Chapter 7: 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 Phospholipids Proteins Question: How are the materials arranged?

Phospholipid Bilayer Phospholipids Hydrophilic heads Hydrophobic tails

Membrane Models

Davson-Danielli Model -1935 phospholipid 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 phospholipids and proteins behave in a membrane.

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

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

“Mosaic” Proteins: float in a sea of phospholipids. 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 phospholipid bilayer. Peripheral - not embedded in the phospholipid 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, especially their R groups.

Hydrophilic Amino Acids Hydrophobic Amino Acids Hydrophilic Amino Acids

Membranes are Bifacial The phospholipid 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 glycophospholipids and glycoproteins on external surface. Function - recognition of "self" vs "other”.

Question How do materials get across a cell's membrane? Non-polar materials or hydrophobic materials do cross easily, especially if small. Ex – CO2, O2

But… phospholipid bilayer is hydrophobic, so hydrophilic materials don't cross easily. (ions, water soluble, polar molecules etc.) Large molecules don't cross easily. Too big to get through the membrane.

Mechanisms to Cross Membranes 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 Low

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 if its high concentration to an area of its low concentration. No cell energy is used.

Tonicity The concentration of outside water relative to a cell’s inside water. 1. Isotonic (same) 2. Hypotonic (below, cell is below outside) 3. Hypertonic (above, cell is above outside)

Isotonic 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 Cell's water is lower than the outside water (more solutes). Therefore, water moves in to the cell. Increase in volume causes the cell to swell, may burst or the cell is turgid if a cell wall prevents rupture.

Hypertonic Cell's water is higher than the outside water (less solutes) Therefore, water moves out of the cell to the outside Volume of the cell shrinks, or plasmolysis occurs.

Problems in tonicity Common to AP exams. Usually depend on being able to define “high” and “low” water or solute concentrations. U-tube problems.

U-tube Problems Let’s do some common examples.

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

Water Transport - Aquaporins http://www.bing.com/videos/search?q=Aquaporin+Water+Channel&Form=VQFRVP&adlt=strict#view=detail&mid=77A81D420E62FB842ED577A81D420E62FB842ED5

Aquaporins Recently found facilitated diffusion channels for osmosis. GFP labeled Aquaporins

Active Transport Movement across membranes that DOES require cellular energy.

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

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

Endocytosis Moves bulk materials in to cells. Several types known.

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

Carbohydrates

Forming vesicles

Chapter 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.