Movement of substances
Diffusion is the net (overall) movement of particles from an area of high concentration to an area of lower concentration.
When you put a drop of red dye into a glass of water, the dye diffuses until it is at equal concentration everywhere in solution.
A difference in concentration of a particle between two areas is called a concentration gradient. Diffusion occurs down a concentration gradient (from high to low concentration).
Diffusion is a passive process; it does not require energy from the cell.
Diffusion is fundamental to how organisms function:
- Proteins diffuse within the cytoplasm of cells.
- Carbon dioxide diffuses from the air into cells in the leaf for photosynthesis.
- Oxygen diffuses from the blood into tissues for respiration.
Organisms can control concentration gradients to determine the direction of diffusion.
Osmosis is the diffusion of water molecules across a semipermeable membrane.
Osmosis is a special kind of diffusion that applies only to water molecules.
Like diffusion, osmosis is a passive process:
Water diffuses from an area of low solute concentration (high water potential) to an area of high solute concentration (low water potential).
Low solute concentration = dilute solution = high water potential!
High solute concentration = concentrated solution = low water potential!
Zero solute concentration = pure water = zero water potential!
A semipermeable membrane only allows certain molecules to pass through. In the case of osmosis, water is able to move through, but large molecules like proteins are not.
Visking tubing is often used as a semi-permeable membrane in experiments.
Tonicity describes the relative solute concentration of a cell and its surroundings.
There are three types of tonicity:
- A hypotonic solution has a low solute concentration relative to the cell. When a cell is in a hypotonic solution, water will enter.
- A hypertonic solution has a high solute concentration relative to the cell. When a cell is in a hypertonic solution, water will leave.
- Isotonic solutions have the same concentration as the cell. There is no net movement of water in or out of the cell.
An animal cell will burst when in a hypotonic solution. Plant cells will not burst, because they are protected by cell walls. Instead, they become turgid (stiff).
Stems of non-woody plants can stand upright due to turgidity.
Animal cells shrivel and become flaccid (loose) in hypertonic solutions. Plant cells first become flaccid and then undergo plasmolysis. Plasmolysis is the shrinking of the cytoplasm away from the cell wall.
Active transport is an energy-consuming process that transports molecules against their concentration gradient.
Active transport moves molecules from an area of low concentration to an area of high concentration.
This is the opposite direction to movement by diffusion!
Cells require molecules that have a higher concentration inside the cell than outside. These molecules cannot diffuse into the cell:
- Root hair cells actively transport ions from the soil. This is important for water uptake in plants.
- Villi in the small intestine actively transport glucose from the gut. This maximises nutrient absorption from food.