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Membrane Structure

All biological membranes are composed mainly of lipid and protein molecules

• PHOSPHOLIPIDS – the most abundant• CHOLESTEROL – responsible for stabilising the

membrane• GLYCOLIPIDS – found at the external surface of the

membrane

The three major types of lipids in cell membranes are:

All of the lipids are described as being AMPHIPATHIC as they have a HYDROPHILIC (‘water-loving’) end and a HYDROPHOBIC (‘water-hating’) end to the molecule

The PROTEINS within the membrane are largely concerned with the transport of molecules across the membrane

The phospholipid moleculehas a polar phosphate –containing head group andtwo hydrophobic fatty acidtails

The tails vary in length andmay have one or more double bonds

Each double bond createsa kink in the tail

The differences in tail lengthand the presence of doublebonds are important forinfluencing the FLUIDITYof the membrane

Kink due to the presenceof a double bond

The hydrophilic headconsists of a phosphateand glycerol group

Two non-polarhydrophobic tail groupsare bonded to the hydrophilic head group

The FLUID MOSAIC MODEL envisages the membrane as a sea of phospholipids within which a mosaic of proteins float like icebergs

The FLUID MOSAIC MODEL proposes a double layer of

phospholipids with PROTEINS penetrating this layer to

different extents

The proteins are globular in nature and form a MOSAIC in

the fluid-like lipid bilayer

Extrinsic proteins (are partially

embeddedin the bilayer)

Lipid bilayer

Intrinsic proteins(extend right across

the bilayer)

The following two dimensional view of the cell membrane illustrates additionalchemical components of the membrane

Bimolecularphospholipid

layer

The lipid bilayeris asymmetrical inthat certain proteinand lipid moleculesat the external surfacecontain carbohydratechains as part of their structure

Glycolipids andglycoproteins formpart of the externalstructure of themembrane

Glycolipids play apart in communicationbetween cells and cell to cell recognition

Many glycoproteins function as surface antigensenabling cells to distinguishself from ‘non-self’

Cholesterol molecules are positioned within the bilayer close to the fatty acid chains;

these molecules partially immobilise these chains and help to stabilise the membrane

glycolipid carbohydrategroup

glycoprotein

Extrinsicprotein

Intrinsicprotein

cholesterol stabilisingthe membrane

Transport across Membranes

The transfer of relatively small molecules across cell membranes may occur in a variety of ways

The PASSIVE TRANSPORT of materials occurs in two ways:1. SIMPLE DIFFUSION where molecules diffuse across the

the lipid bilayer or through channel proteins in the directionof their concentration gradient and

2. FACILITATED DIFFUSION where protein carrier moleculeswithin the membrane assist the passage of substances acrossthe membrane in the direction of their concentration gradient

Cellular energy is NOT required for passive methods of transport and relies largely on the random movement

of molecules and ions

The ACTIVE TRANSPORT of materials also involves carrier proteinsassisting the molecules across the membrane. In this case molecules

are transported against their concentration gradient and cellular energy is required for this to be achieved

energyenergyenergy

Passive and active methods of transport are used by cellsfor the transfer of molecules and ions across membranes

simplediffusion

facilitateddiffusion

Passive Transport Active Transport

The rate of diffusion is affected by a variety of factors that include:• Temperature• Surface area• Steepness of the concentration gradient• Distance over which diffusion is taking place

Three of these factors are expressed in FICK’S LAW, which states that:

Rate of diffusion = surface area x steepness of concentration gradient

thickness of membrane

The larger the surface area

The steeper the concentration gradient

The thinner the membrane or diffusion barrier

The faster is therate of diffusion

Surface Area and Simple Diffusion

Facilitated diffusion is a carrier-assisted transport mechanismin which molecules are transferred across membranes

along their concentration gradients

Intrinsic globular proteinswithin the membrane

function as carriers for the active transport of molecules

across membranes

Active transport is an energy –requiring transport system that

is able to transfer materialagainst a concentration gradient

Many different ions are activelytransported across membranes

as is glucose when being absorbedfrom the gut into the blood stream

Hydrolysis of ATP provides the energyfor the protein carriers to change shape

and transport the molecules acrossthe membrane

energyenergyenergy

ATP ADP

Many of the substances that enter cells are too large to be transported through

the bilayer or by transport proteins

The bulk transfer of materials INTO the cell occurs by ENDOCYTOSIS

The bulk transfer of materials OUT OF the cell occurs by EXOCYTOSIS

The Bulk Transfer of Materials

Osmosis is a special kind of diffusion by which water molecules are transported across partially permeable membranes

Partially permeable membrane withpores created by channel proteins

solute moleculestoo large to passthrough pores

in the membrane

The term WATER POTENTIAL () is used when describing the tendency of water molecules to move from one place to another

To move requires energy; water potential is a measure of the free energy availablefor water molecules to move

The free energy available for water molecules in PURE WATER to do work and move is GREATER than that for water molecules in a

SOLUTION (water plus dissolved solutes)

As pure water contains no dissolved solutes, the water potential of pure water is defined as ZERO – zero is the highest water potential possible

The presence of dissolved solutes hinders the ability of water molecules to move

The presence of any substances dissolved in water LOWERS THE WATER POTENTIAL

THE WATER POTENTIAL OF A SOLUTION ISALWAYS LESS THAN ZERO AND IS THEREFORE A NEGATIVE VALUE

Water potential is measured in units called pascals and the definition of osmosis in terms of water potential is:

Osmosis is the net movement of water molecules through a partially permeablemembrane from a region of high water potential to a region of lower

water potential i.e. movement DOWN a water potential gradient

In the situation shown below, two solutions with different water potential values are separated by a membrane

Pure water has a water potentialof ZERO

The presence of solute molecules in this solution lowers the water potential, e.g. -4

This fully turgid plantcell has been placed ina hypotonic solution

A hypotonic solution isone that is less concentrated

than the protoplast of theplant cell and thus has a

higher water potential than the cell

The net movement of water byosmosis is into the cell and theprotoplast swells and presses

against the cell wall

This plant cell has beenplaced in an isotonic solution

An isotonic solution isone that has the same

concentration as the protoplast of the plant cell and thus has the

same water potential

The cell displays incipientplasmolysis where the membrane

is just beginning to pull awayfrom the cell wall

There is no net movement ofwater in this case and nopressure potential as the protoplast ceases to press

against the cell wall

This plant cell has beenplaced in a hypertonic solution

A hypertonic solution is one that is more concentrated than the protoplast of the plant cell

and thus has a lower water potential than the cell

The net movement of waterby osmosis is out of the celland the protoplast shrinks

The protoplast is pulledcompletely away from the

cell wall and the cell isfully plasmolysed (flaccid)

Consider two adjacent plant cells (A and B) in a tissue where each cell possesses a different water potential

CELL B has the higher water potential (less negative) and therefore water will move down the water potential gradient from Cell B to CELL A

In which direction will water move as a consequence of osmosis?