Intro To Cell Physiology And Pharmacology

Question 1

Define a semi-permeable membrane

Answer 1

A layer through which only allowed substances can pass

Question 2

What molecules pass freely through the membrane?

Answer 2

Hydrophobic molecules
Small uncharged polar molecules (less freely)

Question 3

What molecules don’t pass freely through the membrane?

Answer 3

Large uncharged polar molecules (small amount pass through)
Ions

Question 4

What is fick’s law?

Answer 4

J = P(C1 -C2)

Where:
J = net rate of transport
P = permeability coefficient
C1 and C2 = concentration gradients on side 1 and 2
Rate of passive transport (cm/sec) increases linearly with increasing concentration gradient

Question 5

Compare and contrast simple diffusion, facilitated diffusion and active transport

Answer 5

Simple diffusion: no proteins involved, no energy required, solute moves by diffusion down concentration or electrical gradient.

Facilitated diffusion: channel or carrier protein involved. No energy required. Solute moves by diffusion down concentration or electrical gradients.

Active transport: carrier protein involved. Cellular energy required. Solute moves across the membrane against its concentration gradient.

Question 6

Describe non-gated and gated pores/channels

Answer 6

Gated channels (pores) - conditionally open
Non-gated pores (channels)- always open
Both have hydrophilic centres

Pores allow bi-directional transport
Some channels are ligand-gated, requiring the binding of a molecule to enable ions to pass through (such as ACh in nicotinic acetylcholine receptor in neuromuscular junction)
Some channels are voltage-gated, requiring membrane depolarisation to allow ions through (e.g. Ca2+ channels)

Question 7

What is a uniport transport protein

Answer 7

Transports one substance in one direction
Can be passive or active transport

Question 8

What is a symport transport protein?

Answer 8

Transports 2 different substances in same direction
Can be passive or active transport

Question 9

What is an antiport transport protein?

Answer 9

Transports 2 different substances in opposite directions
Can be passive or active transports

Question 10

What is included in co-transporters?

Answer 10

Symports and antiports can both be considered co-transporters

Question 11

Give an example of 1º active transport

Answer 11

Na+ K+ ATPase - maintains cellular concentrations of Na+ and K+ (antiport)

Question 12

Give an example of 2º active transport

Answer 12

Na+ Ca2+ exchange - inwards flow of Na+ down conc. gradient drives outward flow of Ca2+ up its conc gradient (ANTIPORT)

Na+/H+ exchanger - inward flow of Na+ down its conc gradient leads to cell alkalisation by removing H+ (ANTIPORT)

Question 13

Give an example of a symport

Answer 13

Na+/glucose co-transport (small intestine and kidney) - entry of 1 or 2 Na+ ions provides the energy for the entry of 1 glucose molecule against the conc gradient

Question 14

Give some examples of important physiological roles of transport processes

Answer 14

Maintenance of ionic composition
Maintenance of intracellular pH
Regulation of cell volume
Concentration of metabolic fuels and building blocks
Expulsion of metabolic waste products and toxic substances
Generation of ion gradients necessary for the electrical excitability of nerve and muscle (excitable tissues)

Question 15

Where does the energy for active transport come from?

Answer 15

Directly or indirectly ATP hydrolysis - some cells spend 30-50% of their ATP on active transport e.g. nerves, muscle cells, erythrocytes, enterocytes, hepatocytes

Question 16

What does the sodium potassium pump do?

Answer 16

Transports 3Na+ out and 2K+ in per ATP molecule hydrolysed
Moves both Na+ and K+ against conc gradient
Necessary for electrical excitability - only generates approx 5-10mV of resting membrane potential. Drives 2º active transport - control of pHi, regulation of cell volume and [Ca2+], absorption of Na+ in epithelia, nutrient uptake e.g. glucose or amino acids from the small intestine

Question 17

What is 1º active transport?

Answer 17

Uses ATP as direct energy source

Question 18

What is 2º active transport?

Answer 18

Uses energy present in an electrochemical gradient

Question 19

Give some features of the sodium-potassium pump (Na+ K+ ATPase)

Answer 19

Antiport protein
1º active transport
25% basal metabolic rate used to keep this pump working
Known as a P-type ATPase - ATP phosphorylates aspartate, producing a phosphoenzyme intermediate
Beta subunit glycoprotein directs pump to cell surface