 All cells contain cell membranes  Cell membranes support and protect the cell, and regulate what enters and exits the cell.  Cell Membranes are made up of a lipid bilayer—a double layer of lipids with proteins embedded.

 The cell membrane is selectively permeable— some substances can pass through it but others cannot.

 The Fluid Mosaic Model—used to describe the cell membrane. Proteins embedded in the lipid bilayer can move around and “float” among the lipids. There are so many molecules that make up the cell membrane that it looks like a mosaic.

 All cells exist in a liquid environment.  The cell membrane must keep the cell’s internal environment constant. (homeostasis)  The cell membrane regulates the movement of molecules in and out of the cell.  Insulin maintains homeostasis

 Passive transport—the movement of materials across the cell membrane without the use of energy  Active transport—the movement of materials across the cell membrane against the concentration gradient. Active transport requires energy.

 Diffusion—the movement of molecules from an area of high concentration to an area of low concentration.  Substances will move toward the area where it is less concentrated until it is evenly distributed.  Equilibrium—When the concentration of a substance on both sides of the cell membrane is the same.

 The concentration gradient is the direction particles move in order to reach equilibrium. The concentration gradient is always from high concentrations to low concentrations.

 Facilitated diffusion—When channel proteins embedded in the cell membrane facilitate, or help, the diffusion of specific materials across the cell membrane.  There are hundreds of different channel proteins that allow specific substances across the cell membrane. Ex: Channel proteins help glucose molecules move into a cell

 Osmosis—The diffusion of water through a selectively permeable membrane.  Water will move across the membrane until equilibrium is reached.

 Isotonic solution—When the concentrations of water and dissolved material (solute) are the same on both sides of the membrane.  Equilibrium has been reached in an isotonic solution.  Water will continue to move in both directions across the membrane but there is no net movement of water.

 Hypertonic solution—”above strength” When there is a greater amount of solute outside of the cell than inside the cell.  Water will move out of the cell until equilibrium is reached.  Plasmolysis—cell shrinkage due to water loss

 Hypotonic solution—”below strength” When there is less solute outside the cell than inside the cell.  Water will move into the cell until equilibrium is reached.  This can cause an animal cell to swell or even burst if it is placed in fresh water.  Plant cells have cell walls, so are not as vulnerable to bursting.

 Remember, active transport requires energy. It involves moving molecules against the concentration gradient.  Movement of molecules from low to high concentrations requires energy.  2 types of active transport:  Molecular (small molecules)  Bulk (larger molecules or clumps of molecules)

 Small molecules and ions are carried across the cell membrane by protein pumps.  Much of the cell’s energy is used transporting molecules from low concentrations to high concentration.  This allows cells to concentrate substances in a particular location and to transport materials against the concentration gradient.

 Endocytosis—”into the cell” When a cell takes in large molecules, clumps of food, or other cells. The cell membrane folds in on itself, forming a pocket.  Phagocytosis—when a cell takes in food or other cells  Pinocytosis—when a cell takes in a large amount of water by pinching its cell membrane off into vacuoles

 Exocytosis—”out of the cell” When cells release large amounts of material. The membrane of the vacuole fuses with the cell membrane, forcing material out of the cell.