Definition: water potential is the tendency of water molecules to move from 1 place to another. (dilute solution has higher WP than concentrated solution) #BiologyDefinitions Definition: Osmosis is the process where water molecules move from a region of higher water potential to a region of lower water potential through a partially permeable membrane, down a water potential gradient When PPM separates 2 solutions of different water potentials - water potential gradient is established.
Factors Affecting Rate of Osmosis
- Water potential gradient
- Distance over which water molecules move
- Surface area to volume ratio
*EXAM TECHNIQUES:
- Must say - DOWN the WP/concentration gradient OR AGAINST the concentration gradient
- Osmosis - must say water molecules ONLY
- Diffusion - must add it can occur across PPM too
- Water moving into cell sap of plant in large central vacuole via cell membrane (PPM)
- Water moved into cytoplasm of animal cell through cell membrane (PPM)
- State exactly where the WP is, vacuole or cytoplasm. Else you get 0.
- DO NOT SAY ‘cell increase in size’. Say: ‘cytoplasm/vacuole increases in size’
What happens to cells in solution with higher water potential? Follow answer phrasing EXACTLY. Overview - water molecules move from region of higher WP (outside cell) to region of lower WP (inside cell)
Plant cell:
Expands or swells, becomes turgid
- Cell sap in the large central vacuole of the plant cell has a lower water potential compared to the outside of the cell.
- By osmosis, there is net movement of water molecules moving from outside the cell to the cell sap in the large central vacuole in the plant cell through the partially permeable cell membrane.
- As water molecules enter the cell, the large central vacuole increases in size and pushes the cytoplasm against the cell wall.
- The cell expands or swells, and has become turgid.
does not burst, as it is protected by inelastic and rigid cell wall. refer to function of cell wall - Cell Wall turgidity of cell with water - known as turgor pressure exerted by the water in the vacuole - known as turgor pressure
Animal cell:
Expands or swells, and the cell membrane eventually bursts
- Cytoplasm of the animal cell has a lower water potential compared to the outside of the cell
- By osmosis, there is net movement of water molecules moving from outside the cell to the cytoplasm of the animal cell through the partially permeable cell membrane.
- As water molecules enter the cell, the cytoplasm increases in size and pushes against the cell membrane.
- Unlike plant cells, the animal cell does not have an inelastic and rigid cell wall to protect it from injuries. Thus, the cell membrane bursts.
Note: water enters cytoplasm rather than vacuole in animal cells as there is no large central vacuole in animal cells.
What happens to cell in solution with same water potential → no change in size or shape →movement of water molecules in both directions is the same → there is no net movement of water molecules in or out of the cell. → equilibrium
What happens to cell in solution with lower water potential: Overview: water molecules move from region of higher WP (inside the cell) to region of lower WP (outside of cell)
Plant cell:
decrease in size and become flaccid/limp
- Cell sap in the large central vacuole of the plant cell has a higher WP than the outside of the cell.
- By osmosis, there is net movement of water molecules from the cell sap in the large central vacuole of the plant cell to the outside of the plant cell through the partially permeable cell membrane.
- Cell decreases in size and becomes flaccid/limp
- As water molecules move out of the cell through the partially permeable cell membrane, vacuole decreases in size. → cytoplasm shrinks away from the cell wall (cell membrane shrinks with the cytoplasm too!) → shrinkage of cytoplasm and cell membrane away from the cell wall is known as plasmolysis → The cell is said to be plasmolyzed and (can be restored to ori. State by placing it in solution with higher WP)
Animal cell: → cell shrinks and spikes appear on the cell
- Cytoplasm of animal cell has a higher WP than the outside of the cell
- By osmosis, there is net movement of water molecules moving from the cytoplasm of the animal cell to the outside of the animal cell through the partially permeable cell membrane
- Cytoplasm shrinks away from the cell membrane → this is called crenation
- Animal cell becomes dehydrated and will eventually die