Biology For Dummies
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The plasma membrane surrounding animal cells is where the exchange of substances inside and outside of cells takes place. Some substances need to move from the extracellular fluid outside cells to the inside of the cell, and some substances need to move from the inside of the cell to the extracellular fluid.

Some of the proteins that are stuck in the plasma membrane help to form openings (channels) in the membrane. Through these channels, some substances such as hormones or ions are allowed to pass through. They either are “recognized” by a receptor (a protein molecule) within the cell membrane, or they attach to a carrier molecule, which is allowed through the channels. Because the plasma membrane is choosy about what substances can pass through it, it is said to be selectively permeable.

Permeability describes the ease with which substances can pass through a border, such as a cell membrane. Permeable means that most substances can easily pass through the membrane. Impermeable means that substances cannot pass through the membrane. Selectively permeable or semipermeable means that only certain substances are able to pass through the membrane.

Transporting substances across the plasma membrane can require that the cell use some of its energy. If energy is used, the transport is called active. If molecules can pass through the plasma membrane without using energy, the molecules are using passive transport.

Helping the molecules across: Active transport

Sometimes, the molecules are just too big to easily flow across the plasma membranes or dissolve in the water so that they can be filtered through the membrane. In these cases, the cells must put out a little energy to help get molecules in or out of the cell.

Embedded in the plasma membrane are protein molecules, some of which form channels through which other molecules can pass. Some proteins act as carriers — that is, they are “paid” in energy to let a molecule attach to itself and then transport that molecule inside the cell.

Passive transport of molecules

A membrane can allow molecules to be passively transported through it in three ways: diffusion, osmosis, and filtration.

  • Diffusion: Sometimes organisms need to move molecules from an area where they are highly concentrated to an area where the molecules are less concentrated. This transport is much more easily done than moving molecules from a low concentration to a high concentration. To go from a high concentration to a low concentration, in essence the molecules need to only “spread” themselves, or diffuse, across the membrane separating the areas of concentration.

    In the human body, this action occurs in the lungs. You breathe in air, and oxygen gets into the tiniest air sacs of the lungs, the alveoli. Surrounding the tiniest air sacs of the lungs are the tiniest blood vessels — capillaries. The capillaries in the lungs, called pulmonary capillaries, contain the lowest concentration of oxygen in the body, because by the time the blood gets to the tiniest vessels, most of the oxygen has been used up by other organs and tissues.

    So, the tiniest air sacs of the lungs have a higher concentration of oxygen than do the capillaries. That means that the oxygen from the alveoli of the lungs can spread across the membrane between the air sac and the capillary, getting into the bloodstream.

  • Osmosis: This term is used when talking about water molecules diffusing across a membrane. Basically, the diffusion of water (osmosis) works as described in the preceding bullet. However, with osmosis, the concentration of substances in the water is taken into consideration. If a solution is isotonic, that means the concentrations of the substances (solutes) and water (solvent) on both sides of the membrane are equal. If one solution is hypotonic, there is a lower concentration of substances (and more water) in it when compared to another solution. If a solution is hypertonic, there is a higher concentration of substances in it (and less water) when compared to another solution.

    For example, the blood in your body contains a certain amount of salt. The normal concentration is isotonic. If suddenly there is too high a concentration of salt, the blood becomes hypertonic (too many salt molecules). This excess of salt forces water out of the blood cells in an attempt to even things out. But the effect this action has is actually that of shrinking the blood cells.

    This shrinking of cells is called crenation (not cremation). If too much fluid is in the bloodstream, the blood cells have too few molecules of salt in comparison, making them hypotonic. Then, the blood cells take in water in an attempt to normalize the blood and make it isotonic. However, if the blood cells need to take in too much water to bring everything back into balance, they can swell until they burst. This bursting of cells is called hemolysis (hemo = blood; lysis = break apart).

  • Filtration: The last form of passive transport is used most often in the capillaries. Capillaries are so thin (their membranes are only one cell thick) that diffusion easily takes place through them. But remember that animals have a blood pressure. The pressure at which the blood flows through the capillaries is enough force to push water and small solutes that have dissolved in the water right through the capillary membrane. So, in essence, the capillary membrane acts as filter paper, allowing fluid to surround the body’s cells and keeping large molecules from getting into the tissue fluid.

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René Fester Kratz, PhD, teaches biology at Everett Community College. Kratz has a doctorate in botany from the University of Washington. She works with other scientists and K–12 teachers to develop science curricula that align with national learning standards and the latest research on human learning.

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