lecture 3-membrane structure

Biology 11- Lecture 3- Membrane Structure

1

Membrane Structure and Function

This handout is for lecture use only and not for commercial reproduction and

distribution.

boundary that separates the living cell from its surroundings

exhibits selective permeability

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

Plasma membrane

Hydrophilichead

WATER

Hydrophobictail

WATER

phospholipids are the most abundant lipid in the plasma membrane

phospholipids are amphipathic molecules (hydrophobic and hydrophilic regions)

Plasma membrane structureA typical phospholipid: phosphatidylcholine

Phospholipid

bilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

fluid mosaic model

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane


2

Lateral movement

(107 times per second)Flip-flop ( once per month)

The fluidity of the membranes

most of the lipids, and some proteins, drift laterally

rarely does a molecule flip-flop transversely

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails


Cholesterol

(c) Cholesterol within the animal cell membrane


warm temperatures (37C), restrains movement of phospholipids

cool temperatures, 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 membranes specific functions


Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

Glycolipid

EXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Peripheral proteins - bound to the surface of the membrane

Integral proteins - penetrate the hydrophobic core


Membrane Proteins and Their Functions


3

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

transmembrane proteins - integral proteins that span the membrane

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

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

Six major functions of membrane proteins

Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response.

(d) Cell-cell recognition

Glyco-

protein

(e) Intercellular joining (f) Attachment to

the cytoskeleton

and extracellular

matrix (ECM)

Six major functions of membrane proteins

cell-cell recognition

Glyco-

protein

The Role of Membrane Carbohydrates in Cell-Cell Recognition

cells recognize each other by binding to surface molecules

may be covalently bonded to lipids (glycolipids) or more

commonly to proteins

(glycoproteins)

carbohydrates on the external side of the plasma membrane

vary among species,

individuals, and even cell types

in an individual

Membrane structure results in selective permeability

a cell must exchange materials with its surroundings

selectively permeable, regulating the cells molecular traffic


Selective permeability means that the cell membrane has some control over what can cross it, so that only certain molecules either enter or leave the cell.

The Permeability of the Lipid Bilayer

hydrophobic/nonpolar molecules (hydrocarbons), can dissolve in the lipid bilayer and pass through the membrane rapidly

polar molecules (sugars) do not cross the membrane easily



4

Transport Proteins

- allow passage of hydrophilic substances across the membrane

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

2. aquaporins facilitate the passage of water


3. carrier proteins bind to molecules and change shape to shuttle them across the membrane

- a transport protein is specific for the substance it moves


Transport Proteins

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

Passive transport

diffusion of a substance across a membrane with no energy investment

Diffusion is the movement of molecules from a high concentration to a low concentration.

(b) Diffusion of two solutes

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

substances diffuse down their concentration gradient

Lower

concentrationof solute (sugar)

H2O

Higher

concentrationof sugar

Selectivelypermeable

membrane

Same concentration

of sugar

Osmosis

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

Osmosis

Osmosis is the movement of solvent moleculesthrough a selectively-permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides.

Water Balance of Cells

Tonicity - ability of a solution to cause a cell to gain or lose water

Isotonic solution - solute concentration is the same as that inside the cell

Hypertonic solution - solute concentration is greater than that inside the cell

Hypotonic solution - solute concentration is less than that inside the cell



5

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

(swollen) (inelastic)

(plasma membrane pulls away from the cell wall )

Filling vacuole 50 m

(a) A contractile vacuole fills with fluid that enters froma system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling

fluid from the cell.

