Membranes

Phospholipids Have two regions, with different properties: 2 hydrocarbon tails which are Hydrophobic A phosphate head, that is negatively charged to which Hydrophilic

Phospholipids Bilayer When mixed in water they become arranged in double layers Heads face outward and tails inward This is a stable structure because of the bonds that form between phosphate heads and the surrounding water. Tails form hydrophobic interactions This is a weak intermolecular interaction Look at the combinations of bonds collectively as strong

Phospholipids Bilayer

Membrane Structure Phospholipids Proteins: Integral Proteins – embedded in phospholipids Peripheral Proteins – loosely attached to the surface of the membrane Glycoproteins – proteins in the plasma membrane that have short chains of sugar molecules attached to the outside of the membrane

Proteins

Membrane Protein Functions Hormone Binding Site Insulin receptor Immobilized enzymes with the active site on the outside In the small intestine Cell Adhesion Cell to Cell communication Channels for passive transport Pumps for active transport Uses ATP

Membrane Protein Functions

Diffusion the passive movement of particles from a region of high concentration to a region of low concentration (down a concentration gradient), until there is an equal distribution.

Diffusion Diffusion can only occur across the membrane if the phospholipids bilayer is permeable to the particles Hydrophobic center does not let ions with + or – charges to pass easily Polar molecules with partial + / - charges over the surface can diffuse at slow rates Small particles can pass more easily thatn large ones

Diffusion Concentration Gradient Simple diffusion only happens if the concentration of the particle is higher on one side of the membrane than the other

Facilitated Diffusion Channels in the membrane in which ions and other particles can pass into or out the cell Channels can be a single or group of protein molecules The diameter and properties (like charge) ensure that only one type of particle passes

Osmosis Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration (high water concentration) to a region of higher solute concentration (low water concentration).

Osmosis Lower concentration of solute Higher concentration of solute Equal concentration of solute H2OH2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

Osmosis Osmosis can only occur if there are substances in the water – solutes. The water molecules that bond to solutes cannot move freely, reducing the effective concentration of water Regions with higher solute concentration therefore have a lower water concentration This tends to cause water to move from regions of lower to higher concentrations

Passive Transport Requires no energy Moves from down the concentration gradient Some molecules pass through the membrane Some molecules use channels for facilitated diffusion

Active Transport Cells sometimes take in substances even when there is a higher concentration of the substance inside the cell than outside The substance is absorbed against the concentration gradient Cells can even pump cells out even though there is a larger concentration outside Energy is needed for this to occur ATP is required

Transfer Proteins

Endocytosis The mass movement INTO the cell by the membrane ‘pinching’ into a vacuole

Endocytosis Endocytosis can occur in three ways Phagocytosis - solids Pinocytosis - liquids Receptor-mediated endocytosis

Exocytosis The mass movement OUT of the cell by the fusion of a vacuole and the membrane Both are possible because the of the fluid properties of the membrane (able to break and reform easily, phospholipids not attached just attracted) Example of endocytosis can be found in secretory cells (saliva)