Plasma Membrane Functions Professor Collier Spring Semester 2012

The Fluid Mosaic Model The fluid mosaic model (S.J Singer) -- semi-permeable --fluid portion is a double layer of phospholipids, called the phospholipid bilayer Phospholipids contain a hydrophilic head and a nonpolar hydrophobic tail Cholesterol embedded in the membrane makes it stronger and less fluid

Plasma Membrane Jobs Isolate the cytoplasm from the external environment Regulate the exchange of substances Communicate with other cells Identification

Functions of Membrane Proteins Proteins embedded in membrane serve different functions: 1. Channel Proteins - form small openings for molecules to difuse through 2. Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell 3. Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins) 4. Cell Recognition Proteins - ID tags, to idenitfy cells to the body's immune system 5. Enzymatic Proteins - carry out metabolic reactions

Diffusion The net movement of substances from an area of higher concentration to an area of lower concentration. A state of equilibrium is attained where molecules are uniformly distributed but continue to move randomly.

Osmosis The diffusion of water molecules across a selective permeable membrane. Water moves into causing hydrostatic pressure (osmotic pressure). Turgor pressure is the osmotic pressure that develops when water enters plants and microorganisms

Osmotic Effects

Vesicular Transport Endocytosis The capture of a substance outside of the cell when the plasma membrane merges to engulf it. Phagocytosis: occurs when undissolved material enters the cell, i.e. white blood cell attacks bacteria. Pinocytosis: occurs when dissolved substances enter the cell. Receptor –mediated endocytosis: occurs when specific molecules in the fluid surrounding the cell binds to specific receptors

Vesicular Transport Exocytosis The process of vesicles fusing with plasma membrane and releasing its contents outside cell.

Active Transport The movement of solutes against the gradient and requires the use of energy: ATP. Transport proteins in the membrane transfer solutes such as Na+, K+, Cl-, H+, amino acids, and monosaccharides across the membrane.

Active Transport