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Cell membranes are involved in the transport of various substances within cells, and use several processes by which they carry out their functions. All cells acquire the molecules and ions they need through their plasma membranes. In eukaryotic cells, there is also transport in and out of membrane-bounded organelles such as the nucleus, ER, and mitochondria. One of these types of cellular transport is osmosis. Osmosis is the diffusion of water from a region of high water concentration to a low water concentration through a semi-permeable cell membrane without the use of energy. This cell membrane is not able to allow large particles such as starch pass it, yet the fluidity of the membrane allows water to pass through due to its size and structure. Water moves toward areas of lower concentration until its water potential is zero, which the cell membrane regulates. Osmosis is the transport of water from high to low osmotic potential, low to high solute concentration, and low to high osmotic pressure. Solutions can be isotonic (where the solution has the same solute concentration as the cell), hypertonic (where the solution has more solute concentration than the cell), or hypotonic (where the solution has a lower concentration of solute than the cell). Through the water potential of each of these solutions, the cell membrane is able to control osmosis and osmotic pressure.
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