Chapter 7 Section 3: Cell Boundaries

Cell Membrane Regulates what enters and leaves the cell. Provides protection and support.

Composition Lipid-bilayer: double-layered sheet of lipids. Gives cell membrane flexible structure Forms strong barrier between the cell and surroundings.

Other Components Most cell membranes contain proteins embedded in the lipid bilayer: Integral proteins. Carbs attached to these proteins. Form channels and pumps that help move material across membrane. Carbs act as chemical ID cards—allow cells to identify one another.

Diffusion Through Cell Boundaries Cell membrane regulates movement of dissolved molecules from the liquid on one side of the membrane to the other.

Diffusion Movement of molecules from an area of high concentration to an area of low concentration Equilibrium: reached when concentration of solute is same throughout a system. Passive Transport: does NOT require energy

Permeability If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it. Cell membrane = Selectively permeable: some substances can pass across it and some cannot.

Osmosis Diffusion of water through a selectively permeable membrane.

Osmosis When water is more concentrated on one side of the membrane, water will move to an area of lower concentration in order to re-establish equilibrium. Example: More salt outside of the cell = movement of water _________________

Example of Osmosis

Different Concentrations Isotonic: concentration of solutes is the same inside and outside cell. Hypertonic: solution has a higher solute concentration than the cell. Hypotonic: solution has a lower solute concentration than the cell.

Osmotic Pressure Osmosis exerts a pressure (osmotic pressure) on the hypertonic side of a selectively permeable membrane. (This could cause water to rush into cells and cells could bust) This does not happen because animal cells are in isotonic fluids: example: blood. Plant and Bacteria cells: exposed to tremendous osmotic pressure Rigid cell wall keeps plant and bacteria cells from bursting.

Facilitated Diffusion Protein channels allow certain molecules through the cell membrane easily. Example: Glucose Does not require energy Occurs when concentration is higher on one side of the membrane than the other.

Facilitated Diffusion

Active Transport Requires energy Small molecules or ions: transport proteins or pumps Molecular Transport Larger molecules: changes in the shape of the cell membrane Endocytosis Phagocytosis Pinocytosis Exocytosis

Molecular Transport Many cells use proteins to move calcium, potassium and sodium ions across cell membranes. Works against concentration gradient: Low  High

Endocytosis Taking material into the cell by means of infoldings, or pockets, of the cell membrane. Pocket breaks loose and forms vacuole in cytoplasm.

Two Types of Endocytosis Phagocytosis: cytoplasm and membrane engulf food and store it in a vacuole. Pinocytosis: cytoplasm and membrane engulf liquid and store it in a vacuole.

Exocytosis Membrane of vacuole surrounding material fuses with cell membrane, forcing contents out of the cell.