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Diffusion
Small molecules can pass through the cell membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration(larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance. The amount of a substance in relation to the total volume is the concentration. During diffusion, molecules are said to flow down their concentration gradient, flowing from an area of high concentration to an area of low concentration. Molecules flowing down a concentration gradient is a natural process and does not require energy. Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration, and molecules flow equally in both directions across the semipermeable membrane. At equilibrium, equal amounts of a molecule are entering and leaving a cell. |
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Credit: Hana Zavadska, based on image by Mariana Ruiz Villarreal (http://commons.wikimedia.org/wiki/File:Scheme_facilitated_diffusion_in_cell_membrane-en.svg)
Source: CK-12 Foundation License: CC BY-NC 3.0 |
Osmosis
The diffusion of water across a membrane because of a difference in concentration is called osmosis. Water is attracted to the molecules dissolved in water. In osmosis, the water moves rather than the dissolved molecules. Because water is attracted to the dissolved molecules, the water will move from areas of low dissolved molecule concentration to areas that have higher dissolved molecule concentration. Let's explore three different situations and analyze the flow of water.
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Credit: Mariana Ruiz Villarreal (LadyofHats)
Source: http://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg License: Public Domain This text is adapted under a Creative Commons 4.0 license. You can find the original source here.
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These pictures are of Elodea under a microscope. Both are viewed at 400x. In the picture on the left the elodea has been placed in a 5% salt solution. In the photo on the right the elodea has been placed in a 10% salt solution.
In these photos, salt water outside the cell has caused water to move out of the cell. As water moves out of the cell the cell membrane shrivels up and the chloroplasts are forced into a smaller and smaller space, clumping them together in the center of the cell. |
5% salt solution
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10% salt solution
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Active Transport
During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules. As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport. |
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The sodium-potassium pump moves sodium ions to the outside of the cell and potassium ions to the inside of the cell, areas where these ions are already highly concentrated. ATP is required for the protein to change shape. ATP is converted into ADP (adenosine diphosphate) during active transport.
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