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.

Through the Cell Membrane Diffusion Osmosis Facilitated Diffusion Active Transport 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 O2, CO2 and H2O 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