Chapter 7

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

Biology Eighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 7Chapter 7

Membrane Structure and Function

Plasma Membrane

• the boundary that separates the living cell from its surroundings

• An amphipathic molecules

– contains hydrophobic and hydrophilic regions

• Exhibits selective permeability

– allows some substances to cross a membrane more easily than others

• Cellular membranes are fluid mosaics of lipids and proteins


Fig. 7-2

Hydrophilichead

WATER

Hydrophobictail

WATER

Amphipathic Molecules

Plasma membrane structure

Fig. 7-3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Membranes are fluid mosaics of lipids and proteins

Cell membrane movement

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Fig. 7-5b

Fluid

Unsaturated hydrocarbons

Viscous

Saturated hydrocarbon tails

Membrane fluidity

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

• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

****

Fig. 7-5c

Cholesterol

Cholesterol’s affect on membrane fluidity

• The steroid cholesterol has different effects on membrane fluidity at different temperatures

• At warm temperatures cholesterol restrains movement of phospholipids

• At cool temperatures it maintains fluidity by preventing tight packing

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s specific functions

• Peripheral proteins are bound to the surface of the membrane

• Integral proteins penetrate the hydrophobic core

– Integral proteins that span the membrane are called transmembrane proteins

– The hydrophobic regions of an integral protein consist of nonpolar amino acids, often coiled into alpha helices


Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins Integral

proteinCYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Fig. 7-8

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

Transmembrane proteins consist of helices

A transmembrane protein is an example of an integral protein

• Six major functions of membrane proteins:

– Transport

– Enzymatic activity

– Signal transduction

– Cell-cell recognition

– Intercellular joining

– Attachment to the cytoskeleton and extracellular matrix (ECM)


KNOW THESE!!!

Fig. 7-9

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces

• Membranes are built by the ER and Golgi

– The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built


Fig. 7-10

ER1

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

2Golgiapparatus

Vesicle

3

4

Secretedprotein

Transmembraneglycoprotein

Membrane glycolipid

Synthesis of a membraneComponents via the

endomembrane system

1. ER produces transmembrane glycoprotiens and secretory proteins

2. The golgi apparatus packages transmembrane glycoproteins, secretory proteins and membrane glycolipids

3. Vessicles containing membrane components fuse with the membrane to release those components from the cell

The Permeability of the Lipid Bilayer

• 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

• Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and pass through the membrane rapidly

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


Transport Proteins

• A transport protein is specific for the substance it moves

• Transport proteins allow passage of hydrophilic substances across the membrane

– Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel

– Aquaporins facilitate the passage of water

– Carrier proteins bind to molecules and change shape to shuttle them across the membrane

• Active transport requires energy while passive transport does not


Fig. 7-17Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Molecules flow with the concentration gradient and

therefore do not require energy

Molecules flow against the concentration

gradient which requires energy

Passive Transport

• Transport of molecules in/out of cell does not require energy

• Examples

– Diffusion

– Facilitated diffusion

– Osmosis

• Animal cells (without cell walls)

• Plant cells (with cell walls)

Diffusion

• the tendency for molecules to spread out evenly into the available space

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

• Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another

– No energy required

– Sustances move to less concentrated area


Fig. 7-11

(a) Diffusion of one solute

Molecules of dye Membrane (cross section)

Net diffusion Net diffusion Equilibrium

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

Facilitated Diffusion

• Passive transport aided by proteins

• Transport proteins speed the passive movement of molecules across the plasma membrane

• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

– Ex: Aquaporins

– Ex: Ion channels

• Carrier proteins undergo a subtle change in shape that translocates the solute across the membrane

• No energy required


Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

• Water diffuses across a membrane to balance the concentrations of different solutes

– Low concentration solute high concentration solute

• No energy required


Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Water moves from low concentration of solute to

high concentration of solute

Osmosis in Animal Cells

• Tonicity is the ability of a solution to cause a cell to gain or lose water (always compared to inside the cell)– In an isotonic solution

• Solute concentration is the same as that inside the cell

• Outside cell = inside cell

• No net water movement across the plasma membrane

– In a hypertonic solution

• Solute concentration is greater than that inside the cell

• Outside cell > inside cell

• Cell loses water

– In a hypotonic solution

• Solute concentration is less than that inside the cell

• Outside cell < inside the cell

• Cell gains water

Osmosis in Plant Cells

• Cell walls help maintain water balance

• In a hypotonic solution

– Plant cell swells until the wall opposes uptake (water travels to the higher concentration of solute)

– The cell is now turgid (firm)

• In an isotonic solution

– No net movement of water into the cell

– Cell becomes flaccid (limp), and the plant may wilt

• In a hypertonic solution

– Plant cells lose water (water travels to higher concentration of solute)

– The membrane pulls away from the wall, a usually lethal effect called plasmolysis


Fig. 7-13

Hypotonic solution

(a) Animal cell

(b) Plant cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

***Water moves to higher concentration of solute***

Need to know which condition is normal for plant and animal cells

Active Transport

• Active transport

– moves substances against their concentration gradient

– requires energy (ATP)

– allows cells to maintain concentration gradients that differ from their surroundings

• Examples

– Sodium-potassium pump (animals)

– Proton pump (plants)


K+

K+ 6

K+

K+

5 4

K+

K+

P P

2

[Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

ATP ADP P

Na+ Na+

Na+

P 31

Fig. 7-16-7

The Sodium-Potassium Pump: 3 Na+ out and 2 K+ in

ATP hydrolysis

Fig. 7-18

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

The Proton Pump pumps H+ against their concentration gradient

Bulk transport across the plasma membrane

• Small molecules and water enter/leave the cell through the lipid bilayer or by transport proteins

• Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles

• Bulk transport requires energy

• Exocytosis vs Endocytosis


Exocytosis

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents

• Many secretory cells use exocytosis to export their products


Endocytosis

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

• Endocytosis is a reversal of exocytosis, involving different proteins

• There are three types of endocytosis:

– Phagocytosis (“cellular eating”)

– Pinocytosis (“cellular drinking”)

– Receptor-mediated endocytosis


Fig. 7-20

PHAGOCYTOSISa cell engulfs a particle

in a vacuole

EXTRACELLULARFLUID

CYTOPLASM

Pseudopodium

“Food”orother particle

Foodvacuole

PINOCYTOSISextracellular fluid is

“gulped” into tiny vesicles

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS

binding of ligands to receptors triggers vesicle formation

Receptor

Coat protein Coatedvesicle

Coatedpit

Ligand

Types of Endocytosis

Chapter 7

Technology

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