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Transcript
Passive Transport in the Cell Osmosis and Diffusion Homeostasis The goal of all cells at all times is to stay in balance. This is referred to as HOMEOSTASIS. Cells and organisms will do whatever it takes to keep the inside in a state of homeostasis regardless of any changes that are happening outside. Brownian Motion All molecules in all substances are vibrating or moving at all times. In a solid, the movements are very small. In a liquid, such as the fluids both surrounding and inside a cell, these movements are bigger. As the molecules move around, they pass through the cell membrane. They always move from a region where there is more to a region where there is less. This is called the concentration gradient. http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animat ion__how_diffusion_works.html Diffusion This is the movement of molecules such as Oxygen, Sugar, and Carbon Dioxide from one region to another. Because these are relatively small molecules, they can pass through the cell membrane until they are equal on the inside and the outside. This is referred to as dynamic equilibrium. Diffusion works best for small cells, and this is part of the reason why all cells are microscopic. Osmosis Osmosis specifically refers to the movement of water molecules. When water concentration is equal on the inside and outside of the cell, we refer to this as isotonic. When water moves into the cell, we refer to it as hypotonic. Animal cells that have a lot of water move in will burst. Plant cells that have a lot of water move in will press against the cell wall and become very firm. When water moves out of a cell, it is referred to as hypertonic. Animal cells that have water move out will shrink or shrivel. Plant cells will do the same thing, but within the cell wall. Note: Diffusion, osmosis, and facilitated diffusion (in which a carrier protein helps larger or charged molecules cross the cell membrane, p. 56) are all PASSIVE, meaning that the cell does not have to use any energy to move materials in or out. This is very beneficial to the cell, but movements like these must always go in the direction of the concentration gradient – from high concentration to low concentration. What if the cell needs to move materials AGAINST the concentration gradient?