Membrane Structure and Function - Chapter 7 docx

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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 7

Membrane Structure and

Function

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• The plasma membrane is the boundary that

separates the living cell from its surroundings

permeability , allowing some substances to cross

it more easily than others

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Fig 7-1

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of lipids and proteins

plasma membrane

containing hydrophobic and hydrophilic regions

is a fluid structure with a “mosaic” of various

proteins embedded in it

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Membrane Models: Scientific Inquiry

found to be made of proteins and lipids

that it must be a phospholipid bilayer

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• In 1935, Hugh Davson and James Danielli proposed a

sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

• Later studies found problems with this model,

particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

• In 1972, J Singer and G Nicolson proposed that the

membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to

water

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• Freeze-fracture studies of the plasma membrane

supported the fluid mosaic model

technique that splits a membrane along the middle

of the phospholipid bilayer

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Extracellular layer

Knife Proteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer Cytoplasmic layer

Plasma membrane

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The Fluidity of Membranes

within the bilayer

the membrane

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Lateral movement (~10 7 times per second)

Flip-flop (~ once per month) (a) Movement of phospholipids

(b) Membrane fluidity

Unsaturated hydrocarbon tails with kinks Saturated hydrocarbon tails

(c) Cholesterol within the animal cell membrane

Cholesterol

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Fig 7-5a

(a) Movement of phospholipids

Lateral movement (∼10 7 times per second)

Flip-flop (∼ once per month)

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• As temperatures cool, membranes switch from a

fluid state to a solid state

depends on the types of lipids

fluid that those rich in saturated fatty acids

usually about as fluid as salad oil

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hydro-• The steroid cholesterol has different effects on

membrane fluidity at different temperatures

restrains movement of phospholipids

preventing tight packing

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Cholesterol (c) Cholesterol within the animal cell membrane

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Membrane Proteins and Their Functions

embedded in the fluid matrix of the lipid bilayer

functions

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Fibers of extracellular matrix (ECM)

protein

Glyco-Microfilaments

of cytoskeleton

Cholestero l

Periphera l

MEMBRANE

Carbohydrate

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• Peripheral proteins are bound to the surface of

the membrane

transmembrane proteins

consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

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EXTRACELLULAR SIDE

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• Six major functions of membrane proteins:

– Attachment to the cytoskeleton and

extracellular matrix (ECM)

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(d) Cell-cell recognition

protein

Glyco-(e) Intercellular joining (f) Attachment to

the cytoskeleton and extracellular matrix (ECM)

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Receptor

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(d) Cell-cell recognition

protein

the cytoskeleton and extracellular matrix (ECM)

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The Role of Membrane Carbohydrates in Cell-Cell

Recognition

molecules, often carbohydrates, on the plasma

membrane

to lipids (forming glycolipids ) or more commonly

to proteins (forming glycoproteins )

membrane vary among species, individuals, and

even cell types in an individual

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• Membranes have distinct inside and outside faces

and associated carbohydrates in the plasma

membrane is determined when the membrane is built by the ER and Golgi apparatus

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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

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in selective permeability

surroundings, a process controlled by the plasma membrane

regulating the cell’s molecular traffic

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The Permeability of the Lipid Bilayer

hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

membrane easily

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• Transport proteins allow passage of hydrophilic

substances across the membrane

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

passage of water

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• Other transport proteins, called carrier proteins,

bind to molecules and change shape to shuttle them across the membrane

moves

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substance across a membrane with no energy

investment

out evenly into the available space

of a population of molecules may exhibit a net

movement in one direction

one way as cross in the other direction

Animation: Membrane Selectivity Animation: Diffusion

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Net diffusion Net diffusion

Equilibriu m

(b) Diffusion of two solutes

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Molecules of dye Membrane (cross section)

WATER

Net

(a) Diffusion of one solute

Equilibrium

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• Substances diffuse down their concentration

gradient, the difference in concentration of a

substance from one area to another

the concentration gradient

membrane is passive transport because it

requires no energy from the cell to make it happen

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(b) Diffusion of two solutes

Net diffusion

Net diffusion Net diffusion

Equilibrium Equilibrium

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Effects of Osmosis on Water Balance

selectively permeable membrane

of lower solute concentration to the region of higher solute concentration

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of sugar

Osmosis

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Water Balance of Cells Without Walls

to gain or lose water

as that inside the cell; no net water movement

across the plasma membrane

greater than that inside the cell; cell loses water

than that inside the cell; cell gains water

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Isotonic solution

Flaccid

H 2 O

H 2 O Shriveled

Plasmolyzed Hypertonic solution

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• Hypertonic or hypotonic environments create

osmotic problems for organisms

a necessary adaptation for life in such environments

pond water environment, has a contractile vacuole that acts as a pump

Video: Chlamydomonas Video: Paramecium Vacuole

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(a) A contractile vacuole fills with fluid that enters from

a system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling fluid from the cell.

