1. Diffusion, osmosis and active transport

2. INDEX Diffusion “Test yourself!” – Diffusion Osmosis Osmosis in cells
Video – Diffusion and osmosis activity
Active Transport

3. Diffusion
Diffusion occurs when particles spread. They travel from an area of high concentration to an area of low concentration. Diffusion happens when the particles are free to move; this is true in gases or liquids.
The “scent particles” from this hamburger are in high concentration here:
Eventually they will “diffuse” out into this area of low concentration:

4. Examples of diffusion in living systems
Location
Substances
From To
Adaption
lung
oxygen
air space
red blood cells
alveoli (moist lining, good blood supply)
carbon dioxide
blood plasma
digestive system
food molecules eg glucose
stomach, intestine
blood
villi, microvilli, good blood supply
uterus
food molecules and oxygen
mother's blood supply
foetus's blood supply
placenta with villi, thin walls and good blood supply
leaf
leaf cell
thin leaves, with air spaces and spongy layer

5. Diffusion in Uterus

6. Diffusion in leaves

7. Test yourself!

8. Osmosis
Osmosis is a “special kind of diffusion”. It’s when water diffuses from a concentrated area to a less concentrated area through a partially permeable membrane (i.e. one that allows water to move through but not anything else):
In this example the water molecules will move from left to right (along the concentration gradient) and gradually dilute the sugar solution.
Water
Sugar solution

9. Strong sugar solution Medium sugar solution Weak sugar solution
Potato cells
Weak sugar solution

10. Important vocabulary - higher tier
Lysis – bursting an animal cell by osmosis
Crenation – shrinking an animal cell by osmosis
Turgid – a plant cell fully inflated with water
Plasmolysed – a plant cell that has lost water causing the cell membrane to be pulled away from the inside of the cell wall
Flaccid – a plant cell that is limp through a reduction of pressure inside the cell.

11. Osmosis in cells Osmosis takes place in all cells
Osmosis in cells Osmosis takes place in all cells. The cell membrane is partially permeable.
Osmosis in red blood cells.
If a red blood cell is placed in water, water enters the cell by osmosis. Because the membrane is quite weak the cell will burst as the volume and therefore the pressure in the cell increases. Red blood cells shrink when placed in concentrated solutions of sugar as water moves out of them by osmosis. This makes the cells appear wrinkled when viewed through a microscope.
This does not happen inside the body because the kidneys make sure the concentration of the blood stays about the same as the concentration of the solution inside the red blood cell.

12. Osmosis Osmosis in plant cells
Plant cells have a strong rigid cell wall on the outside of the cell membrane. This stops the cell bursting when it absorbs water by osmosis. The increase in pressure makes the cell rigid. This is useful as plants do not have a skeleton. Instead the leaves and shoots can be supported by the pressure of water in their cells. If plant cells lose too much water by osmosis they become less rigid and eventually the cell membrane shrinks away from the cell wall.

13. Osmosis
Loss of water makes the cell limp and shrinks the cell membrane away from the cell wall.
Water entering the cell by
osmosis inflates the cell and makes it rigid

14. Here’s a video for you to understand diffusion and osmosis activity better:

15. Active Transport
Active transport is when substances are absorbed against a concentration gradient (like a ball being rolled up a hill). This takes place in the opposite direction to normal diffusion.
This is how nitrates are taken in by root hair cells:
Root hair cell already has a large concentration of nitrate ions
Root hair cells “pull” more nitrate ions in
This process needs energy (just like rolling a ball up a hill would). This energy is provided by respiration.

16. Active Transport
In humans, active transport takes place during the digestion of food in the small
intestine. Carbohydrates are
broken down into simple sugars
such as glucose. The glucose is
absorbed by active transport into
the villi, to be passed into the
bloodstream and taken around
the body.