Osmosis Flashcards
What is osmosis?
Diffusion of water through partially permeable membrane from dilute to concentrated solution
Passive - no energy required
What is water potential?
Pressure exerted by water molecules as they collide with membrane
Only solvents moved - solutes are too big
What is the water potential of water?
0kPa, at 25’C and atmospheric pressure 100kPa
What does presence of solutes do to water potential?
Decreases water potential, so all solutions have negative water potential
More concentrated the solution, more negative the water potential
What does high water potential mean?
More water molecules, less
More pressure from water molecules
Water molecules move until when?
Equilibrium is reached, where water potential is equal on both sides.
Water molecules will move equally between 2 solutions
What are the effects of osmosis?
- Movement of water into solution increases volume. In an enclosed space, such as a cell, this will increase hydrostatic pressure, fatal to cells
- Water moving out of cell decreases volume of cell
What happens when water potential of external solution is HIGHER than cell solution?
- Water moves into cell by osmosis, increasing hydrostatic pressure in cell
- Cell has thin cell surface membrane (7nm) + no cell walls. Cant stretch + withstand pressure
- Cytolysis = Splitting of cells, making them burst
What happens when water potential of external solution is LOWER than cell solution?
- Cell loses water to solution by osmosis, down WP gradient
- Causes crenation = reduction in volume of cells, causing them to shrivel/‘pucker’
How do u prevent cytolysis or crenation?
Multicellular organisms usually have control mechanisms to make sure water potential of external solutions matches that of cytoplasm
What happens when water potential of external solution is equal to cell solution? PLANT CELLS
Water constantly enters + leaves cell, no change
What happens when WP of external solution is higher than cell solution? PLANT CELLS
- Increased hydrostatic pressure pushes membrane against cell wall
- Pressure against cell wall = turgor
- As turgor pressure increases, it resists entry of further water = cell is turgid
What happens when WP of external solution is lower than cell solution? PLANT CELLS
- Water lost from cells by osmosis
- Leads to reduction in vol of cytoplasm
- Eventually pulls cell surface membrane away from cell wall - cell is plasmolysed
Explain plasmolysed:
Cell surface membrane pulled away from cell wall
What is turgor?
Pressure against cell
