Chapter 5 Cell Membranes

5.1 Structure of Membranes Fluid Mosaic Model-globular proteins are inserted into the lipid bilayer with their non-polar segments in contact with non-polar region and their polar portions protruding out from the membrane surface. “Proteins float in fluid bilayer.” Components 1.Lipid bilayer- every cell membrane has a phospholipid bilayer. 2.Transmembrane proteins-float on or in bilayer-provide passageways that allow substances & info to cross membrane. 3.Network of supporting fibers-intracellular proteins that reinforce membrane’s shape. Ex: RBC-spectrin (scaffold protein)

5.1 Cont’d 4. Exterior Proteins & Glycolipids-cell surface markers Glycoproteins- “self” recognition (immune system) Glycolipids- tissue recognition (A, B, O blood markers)

5.2 Phospholipids: The Membrane’s Foundation Lipid Bilayer-composed of phospholipids. When placed in water the polar molecules repel the long non-polar tails, causing them to orient themselves away from water. The nonpolar interior impedes the passage of any water soluble substance through the bilayer Membrane is equipped with proteins to solve this problem

5.2 Cont’d Bilayer is fluid-individual phospholipids and unanchored proteins can move around membrane. Some membranes are more fluid that others; some have tails that are attracted to one another and line up close together……less fluid.

5.3 Proteins: Multifunctional Components Kinds of Proteins 1.Transporters-channels or carriers that allow certain substances through membrane 2.Enzymes-carry out chemical reactions on interior surface of plasma membrane 3.Cell surface receptors-act as antennae; sensitive to chemical messages 4.Cell surface identity markers- ID tags so cells can “recognize” one another 5.Cell adhesion protein-glue themselves to one another. 6.Attachment to cytoskeleton-linking proteins that anchor to cytoskeleton

5.4 Passive transport across membranes 1. Diffusion-movement of molecules from and area of high concentration to an area of low concentration. Will continue until molecules reach equilibrium. Passive form or transport-no energy required. Animation: How Diffusion Works 2. Facilitated Diffusion Some molecules, because they are polar, cannot get through non-polar portion of membrane. They go through specific channels. Ion channels possess hydrated interiors so that ions can go through without coming in contact with hydrophobic region. Concentration & voltage determine net movement of ions.

5.4 cont’d Carrier mediated transport process 1. Passive-determined by net concentration 2. Its specific-certain carriers transport only certain molecules 3. It saturates-relevant protein carriers are in use, increases in the concentration gradient do not increase the transport rate. Passive Transport – YouTube

5.4 cont’d 3. Osmosis- net movement of water from an area of high concentration to low concentration. Water moves through aquaporins Cytoplasm is an aqueous solution; water is the solvent and dissolved substances are the solutes. Concentration of all solutes determines osmotic concentration. If they are unequal, the higher one is hypertonic (hyperosmotic); the lower one is hypotonic (hypoosmotic); if they are equal, it is isotonic (isoosmotic)

6.3 cont’d Osmotic pressure-pressure that must be applied to stop the osmotic movement of water across a membrane. If the membrane is strong enough, the cell would reach equilibrium. If the pressure inside the cell (hydrostatic) is equal to the osmotic pressure (outside), equilibrium is reached. `

6.3 cont’d Some cells have adapted to how they maintain their osmotic pressure. 1.Extrusion-removal of water ex: contractile vacuoles 2.Isosmotic solution- same environment outside that is inside ex: organisms that live in ocean adjust concentration internally to match surroundings 3.Turgor pressure-presses plasma membrane firmly against the interior of the cell ex: plants

5.5 Active Transport Across Membranes Active Transport-movement of substances against a concentration gradient; requires ATP and highly selective protein carriers. 1.Na + /K + Pump – Actively uses ATP – more than 1/3 of all energy expended by animal cell that is not actively dividing is used for this process -Most cells have a low Na + /high K + concentration inside the cell. They maintain this by actively pumping Na+ out and K + in. -This is done in a six step process -3 Na + out/2 K + in Video About Cotransport & Na-K pump.flv | Encyclopedia.com

5.5 Active Transport 2. Cotransport-molecules move hand and hand with Na + that are moving down their concentration gradient -Na + and specific sugars or amino acids simultaneously bind to the same transmembrane proteins (symport) on the outside of the cell. Cotransport (Symport and Antiport)Cotransport (Symport and Antiport) 3. Countertransport-inward movement of Na + is coupled with the outward movement of Ca 2+ or H + -Both bind to the same transport protein (antiport) but bind on opposite sides and are moved in opposite directions. Countertransport & Na-K pump.flv - YouTube

5.6 Bulk Transport Bulk Transport 1.Endocytosis-plasma membrane extends outward and envelops food particles Phagocytosis-material taken in is particulate (made up of discrete particles); “cell eating” Pinocytosis-material taken in is liquid; “cell drinking” Receptor mediated-molecules to be transported first bind to specific receptors on membrane – Target molecule hits receptor – Clathrin is a trigger to start endocytosis – Vesicle or clathrin basket if formed, bringing in target molecule – Ex: LDL brings in cholesterol which is important in determing stiffness of cell membranes – Receptor mediated endocytosis - YouTube Receptor mediated endocytosis - YouTube

5.6 cont’d 2. Exocytosis-discharge of material from vesicles at the cell surface ex: mechanism for animal cells to secrete hormones, neurotransmitters and digestive enzymes