cmb lect 3 2011 colour 2 slides per page.pdf

8/14/2019 CMB Lect 3 2011 colour 2 slides per page.pdf

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LECTURE 3

Cell membranes, cell

BIOSCI 101: Cellular and Molecular Biology

unc ons an ce u ar

interactions

Cell Membranes

•

- encloses the cell, defines its boundaries andmaintains differences between the cytosol andthe extracellular environment.

• Other membranes in the cell


- surround organelles (eg mitochondria,chloroplasts, nuclei), form ER and Golgi

- control passage of materials among intracellularcompartments



Cell membranes (contd)

• Chemical composition of membranes

a li ids

- most abundant lipids in membranes arephospholipids

- amphipathic ie hydrophilic heads andhydrophobic tails

- lipid bilayer is the basic structure of cellmembranes, 7-8 nm thick


(b) proteins

- may be detected chemically and seen inelectron micrographs using freeze-fracturetechniques

The phospholipid bilayer

BIOSCI 101: Cellular and Molecular BiologyCampbell 7.2



Freeze fracturing membranes



•

- 1930’s (Davson and Danielli) - bilayer withlayer of protein on each surface.

- 1972 (Singer and Nicholson) - Fluid Mosaicmodel. The membrane is described as a mosaic


o pro e n mo ecu es oa ng n a p osp o pbilayer



The fluid mosaic model formembrane structure

Phospholipid

bilayer


Hydrophobic regionsof protein Hydrophilicregions of protein


Important features of the fluid mosaic model:

- lipid bilayer is fluid; molecules are able todiffuse rapidly within their own monolayer butvery rarely transfer spontaneously from onemonolayer to the other

- in eukaryotes, cholesterol molecules moderatethe fluidit of the bila er


- the two halves of the bilayer are asymmetricalie may contain different species of phospholipids.Glycolipids are found on the outer half of thebilayer



Movement of phospholipids inmembranes


107 times per second.

-

is rare ( once per month).

Fluidity of membranes

Fluid Viscous

Unsaturated hydrocarbon

tailsSaturated hydrocarbon tails

BIOSCI 101: Cellular and Molecular BiologyCampbell 7.8a

(a) Unsaturated versus saturated hydrocarbon tails



Cholesterol in the membrane

(b) Cholesterol w ithin the animal

cell membrane

BIOSCI 101: Cellular and Molecular BiologyCampbell 7.8b


-hydrophobic core of the bilayer, peripheralmembrane proteins are loosely bound to thesurface

- proteins float in the lipid bilayer and some mayextend right through it

-


as receptors

- proteins can diffuse laterally in the bilayer (cellfusion expts)



Evidence for movement ofmembrane proteins


The plasma membrane

Fibers of extra-

cellular matrix (ECM)

Glyco-

proteinCarbohydrate

GlycolipidEXTRACELLULAR

SIDE OF

MEMBRANE


Microfilaments

of cytoskeleton

CYTOPLASMIC SIDE

OF MEMBRANE

Integral

protein

Peripheral

proteins

Cholesterol



Cell membranes (contd)•

- intercellular junctions (gap junctions)

- transport (Na/K Atp'ase)

- cellular recognition (histocompatibility antigens)

- enzymes (mitochondria)


- receptors (hormones)

- anchors (cytoskeleton)

Functions of membrane proteins (1)

Signaling molecule

Enzymes

ATP


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



Functions of membrane proteins (2)

Glyco-

protein


(d) Cell-cell recogniti on (e) Intercellular joining (f) Attachment to

the cytoskeleton

and extracellular matrix (ECM)

Functions of membrane proteins

• a) Intercellular Junctions

i) Desmosomes - provide structural strength(spot welds)

ii) Tight junctions - water-tight seal

iii) Gap junctions - communication channels, 6subunits around a 2nm diameter hole.


iv)Plasmodesmata - channels which traverseplant cell walls thereby directly connecting thecytoplasm of adjacent cells



Tight junctions prevent

fluid from moving

across a layer of cellsTight junction

TEM0.5 m

Intercellular junctions inanimal cells

Tight junction

TEM1 m

Desmosome

Intermediate

filaments

Gap

jun cti on


T E M

0.1 m

Extracellular

matrixPlasma membranes

of adjacent cells

Space

between cells

Ions or small

molecules

Gap junction




Plasmodesmata form channels through

the cell walls of plants

Cell walls

Interior

of cell

Interior

of cell


0.5 m Plasmodesmata Plasma membranes

Functions of membrane proteins (contd)

• b) Membrane Transport

- membranes are selectively permeable., 2

and small, uncharged polar molecules (CO2)can pass rapidly through the cell membranebut it is relatively impermeable to large,uncharged polar molecules (glucose) and ions

(H+, Na+, K+, Cl-)

- water moves to equalise concentrations of


solutes on each side of the membrane(osmosis)

- compounds can cross the membrane bymoving through the bilayer, through a proteinor by endo/exocytosis.



Passive and active transport



• b) Membrane Transport (contd)

- represents a movement of molecules fromhigh concentration to low concentration(requires a favourable gradient to bemaintained)






transport proteins

Transport proteins:

- provide great selectivity

- remain unchanged

- are recycled after each assisted entry and/or


exit


• in facilitated diffusion no energy is expended egentry of glucose into an animal cell. Glucose israpidly broken down when it enters the cell so asteep concentration gradient is maintained but rateof entry is limited by the number of specificlucose-trans ortin molecules in the membrane


• water is also now known to diffuse through achannel protein (aquaporin) rather than the lipidbilayer itself



Two models for facilitated diffusion



• (ii) facilitated diffusion and active transport (contd)

- for active transport, energy is required in theform usually of ATP eg the sodium /potassium pump. This allows cells to maintaina low internal concentration of Na and high K.

- three Na pumped out for every two K in andthe protein responsible is known as the Na /K stimulated ATPase (consumes 30% of the


ATP supply of a resting cell)



Active transport by the Na/K pump

EXTRACELLULAR

FLUID [Na ] high

[K ] low

Na

Na

Na

Na

Na

Na

Na

[Na ] low

[K ] high

CYTOPLASM Na

1 2 3

Na

K

PP

ATP

ADP


456

K

K

K

K K

PP i



(iii) Exo- and endocytosis

- exocytosis is associated with secretion

- involves fusion of vesicles with the plasmamembrane eg vesicles from Golgi

- endocytosis is associated with entry ofmacromolecules and particulate matter


- achieved by phagocytosis, pinocytosis orreceptor-mediated endocytosis



Pseudopodium

Solutes

EXTRACELLULAR

FLUID

Pseudopodiumof amoeba

Phagocytosis

Phagocytosis“Food”

or other

particle

Bacterium

Food vacuole

An amoeba engul fin g a bacter ium

via phagocytosi s (TEM).

1 m


Foodvacuole

CYTOPLASM

Plasma 5 m

Pinocytosis

PinocytosisPinocytosis vesicles forming

in a cell lining a small blood

vessel (TEM).

0 .


Vesicle



Receptor-mediated endocytosis

Receptor-Mediated Endocytosis

Receptor

m

Ligand

Coat proteins

Coated

pit

Coat

proteins

Plasma

membrane


Top: A coated p it. Bottom: Acoated vesicle forming duringreceptor-mediated endocytosi s(TEMs).

0 . 2

Coated

vesicle

Past question

a n s o mo ecu es pass roug a cemembrane most easily?

1. ionic2. small and hydrophobic3. large and hydrophobic


.5. large and polar

cmb lect 3 2011 colour 2 slides per page.pdf

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