biology igcse topic 3

1. Topic Three
Concentrationgradients
The idea of concentrations and gradients within them is
important when understanding the movement of
substances across cell membranes.
Concentration
When sucrose is dissolved in water:
the solute is sucrose
water is the solvent
The more particles there are in a certain volume, the more
concentrated those particles are.
Solutionone
Low solute concentration

2. Solutiontwo
High solute concentration
A solution with a low solute concentration has a high water
concentration, and a high water potential. Pure water has
the highest water potential.
A concentration gradient exists when there is a region of
high concentration leading to a region of low
concentration:
going from high to low concentration is going
down the concentration gradient
going from low to high concentration is going
against the concentration gradient
Diffusion
Dissolved or gaseous substances have to pass through
the cell membrane to get into or out of a cell. Diffusion is
one of the processes that allows this to happen.
Diffusion occurs when particles spread. They move from a

3. region where they are in high concentration to a region
where they are in low concentration. Diffusion happens
when the particles are free to move. This is true in gases
and for particles dissolved in solutions - but diffusion does
not occur in solids.
Particles diffuse down a concentration gradient, from an
area of high concentration to an area of low concentration.
This is how the smell of cooking travels around the house
from the kitchen, for example.
Diffusionexperiment
Potassium permanganate is placed into a beaker of water

4. Particles diffuse from an area of high concentration to an
area of low concentration
The contents of the beaker are now all the same
concentration
Examplesofdiffusioninlivingorganisms
Products of digestion, dissolved in water, can pass across
the wall of the small intestine by diffusion. Their
concentration is higher in the small intestine than their
concentration in the blood, so there is a concentration
gradient from the intestine to the blood.
Oxygen and carbon dioxide, dissolved in water, are
exchanged by diffusion in the lungs:
. oxygen moves down a concentration gradient
from the air in the alveoli to the blood
. carbon dioxide moves down a concentration

5. gradient from the blood to the air in the alveoli
The dissolved substances will only continue to diffuse
while there is a concentration gradient.
Gas exchange in the lungs happening in the alveoli
Osmosis
Osmosis is the diffusion of water molecules, from a
region of higher concentration to a region of lower
concentration, through a partially permeable membrane. A
dilute solution contains a high concentration of water
molecules, while a concentrated solution contains a low
concentration of water molecules.
Partially permeable membranes are also called
selectively permeable membranes or semi-permeable
membranes. They let some substances pass through
them, but not others.
The slideshow shows an example of osmosis:

6. Osmosisexperiment
The beaker contains water and sugar molecules
Water molecules pass through from solution one to two

7. Eventually the level on the more concentrated side of the
membrane rises, while the one on the less concentrated
side falls.
When the concentration is the same on both sides of the
membrane, the movement of water molecules will be the
same in both directions. At this point, the net exchange of
water is zero and there is no further change in the liquid
levels.
Osmosisincells
The results of osmosis are different in plant and animal
cells.
Plantcells
Plant cells have a strong cellulose cell wall on the outside
of the cell membrane. This supports the cell and stops it
bursting when it gains water by osmosis.
A plant cell in a dilute solution (higher water potential
than the cell contents)
Water enters the cell by osmosis. The
cytoplasm pushes against the cell wall
and the cell becomes turgid.

8. A plant cell in a concentrated solution (lower water
potential than the cell contents)
Water leaves the cell by osmosis. The
cytoplasm pulls away from the cell wall
(plasmolysis) and the cell becomes
flaccid and the plant wilts.
Turgid plant cells play an important part in supporting the
plant.
Animalcells
Animal cells do not have a cell wall. They change size and
shape when put into solutions that are at a different
concentration to the cell contents.
For example, red blood cells:
gain water, swell and burst in a more dilute
solution (this is called haemolysis)
lose water and shrink in a more concentrated
solution (they become crenated or wrinkled)
These things do not happen inside the body.
Osmoregulation involving the kidneys ensures that the
concentration of the blood stays about the same as the
concentration of the cell contents.
Activetransport

9. Active transport is the movement of dissolved molecules
into or out of a cell through the cell membrane, from a
region of lower concentration to a region of higher
concentration. The particles move against the
concentration gradient, using energy released during
respiration.
Sometimes dissolved molecules are at a higher
concentration inside the cell than outside, but, because
the organism needs these molecules, they still have to be
absorbed. Carrier proteins pick up specific molecules
and take them through the cell membrane against the
concentration gradient.
Examples of active transport include:
uptake of glucose by epithelial cells in the villi
of the small intestine
uptake of ions from soil water by root hair cells
in plants
Activetransportvsdiffusionandosmosis
Diffusion Osmosis
Active
transport

10. Down a
concentration
gradient
✓ ✓ ✗
Against a
concentration
gradient
✗ ✗ ✓
Energy needed
✗ ✗ ✓
Substance moved
Dissolved
solutes
Water Dissolved
solutes
Notes
Gases and
dissolved
gases also
diffuse
Partially
permeable
membrane
needed
Carrier
protein
needed