Cell membranes and exchange of material

HBS 3A

Exchange of material

Occurs at the cell membrane

Involves the intake of essential materials eg nutrients and oxygen and the removal of unwanted materials eg wastes

The processes involved include diffusion, osmosis and active transport

Structure of the cell membrane

Fluid mosaic model

Describe the roles of:• PhospholipidsWaterproof barrier, acts as a bubble, main

constituent

• Proteins Act as carriers, receptor sites, antigens,

channels

• CarbohydratesAct as receptor sites, antigens

Exchange of material

Small particles eg water, ions can pass through the gaps between the phospholipids

Larger molecules pass through channels in the proteins or are carried by special proteins

Movement of particles - diffusion

Occurs without energy

Particles will move from higher concentrations to lower concentrations

Movement through membranes

Diffusion through membranesMembranes are differentially permeablePermeable substances (eg small particles) move throughImpermeable substances (eg large particles) can’t move throughMovement is from areas of high concentration to areas of low

concentrationEnergy is not needed

Facilitated diffusion

Allows larger or less permeable substances to cross

Involves channel proteins

Energy is not needed

OsmosisMovement of waterThis occurs from areas of high water concentration (low

concentration of solute) to areas of low water concentration (high concentration of solute)

Concentration gradient

This refers to the difference in concentration across membranes

The steeper the gradient,

the greater the movement

Osmotic pressure refers to the pull of water due to concentration gradients

Active transport

Movement against a concentration gradient.Energy is needed

Ion pumps

Involves special proteins that act as pumps

Allows cells to maintain ion concentrations that are very different from the external environment

Energy is used

EndocytosisMovement due to folding of membrane and bringing in the

trapped substances Energy is needed Phagocytosis – solid substances trappedPinocytosis – liquids are trapped

Exocytosis

Movement due to vacuoles moving to membrane and attaching to release contents

Energy is needed