1. Diffusion Most common type of passive transport.
Diffusion – is the random movement of particles (atoms, ions, molecules) from a region of high concentration to low concentration, down a concentration gradient.
Molecules diffuse down a concentration gradient.
Diffusion stops when molecules dispersed evenly (with no concentration gradient), and a state of equilibrium is reached.

2. Process of diffusion
When crystals of dye are placed in water, they are concentrated in one area.
Dissolved substance diffuse throughout liquid in which they are dissolved.

3. Example of diffusion: Gas exchange in lungs
Oxygen diffuses from the alveoli into the capillaries because there is a higher concentration of oxygen in the alveoli than in the blood of the capillaries.

4. Why is diffusion important?
Diffusion is important for:
Gaseous exchange (oxygen, carbon dioxide) during respiration and photosynthesis
Excreting waste products e.g. ammonia, water, mineral salts
Absorption of digested food into blood through walls of small intestine.
Enables animals to detect food by smell.

5. What is the difference between
Osmosis
A form of passive transport process
Osmosis – diffusion/movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
A partially/selectively permeable membrane only allows certain molecules to pass through it but not others.
What is the difference between
diffusion and osmosis?

6. Osmosis demonstration
A thistle tube, covered at the base with differentially permeable membrane, contains a 10% sugar solution.

7. Osmosis and Plant cells
In plant cells, cell sap contains dissolved salts and sugar. If cell sap has lower water potential than that of surrounding solution, water enters by osmosis.
Plant cell will swell and become firm / turgid.
Plant cell walls prevent cells from bursting.
Turgor pressure - outward pressure which cell sap exerts against inside wall of cell.
Turgor helps to support soft tissues in plants

8. Osmosis and Plant cells
If cell sap has higher water potential than surrounding solution, water moves out of the vacuole and cytoplasm shrinks away from the cell wall.
Cell loses its turgor, shrinks and becomes flaccid or soft. The cell becomes plasmolysed.
Plasmolysis - shrinkage of cytoplasm away from the cell wall when plant cells are immersed in a solution of low water potential.
Plasmolysis causes land plants to wilt, in non-woody parts of plants e.g. leaves, shoots

9. Dilute vs Concentrated solutionsB
Hypotonic - Dilute solution A ( higher water potential) compared to concentrated sugar solution B ( lower water potential)
Hypertonic - Solution B has water potential compared to solution A
Isotonic - when both solutions have the same water potential (‘iso’: same as; ‘tonicity’: strength of solution).
(Terms apply to animal systems only.)

10. Osmosis in plant and animal cells
Arrows indicate the direction of movement of water. In an isotonic solution, there is no net movement of water, and the cell neither gains nor loses water.

11. In hypotonic solution, a cell gains water
In hypotonic solution, a cell gains water. The animal cell may undergo lysis (burst). In the plant cell, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.

12. In a hypertonic solution, a cell loses water
In a hypertonic solution, a cell loses water. Animal cells shrivel (undergo crenation). Plant cell vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts can be seen in the center of the cell.

13. Osmosis and Plant cells

14. Osmosis and Animal cells

15. Active Transport
Active transport - molecules move from a region of low concentration to a high concentration (against a concentration gradient) using energy from respiration
Only in living cells
Cell contains numerous mitochondria, with high respiratory rate to provide energy for this process
Examples:
Absorption of dissolved mineral salts by root hairs
Absorption of glucose and amino acids by cells in small intestine

16. Active Transport
Presence of microvilli increases surface area for active transport of glucose into cells of small intestine.
Small cell has larger surface area:volume ratio than a large cell of same shape.
Cells are modified to increase surface area: volume ratio e.g. root hair cells, microvilli in small intestine and flattened, biconcave shape of red blood cells.
Accumulation of iodine by
marine organisms