Biology - Movement of substances

4 1. Diffusion Definition Diffusion is the net movement of molecules, ions or atoms from a region of higher concentration to a region of lower concentration. Concepts - Diffusion occurs down the concentration gradient, no need energy - No diffusion when dynamic equilibrium is reached/ both regions have same concentration. But particles are still in constant random motion. Factors - Higher temperature, ↑ diffusion rate - Steeper concentration gradient, ↑ diffusion rate - Shorter diffusion distance (eg. one-cell thick layer), ↑ diffusion rate - Higher SA:VR (eg. smaller piece), ↑ diffusion rate Example Plant Animal With membrane Absorption of mineral salts from soil (in root) Exchange of gases across alveoli and blood capillaries Without membrane Loss of water vapour from the stomata (in leaf) Neurotransmitters move across synpase/between neurones Application Transdermal medicine patch Advantage : Painless, quick termination, suitable for drugs that is difficult to absorbed by digestive system into blood. Disadvantage: Unsuitable for sensitive skin, may cause local irritation. Not suitable drugs that can irritates the skin or low skin permeability. Note: Visking tubing is a type of artifical membrane. MOVEMENT OF SUBSTANCES

5 2. Osmosis Definition Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential through partially permeable membrane. Concepts - Osmosis occurs down the water potential gradient, no need energy - If partially permeable membrane is destroyed by heat/pH/ethanol(alcohol) 1. No osmosis 2. Cell contents also leaves, causing cell mass and size decreases greatly - Water potential: (lowest) concentrated < dilute < distilled water (highest) 80% sucrose < 20% sucrose < 0% sucrose solution - Water enter the cell can cause increase in mass, volume, length & pressure Effect on cells Phrasing: 1) compare water potential 2) water enter/leave the cell 3) effect on cell Animal cell Plant cell Pure water - Cell has lower water potential - Water molecules enters - Cell swells/expands and bursts (as cell surface membrane is overstretched due to increasing pressure) - Cell has lower water potential - Water molecules enters - Cell becomes firm and turgid, turgor pressure exerted. (as cell wall exerts opposing pressure, prevent plant cell from bursting) Salt water - Cell has higher water potential - Water molecules leaves - Cell shrinks and crenates - Cell has higher water potential - Water molecules leaves - Large central vacuole decrease in size and plasmolysis occurs (cell surface membrane and cytoplasm pulls away from cell wall). Cell becomes soft and flaccid/ plasmolysed. Pure water in animal cell: - Pure water has higher water potential than cytoplasm in red blood cell - Water molecules enters cytoplasm of red blood cell by osmosis through partially permeable membrane - Red blood cells swells/expands and bursts as partially permeable membrane is overstretched due to increasing pressure Salt water in animal cell: - Salt water has lower water potential than cytoplasm in red blood cell - Water molecules leaves cytoplasm of red blood cell by osmosis through partially permeable membrane - Red blood cells shrinks and crenates Pure water in plant cell: - Pure water has higher water potential than cell sap in plant cell - Water molecules enters cell sap of plant cell by osmosis through partially permeable membrane - Plant cell becomes firm and turgid, turgor pressure exerted. Cell wall exerts opposing pressure to prevent cell burst. Salt water in plant cell: - Salt water has lower water potential than cell sap in plant cell - Water molecules leaves the cell sap of plant cell by osmosis through partially permeable membrane - Large central vacuole decrease in size, cell surface membrane and cytoplasm pulls away from cell wall, plant cell becomes soft and flaccid/ plasmolysed. Process: Plasmolysis.

6 Application Find water potential of cell sap in plant cell : Water potential of cell sap in plant cell → No net movement of water = no change in mass = both region same water potential. Turgor in plant tissue with dandelion stalk: • cutting the strip lengthwise causes them to curl outwards immediately due to the pressure in the cortex cells(white). Cortex cells are prevented from expanding by epidermis and inelastic cuticle layer (green). • A & C: Petroleum jelly prevents movment of water in cell, no unchange in tissue. • B: water enters by osmosis, causing cortex cell to expand more than on the inner side due to the restraint exerted by outer epidermis and inelastic cuticle layer, cause it to curl outwards. • D: water leaves by osmosis, causing cortex cell to shrinks. Pressure exerted by outer epidermis and inelastic cuticle layer cause inner side to curl inwards. Osmosis Diffusion High water potential to low water potential High concentration to low concentration Energy not required Energy not required Partially permeable membrane needed Partially permeable membrane may or may not be needed Involve water molecules only Involve any particles except water molecules

7 3. Movement of substances in nutrient uptake and gas exchange Nutrient uptake Gas exchange Digested food substances such as glucose, and amino acids diffuse into epithelial cells of villi in the small intestine O2 dissolves into the thin film of moisture lining the alveolar air space, then diffuses into the blood plasma Conversely, CO2 diffuses from the blood plasma into the alveoli air space Mineral salts are transported from the soil into root hair cells by diffusion, to maintain a high concentration of mineral salts in their vacuoles This creates a region of lower water potential than the surrounding soil, so that water flows in by osmosis O2 diffuses from the intercellular air spaces of the leaf to the surrounding air, through the stomata CO2 diffuses from the surrounding air into the intercellular air spaces of the leaf, through the stomata