Passive and Active Transport

SIMPLE DIFFUSION

1. The simplest type of passive transport, diffusion does not require the cell to use energy. Only small molecules can cross the cell membrane by simple diffusion.

2. Diffusion is the movement of molecules from an area of high concentration to one of low concentration.

3. This difference in the concentration of molecules across a space is called the concentration gradient.

4. Diffusion is driven by the kinetic energy of the molecules. Because of their KE, molecules are in constant motion. Diffusion occurs when molecules move randomly away from each other in a liquid or gas.

5. The rate of diffusion depends on the temperature, size and the type of molecules that are diffusing.

6. Molecules diffuse faster at higher temperatures than at lower temperatures, and smaller molecules diffuse faster than large molecules.

7. Most transport of materials into and out of cells occurs by diffusion.

OSMOSIS

1.Osmosis is “The process by which water molecules diffuse across a partially permeable membrane from a region of higher water potential to a region of lower water potential.”

2. Water moves by diffusion, like any other molecule, from a region of high concentration to one of low concentration, i.e. down its concentration gradient. Confusion occurs because ‘concentration’ normally refers to the solute concentration, whereas, in this case, we are referring to the solvent concentration.

3. For this reason, the use of the term ‘water potential’ is essential; water then simply moves ‘down the water potential gradient’ – easy!

4. The water potential of pure water is zero (0), so, since a solution must always be less than 100% pure water, all solutions (and cells) have a negative (-) water potential.

5. Insoluble molecules do not affect the solute potential (obviously), so have no osmotic effect. Such molecules are used for storage – starch, lipids, and very large proteins (albumin).

6.Water potential is actually a pressure, and is measured in Pascals (Pa), but since these are so small the normal unit is the kilo Pascal (kPa). Typical plant values are -200 to -2000kPa.

FACILITATED DIFFUSION

1. Passive transport across a membrane requires no energy input from the cell and always goes down the concentration gradient. Simple diffusion and osmosis are examples of passive transport.

2. However, most molecules cannot cross the membrane by simple diffusion; to do so, the molecule must either be very small (water, carbon dioxide) or be soluble in both water and lipid (ethanol).

3. Some molecules are carried across the membrane by carrier proteins which are embedded in the cell membrane.

 

4. Carrier proteins often change shape when molecules attach to them, and this change in shape enables the molecule to cross the membrane.

5. Because the carrier protein has to fit around the molecule, it is specific to one molecule, or related class of molecules.

6. This use of carrier proteins to cross the membrane is known as facilitated diffusion, and can be used by those molecules to cross the membrane in either direction – into or out of the cell.

7. Like simple diffusion, facilitated diffusion always goes down the concentration gradient, and therefore continues until equilibrium is reached, for that molecule.

8. A good example of facilitated diffusion is the transport of glucose into the cell. Once inside the cell, the glucose is immediately turned into glucose phosphate, for which no carrier protein exists. Glucose will thus continue to enter the cell, since equilibrium can never be reached!

9. Facilitated diffusion is therefore another form of passive transport, since it requires no energy input from the cell.

10. Some molecules, mainly ions (e.g. Na+, K+) cross the membrane through tunnels made of protein called ion channels.

11. Some ion channels are always open, but others (e.g. in neurones) have ‘gates’ that open to allow ions to pass or close to stop their passage.

12. Gates open and close in response to conditions in the external environment, or in the cell. It is the opening and closing of the sodium and potassium gates that allows a nerve impulse to be formed and passed along a neurone.

 

ACTIVE TRANSPORT (using ATP energy)

CELL MEMBRANE PUMPS

1. Cells often move molecules across the membrane against the concentration gradient, i.e. from an area of low concentration to an area of high concentration.

2. This requires energy (uses ATP), and is known as active transport.

3. Active transport involves the use of carrier proteins, similar to those of facilitated diffusion, but these carrier proteins act as pumps, using the energy from splitting ATP to pump specific molecules against the concentration gradient.

4. These carrier proteins are known as membrane pumps, and are particularly important in maintaining the Na+ /K+ ion balance between Eukaryotic cells and their external environment.

5. The sodium/potassium (Na+ /K+) pump maintains a high concentration of Na+ ions outside the cell, and a high concentration of K+ ions inside the cell. This is particularly important in muscle contractions, nerve impulses and the absorption of nutrients from the gut.

6. The Na+/K+ ion pump moves Na+ ions out of the cell, and K+ ions into the cell, against their concentration gradient, using ATP to supply the energy needed.

7. In plants, active transport enables roots to absorb mineral ions from the soil, which are therefore more concentrated inside plant cells than in the soil.

8. This requires ATP energy from aerobic respiration, and therefore roots need oxygen to allow mineral uptake and a waterlogged (thus anaerobic) soil will kill most roots.