Membrane Structure and Function

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

Membrane Composition Lipids Proteins Question: How are the materials arranged?

Membrane Models

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

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

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

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

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

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

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

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

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

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

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

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

Hydrophilic Amino Acids Hydrophobic Amino Acids Hydrophilic Amino Acids

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

Carbohydrates

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

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

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

Mechanisms 1. Passive Transport 2. Active Transport

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

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

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

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

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

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

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

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

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

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

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

Osmosis – an update Osmosis is a facilitated diffusion process through proteins called aquaporins. Water is too polar to slip through the phospholipid bilayer.

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

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

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

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

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

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

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

H+ pumps - application Plants were genetically engineered to make more H+ pumps in the tonoplast. Result – a higher osmotic gradient. Use – salt tolerant plants

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

Endocytosis Moves bulk materials into cells. Several types known.

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

Forming vesicles

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