Chapter 7 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 u phospholipids u Proteins u Cholesterol u Question: How are the materials arranged?

Phospholipid Bilayer u Phospholipids u Hydrophilic heads u Hydrophobic tails

Membrane Models

Fluid Mosaic Model 1972 u New model to fit the new evidence with membranes.

Fluid Mosaic Model u 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.

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

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 - are attached to the membrane surface.

I or P? (mark these on the list above) u Transport –Channels and pumps (Integral) u Enzymes- I or P u Receptors- P or sometimes I u Cell adhesion- P or I u Cell to cell recognition- P (glycoproteins) u Attachment to cytoskeleton- I

Question? u How do the integral proteins stick to the membrane? u By the solubility of their amino acids. u Non-polar will be in the hydrophobic area

Hydrophilic Amino Acids Hydrophobic Amino Acids Hydrophilic Amino Acids

Membranes are Bifacial u Inside layer is different than outer layer 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 – cell to cell recognition

Cholesterol u Keeps membranes the right fluidity level u Not too flimsy u Not too rigid

u End of Part 1 u Create a Paper Model of a membrane

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

Problems u phospholipid 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 Affect Diffusion 1. Concentration 2. Temperature 3. Pressure 4. Particle size 5. Mixing

Osmosis u Diffusion of water. u Water moving from an area of 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 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 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 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 RequiresTransport protein that helps materials through the cell membrane. u Doesn't require energy (ATP).

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 + - K + pump u Moves 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

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.