CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece

5
Membrane
Transport and
Cell Signaling
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
Edited by Rena Quinlan, Ph.D.
© 2014 Pearson Education, Inc.

Overview: Life at the Edge
 The plasma membrane separates the living cell from its surroundings
 The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
Glycolipid

EXTRACELLULAR
SIDE OF
MEMBRANE

Peripheral proteins Integral protein Figure 5.2b
© 2014 Pearson Education, Inc.

CYTOPLASMIC SIDE
OF MEMBRANE

CONCEPT 5.1: Cellular membranes are fluid mosaics of lipids and proteins
 Phospholipids are the most abundant lipid in most membranes  Phospholipids are amphipathic molecules, containing both hydrophobic and hydrophilic regions
 A phospholipid bilayer can exist as a stable boundary between two aqueous compartments
© 2014 Pearson Education, Inc.

Hydrophilic head WATER

WATER
Hydrophobic
tail

Figure 5.3

 Most membrane proteins are also amphipathic and reside in the bilayer with their hydrophilic portions protruding
 The Fluid Mosaic Model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
Fibers of extracellular matrix (ECM)

Glycoprotein

Carbohydrate

Glycolipid

EXTRACELLULAR
SIDE OF
MEMBRANE

Cholesterol
Microfilaments
of cytoskeleton
Fig. 5.2
© 2014 Pearson Education, Inc.

Peripheral proteins Integral protein CYTOPLASMIC SIDE
OF MEMBRANE

 As temperatures cool, membranes switch from a fluid state to a solid state

Fluid

Viscous

Unsaturated hydrocarbon tails
(kinked) prevent packing enhancing membrane fluidity.

Saturated hydrocarbon tails pack together, increasing membrane viscosity (friction) .

 The temperature at which a membrane solidifies depends on the types of lipids  A membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails (because of kinks in tails where double bonds are located, unsaturated tails cannot pack as tightly together as saturated tails
– makes the membrane more fluid)
 Membranes must be fluid to work properly; they are usually about as fluid as salad oil

(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol helps maintain membrane fluidity by disrupting the regular packing of phospholipids. Cholesterol lowers the temperature required for the membrane to solidify – thereby helping membranes resist changes in fluidity when the temperature changes. .

Cholesterol

Figure 5.5. Factors that affect membrane fluidity.
© 2014 Pearson Education, Inc.

Membrane Proteins and Their Functions
 A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer
 Proteins determine most of the membrane’s specific functions © 2014 Pearson Education, Inc.

N-terminus

 Integral proteins penetrate the hydrophobic interior of the lipid bilayer

EXTRACELLULAR
SIDE

 Integral proteins that span the entire length of the membrane are called transmembrane proteins
 The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into  helices
 Peripheral proteins are loosely bound to the surface of the membrane

© 2014 Pearson Education, Inc.

 helix
C-terminus

Fig. 5.6. The structure of a transmembrane protein

CYTOPLASMIC
SIDE

Six Major Functions of Membrane Proteins
Enzymes

ATP
a). Transport. Left: protein may provide hydrophilic channel that is selective for a particular solute. Right: other transport proteins shuttle substances across membrane by changing shape – some use ATP to actively pump substances across membrane.

(b) Enzymatic activity – several membrane proteins may be organized in a team that carry out sequential steps in a pathway.

(c) Attachment to the cytoskeleton and extra-cellular matrix (ECM): helps maintain cell shape and stabilizes