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.

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.

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.

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.

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?

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

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

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

Dilute vs Concentrated solutions B 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.)

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.

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.

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.

Osmosis and Plant cells

Osmosis and Animal cells

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

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