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Transport into and out of cellsFor an organism to function, substances must move into and out of cells. Three processes contribute to this movement – diffusion, osmosis and active transport. If a crystal of a coloured chemical, eg potassium manganate(VII), is placed in water, the particles spread out and mix with the water particles. The particles have moved from a region of high
concentration in the crystal to a low concentration in the water. This difference in concentration is called a
concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration. As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells. The rate of diffusion can be affected by a number of factors: For a
bacterium, substances diffuse into and out of the bacterial cell across its surface. Once inside, because of the bacterium's size, substances will need to diffuse 1 μm or less to where they are needed, for
example, for respiration. For simple multicellularorganisms, such as small plants like mosses, substances diffuse into the leaves and roots over their surface. Again, once inside the plant, they don't need to move far. Substances move into and around the moss plants by diffusion and osmosis. Simple organisms take in substances over their body surface. Their needs are determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume. For example, the surface area to volume ratio of a puppy is several times greater than that of an adult dog. QuestionSuggest why puppies are more at risk of losing body heat than adult dogs. Dogs lose heat over their body surface. Puppies have a larger surface area to volume ratio than adult dogs, so will lose heat more readily.
GCSE Subjects
How can you speed up the rate of diffusion?The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly. The greater the surface area, the faster the rate of diffusion.
How do you make a cell diffuse faster?Accelerating Diffusion
Increasing the temperature of a solution, which increases the amount of heat energy available to facilitate diffusion, is one way to get larger molecules to diffuse more quickly. Larger molecules will also diffuse faster when the concentration gradient is steeper.
What 3 factors affect diffusion speed?Diffusion of substances plays an important role in cellular transport in plants. The rate of diffusion is affected by the concentration gradient, membrane permeability, temperature, and pressure. Diffusion takes place as long as there is a difference between the concentrations of a substance across a barrier.
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