Cells, membranes Cells, membranes and environmentsand environments

4.4 Movement across 4.4 Movement across membranesmembranes

“Cells must be able to exchange substances with their environment (Figure 4.10a).”

Movement depends on Nature of substance being moved Surface area Composition of membrane Concentration Gradient Temperature

4.4 Movement across 4.4 Movement across membranesmembranes

Different types of substances cross membranes in different ways Membranes are made of lipids and are

therefore impermeable to most water-soluble molecules, ions (have overall positive or negative charge), charged molecules (no overall charge, but charged regions).

These molecules cross the membrane via specific channels in the membrane; the channels are made from proteins (ion channels)

4.4 Movement across 4.4 Movement across membranesmembranes

Lipid-soluble molecules of various sizes (e.g. chloroform, alcohol, cocaine, heroin, etc.) dissolve through lipid membranes

Very small molecules (e.g. water and urea) pass between the lipid molecules of the membrane

Small uncharged molecules (e.g. O2 and CO2) pass directly through membrane

Large water soluble molecules (e.g. amino acids, simple sugars) pass through via protein channels

4.4 Movement across 4.4 Movement across membranesmembranes

Diffusion: “The molecules in a solution move about at high speed in random directions.” Solute – substance being dissolved. Solvent – substance the solute is being dissolved

in.

Solute molecules move from an area of high solute concentration to low solute concentration by the random movement of both the solute and solvent molecules.

The difference between the solute concentrations is called a concentration gradient.

The larger the gradient, the quicker the diffusion

This process is passive, no energy is required

4.4 Movement across 4.4 Movement across membranesmembranes

Diffusion across a membrane: The molecules of a solute are continuously moving across membranes. If the concentration on both sides is equal, then there is no net movement.

If there is a difference in concentration on both side of the plasma membrane, then there is a concentration gradient into the more dilute solution. This results in a net movement.

Increasing temperature increases the rate of diffusionThis process is passive, no energy is required.

4.4 Movement across 4.4 Movement across membranesmembranes

Facilitated Diffusion: Protein channels enhance the diffusion of particular ions and molecules. This process is passive, no energy is required.

Transport is more rapid

Channels are selective

Channels can be come saturated as the concentration increases

Transport may be inhibited by other molecules

4.4 Movement across 4.4 Movement across membranesmembranes

Osmosis: Special type of diffusion involving free water molecules

Net movement occurs from the side with the lower solute concentration to the side with the higher solute concentration

Osmotic gradient

Passive, no energy required

4.4 Movement across 4.4 Movement across membranesmembranes

Active Transport: Active, requires energy

Moves molecules against the concentration gradient