Membrane Structure and Function

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Fig. 7-10

ER

1

Transmembrane

glycoproteins

Secretory

protein

Glycolipid

2

Golgi

apparatus

Vesicle

3

4

Secreted

protein

Transmembrane

glycoprotein

Plasma membrane:

Cytoplasmic face

Extracellular face

Membrane glycolipid

Slide 29

Concept 7.2: Membrane structure results in selective permeability

A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

Plasma membranes are selectively permeable, regulating the cell’s molecular traffic

Slide 30

The Permeability of the Lipid Bilayer

Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

Polar molecules, such as sugars, do not cross the membrane easily

Slide 31

Transport Proteins

Transport proteins allow passage of hydrophilic substances across the membrane

Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

Channel proteins called aquaporins facilitate the passage of water

Slide 32

Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane

A transport protein is specific for the substance it moves

Slide 33

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment

Diffusion is the tendency for molecules to spread out evenly into the available space

Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction

At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Animation: Membrane Selectivity

Animation: Diffusion

Slide 34

Fig. 7-11

Molecules of dye

Membrane (cross section)

WATER

Net diffusion

Net diffusion

Equilibrium

(a) Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

Slide 35

Molecules of dye

Fig. 7-11a

Membrane (cross section)

WATER

Net diffusion

Net diffusion

(a) Diffusion of one solute

Equilibrium

Slide 36