VS1: Introduction to homeostasis

Question 1

Define ‘homeostasis’

Answer 1

The tendency towards a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes

Question 2

Give a brief history of the concept of homeostasis.

Answer 2

• Claude Bernard introduced the term ‘milieu interieur’ (internal environment) in 1860s
• Walter Bradford Cannon developed the concept of homeostasis in 1920s

Question 3

Why is homeostasis important, making reference to internal and external conditions?

Answer 3

Parameters are constantly changing, which affects chemical reactions

Internal conditions must remain relatively constant for reactions to take place, even if external conditions (e.g. chemical composition, temperature) change

Entropy of interal conditions is low, whilst entropy of external conditions is high

Question 4

Describe the general homeostatic principle.

Answer 4

The variable is kept in a steady-state balance:

1. The set-point is disturbed
2. Sensors detect disturbance
3. Effector action counteracts disturbance and the set-point is obtained again

Question 5

What variables need to be controlled in organisms?

Answer 5

Nutrient/water intake

Waste prouct excretion

Body temperature

Extracellular fluid composition

Question 6

Why are cells compartmentalised?

Answer 6

Each compartment has specific functions that require specific conditions, e.g. pH, Ca²⁺ concentration

Question 7

What are the pH values for:

• mitochondrial matrix
• extracellular fluid
• lysosomes, endosomes
• scecretory vesicles
• cytosol?

Answer 7

• Mitochondrial matrix = 7.7
• Extracellular fluid = 7.4
• Lysosomes, endosomes = 4.6 – 6
• Secretory vesicles = 5 – 6
• Cytosol = 7.1

Question 8

What are the Ca²⁺ concentrations of:

• mitochondrial matrix
• extracellular gluid
• endoplasmic/sarcoplasmic reticulum
• cytosol?

Answer 8

• Mitochonidrial matrix = >1mM
• Extracellular fluid = approx. 2mM
• Endoplasmic/sarcoplasmic reticulum = >1mM
• Cytosol = 10 – 100nM

Question 9

Why have multicellular organisms evolved to have an extracellular fluid high in sodium and chloride?

Answer 9

The extracellular fluid provides a ‘personal ocean’ that replicates evolutionary conditions in salt water

Question 10

What are the roles of epiithelia?

Answer 10

• Act as an interface between internal environment of body and external space (e.g. lungs, skin, intestine)
• Maintain constant composition and volume of extracellular fluid

Question 11

What is the purpose of extracellular fluid?

Answer 11

Provides a stable environment for cells which is important for their health

Question 12

How is homeostasis achieved?

Answer 12

Regulated exchange of molecules/ions between:

• intracellular fluid (cytosol) and extracellular fluid
• cytosol and intracellular compartments

which is possible due to the nature of cell membranes (lipids and proteins)

Question 13

To what type of molecule are lipid bilayers permeable?

Answer 13

Hydrophobic molecules

Gases

Small uncharged molecules

Question 14

How is the permeability of lipid bilayers and membranes measured?

Answer 14

Measured by membrane permeability coefficient, where diffusion rate is proportional to concentration difference:

J = P x (C₁ – C₂)

J = net rate of solute movement
P = permeability coefficient
C₁, C₂ = concentrations on both sides of the membrane

Question 15

Why do cell membranes require specialised transporters whilst pure lipid bilayers do not?

Answer 15

Cell membranes have a much higher permeability for ions, glucose, and other molecules than pure lipid bilayers

Question 16

What are the five types of membrane proteins that can act as transporters?

Answer 16

1. Pores
2. Ion channels
3. Solute carriers
4. Pumps (ATP-dependent transporters)
5. Vesicular transport