Chapter 6 Membrane Structure and Function Plasma Membrane

Chapter 6 Membrane Structure and Function

Plasma Membrane u. The membrane at the boundary of every cell. u. Functions as a selective barrier for the passage of materials in and out of cells.

Membrane Composition uphospholipids u. Proteins u. Question: How are the materials arranged?

Phosphophospholipid Bilayer u. Phospholipids u. Hydrophilic heads u. Hydrophobic tails

Membrane Models

Davson-Danielli Model 1935 uphospholipid bilayer. u. Proteins coat the surfaces. u. Sometimes called the “sandwich” model.

Evidence u. Biochemical work. u. TEM pictures show the membrane as a double line.

Problems u. Not all membranes in a cell were the same. u. How could the proteins stay in place? u. Result - the model was questioned and tested by scientific process.

Fluid Mosaic Model 1972 u. New model to fit the new evidence with membranes. u. Example of “Science as a Process”.

Fluid Mosaic Model u. Refers to the way the phospholipids and proteins behave in a membrane.

“Fluid” u. Refers to the phospholipid bilayer. u. Molecules are not bonded together, so are free to shift. u Must remain "fluid" for membranes to function.

Ways to keep the membrane “fluid” uphospholipid changes or shifts: u. Cold hardening of plants (shift to unsaturated fatty acids). u. Hibernating animals (Cholesterol increase).

“Mosaic” u. Proteins: float in a sea of phospholipids. u. Proteins form a collage or mosaic pattern that shifts over time.

Evidence u. TEM pictures of fractured membranes. u. Cell fusion studies. u. Tagging of membrane proteins by antibodies.

Protein Function in Membranes u. Transport. u. Enzymatic activity. u. Receptor sites for signals. u. Cell adhesion. u. Cell-cell recognition. u. Attachment to the cytoskeleton.

Types of Membrane Proteins u. Integral - inserted into the phospholipid bilayer. u. Peripheral - not embedded in the phospholipid bilayer, but are attached to the membrane surface.

Question? u. How do the integral proteins stick to the membrane? u. By the solubility of their amino acids.

Hydrophilic Amino Acids Hydrophobic Amino Acids Hydrophilic Amino Acids

Membranes are Bifacial u. The phospholipid composition of the two layers is different. u. The proteins have specific orientations. u. Carbohydrates are found only on the outer surface.

Carbohydrates

Membrane Carbohydrates u. Branched oligosaccharides form glycophospholipids and glycoproteins on external surface. u. Function - recognition of "self" vs "other”.

Question u. How do materials get across a cell's membrane?

Problems uphospholipid bilayer is hydrophobic. Hydrophilic materials don't cross easily. u. Large molecules don't cross easily. Too big to get through the membrane.

Mechanisms 1. Passive Transport 2. Active Transport

Passive Transport u. Movement across membranes that does NOT require cellular energy.

Types of Passive Transport 1. Diffusion 2. Osmosis 3. Facilitated Diffusion

Diffusion u. The net movement of atoms, ions or molecules down a concentration gradient. u. Movement is from: High Low

Equilibrium u. When the concentration is equal on both sides. u. There is no net movement of materials.

Factors that Effect Diffusion 1. 2. 3. 4. 5. Concentration Temperature Pressure Particle size Mixing

Osmosis u. Diffusion of water. u. Water moving from an area if its high concentration to an area of its low concentration. u. No cell energy is used.

Tonicity u. The concentration of water relative to a cell. 1. Isotonic (same) 2. Hypotonic (below) 3. Hypertonic (above)

Isotonic u. Isosmotic solution. u. Cell and water are equal in solute concentration. u. No net movement of water in or out of the cell. u. No change in cell size.

Hypotonic u. Hypoosmotic solution u. Cell's water is lower than the outside water (more solutes). u. Water moves into the cell. u. Cell swells, may burst or the cell is turgid.

Hypertonic u. Hyperosmotic solution u. Cell's water is higher than the outside water (less solutes) u. Water moves out of the cell. u. Cell shrinks or plasmolysis occurs.

Facilitated Diffusion u. Transport protein that helps materials through the cell membrane. u. Doesn't require energy (ATP). u. Works on a downhill concentration gradient.

Aquaporins u. Newly found channels for osmosis. GFP labeled Aquaporins

Active Transport u. Movement across membranes that DOES require cellular energy.

Types of Active Transport 1. Carrier-Mediated 2. Endocytosis 3. Exocytosis

Carrier-Mediated Transport u. General term for the active transport of materials into cells AGAINST the concentration gradient. u. Movement is: low high

Examples 1. Na+- K+ pump 2. Electrogenic or H+ pumps 3. Cotransport

+ Na u. Moves + K pump Na+ ions out of cells while moving K+ ions in.

Electrogenic or H+ pumps u. Also called Proton pumps. u. Create voltages across membranes for other cell processes. u. Used by plants, fungi and bacteria.

Cotransport u. Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane. u. Example - Sucrose transport

Exocytosis u. Moves bulk material out of cells. u. Example - secretion of enzymes.

Endocytosis u. Moves bulk materials into cells. u. Several types known.

Types 1. Pinocytosis - liquids 2. Phagocytosis - solids 3. Receptor Mediated - uses receptors to "catch" specific kinds of molecules.

Carbohydrates

Forming vesicles

Summary u. Know membrane structure. u. Be able to discuss the various methods by which cells move materials through membranes. u. Be able to solve problems in osmosis.

AP Lab 1 u. Please come in tomorrow with your prelab done and ready to go! u We will start lab at 7: 30!!