How cell membrane helps in maintaining equibilirum while exchanging materials in the envirnment
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Answer:
The first point is to that biology uses the term dynamic disequilibrium, which clarifies that: 1) the balance is not constant over time (it changes as the cell does its functions), and 2) the concentration of chemicals (ions, etc.) are kept unbalanced between the outside and inside of the cell.
For most cases, it is most appropriate to think of the cell membrane as a barrier. Outside a cell is water-like (aqueous) and inside the cell is aqueous, but the cell membrane is fatty. Many things outside the cell cannot enter the cell because they cannot cross through the membrane. This is where channels and pumps become important. If the cell is going to let in a particular ion/compound that is in higher concentration outside the cell then a protein is opened that is specially shaped to let in that ion/compound. Later, if he cell wanted to remove the same ion it would use another protein called a pump that would use the energy of a chemical called ATP to force out the ion against the gradient. Alternatively, the cell membrane contains proteins that allow one ion out of the cell at the same time it pushes one ion into the cell (antiporter).
A few things can cross the cell membrane freely. One of these classes of chemicals is hormones. They are able to interact well enough with water that they can be in aqueous environments, but they are not so charged to not be able to move across the cell membrane. Several hormones are in this class of chemicals.
The last way things are let in and out of the cell are by the vesicular system. In this case, a portion of the outside of the cell is internalized (endosome) or a portion of the internal of the cell is externalized (exosome). These vesicles are surrounded by a lipid membrane like the cell membrane. Formation and management of these vesicles is tightly controlled.