The Cell Membrane

Function  Regulates the movement of materials from one environment to the other.  Transports raw materials into the cell and waste out of the cell.  Prevents the entry of unwanted matter and the escape of needed materials.  Maintain a steady environment: Homeostasis

Structure:  Composed of a phospholipid bilayer with a collage of many different proteins, lipids and carbohydrates.  A Phospholipid is composed of 1 glycerol molecule, 2 fatty acids and 1 phosphate group. This structure causes hydrophilic and hydrophobic regions.

The Fluid-Mosaic Membrane Model  Membranes are not static; they have a fluid consistency.  Most membrane lipids and proteins can drift about laterally in the plane of the membrane.  Cholesterol enhances membrane fluidity, allows animal membranes to function in a wide range of temperatures and also makes the membrane less permeable to biological molecules.

Membrane Proteins: Two Types:  Integral: Proteins that insert into the membrane (transmembrane proteins)  Peripheral: Proteins attached to the surface of the cell membrane. Function:  Transportation  Enzymes  Receptor sites  Cell adhesion  Attachment to the cytoskeleton

Carbohydrates:  Usually branched molecules of 15 or less sugar units.  Some are bonded to lipids: Glycolipids.  Most are bonded to proteins: Glycoproteins.  Function: Cell-cell recognition.

Construction of a Cell Membrane Copy the link below to create your own cell membrane. online.com/objects/index_tj.asp?objID=A P1101

Through the Cell Membrane 1.Diffusion 2.Osmosis 3.Facilitated Diffusion 4.Active Transport 5.Bulk Transport

1. Diffusion:  Passive movement of molecules from a region of high concentration to a region of low concentration.  (Concentration gradient is the difference in concentration between the two regions)  Small, uncharged molecules like O 2, CO 2 and H 2 O can move easily through the membrane.  Works well over short distances. Once molecules enter the cell the rate of diffusion slows.  Limits cell size.

Passive Transport

2. Osmosis  Diffusion of the solvent across a semi-permeable membrane separating two solutions. (Diffusion of water)  Water molecules move from a region of high concentration to a region of low concentration.  Direction depends on the relative concentration of water molecules on either side of the cell membrane.  Isotonic: Water inside the cell equals the water outside the cell and equal amounts of water move in and out of the cell.  Hypotonic: Water outside the cell is greater than that inside the cell, water moves into the cell, may cause cell to burst (lysis)  Hypertonic: Water inside the cell is greater than outside. Water moves out of the cell, may cause the cell to shrink (plasmolysis)

Osmosis: Hypotonic

3. Facilitated Diffusion  Assists with the movement of large molecules like glucose.  Passive movement of a substance into or out of the cell by means of carrier proteins or channel proteins.  Moves molecules from high to low regions of concentration.  Carrier proteins: Transports non-charged molecules with a specific shape.  Channel proteins: Tunnel shape that transports small charged molecules.

4. Active Transport  The process of moving substances against their concentration gradients. Requires energy.  Examples:  Kidney cells pump glucose and amino acids out of the urine and back into the blood.  Intestinal cells pump in nutrients from the gut.  Root cells pump in nutrients from the soil.  Gill cells in fish pump out sodium ions.  Active Transport Pump:  Sodium-potassium pump  3 sodium ions inside the cell and 2 potassium ions outside the cell bind to the pump.  This allows the release of energy from ATP and causes the protein complex to change shape.  The change in shape allow the Na + and K+ ions to move across and be released.

Active Transport Pump

5. Bulk Transport 1. Endocytosis: The cell membrane folds inward, traps and encloses a small amount of matter from the extracellular fluid. 3 types:

Endocytosis  Pinocytosis: The intake of a small droplet of extracellular fluid. This occurs in nearly all cell types.  Phagocytosis: The intake of a large droplet of extracellular fluid. This occurs in specialized cells.  Receptor-assisted endocytosis: The intake of specific molecules that attach to special proteins in the cell membrane. These proteins are uniquely shaped to fit the shape of a specific molecule.

Bulk Transport 2. Exocytosis: The reverse of endocytosis: A vesicle from inside the cell moves to the cell membrane. The vesicle fuses to the membrane and the contents are secreted