Introduction to cellular homeostasis

Question 1

Homeostasis

Answer 1

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

Question 2

Importance of homeostasis

Answer 2

Maintaining a constant internal chemical composition and temperature vs variable external conditions.

Question 3

General homeostatic principle

Answer 3

Variable set point disturbed -> sensor detects disturbance -> effector action counteracts imbalance

Question 4

Biological variables controlled by homeostasis

Answer 4

• Nutrient/water intake
• Waste product excretion
• Body temperature
• Extracellular fluid composition

Question 5

Parameters that need to be kept in narrow range for optimal cell functon

Answer 5

• Water level
• Ion concentrations
• Solute concentrations
• pH

Question 6

Na+ cellular fluid concentrations

Answer 6

Intracellular fluid: 10-15 mmol/L

Extracellular fluid: 135-147 mmol/L

Question 7

K+ cellular fluid concentrations

Answer 7

Intracellular fluid: 120-150 mmol/L

Extracellular fluid: 3.5-5 mmol/L

Question 8

Cl- cellular fluid concentrations

Answer 8

Intracellular fluid: 20-30 mmol/L

Extracellular fluid: 95-105 mmol/L

Question 9

HCO3- cellular fluid concentrations

Answer 9

Intracellular fluid: 12-16 mmol/L

Extracellular fluid: 22-28 mmol/L

Question 10

Ca2+ cellular fluid concentrations

Answer 10

Intracellular fluid: ~10-7 mmol/L

Extracellular fluid: 1.1-1.4 mmol/L

Question 11

Why are Na+ and Cl- so concentrated in extracellular fluid?

Answer 11

High NaCl concentration is the result of evolution from organisms living in salty sea water.

Question 12

Epithelia

Answer 12

Interface between internal environment of body and external space (skin, lungs, intestines).

• > maintains constant composition and volume of ECF
• > provides stable environment for cells

Question 13

How is homeostasis achieved?

Answer 13

Regulation of exchange of molecules/ions between both cytosol and ECF along with cytosol and intracellular compartments.
-> possible due to nature of cell membrane (phospholipid bilayer & membrane proteins)

Question 14

What are phospholipid bilayers permeable to?

Answer 14

• hydrophobic molecules
• gases
• small uncharged molecules

Question 15

Membrane permeability coefficient

Answer 15

J = P x (C1-C2)
J: net rate of solute movement
P: permeability coefficient
C1, C2: concentrations on both sides of membrane

Question 16

Why are cell membranes much more permeable to ions, glucose and other molecules compared to pure lipid bilayers?

Answer 16

They contain specialist transporters.