Diffusion, osmosis and active uptake (Combined Science)
Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration.
Diffusion occurs along the concentration gradient.
A concentration gradient is the difference in concentration between a region of higher concentration of a substance and a region of lower concentration of the substance.
Whenever a concentration gradient exists, diffusion will take place until the particles are evenly distributed throughout the region.
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Osmosis is a special type of diffusion, but however take note of the following key aspects:
- diffusion is the movement of any type of molecules, whilst osmosis is specifically the movement of water molecules.
- diffusion occurs anywhere, whilst osmosis occurs across a partially permeable membrane.
A semi-permeable, partially permeable or selectively permeable membrane is a membrane that allows certain substances to pass through it whilst blocking others.
Water potential is a measure of the tendency of water molecules to move from one region to another. The more concentrated a solution is, the lower the number of freely moving water molecules present, hence the lower the water potential of the solution. Therefore, a dilute solution has a higher water potential than a concentrated solution and pure water has the highest water potential. This means that water will move from a dilute solution to a concentrated solution during osmosis.
Turgidity of plant cells
A plant cell has a large central vacuole which occupies most of its space. When a plant cell is immersed in a solution of higher water potential, such as pure water, water molecules enter the cell by osmosis. This causes the vacuole to increase in size exerting pressure on the cell wall. The cellulose cell wall of a plant cell is strong and rigid enough to withstand to pressure and prevent the cell from bursting. The cell becomes very firm or turgid.
Turgidity is the firmness of a cell when the cell is fully expanded due to the absorption of water.
Turgid cells are firm and support the plant to maintain its shape, resilient and strength
Plasmolysis of plant cells
When a plant cell is immersed in a solution with a lower water potential, such as a highly concentrated solution, water diffuses out of the cell, and the turgor pressure of the cell is lost. The vacuole shrinks and the cell stops exerting pressure on the cell wall. This causes the cell to become flaccid and the cell membrane to pull away from the cell wall.
The phenomenon in which the cell surface membrane pulls away from the cell wall is called plasmolysis.
Plasmolysis is the shrinking of the cell membrane away from the cell wall of a plant due to loss of water by osmosis.
This causes the cell membrane to pull away from the cell wall. Further water loss results in the cell wall totally collapsing. This results in the wilting of the plant.
Besides absorbing water, plants also need to absorb mineral salts from the soil. Some of these salts cannot be absorbed by diffusion because they are in very dilute solution in the soil than in the plant sap. Active transport (also known as active uptake) enables the cells of plant roots to take up salts from this dilute solution against the concentration gradient. However, unlike diffusion which requires only a concentration gradient, active uptake requires energy.
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using the energy from respiration.