Active transport is the energy-assisted movement of substances against a diffusion or electrical gradient. This process requires enzymes and a ‘proton-pump’ embedded in the plasma membrane. The pumps are energized by ATP molecules—a cellular energy storage molecule.
Imbibition is the swelling of tissues, alive or dead, to several times their original volume. This is a result of the electrical charges on materials in suspension (colloidal) such as minerals, cellulose and starches attracting highly polar water molecules which then move into the cell. This swelling process is the initial step in the germination of seeds.
Plasmolysis is the loss of water via osmosis and accompanying shrinkage of the protoplasm away from the cell wall. When this occurs, the cell is said to be plasmolyzed. This process can be reversed if the cell is placed in freshwater and the cell is allowed to regain its turgor pressure. However, as with anything living, there is a point of no return and permanent or fatal damage to the cell can occur.
Osmosis in plant cells is basically the diffusion of molecules through a semipermeable, or differentially permeable, membrane from a region of higher solute concentration to a region of lower solute concentration. The application of pressure can prevent osmosis from occurring. Plant physiologists like to describe osmosis more precisely in terms of potentials. Osmotic potential is the minimum pressure required to prevent fluid from moving as a result of osmosis. Fluid will enter the cell via osmosis until the osmotic potential is balanced by the cell wall resistance to expansion. Any water gained by osmosis may help keep a plant cell rigid or turgid.
The turgor pressure that develops against the cell walls as a result of water entering the cell’s vacuole. This pressure is also referred to as the pressure potential. The crunch when you bite into a celery stick is as a result of the violation of the cell’s turgor pressure. The osmotic potential and pressure potential combined make up the water potential of a plant cell. If there are two cells next to each other of different water potentials, water will move from the cell with the higher water potential to the cell with the lower water potential. Water enters plant cells from the environment via osmosis. Water moves because the overall water potential in the soil is higher than the water potential in the roots and plant parts. If the soil is desiccated then there will be no net movement into the plant cells and the plant will die.
Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration. Think of it as a rebalancing. The molecules or ions are said to be moving along a diffusion gradient. If molecules or ions moving in the opposite direction are said to be moving against a diffusion gradient. Diffusion will continue until a state of equilibrium is reached. Rates of diffusion are affected by temperature and the density of the involved molecules among other things. In the leaves, water diffuses out via the stomata into the atmosphere.