How Substances Cross the Cell Membrane

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Submitted By katekitty01
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How substances cross the cell membrane.
A cell’s membrane has many functions including forming a boundary in order to organise the contents and keep them separate from the extracellular environment. Yet in order for the cell to ‘live’ and perform its particular function, it needs to be able to take in substances and nutrients and to get rid of waste products. But with such a vast array of substances in the extracellular environment, it is a difficult task to control the influx and out-pouring of substances. There are several ways that this is accomplished and these are set out below.
Cell, or plasma, membranes are formed from two layers of phospholipids which have a polar, hydrophilic head and a nonpolar, hydrophobic tail. This is known as the fluid mosaic model. It is this feature of membranes which makes them semipermeable and dictates how substances may cross them. Figure 1 shows a simplified structure of the cell membrane.

Small, nonpolar molecules such as (O2), carbon dioxide (CO2) and nitrogen (N2) are able to diffuse passively across the membrane and do so down their concentration gradients or electrochemical gradients in the case of ions. However larger or charged molecules and ions require assistance of some type as they are too large or have a charge on them which prevents them crossing the hydrophobic interior of the membrane.

In many cases a substance crosses the membrane with the assistance of membrane proteins, this is known as facilitated diffusion. Again the substance diffuses down its concentration or electrochemical gradient but in order to get through the hydrophobic section of the membrane it will make use of a membrane protein channel. The membrane protein which is integral to the membrane, has a receptor on it which will bind with a particular signalling molecule when the substance in question is required in the cell. When this…...

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