Potato Osmosis Lab

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Introduction All cells have plasma membranes which are selectively permeable meaning only some materials are able to move across it either to enter or exit the cell. The ability of a molecule to cross the plasma membrane depends on its size, hydrophobicity and charge as well as the relative concentration inside and outside of that cell (Biology Department, 2010). When molecules move from high concentration to low concentration it is called diffusion. More specifically when water diffuses across a cell membrane it is called osmosis. If the concentration of water in a cell is lower than the concentration of water in the solute the solution is said to be hypotonic and the cells swell because they will take up water. It is possible for animal cells to swell to the point of bursting because they do not have cell walls to limit the amount of water that enters. Plant cells hold the excess water in their large vacuoles and have cell walls so they remain rigid and further uptake of water will not happen. When the concentration of water in a cell is higher than the concentration of water in the solute the solution is said to be hypertonic and the cells will shrink because the water will diffuse from the cell into the solution. A plant whose cells are in a hypertonic solution will appear droopy because there is a loss of turgor pressure in each cell. When the concentration of water is the same in the cell and the solute the cell is said to be isotonic and is at equilibrium. There is no net movement of water across the membrane. (Biology Department, 2010). Surface area to volume ratio has a direct effect on the amount of water (as well as nutrients and waste) that can diffuse through the plasma membrane. It is a big factor in determining the size of a cell (Biology Department, 2010). This is because the rate that substances can diffuse across the plasma membrane

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