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: Phospholipids Waterproof barrier, acts as a bubble, main constituent Proteins Act as carriers, receptor sites, antigens, channels Carbohydrates Act 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 membranes Membranes are differentially permeable Permeable substances (eg small particles) move through Impermeable substances (eg large particles) can’t move through Movement is from areas of high concentration to areas of low concentration Energy is not needed

Facilitated diffusion Allows larger or less permeable substances to cross Involves channel proteins Energy is not needed

Osmosis Movement of water This 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

Endocytosis Movement due to folding of membrane and bringing in the trapped substances Energy is needed Phagocytosis – solid substances trapped Pinocytosis – liquids are trapped

Exocytosis Movement due to vacuoles moving to membrane and attaching to release contents Energy is needed