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Biology-Life Sciences

Standard A: Plasma Membranes
Standard A: Plasma Membranes
Standard B: Enzymes
Standard C: Prokaryotic Cells, Eukaryotic Cells, and Viruses
Standard D: Central Dogma of Molecular Biology
Standard E : The endoplasmic reticulum and golgi apparatus
Standard F: Chloroplasts
Standard G: Mitochondria
Standard H: Macromolecules
Standard J : The Cytoskeleton and Cell Wall
Standard I : ATP production
Exemplar Chart




All cells are enclosed with semipermeable membranes.  The semipermeable membranes in cells are plasma membranes, which is a flexible boundary between a  cell and its environment.  The plasma membrane allows a steady supply of nutrients to come into a cell.  For example, the plasma membrane allows oxygen, amino acids, water, and glucose to come in, but does not allow carbon dioxide, waste and water to enter




The plasma membrane is made up of a phospholipid bilayer.  A phospholipid bilayer has two layers of phospholipids back-to-back.  A phospholipid is made up of a glycerol backbone, two fatty acid chains, and a phosphate group.




Osmosis is the diffusion of water across a semipermeable membrane.  This means that the plasma membrane controls how much water gets to enter the cell.  Osmosis affects cells in different ways. 


***In isotonic solutions, the concentration of dissolved substances in the solution is the same as a the level of concentration of dissolved substances inside the cell. 


***In hypotonic solutions, the concentration of dissolved substances in the solution is lower in the solution outside the cell than the concentration inside the cell.  Cells in the solution experience osmosis that sometimes causes the cell to swell and burst as water enters the cell.

***In hypertonic solutions, the concentration of dissolved substances outside the cell is higher than the concentration inside the cell.  Cells in this solution experience osmosis that causes water to flow out. Animal cells shrink because of decreased pressure.

Diffusion is the random movement of particles from an area of high concentration to an area of low concentration, eventually resulting in even distribution.  Diffusion is a slow process because it relies on the random motion of atoms and molecules.  Concentration, temperature, and pressure all affect the rate of diffusion.  After two solutions are distributed evenly, no further reaction can occur (called dynamic equilibrium).

Active Transport and Passive Transport
     Passive transport is the movement of particles across cell membranes by diffusion or osmosis.  This requires no energy because particles are moving in the direction of diffusion.  Passive transport of materials across the membrane using transport proteins is called facilitated diffusion.  Transport proteins help substances move thorugh the plasma membrane.
     The energy-expanding process where cells transport materials across the cell membrane against a concentration gradient is called active transport.  Active transport requires energy from the cell because it has to counteract the force of diffusion that is moving particles in the opposite direction.
**pm proteins are proteins found in the plasma membrane and they are used for both active and passive transport