Transport Across Cell Membranes

Question 1

How does glucose enter the bloodstream?

Answer 1

Leave intestine, enters blood, crosses 4 lipid membranes and interstitial space

Question 2

What determines whether a molecule/ion/gas will cross the cell membrane?

Answer 2

Chemical gradient
Electrical gradient
Permeability

Question 3

What are the forms of passive transport?

Answer 3

Simple diffusion
Simple diffusion through protein channels
Facilitated diffusion

Question 4

What passes through simple diffusion?

Answer 4

Nonpolar molecules

Question 5

What passes through simple diffusion through protein channels?

Answer 5

Inorganic ions and water

Question 6

What passes through facilitated diffusion?

Answer 6

Small organic molecules
Glucose

Question 7

Explain Brownian Motion.

Answer 7

Molecules of a solution are in a constant state of motion as a result of their thermal energy

Question 8

How do gated channels open when closed?

Answer 8

Chemical, mechanical and electrical signals

Question 9

According to Fick’s Law of Diffusion, what is diffusion rate proportional to?

Answer 9

Concentration gradientmembrane permeabilitysurface area/membrane thickness

Question 10

What is membrane permeability proportional to?

Answer 10

Lipid solubility/molecular size

Question 11

What factors affect the rate of diffusion through cell membranes?

Answer 11

Lipid solubility
Molecular size
Cell membrane thickness
Concentration gradient
Membrane surface area
Composition of lipid layer

Question 12

How do ions passively move through channels?

Answer 12

Channels form pores in membranes
Specific for single substance
Flux of ions is passive and is dictated by concentration gradient

Question 13

How do uniporters work?

Answer 13

Bind and transport molecules of one type down concentration gradient
Regulated by insertion/removal of uniporter from membrane

Question 14

Why does glucose use a transporter rather than a channel?

Answer 14

Charge, size and shape
Glucose channel would allow movement of other ions and water
Loss of selectively permeable nature of cell membrane

Question 15

What is primary active transport?

Answer 15

Transport by ion pumps and is ATP dependent

Question 16

What is secondary active transport?

Answer 16

Symporters and antiporters
Uses energy stored in concentration gradients by primary active transport

Question 17

What is the osmolarity of plasma?

Answer 17

150 mM Na+
5mM K+
2mM Ca++
130mM other solutes

Question 18

How do you calculate osmolarity in biological fluids?

Answer 18

Measure depression of freezing point - affected by total concentration of solution
One mole of solute depresses the freezing points of water by 1.86oC
Depression in freezing point/-1.86 = x Osm = 1000x mOsm

Question 19

What is the solute composition of ECF?

Answer 19

150mM Na+
5mM K+
2mM Ca++

Question 20

What is the solute composition of ICF?

Answer 20

10mM Na+
150mM K+
0.0004 mM Ca++

Question 21

Is tonicity or osmolarity more physiologically relevant and why?

Answer 21

Tonicity as it accounts for cell membrane permeability

Question 22

What is osmolality?

Answer 22

The number of solute particles in 1kg of solvent

Question 23

In the case of dehydration, how is blood osmolality regulated?

Answer 23

Blood volume decrease and plasma osmolality increase
Osmoreceptors are activated by this increase
Thirst is stimulated, releasing vasopressin
Water retention promoted

Question 24

How does glucose cross epithelial cells and enter interstitial space?

Answer 24

ICF [Na+] is reduced by the Na+ pump on the basolateral membrane
Low [Na+] creates a gradient for Na+ to enter, but there are no suitable channels on apical membrane
Na+/Glu symporters harness the energy from Na+ gradient to allow Na+ and glucose to enter
Glucose leaves the basolateral membrane using a glucose uniporter

Question 25

What must be in place on epithelia to make glucose enter blood?

Answer 25

Sodium pump must be on basolateral membrane Na/Glu symporter must be on apical membrane Glucose uniporter must be on basolateral membrane

Question 26

How do antiporters and symporters move molecules through the plasma membrane?

Answer 26

Uses energy stored in concentration gradients established by primary active transport

Question 27

What does the route of water in transport through epithelia depend on?

Answer 27

Type of epithelia Location within the body

Question 28

What is the paracellular pathway of water transport?

Answer 28

Leaky tight junctions Requires osmotic gradient, but essentially a constitutive process

Question 29

How does absorption work?

Answer 29

Na+ pump extrudes Na+ via basolateral membrane Na+/Glu symporter allows Na+ to cross apical membrane Na+ is pumped out of basolateral membrane and glucose leaves via uniporter

Question 30

How is water absorbed into a cell?

Answer 30

Uses the Na+/Glu movement which creates a large osmotic gradient across the cell Water moves paracellularly from lumen to interstitial space

Question 31

How does oral rehydration therapy work?

Answer 31

Increased interstitial fluid [Na+] is required for reabsorption of water Glucose accelerates absorption of solute and water

Question 32

How is secretion of H2O work?

Answer 32

Opposite of absorption Export Cl- H2O and Na+ will follow