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Diffusion, osmosis and active transport INDEX • • • • • • Diffusion “Test yourself!” – Diffusion Osmosis Osmosis in cells Video – Diffusion and osmosis activity Active Transport Diffusion Diffusion occurs when particles spread. They travel from an area of high concentration to an area of low concentration. Diffusion happens when the particles are free to move; this is true in gases or liquids. The “scent particles” from this hamburger are in high concentration here: Eventually they will “diffuse” out into this area of low concentration: Examples of diffusion in living systems Location Substances From To Adaption lung oxygen air space red blood cells alveoli (moist lining, good blood supply) lung carbon dioxide blood plasma air space alveoli (moist lining, good blood supply) blood villi, microvilli, good blood supply placenta with villi, thin walls and good blood supply thin leaves, with air spaces and digestive system food molecules stomach, eg glucose intestine uterus food molecules mother's blood foetus's blood and oxygen supply supply leaf oxygen leaf cell air space Diffusion in Uterus Diffusion in leaves Test yourself! http://www.bbc.co.uk/apps/ifl/schools/gcsebitesize/science/ quizengine?quiz=add_gateway_diffusionh;templateStyle=scie nce Osmosis Osmosis is a “special kind of diffusion”. It’s when water diffuses from a concentrated area to a less concentrated area through a partially permeable membrane (i.e. one that allows water to move through but not anything else): Water Sugar solution In this example the water molecules will move from left to right (along the concentration gradient) and gradually dilute the sugar solution. Potato cells Strong sugar solution Medium sugar solution Weak sugar solution Important vocabulary - higher tier Lysis – bursting an animal cell by osmosis Crenation – shrinking an animal cell by osmosis Turgid – a plant cell fully inflated with water Plasmolysed – a plant cell that has lost water causing the cell membrane to be pulled away from the inside of the cell wall Flaccid – a plant cell that is limp through a reduction of pressure inside the cell. Osmosis in cells Osmosis takes place in all cells. The cell membrane is partially permeable. Osmosis in red blood cells. If a red blood cell is placed in water, water enters the cell by osmosis. Because the membrane is quite weak the cell will burst as the volume and therefore the pressure in the cell increases. Red blood cells shrink when placed in concentrated solutions of sugar as water moves out of them by osmosis. This makes the cells appear wrinkled when viewed through a microscope. This does not happen inside the body because the kidneys make sure the concentration of the blood stays about the same as the concentration of the solution inside the red blood cell. Osmosis Osmosis in plant cells Plant cells have a strong rigid cell wall on the outside of the cell membrane. This stops the cell bursting when it absorbs water by osmosis. The increase in pressure makes the cell rigid. This is useful as plants do not have a skeleton. Instead the leaves and shoots can be supported by the pressure of water in their cells. If plant cells lose too much water by osmosis they become less rigid and eventually the cell membrane shrinks away from the cell wall. Osmosis Water entering the cell by osmosis inflates the cell and makes it rigid Loss of water makes the cell limp and shrinks the cell membrane away from the cell wall. Here’s a video for you to understand diffusion and osmosis activity better: http://www.bbc.co.uk/schools/gcsebitesize/scie nce/add_aqa_pre_2011/cells/osmosisact.shtml Active Transport Active transport is when substances are absorbed against a concentration gradient (like a ball being rolled up a hill). This takes place in the opposite direction to normal diffusion. This is how nitrates are taken in by root hair cells: Root hair cell already has a large concentration of nitrate ions Root hair cells “pull” more nitrate ions in This process needs energy (just like rolling a ball up a hill would). This energy is provided by respiration. Active Transport In humans, active transport takes place during the digestion of food in the small intestine. Carbohydrates are broken down into simple sugars such as glucose. The glucose is absorbed by active transport into the villi, to be passed into the bloodstream and taken around the body.
