Cells, membranes and environments

4.4 Movement across membranes  “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 membranes  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 membranes  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. O 2 and CO 2 ) pass directly through membrane  Large water soluble molecules (e.g. amino acids, simple sugars) pass through via protein channels

4.4 Movement across membranes  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 membranes  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 diffusion This process is passive, no energy is required.

4.4 Movement across membranes  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 membranes  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 membranes  Active Transport: Active, requires energy  Moves molecules against the concentration gradient