Transport across membranes: Osmosis Flashcards
What type of molecules does osmosis involve?
Water molecules
What is osmosis?
Osmosis is the net movement of water from a region where it has a higher water potential to a region where it has a lower water potential, through a semipermeable membrane
What is water potential represented by and what is it measured in?
Water potential is measured in units of pressure, kiloPascals and represented by by the greek letter Psi
What is water potential?
Water potential is the pressure created by water molecules
What is a solute? What added to a solute forms a solvent?
A solute is any substance that is dissolved in a solvent. A solvent and a solute come together to form a solvent
Under standard conditions, what are the values of pressure, water potential and temperature?
Water potential is zero, pressure is 100 kiloPascals and temperature is 25 degrees celscius
How do you determine the water potential of cells or tissues?
To find the water potential of cells/tissues you place them in a series of solutions with different water potentials. When there is no net gain or loss of water from the cell/tissue, you will know the water potential inside the cells/tissues is the same as the external solution
What lowers the water potential of a solution?
By adding the a solute to a solution, the solution will get a lower water potential level
How does osmosis occur?
The water molecules within the solution with a low concentration of solute molecules (high water potential) will diffuse, down the water potential gradient to the solution with a high concentration of solute molecules (lower water potential).
Once the water potential on either side of the plasma membrane is equal, an equilibrium will be established and there will be no net movement of water
Does the selectively permeable membrane allow water molecules to diffuse through or the solute molecules?
The selectively permeable membrane only allows water molecules across, not solute molecules
Why do animal cells burst when placed in pure water? (red blood cells)
When red blood cells are placed in pure water they absorb water through osmosis because of its low water potential. The cell surface membrane can then not stretch to a great extent leading to the cell bursting and releasing its contents. This is referred to as haemolysis in red blood cells
What occurs when a red blood cell is placed in a solution with a water potential lower than itself?
When a red blood cell is placed in a solution with a water potential lower than its own, water leaves by osmosis and the cell shrinks and shrivels
How do animal cells prevent themselves from bursting?
To prevent their cells from bursting, animal cells normally live in a liquid which has the same water potential as the cells
For example, red blood cells and blood plasma have the same water potential
Why does the plant cell not burst as easily as the animal cell?
Plant cells cannot control the fluid composition around their cells so when water enters through osmosis the protoplast swells and starts to press on the cell wall. The cell wall cannot expand too much, so pressure build up on it resisting the entry of further water molecules. This eventually leads to the plant cell becoming turgid
What occurs when a plant is described as being in incipient plasmolysis?
When a plant is placed in a solution with a lower water potential than itself, water leaves by osmosis decreasing the volume of the cell and the cell reaches a stage where the protoplast no longer presses on the cellulose cell wall
