High School Biology/Lessons/Lesson 3

{| cellspacing="0" cellpadding="0" style="margin:0em 0em 1em 0em; width:100%" Lesson 3-1 in High School Biology
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Diffusion

 * Every living cell exists in a liquid environment that it needs to survive. It may not always seem that way; yet even in the dust and heat of a desert, the cells of cactus plants, scorpions, and vultures are bathed in liquid. One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.


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Osmosis

 * Although many substances can diffuse across biological membranes, some are too large or too strongly charged to cross the lipid bilayer. If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it. A membrane is impermeable to substances that cannot pass across it. Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot.


 * Water passes quite easily across most membranes, even though many solute molecules cannot. An important process known as Osmosis  is the result. Osmosis is the diffusion of water through a selectively permeable membrane.


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Active Transport

 * As powerful as diffusion is, cells sometimes must move materials in the opposite direction—against a concentration difference. This is accomplished by a process known as active transport. As its name implies, active transport requires energy. The active transport  of small molecules or ions across a cell membrane is generally carried out by transport proteins or “pumps” that are found in the membrane itself. Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis. The transport of these larger materials sometimes involves changes in the shape of the cell membrane.


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The Next Lesson

 * Continue to Lesson 3- Enzymes.

Contact Your Instructor

 * Your instructor for this lesson was : Avid Insight  (Talk). To email me, you simply @undefined.  [[Image:Crystal Clear app xfmail.png|32px]]


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