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Osmosis and cells play integral roles in biological life. Osmosis is the traveling of water across a membrane. It is important for a cell’s survival to regulate osmosis in order to maintain an optimal internal environment, according to Student Study Guide for Campbell’s BIOLOGY, Fourth Edition by Martha R. Taylor. In order to regulate osmosis, a cell uses a fluid mosaic of lipids, proteins, and carbohydrates. This fluid structure is known as the cell membrane.
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Selective Permeability of Cell Membrane
Biological cell membranes are selectively permeable, which means that the ease and rate of small molecules passing through membranes vary widely. The plasma membrane regulates exchange of nutrients, oxygen, inorganic ions, waste products, and water. The cellular membrane’s molecular organization controls permeability. A cell's plasma membrane consists of a bilayer of lipid molecules, which allow certain molecules to pass more easily than others. Additionally, transport proteins may aid certain molecules to cross the plasma membrane. These proteins either provide a channel or physically bind and transport the specific molecule across the membrane.
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Diffusion Across a Cell Membrane
Diffusion is the movement of a substance across a membrane. Substances diffuse across cell membranes in a process known as passive transport. This means that the cell does not expend any energy in transporting substances across the cell membrane. Instead, substances move down their concentration gradient as a result of random thermal motion.
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Hypotonic Solution vs. Hypertonic Solution
Osmosis is the diffusion of water across a selectively permeable membrane. In order to grasp the mechanisms of osmosis, one must understand the difference between a hypotonic solution and a hypertonic solution.
A hypotonic solution is a solution with a lesser concentration of solutes and greater concentration of unbound water.
Alternatively, a hypertonic solution has a greater concentration of solutes and a lesser concentration of unbound water.
The direction of osmosis is a function of difference in total solute concentration, regardless of types of solute molecules.
Water moves down its own concentration gradient, which means from a hypotonic solution to a hypertonic solution.
When there is an equal solute concentration, this is known as an isotonic solution. There is no net water movement in an isotonic solution.
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Water Balance of Cells With Rigid Walls
Rigid cellular walls are necessary for some types of cells to survive in hypotonic environments. These rigid walls surround the cellular membranes of plants, fungi, prokaryotes, and some protists. When water moves into a plant cell, it swells against its rigid wall. When a cell is in this state it is known as a turgid cell. Plant cells are referred to as flaccid when in an isotonic fluid. The plant cell may pull its plasma membrane away from its cell wall in a hypertonic environment. This process is known as plasmolysis.
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Water Balance of Cells Without Rigid Walls
Unlike plants, animal cells do not have rigid walls surrounding their cellular membranes. If an animal cell is placed in a hypotonic environment, the cell will gain water, swell, and possibly burst. A cell without a rigid wall will lose water and shrivel if placed in a hypertonic environment. A cell without rigid walls may require an isotonic environment to live. Alternatively, this type of cell may also survive through the use of adaptations for osmoregulation. This allows cells to actively regulate the flow of water across the membrane.
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The Process Varies
Osmosis is a vital process for all living organisms. Although the general mechanisms of osmosis are the same in most cell membrane functions, the regulation of this process varies widely among living cell types. Some types of cell membranes are optimized for hypotonic solutions, while others prefer hypertonic or isotonic environments. Osmosis plays varying roles depending upon cell structure.