counter metabolic acidosis will increase RQ as more CO₂ is excreted by the lungs.
Non-Metabolic
Mainly due to drugs such aas opioids, inducing hypoventilation and reducing the amount of CO₂ excreted, lowering the RQ