4.3 Osmosis Flashcards
Definition of Osmosis
. Movement of water from a region of high water potential to a region of low water potential.
. This occurs through a selectively permeable membrane
What does selectively permeable mean
. Eg in plasma membranes they are permeable to water molecules and a few other small molecules, but not to larger molecules
What is a solute
What is a solution
. A substance that is dissolved in a solvent eg water
. The solute and solvent together form a solution
What is water potential
What is it measured in
Under standard conditions (25degrees, and pressure 100kpa) what is the water potential
. The pressure created by water molecules
. Measured in kiloPascals (kPa)
Water potential would be zero
What happens to the water potential if a solute is added to pure water
What must the water potential always be in a solution
So what does this mean for the movement of water
. The addition of a solute to pure water lowers its water potential
. The more solute you add, the lower its water potential.
It must always be less than zero, so negative because pure water has a water potential of 0
Water will always move from an area of higher (less negative) water potential to an area of lower(more negative) water potential.
What is one simple way of finding the water potential of cells and tissues
. Place them in lots of solutions of different water potentials.
. Where there is no net gain or loss of water from the cells or tissues, the water potential inside the cells or tissues must be the same as that as the external solution
What happens with the particles when a selectively permeable membrane separates two solutions.
If on the left side is the low concentration of solute molecules and on the right has a high concentration of solute molecules.
Give their characteristics
. Both the solute and water molecules are in random motion due to their kinetic energy
. However the selectively permeable membrane only lets water molecules across it, not solute molecules.
. The water molecules diffuse from the left side which has higher water potential, to the right side which has lower water potential.
So down the potential gradient.
. At the point where the water potentials are equal on both sides, a dynamic equilibrium is reached and there is no net movement of water
What would happen if a red blood cell from animals was placed in water of a higher water potential.
. It will absorb water by osmosis because it has a lower water potential.
. The cell surface membrane of it is very thin (7nm) and although they are flexible, they can’t stretch to any great extent.
. So they swell up
. They do not become turgid though because they don’t have a cell wall
. As a result the membrane will break, bursting the cell and releasing its contents.
This is called haemolysis.
What is done to prevent red blood cells bursting in animals
Animal cells usually live in a liquid of the same water potential as the cells
Eg liquid in blood plasma as this and red blood cells have the same water potential
What happens if an animal cell is placed in water with a lower (more negative) water potential
. The water will move out of the cell
. So the cell shrinks and shrivels up
It doesn’t become flaccid though because there is no cell wall
. The haemoglobin is more concentrated so will give cell a darker appearance
What happens in a plant cell if it is placed in water of a higher
(less negative) water potential
.The water will enter the cell
. The plant cell will swell up and become turgid
However unlike animal cells, they can control the composition of fluid around their cells.
What does the protoplast of a plant cell do for managing water potential
- When lots of water is entering the cell through osmosis, the protoplast swells up and presses on the cell wall
- Because the cell wall is capable of only very limited expansion, a pressure builds upon it that resists the entry of further water
. So the plant becomes turgid
What happens in a plant cell if it is placed in a solution with a lower
(more negative) water potential than the cell
. Water leaves the cell by osmosis
. The protoplast shrinks and pulls away from the cell wall
. So the cell is plasmolysed
The volume of the cell decreases, as the protoplast no longer pushes on the cell wall so incipient plasmolysis occurs.
(so 50% of the cells are plasmolysed) so osmotic potential inside cell matches that of the medium
Then, further loss of water causes cell contents to shrink further and the protoplast pulls away from the cell wall so the cell is plasmolysed.
What happens if a plant cell is placed in a solution with the same water potential as it
. Water neither enters or leaves
. So there is no change in the protoplast
.So incipient plasmolysis occurs where the protoplast isn’t pushing on the cell wall and is beginning to pull away from it.
What happens if a red blood cell is placed in a solution with the same water potential of it
. Water neither enters or leaves the cell
. So there is no change in the state of the cell and it stays normal
