1.4 Membrane transport

Passive transport

Particles can move across the membrane by simple diffusion, facilitated diffusion or active transport
- Osmosis
- Diffusing
Passive \= no energy needed

Diffusion

The mov of particles from an area of higher concentration to an area of lower concentration
- Non- polar molecules

Facilitated diffusion

- Carrier proteins are needed for diffusion to happen
- Large molecules can't get across the membrane via simple diffusion
- Faster than simple diffusion
- Size and shape of protein carriers and channels determine what substances can cross the membrane
- Polytopic proteins recognise a specific molecules and help it move across the membrane
- Direction it moves depend on the concentration gradient
- Rate levels off when total saturation of carriers occurs
- Polar molecules

Active transport

Movement of particles from an area of Low concentration to an area of high concentration with the use of energy from ATP
- Polar molecules
- Requires special membrane proteins, called protein pumps

Factors with affect diffusion

Temperature
- Molecules will diffuse faster if the temperature is higher

Particle size
- Smaller molecules diffuse faster than larger ones

Surface area
- An increase in SA allows more molecules to diffuse

Concentration gradient
- The bigger the gradient the faster the diffusion

Solute

A substances which dissolves in a solvent

Solvent

A substance which dissolves solutes

Hypertonic

Less water molecules

Hypotonic

More water molecules

Isotonic

Same amount of water molecules

Osmolarity

The measure of the solute concentration

Osmosis

The passive net movement of water molecules from regions of Low solute concentration to high solute concentration through a selectively permeable membrane

High solute vs. Low solute

High solute —\> Concentrated, + solute than water
Low solute —\> Diluted, + water than solute

Aquaporin

- Found in some cells such as those in the collecting duct
- An integral protein
- Acts as a pore in the membrane that speeds the movement of water molecules

Importance of Osmotic control

- Prevents damage to cells and tissues
- Essential that the osmolarity of the saline solution is the same as in the cytoplasm of the cells of the tissue to prevent any osmosis that would damage the cells

Plasmolyzed

- When too much water is lost from a cell

Flaccid

- When an equal amount of water is lost and gained

Turgid

- When too much water enters the cell

Endocytosis and Exocytosis

- Vesicles move materials within cells
- The fluidity of membranes allows materials to be taken by Endocytosis or taken out by exocytosis

Endocytosis

The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle

Pinocytosis - the taking in of water molecules
Phagocytosis - the taking in of solid molecules

Exocytosis

- The release of substances from a cell when a vesicle joins with the cell plasma membrane
- Can be waste products (excretion) or useful substances (secretion)

The role of vesicles

- Vesicles are small sacs of membrane containing a drop of fluid of varying contents
- These are made by the pinching off portions of the cell membrane

Vesicle process

Endocytosis:
1. Vesicle is formed
2. Moves away from pm & into the cytoplasm
3. Pm encloses and engulfs the solid particles

Exocytosis:
1. Vesicle contains substances to be excreted
2. Approaches pm and fuses
3. Pm pore opens and releases content

Facilitated diffusion with Potassium & Sodium channels

1. Nerve impulse pass through the axon
2. Sodium ions use sodium channels to move into the axon
3. Known as depolarisation
4. Voltage change causes potassium channels to open
5. Allows potassium ions to diffuse out of the axon
6. Known as repolarisation
7. We want a higher concentration of Sodium ions OUTSIDE of the cell and a higher concentration of Potassium INSIDE of the cell

Active transport with Sodium Potassium pumps

- Facilitated diffusing for the moving IN of Na ions from the cell and the moving OUT of K ions
- Sodium- Potassium protein pumps are needed for the active transport

1. The proteins that conduct active transport are often called pumps
- Force molecules or ions to move against the concentration gradient
2. The Na-K pump has a binding site for 3Na and 2K ions
3. 3Na ions are positioned within the carrier protein
- An ATP molecules splits, releasing phosphate
- Phosphate binds to a location on the exterior of the carrier protein causing the protein to change shape
- 3Na ions are realised from the change in shape on the other side of the pm
4. 2K ions position themselves within the carrier protein
- Undergoes another change in shape
- Phosphate molecule is released
- Carrier protein expels the K ions into the interior of the cell
5. Carrier resumes its initial shape, completing the cycle
6. For every 3Na ions leaving the cell, 2K ions enter
7. Both have a positive charge
- Unequal movement causes an electrical gradient top dev across the pm of the cell
- Number of cellular process, including the generation of nerve impulses use this electrical gradient