Membranes160-page6 - • Molecule size(Smaller molecules...

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Membrane Structure and Function - 6 Gradients are important in moving materials through membranes, both passively (without the use of energy by the cell) and actively (transport requiring cell energy). Passive Transport involves moving things through membranes without the expenditure of cell energy down gradients. Passive transport in cells involves the process of diffusion. Simple Diffusion Diffusion is a means of passive transport, since no additional energy is expended for the process. In terms of cellular activity, diffusion: Requires no energy But the cell has no control over diffusion, and the rate of diffusion is pretty slow and can not cover much distance. The Rate of Diffusion can be affected by: Temperature (Higher temperature, faster molecule movement)
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Unformatted text preview: • Molecule size (Smaller molecules often move more easily) • Concentration (Initial rate faster with higher concentration) • Electrical and pressure gradients of the two regions (Greater the gradient differential, the more rapid the diffusion (again, initially)) Materials that may move through membranes by simple diffusion include: • H 2 0 (water) • CO 2 (carbon dioxide) • O 2 (oxygen) • Some small lipid-soluble molecules (alcohol) Note: The movement of water through a differentially permeable membrane in response to solute concentrations, the phenomenon of osmosis, is a special case of diffusion that we shall discuss later...
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