1. Transport in living things

2. 25.1 Diffusion How are you able to smell various aromas from food?
The aromas come from vapour from the food.
Molecules in the vapour move about randomly.
They bump into one another and spread out quickly.
As a result, some molecules enter your nostrils.
The spreading out of molecules is brought about by diffusion.
Diffusion in the overall movement of molecules from a region of higher concentration to a region of lower concentration.

3. Diffusion in non-living systemsA B
permeable membrane
10 % sucrose solution
2 % sucrose solution
sucrose molecules
water molecules
Movement of sucrose molecules from A to B
Movement of water molecules from A to B
Movement of sucrose molecules from A to B
Movement of water molecules from A to B
Hence, there is a net movement of sugar molecules from solution A to solution B and a net movement of water molecules from solution B to solution A. Eventually, both solutions will have the same concentration of sugar and water molecules.
Also, solution B has a higher concentration of water molecules. Thus, more water molecules will diffuse from solution B to solution A.
Since solution A has a higher concentration of sugar, more sugar molecules will diffuse from solution A to solution B.
Both types of molecules are free to move across the membrane as the particles are in constant random motion.
Solutions A and B are separated by a permeable membrane, which allows both small and large molecules to pass through. Both the solvent (water) and the solutes (the sucrose molecules) can pass through it.

4. Diffusion in living organisms
Diffusion in living organisms occurs continuously and it does not always take place across a membrane (e.g. cell membranes).
Diffusion is important in moving materials in and out of the cells.

5. Diffusion in living organisms
Diffusion in the absence of membranes
air spaces
Diffusion occurs during photosynthesis in the absence of membranes.
Carbon dioxide enters the leaves through the stomata, moving into air spaces in the leaves.
This is because the carbon dioxide concentration is higher outside the leaves than in the air spaces inside the leaves.
Similarly, oxygen given off during photosynthesis diffuses out of leaf air spaces as there is a lower concentration of oxygen outside the leaves.
cross-section of part of leaf
high concentration of oxygen
low concentration of carbon dioxide
carbon dioxide diffuses in through stomata
oxygen diffuses out through stomata

6. Diffusion in living organisms
Diffusion through a partially permeable membrane
A partially permeable membrane is a membrane that allows some molecules to pass through but not others.
The cell surface membrane in plants is an example of a partially permeable membrane.
Gases (e.g. oxygen) and smaller molecules (e.g. sugar) diffuse into the plants cells through the membrane.
In humans, gaseous exchange occurs in the lungs by diffusion.
When we breathe in, oxygen enters the air sacs in our lungs.
The oxygen concentration is higher in the air sacs than in the blood surrounding them.
Thus, oxygen diffuses out of the air sacs into the bloodstream.

7. Diffusion in living organisms
Diffusion through a partially permeable membrane
In humans, gaseous exchange occurs in the lungs by diffusion.
During respiration in the cells, carbon dioxide is produced as waste.
Carbon dioxide is carried in the bloodstream leading to the lungs.
It then diffuses out into the air sacs, and leaves the body when we breathe out.

8. Diffusion in living organisms
Diffusion through a partially permeable membrane
Carbon dioxide diffuses out of the bloodstream and into the air sac.
Blood leaving blood vessel has high concentration of oxygen and low concentration of carbon dioxide.
Blood entering blood vessel has high concentration of carbon dioxide and low concentration of oxygen.
lungs
Oxygen diffuses from surrounding air into the air sac and bloodstream.
tube
cluster of air sacs
air sac

9. 25.2 Osmosis
Osmosis is the net movement of water molecules from a region of higher concentration of water molecules (higher water potential) to a region of lower concentration of water molecules (lower water potential) through a partially permeable membrane.

10. Osmosis in non-living systems
Water molecules can pass freely through the membrane A B
Sucrose molecules are too big to pass through its pores.
Partially permeable membrane
10% sucrose solution (lower concentration of water molecules; lower water potential)
5% sucrose solution (higher concentration of water molecules; higher water potential)
During osmosis, the water molecules will flow from solution B with higher concentration of water molecules (higher water potential) to solution A with lower concentration of water molecules (lower water potential).
As water molecules flow, the volume of solution A rises and the volume of solution B drops. When the concentrations of both solutions A and B are the same, there will be no net movement of water molecules.
Solutions A and B are separated by a partially permeable membrane.
The membrane allows smaller water molecules to pass through but not the larger sugar molecules.

