3.3 Osmosis Flashcards
Osmosis is a special case of…
Diffusion
Define osmosis.
The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane.
What is a partially permeable membrane?
A membrane which is permeable to some molecules but not others.
Cell-surface membranes and other plasma membranes such as those around organelles are partially permeable in the way that…
They are permeable to water molecules and a few other small molecules but not to larger molecules.
What is a solute?
Any substance which is dissolved in a solvent e.g water. The solvent and solute together form a solution.
What is water potential represented by?
The Greek letter psi (looks like a fork) measured in pressure. It is the pressure created by water molecules.
Under standard conditions of temperature (25 degrees) and pressure (100kPa) what is pure water said to have?
A water potential of zero
How would you lower pure waters water potential?
Add a solute
The water potential of a solution (water+solute) must always be less than…
Zero
The more solute added to a solution…
The lower it’s water potential
Water will move by osmosis from a region of higher water potential to one of…
Lower water potential
One way of finding the water potential of cells or tissues is…
To place them in a series 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 of the external solution.
The highest value of water potential is…
Zero (pure water)
Animal cells contain a variety of solutes dissolved in their watery cytoplasm. If a red blood cell is placed in pure water…
It will absorb water by osmosis because it has a lower water potential. The cell-surface membrane (very thin- 7nm) cannot stretch and it will break, releasing its contents.
What is it called when red blood cells burst and release their contents?
Haemolysis
What is done to prevent animal cells from bursting?
They are bathed in a liquid which has the same water potential as the cells.
Blood cells have the same water potential as…
Blood plasma
If water potential of external solution and cell solution are equal what happens to the cell?
There is no change - water neither enters nor leaves the cell.
If water potential of external solution is lower than the water potential of the cell solution what happens to the cell?
Water leaves the cell, it shrinks and the haemoglobin is more concentrated, giving the cell a darker appearance.
In plant cells, what is the central vacuole?
The central vacuole contains a solution of salts, sugars and organic acids in water.
In plant cells, what is the protoplast?
It consists of the cell-surface membrane, nucleus, cytoplasm and inner vacuole membrane
What is the cellulose cell wall?
A tough, inelastic covering that is permeable to even large molecules.
Plant cells cannot control the composition of the…
Fluid around them unlike animal cells - they are permanently bathed in almost pure water which is constantly absorbed by the plants roots.
What does water entering a plant cell by osmosis cause the protoplast to do?
The protoplast swells and presses on the cell wall. The cell wall is only capable of very limited expansion - a pressure builds up which on it which resists the entry of further water - the cell is said to be TURGID.
