Cell Bio

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MOVEMENT ACROSS MEMBRANES
Intracellular fluid Extracellular fluid Pond water Blood plasma

K+ A-

ClNa+

Cl+
K+ Na+ Cl-

Na+

Na+
Gill epithelial cell Intestinal epithelial cells

Cl-

Cell membrane Anionic proteins (a) Ion concentration inside a single animal cell

(b) Ion concentration across gill epithelium of a freshwater fish Blood capillary

Intestinal lumen

Glucose from meal

Cross section of small intestine

(c) Glucose transport across intestinal epithelium into the blood system

MEMBRANE STRUCTURE
Extracellular fluid Glycoproteins and -lipids

Phospholipid bilayer Cytoplasm

Integral and peripheral proteins

Cholesterol Cytoskeleton

How do phospholipids build a membrane?

1

Phospholipid structure
Choline+ PhosphateGlycerol
Bends symbolize double bonds – less densely packed lipids. Many positively charged groups can occupy this position.

Polar head

Non-polar fatty acid chains

Nonpolar tails Hydrophilic Hydrophobic

Phospholipid “behavior” in water
Water Micelle Hydrophillic heads

Hydrophobic tails

Phospholipid bilayer

2

Fluidity of membranes

(a)
Lateral Movement (frequent) Flip-flops (rare) Frequent lateral movement of lipids; infrequent flip-flops across membrane leaflets.

Fluid (b)
Unsaturated fatty acid tails with kinks

Viscous

Saturated fatty acid tails

Degree of unsaturated/saturated fatty acid tails (hydrocarbon) determines membrane fluidity.

Phospholipid composition and habitat temperature
% of hydrocarbon tails unsaturated

Phosphatidyl-ethanolamines

Phosphatidyl-cholines

Adaptation temperature (°C)

Fatty acid chains vary in chemical unsaturation = double bonds that produce a bend in the chain. Unsaturated phospholipids are less densely packed and increase membrane fluidity. To maintain membrane fluidity at low temperatures, cold adapted species incorporate more unsaturated lipids into their membrane.

after Logue et al. (2000), J Exp Biol 203, 2105-2115

3

Membrane proteins

PASSIVE AND ACTIVE TRANSPORT
What are equilibrium conditions? A system moves towards its “equilibrium” when there is no further input of energy or matter – has minimal potential for more work.

Passive transport mechanisms work only in the direction of equilibrium.

Active transport mechanisms are capable of working in a direction against the equilibrium.

4

PASSIVE: Diffusion as passive solute transport
High Low glucose concentration

Membrane Diffusion of glucose toward equilibrium

Due to molecular movement glucose passes through the membrane passively. Because there is more glucose on the left side, on average and by chance more molecules pass from left to right. This leads to equal concentrations on both sides – the equilibrium for this system.

PASSIVE: Solute diffusion
High C1 Low C2 solute concentration

J = D A C1 – C 2
X

Fick’s diffusion equation (Adolf Fick, 1829-1901)

X= l en gth

J ~ C1 – C2: Diffusion rate is proportional to concentration gradient JY ~ C solute Y : Diffusion rate for solute Y depends on concentration gradient of the solute Y J ~ 1/X: Diffusion rate is greater when diffusion distance is shorter J ~ A: Diffusion rate is greater when diffusion area (surface) is greater

J=net rate of diffusion

J ~ D: Diffusion rate is proportional to the diffusion coefficient D is a measure of the ease with which a solute moves through a medium separating the two concentrations; temperature dependent; defines permeability.

How do membranes affect diffusion?

5

Boundary layers and diffusion

Outward diffusion from the animal or cell increases the concentration in the environmental solution next to the outer surface. The boundary layer thus created may be very thin yet still have a substantial impact on the rate of diffusion.

6

PASSIVE: Permeability of a membrane for a solute
High Low glucose concentration

J =

P A

C1 – C 2 X

P = DmK

The...
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