11. Osmosis in living organisms
Recall some properties of cells:
Cell membrane of both plant and animal cells is partially permeable.
The plant cell wall is made of cellulose. It is permeable.
The plant cell has a relatively rigid (firm) cell wall.

12. Osmosis in living organisms
What happens to animal cells when they are placed in distilled water?
Animal cells
Animal cells have elastic membranes. When the water molecules flow in, the animal cells will swell and eventually burst.
water moves in
Water potential outside the cell is higher than that in the cytoplasm.
Cell expands and eventually bursts.

13. Osmosis in living organisms
What happens to plant cells when they are placed in distilled water?
Plant cells
Plant cells have strong, rigid cells walls which prevent the cells from expanding too much.
When water molecules flow in, the contents in the cell press the cell wall.
The water creates a pressure on the cell wall of the plant cell. This is called turgor pressure and keeps the plant tissues turgid.
Soft tissues in plants depend on turgor for support. If plants lose too much water, they will wilt.

14. Osmosis in living organisms
Plant cells
water moves in
Water potential outside the cell is higher than that in the cell sap.
Cell expands and becomes turgid. The rigid cellulose cell wall expands slightly only. This prevents the cell from bursting.

15. Osmosis in living organisms
What happens to animal cells when they are placed in concentrated solution?
Animal cells
Animal cells will shrivel up as they lose water.
Water leaves the cell by osmosis
Concentration of water molecules outside the cell is lower than that in the cytoplasm.
Cell shrinks and becomes soft. It is dehydrated.

16. Osmosis in living organisms
What happens to plant cells when they are placed in concentrated solution?
Plant cells
When the concentration of water molecules of the cytoplasm and cell sap is higher than that of the surrounding solution, water leaves the plant cells by osmosis.
The vacuoles shrink and the cell contents pulls away from the cellulose cell walls. The plant cells lose turgor pressure and become flaccid.

17. Osmosis in living organisms
Plant cells
Water leaves the cell by osmosis.
Concentration of the water molecules outside the cell is lower than that in the cell sap.
Cell contents pull away from the cell walls and the cell becomes flaccid.

18. 25.3 The Need for a Transport System
A multicellular organism like a plant or a human being is made up of billions of cells.
Most of these cells lie deep inside the body of the organism.
Nutrients and oxygen need to reach each cell in the body to support the life of the cells.
A transport system ensures that substances are transported fast enough throughout the organism.

19. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
Water and mineral salts are absorbed by the root hairs from the surrounding soil.
Water enters the roots by osmosis.
Mineral salts enter by active transport.

20. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
How is water absorbed by the roots?
There are numerous root hairs at the ends of young roots.
A root hair is an outgrowth of a root hair cell.
It has a cell membrane, cytoplasm and a large central vacuole.
The vacuole contains cell sap, which contains mainly water and dissolved substances such as mineral salts.

21. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
How is water absorbed by the roots?
root hair cell
soil particles
film of water
water entering root hair
lower concentration of water molecules
higher concentration of water molecules

22. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
How is water absorbed by the roots?
The soil solution consists of water with dissolved mineral salts.
It is less concentrated (higher concentration of water molecules) than the cell sap and the cytoplasm.
As the cell membrane is partially permeable, water enters the root hairs by osmosis.

23. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
How are dissolved mineral salts absorbed by the roots?
The concentration of mineral salts in the root hairs is often higher than in the soil solution.
Thus, mineral salts should diffuse out of the root hairs.
However, this does not occur as the root hairs not only prevent the outflow of the salts, but absorb salts from the soil solution.

24. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
How are dissolved mineral salts absorbed by the roots?
xylem
soil particles
root hair absorbing mineral salts from the soil
Flow of mineral salts upwards
The movement of substances against a concentration gradient using energy is called active transport.
The root hairs absorbed dissolved mineral salts from a region of lower concentration to a region of higher salt concentration. This occurs because the root hairs are using energy during the absorption process.

25. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
Why is energy required in active transport?
The higher concentration of molecules is at the top of the slope.
The lower concentration of molecules is at the bottom of the slope.
In diffusion, molecules can roll down the slope without using much energy.
Diffusion

26. 25.4 Absorption of Water and Mineral Salts by Roots of Plants
Why is energy required in active transport?
If the molecules were to move up the slope, energy is needed.
This is what happens in active transport.
Active transport