1. HUMAN AND SOCIAL BIOLOGY Movement of Substances
Across Cell Membrane
DIFFUSION ,OSMOSIS
AND
ACTIVE TRANSPORT

2. Movement of Substances
Cells need to move different substances into and out of the cell in order to function properly.
Diffusion ,osmosis and active transport are two processes by which this takes place

3. Diffusion
The movement of molecules from a region where they are in higher concentration to a region where they are in lower concentration.
All substances are made of particles.

4. In gases and liquids these particles move around randomly.
Diffusion take place when particles are not evenly distributed.
Eventually the particles will spread out evenly (equilibrium is reached).

5. The particles move along a concentration gradient.
The bigger the difference in concentration between two regions, the steeper the concentration gradient.
Diffusion takes place more quickly when there is a steeper concentration gradient.

6. Diffusion and cell membrane
Substances pass into and out of a cell through a cell membrane.
The cell membrane is a barrier
with tiny pores or holes in it.
Small molecules, such as oxygen
,carbon dioxide, water and
glucose, are able to
diffuse through.
Large molecules, such as
starch and proteins cannot.

7. Cell membrane is called partially permeable, as it allow only some substances to pass through.

8. Diffusion In Human Body
After breathing in, oxygen diffuses into the small blood vessels of the lungs.
Carbon will diffuses from blood vessels to the air sacs in the lungs.
Digested food also passes from
the small intestines into the
blood capillaries by diffusion.

9. Concentration Gradient
This is when there is a place of high concentration and a place of low concentration.
Diffusion allows substances to move from a place of high concentration to a place of lower concentration (down a concentration gradient).
Until an equilibrium is reached.

10. Adaptations That Speed up Diffusion
Diffusion distance are short
Membranes in the lungs are very thin for example.
Hence oxygen and carbon
dioxide can diffuse between
the blood and air sacs.

11. Concentration Gradients Are Maintained
Glucose molecules that cross from the gut into the blood are quickly removed by circulating blood.
Hence there will always be a
concentration gradient.

12. Diffusion Surfaces Are Large
The surface area of the air sacs in the lungs are large because of the millions of air sacs which are present.

13. Osmosis
Osmosis is a special case of diffusion.
Osmosis involve the movement of water through a partially permeable membrane.
This means that only some substances can pass through the membrane.
Small molecules, such as
water, can pass through
but larger molecules
cannot.

14. Osmosis is water moving from a region where its concentration is high (in a dilute solution) to where its concentration is low (to a more concentrated solution).

15. Cell in Water
When you place a cell in water, water will enter the cell by osmosis.
The cell will become turgid or stiff.
Animal cells will swell up and burst if too much water enter the cell.
The strong cell wall
stop plant cells from
bursting when water
enters them.

16. Cells in a Concentrated Solution
Water will leaves the cell by osmosis.
Animal cell will shrink
In plant cell, the strong cell
wall will stay the same.
The inside of the cell shrinks away from the cell wall.
Cell is plasmolysed.

17. Active Transport
The movement of a substance across a cell membrane, against concentration gradient, with the use of energy provided by the cell.
At times the cell needs something that is present in low concentration outside of the cell.

18. There are special molecules in the cell membrane that can grab glucose molecules in the cell membranes.
They drag them through the cell membrane and into the cell.
This uses up energy.
The energy is provided by respiration in the cell.