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Water Balance of Cells with Walls

wall opposes uptake; the cell is now turgid (firm)

there is no net movement of water into the cell; the

cell becomes flaccid (limp), and the plant may

wilt

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Video: Plasmolysis Video: Turgid Elodea

Animation: Osmosis

eventually, the membrane pulls away from the wall,

a usually lethal effect called plasmolysis

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Facilitated Diffusion: Passive Transport Aided by Proteins

the passive movement of molecules across the

plasma membrane

specific molecule or ion to cross the membrane

– Aquaporins, for facilitated diffusion of water

– Ion channels that open or close in response to

a stimulus (gated channels)

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FLUID

Channel protein (a) A channel protein

Solute

CYTOPLASM

Solute Carrier protein

(b) A carrier protein

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• Carrier proteins undergo a subtle change in shape

that translocates the solute-binding site across the membrane

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specific transport systems, for example the kidney disease cystinuria

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Concept 7.4: Active transport uses energy to move solutes against their gradients

solute moves down its concentration gradient

against their concentration gradients

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• Active transport moves substances against their

concentration gradient

of ATP

embedded in the membranes

Animation: Active Transport

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• Active transport allows cells to maintain

concentration gradients that differ from their

surroundings

active transport system

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EXTRACELLULAR FLUID

[Na + ] high [K + ] low

Na +

Na + [Na + ] low

[K + ] high CYTOPLASM

Cytoplasmic Na + binds to

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Na + binding stimulates phosphorylation by ATP

2

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Phosphorylation causes the protein to change its

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Fig 7-16-4

extracellular side and triggers release of the phosphate group

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Loss of the phosphate restores the protein’s original shape

K +

5

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FLUID

[Na + ] high [K + ] low

[Na + ] low [K + ] high

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• Membrane potential is the voltage difference

across a membrane

of positive and negative ions

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• Two combined forces, collectively called the

electrochemical gradient, drive the diffusion of

ions across a membrane:

– A chemical force (the ion’s concentration

gradient) – An electrical force (the effect of the membrane

potential on the ion’s movement)

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generates voltage across a membrane

electrogenic pump of animal cells

bacteria is a proton pump

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Fig 7-18

EXTRACELLULAR FLUID

H +

H + Proton pump

+ +

+

H +

+ +

H +

– –

ATP

CYTOPLASM

–

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solute indirectly drives transport of another solute

generated by proton pumps to drive active transport

of nutrients into the cell

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Fig 7-19

Proton pump

– –

+ +

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membrane occurs by exocytosis and endocytosis

through the lipid bilayer or by transport proteins

proteins, cross the membrane in bulk via vesicles

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membrane, fuse with it, and release their contents

products

Animation: Exocytosis

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• 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

Animation: Exocytosis and Endocytosis Introduction

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• In phagocytosis a cell engulfs a particle in a

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FLUID

Pseudopodium

“Food”or other particle

Food vacuole

PINOCYTOSIS

Pseudopodium

of amoeba

Bacterium Food vacuole

An amoeba engulfing a bacterium via phagocytosis (TEM)

Plasma membrane

Vesicle

0.5 µm

Pinocytosis vesicles forming (arrows) in

a cell lining a small blood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS Receptor

Coat protein

Coated vesicle

Coated pit Ligand

Coat protein

Plasma membrane

A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs)

0.25 µm

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Fig 7-20a

PHAGOCYTOSIS CYTOPLASM

EXTRACELLULAR

FLUID

Pseudopodium

“Food” or other particle

Food vacuole Food vacuole

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extracellular fluid is “gulped” into tiny vesicles

Animation: Pinocytosis

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Fig 7-20b

PINOCYTOSIS

Plasma membrane

Vesicle

0.5 µm

Pinocytosis vesicles forming (arrows) in

a cell lining a small blood vessel (TEM)

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• In receptor-mediated endocytosis , binding of

ligands to receptors triggers vesicle formation

a receptor site of another molecule

Animation: Receptor-Mediated Endocytosis

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Coat protein

Plasma membrane

0.25 µm

Coated vesicle

A coated pit and a coated vesicle formed during

mediated endocytosis (TEMs)

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receptor-Facilitated diffusion

Channel

protein

Carrier protein

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Fig 7-UN2

Active transport:

ATP

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0.01 M sucrose 0.01 M glucose 0.01 M fructose

“Cell”

0.03 M sucrose 0.02 M glucose

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Fig 7-UN4

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1 Define the following terms: amphipathic

molecules, aquaporins, diffusion

temperature and membrane composition

terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic,

hypotonic, and isotonic solutions

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4 Explain how transport proteins facilitate diffusion

across a membrane, and name two electrogenic

pumps

across a cell membrane

Tiêu đề	Membrane Structure and Function
Tác giả	Neil Campbell, Jane Reece
Người hướng dẫn	Chris Romero, Erin Barley, Joan Sharp
Trường học	Pearson Education, Inc.
Chuyên ngành	Biology
Thể loại	lecture presentation
Năm xuất bản	2008
Thành phố	Unknown

Định dạng
Số trang	83
Dung lượng	8,12 MB