Cell Cell MembraneMembrane

andandTransportTransport

Cell membrane Cell membrane structurestructure Made of Made of

– PhospholipidsPhospholipids– ProteinsProteins– CholesterolCholesterol– Carbohydrate chains Carbohydrate chains

(glycolipids and (glycolipids and glycoproteins)glycoproteins)

Fluid Mosaic modelFluid Mosaic model

Phospholipid bilayer has been seen Phospholipid bilayer has been seen under electron microscopeunder electron microscope

The rest is what scientists believe to be The rest is what scientists believe to be true so it is called a modeltrue so it is called a model

Fluid – phospholipids and proteins move Fluid – phospholipids and proteins move within their areawithin their area

Mosaic – means pattern which is how the Mosaic – means pattern which is how the proteins lie between the phospholipidsproteins lie between the phospholipids

FeaturesFeatures

Phospholipid bilayer – about 7nmPhospholipid bilayer – about 7nm– Hydrophobic tails point inwardsHydrophobic tails point inwards

Non-polar hydrophobic interiorNon-polar hydrophobic interiorUnsaturated tails mean Unsaturated tails mean membrane more fluidmembrane more fluid

If temperature decreases become If temperature decreases become less fluidless fluid

– Hydrophilic heads point outwardsHydrophilic heads point outwards

ProteinsProteins– Float in phospholipid bilayerFloat in phospholipid bilayer– Extrinsic on outside of membraneExtrinsic on outside of membrane– Intrinsic on insideIntrinsic on inside– Transmembrane span membraneTransmembrane span membrane

Hydrophilic outer regionsHydrophilic outer regionsHydrophobic inner regions Hydrophobic inner regions Sit between phospholipidsSit between phospholipids

GlycolipidsGlycolipids– Branched carbohydrate Branched carbohydrate

attached to phospholipidattached to phospholipid GlycoproteinsGlycoproteins

– Branched carbohydrate Branched carbohydrate attached to proteinattached to protein

CholesterolCholesterol

FunctionFunction

PhospholipidsPhospholipids– Barrier to most water-soluble substancesBarrier to most water-soluble substances– Non-polar regionNon-polar region

ProteinsProteins– TransportTransport

Hydrophilic channels. Good for polar molecules.Hydrophilic channels. Good for polar molecules. Specific to certain substancesSpecific to certain substances

– EnzymesEnzymes– Specific organelle functions(eg involved in Specific organelle functions(eg involved in

respiration in mitochondrial membranes)respiration in mitochondrial membranes)

Glycolipids and glycoproteinsGlycolipids and glycoproteins– Form hydrogen bonds with surrounding Form hydrogen bonds with surrounding

fluidfluid Stabilises membrane structure(restricts Stabilises membrane structure(restricts

movement)movement)

– Act as receptor moleculesAct as receptor molecules Bind with particular substances eg. HormonesBind with particular substances eg. Hormones SpecificSpecific Antigens – for like-cell recognitionAntigens – for like-cell recognition

CholesterolCholesterol– Hydrophilic and hydrophobic regions Hydrophilic and hydrophobic regions – Regulate fluidity and give mechanical Regulate fluidity and give mechanical

stability. stability.

Transport across the Transport across the membranemembrane 4 types:4 types:

– DiffusionDiffusion– OsmosisOsmosis– Active transportActive transport– Bulk transportBulk transport

DiffusionDiffusion

All molecules have kinetic energy All molecules have kinetic energy (ie they move)(ie they move)

≡ ≡ net movement of molecules from net movement of molecules from a region of their higher a region of their higher concentration to a region of their concentration to a region of their lower concentration down a lower concentration down a concentration gradient.concentration gradient.

Diffusion in a liquidDiffusion in a liquid

Molecules that diffuse Molecules that diffuse through cell membranesthrough cell membranes

1.1. OxygenOxygen – Non-polar – Non-polar so diffuses very so diffuses very quickly.quickly.

2.2. Carbon dioxideCarbon dioxide – – Polar but very Polar but very small so diffuses small so diffuses quickly.quickly.

3.3. WaterWater – Polar but – Polar but also very small so also very small so diffuses quickly.diffuses quickly.

Diffusion RateDiffusion Rate

Dependant on:Dependant on:– Steepness of concentration gradientSteepness of concentration gradient– TemperatureTemperature– Surface areaSurface area– Type of moleculeType of molecule

Large and polar molecules slowerLarge and polar molecules slower

Facilitated (to make Facilitated (to make possible) diffusionpossible) diffusion Large polar molecules can’t go through Large polar molecules can’t go through

the bilayer, so therefore……..the bilayer, so therefore…….. Use hydrophilic channels in proteinsUse hydrophilic channels in proteins Eg. Glucose, amino acids, NaEg. Glucose, amino acids, Na++ and Cl and Cl-- Specific (allow only one thing to pass)Specific (allow only one thing to pass) Passive (no energy involved)Passive (no energy involved) Rate depends on number of channels Rate depends on number of channels

availableavailable

OsmosisOsmosis

Diffusion of waterDiffusion of water

≡ ≡ the movement of water molecules from a the movement of water molecules from a region of higher water potential to a region region of higher water potential to a region of lower water potential through a partially of lower water potential through a partially permeable membrane.permeable membrane.

Although water is a polar molecule and Although water is a polar molecule and shouldn’t pass through non-polar shouldn’t pass through non-polar membranes it is small enough to go throughmembranes it is small enough to go through

Water PotentialWater Potential

The tendency of water molecules The tendency of water molecules to move from one place to to move from one place to anotheranother

ψψ

Effect of water Effect of water potential on plant cellspotential on plant cells If water moves into the plant cell If water moves into the plant cell

the vacuole expands and puts the vacuole expands and puts pressure on the cell wall. pressure on the cell wall.

TurgidTurgid

If water moves out the vacuole If water moves out the vacuole shrinks and it becomes flaccidshrinks and it becomes flaccid

If this occurs too much the cell If this occurs too much the cell membrane pulls away from the membrane pulls away from the cell wall.cell wall.

This is called plasmolysis.This is called plasmolysis. The spaces will be filled with The spaces will be filled with

whatever solution the plant cell is whatever solution the plant cell is sitting in.sitting in.

Animal cells and water Animal cells and water potentialpotential In a solution with lower water In a solution with lower water

potential the cell shrinkspotential the cell shrinks In a solution with higher water In a solution with higher water

potential the cell swells until it potential the cell swells until it bursts. bursts.

Pressure PotentialPressure Potential

The tendency of water to move in The tendency of water to move in response to pressureresponse to pressure

Symbol Ψp. Symbol Ψp. In turgid plant cells it usually has In turgid plant cells it usually has

a positive value as the entry of a positive value as the entry of water causes the protoplast to water causes the protoplast to push against the cell wall. push against the cell wall.

Solute PotentialSolute Potential

Solute potential is a measure of Solute potential is a measure of the change in water potential of a the change in water potential of a system due to the presence of system due to the presence of solute molecules. solute molecules.

The more solute molecules are The more solute molecules are present, the lower (and more present, the lower (and more negative) is negative) is ψψs.s.

Solute potential is always Solute potential is always negative. negative.

Active TransportActive Transport

The energy consuming transport The energy consuming transport of molecules against a of molecules against a concentration gradient.concentration gradient.

Bulk TransportBulk Transport

EndocytosisEndocytosis– PhagocytosisPhagocytosis– PinocytosisPinocytosis

ExocytosisExocytosis