2. The Plasma Membrane and Homeostasis

3. Homeostasis – Maintaining a Balance  Cells must keep the proper concentration of nutrients and water and eliminate wastes.  Cells die without homeostasis.  The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

4. phospholipid bilayer

5. PLASMA MEMBRANE  Two layers of phospholipids embedded with proteins  Selectively permeable  Fluid mosaic model – made of similar molecules free to move sideways  Surround organelles and cells  Permeability of membranes differ because each has their own unique proteins and fatty acids  Cholesterol increases the stability of the membrane

6. What’s a Phospholipid?  It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. Polar (water-soluble, hydrophilic) phosphate heads face outPolar (water-soluble, hydrophilic) phosphate heads face out Nonpolar (hydrophobic) fatty acids hang insideNonpolar (hydrophobic) fatty acids hang inside

7. Membrane Proteins  1. Determine what particles can pass through the membrane.  2. Serve as enzymes (may speed reactions).  3. Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system).

8. MEMBRANES OF A CARIBOU In caribou, an arctic animal, membranes of cells near the animal’s hooves have phospholipids with many unsaturated fatty acids. Unsaturated fatty acids remain liquid at low temperatures. The unsaturated lipids allow the caribou's feet and legs to drop to almost 0 0 C in the arctic winters and still maintain plasma membrane function. The cell membranes of the rest of the caribou's body have more saturated lipids.

9. AURIVOIR