Diffusion

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 Dissolved substance diffuse throughout liquid in which they are dissolved.

3. Example of diffusion: Gas exchange in lungs

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. 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

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. A B Dilute vs Concentrated solutions • 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

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