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Movement of Water through membranes
There are three types of membranes
Permeable
allows all substances to pass through’
Impermeable
allows no substances to pass through’
Semi-permeable
allows only some molecules to pass through
eg Cell Wall
eg Cell Membrane
more correctly referred to as a PARTIALLY PERMEABLE solution
Components of a solution
Solute – the solid molecules dissolved in a liquid (usually water)
Solvent – the liquid in which the solids are dissolved
There are various types of solution
Concentrated:
lots of solute in very little solvent (a strong solution)
Dilute:
very little solvent in a lot of solution (a weak solution)
Isotonic:
when two solutions have the same amount of solute and solvent, they
are said to be isotonic to each other
Methods of moving molecules
Diffusion:
the movement of molecules from where they are plentiful to where they
are scarce e.g. the movement of O2 from the air in the alveoli into the
blood capillaries, the movement of digested food across the villi...
Diffusion is passive – it uses no energy
Water molecules are scarce in concentrated solutions and plentiful in
dilute solutions
Osmosis:
a special case of diffusion - it is the diffusion of water molecules
Osmosis is the diffusion of water molecules
from a dilute solution to a concentrated solution
across a partially permeable membrane
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What happens to animal cells in an isotonic solution?

Water molecules diffuse freely in and out through the partially permeable lipoprotein
membrane all the time

There is no change in the volume of the cytosol

OSMOREGULATION ensures that body cells are isotonic to the fluids which
surround them - this is part of the body's control system (see homeostasis)
What happens to animal cells in a strong solution?

Water molecules diffuse out of the cytosol towards the stronger solution

The cytosol loses volume and the lipoprotein membrane shrinks

If this happens in red blood cells, they are said to be CRENATED
What happens to animal cells in a dilute solution?

Water molecules diffuse into the cytosol towards the stronger solution

The cytosol volume swells and pushes against the lipoprotein membrane

If this continues, the lipoprotein membrane may rupture, the cell bursts

If this happens in red blood cells, it is called HAEMOLYSIS
What happens to plant cells in an isotonic solution?

Water molecules diffuse freely in and out through the semi permeable lipoprotein
membrane and permeable cell wall all the time

There is no change in the volume of the cytosol
What happens to plant cells in a strong solution?

Water molecules diffuse out of the cytosol towards the stronger solution

The cytosol loses volume, the lipoprotein membrane shrinks away from the cell wall

The cell is said to be FLACCID – this process is called PLASMOLYSIS

Plasmolysis can cause the plant to wilt
What happens to plant cells in a dilute solution?

Water molecules diffuse into the cytosol towards the stronger solution

The cytosol volume swells and pushes against the lipoprotein membrane

The strong cellulose cell wall prevents the membrane from bursting

The cell is said to be TURGID

If all cells in a plant are turgid, it gives strength to support the plant upright
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