Osmoregulation

- the control of water balance

Facilitated Diffusion

Transport proteins speed the passive movement of molecules


(a) Channel protein

(b) Carrier protein

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

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

channel proteins include:

1. Aquaporins, for facilitated diffusion of water

2. Ion channels that open or close in response to a

stimulus (gated channels)

Solute Carrier protein

(b) A carrier protein

Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

moves substances against their concentration gradient

requires energy, usually in the form of ATP

performed by specific proteins embedded in the membranes


Active transport


6

allows cells to maintain concentration gradients that differ from their surroundings

The sodium-potassium pump is one type of active transport system


Active transport

2

EXTRACELLULAR

FLUID[Na+] high

[K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM

ATP

ADP

P

Na+

Na+

Na+

P

3

6 5 4

PP

1 Cytoplasmic Na+ binds tothe sodium-potassium pump.

Na+ binding stimulatesphosphorylation by ATP.

Phosphorylation causes protein to change shape. Na+ is expelled to the outside.

K+ binds on the extracellular side and triggers release of PO4 group.

Loss of the phosphaterestores the proteins originalshape.

K+ is released, and thecycle repeats.

Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

How Ion Pumps Maintain Membrane Potential

Membrane potential is the voltage difference across a membrane

Voltage is created by differences in the distribution of positive and negative ions


Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:

a chemical force (the ions concentration gradient)

an electrical force (the effect of the membrane potential on the ions movement)



an electrogenic pump is a transport protein that generates voltage across a membrane

the sodium-potassium pump is the major electrogenic pump of animal cells

the main electrogenic pump of plants, fungi, and bacteria is a proton pump




7

EXTRACELLULAR

FLUID

H+

H+

H+

H+

Proton

pump

+

+

+

H+

H+

+

+

H+

ATP

CYTOPLASM

Using ATP for power, a proton pump translocates positive charge in the form of hydrogen ions (protons). The voltage and H+ concentration gradient represent a dual energy source that can drive other

processes, such as the uptake of nutrients.

Cotransport: Coupled Transport by a Membrane Protein

occurs when active transport of a solute indirectly drives transport of another solute

plants use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients (e.g. sugar) into the cell


Proton pump

+

+

+

+

+

+

ATP

H+

H+

H+H+

H+

H+

H+

H+

Diffusion

of H+Sucrose-H+

cotransporter

Sucrose

Sucrose

A carrier protein such as this sucrose-H+ cotransporter is able to use the diffusion of H+

down its electrochemical gradient into the cell to drive the uptake of sucrose. The W gradient is maintained by an ATP-driven proton pump that concentrates H' outside the cell, thus storing potential energy that can be used for active transport, inthis case of sucrose. Thus, ATP is indirectly providing the energy necessary for cotransport.

Bulk transport

small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

large molecules ( e.g. polysaccharides and proteins) cross the membrane in bulk via vesicles

requires energy

occurs by exocytosis and endocytosis


Exocytosis

Transport vesicles migrate to the membrane, fuse with it, and release their contents

used by many secretory cells to export their products


Endocytosis

the cell takes in macromolecules by forming vesiclesfrom the plasma membrane

a reversal of exocytosis, involving different proteins

three types:

phagocytosis (cellular eating)

pinocytosis (cellular drinking)

receptor-mediated endocytosis



8

a cell engulfs a particle in a vacuole

the vacuole fuses with a lysosome to digest the particle


Phagocytosis

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

Food or

other particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacterium

via phagocytosis (TEM)

Pseudopodium

of amoeba

1 m

molecules are taken up when extracellular fluid is gulped into tiny vesicles


PINOCYTOSIS

Plasmamembrane

Vesicle

0.5 m

Pinocytosis vesicles

forming (arrows) in

a cell lining a small

blood vessel (TEM)

Pinocytosis

Receptor

Coat protein

Coatedpit

Ligand

Coatprotein

Plasmamembrane

0.25 m

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)

binding of ligands to receptors triggers

vesicle formation

a ligand is any molecule that binds

specifically to a

receptor site of

another molecule

Receptor-mediated endocytosisPassive transport: Facilitated diffusion

Channel

protein Carrier

protein

Active transport

ATP

lecture 3-membrane structure

Documents

membrane integral proteins

animal cell membrane

plasma membrane phospholipids

membrane hydrophobic

membrane structure3nterminusc

cellcell recognition

living cell

pearson education