Diffusion, Facilitated Diffusion and Osmosis:


Maintaining a constant cell environment:


We know that the cell membrane controls what goes into and out of the cell. We also know that the internal environment of a cell remains relatively constant at all times. This constant balance in the cell is called HOMEOSTASIS. By the cell membrane allowing certain things into the cell and not others it helps to create a chemical difference between the inside of the cell and the outside environment. If the homeostasis of a cell is disturbed the cell will die.


The ability of cells to establish and maintain homeostasis is a special function of the cell membrane. We learned that the membrane is SELECTIVELY PERMEABLE. It allows some things but not others to enter the cell. This is based on the factors of: size, shape and electrical charge.



The movement of molecules or particles from an area of greater concentration to an area of lesser concentration. Diffusion occurs because molecules are in constant motion.




Concentration gradient:

The difference in concentration between a region of high concentration and a region of lower concentration.


Diffusion will only occur if there is a concentration gradient. The result of diffusion is the molecules becoming evenly distributed throughout the available space. When they are evenly distributed, no further change in concentration occurs. Molecules still move. The same amount of molecules moving out of a given area is the same amount of molecules moving into the area. This is called equilibrium.


Diffusion is important in the transport (movement) of molecules into and out of the cell. Depending on the concentration gradient will be whether certain materials will either enter or leave cells by diffusion. The process of diffusion plays an important role in the enrty and exit of molecules in living cells.


Facilitated Diffusion:


Process by which certain molecules diffuse quickly across a cell membrane. This type of transport occurs because of specialized transport specialized transport proteins in the cell membrane. Each type of transport protein is specific to the substance that it carries. It works only in the direction of the concentration gradient. it speeds up the movement across the membrane from a region of high concentration to an area of lower concentration.




The diffusion of water across a semi-permiable membrane from a region of high concentration of water to a region of low concentration of water. "Concentration of water" means the amount of water present in a given volume. Since two things cannot occupy the same space at the same time, the number of particles dissolved in a given amount of water determines the concentration of water molecules present. Example: the highest concentration of water molecules is in pure water- water with nothing else in it. the more particles dissolved in water, the few the water molecules present.

The cytoplasm of cells is made mainly of water that contains a variety of dissolved substances. Water can pass freely through the cell membrane in both directions For example if the concentration of water inside and outside the cell are the same, then there is no water diffusion (osmosis). if the water concentration is higher inside the cell than outside, the water will move out of the cell.

By placing cells into solutions containing different concentrations of water can effect osmosis.


Isotonic soultion: a solution that contains the same concentration of dissolved substances as does a living cell placed in it. the concentration gradient is zero, therefore there is no gain of loss of water by the cell.


Hypotonic solution: a solution that contains a lower concentration of dissolved substances than that of a cell placed in it. Therefore the concentration of water molecules is higher in the hypotonic solution than it is in the cell. since the concentration of water is higher outside the cell than inside there is a net movement of water into the cell by osmosis. when a cell is placed in a hypotonic solution, it swells and bursts.


Hypertonic solution: a solution whose concentration of solutes is higher than that of a cell placed in it. It contains a higher concentration of dissolved substances than the cell. The concentration gradient results in a net movement of water out of the cell. Animal cells will shrink when they are placed in a hypertonic solution. In plant cells the vacuole will collapse which will cause the cytoplasm to shrink within the cell wall. The shrinking of cytoplasm by osmosis is called plasmolysis.

See how different solutions effect a cell


Passive and Active Transport:


Passive transport: a process where materials move across cell membranes without the use of cellular energy


**In diffusion, no cellular energy is used to move substances into or out of the cell. Many substances move into and out of the cell by passive transport**


Active transport: a process in which the movement of materials across a cell membrane requires the use of cellular energy


**active transport usually involves the movement of materials against a concentration gradient. Materials move from an area of low concentration of an area of higher concentration.

you can think of a concentration gradient as a hill. The top of the hill represents the higher concentration, and the bottom of the hill represents the lower concentration. A bike moving downhill along the concentration gradient needs no outside source of energy in order to keep rolling (passive transport: moving from high concentrations to low concentrations). To move the bike uphill or against the concentration gradient, requires energy (active transport: needing energy to move something from a low concentration to a higher concentration).