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Methods of Cell Transport: A Study Guide

written by: Sylvia Cini • edited by: Elizabeth Stannard Gromisch • updated: 6/6/2012

Cells constantly move materials in and out of your body using one of two methods of transportation. They do this to ensure that harmful waste is removed and nutrients are delivered where needed. Every time a cut heals or you get pruney fingers in the tub, you've got cell transport to thank.

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    What Is Cell Transport?

    Your body is made of trillions of cells. Although they are small, each cell is a crucial part of your system--contributing valuable information and ensuring that you have all of the things you need to continue living.

    One important task that is completed by your cells is ensuring homeostasis. Homeostasis is a state of being "in balance." This means that your cells are constantly accepting and releasing nutrients, minerals, water, protein and other substances to make sure that your body has exactly what it needs--and no more.

    The process of moving material into and out of cells is called cell transport. And there are two main types of transport: passive and active.

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    Passive Transport

    Passive transport is a method of cell transport that requires no energy. Material is moving from areas of high concentration to low, so all the cell does is provide an opening or channel through which to escape. Now these openings are not like doorways; not every molecule can pass through freely. This trait is called selective-permeability.

    The oxygen used for cell respiration is transported by a method of passive transport called simple diffusion. Unless the temperature is at or below absolute zero, molecules are constantly in motion. They bump into barriers. They bump into each other, always trying to move from an area of high pressure to an area of low pressure. And since your cells are always using oxygen, there are fewer oxygen molecules inside of your cells than there are outside, making the inside of your cells the perfect spot for molecules on the move.

    Oxygen simply passes through the lipid bilayer, the main part of your cell's outer wall.

    Water also moves in and out of your cells passively, though the process is called osmosis not diffusion. However, water also travels on special "Express" pathways that are built into your cell walls. These pathways are called aquaporins. Aquaporins are proteins that assist in the transport of water molecules. Scientists believe that aquaporins are necessary because water is the #1 import/export of our cells.

    Facilitated diffusion is another type of passive transport in which substances travel through the cell membrane. One substance that undergoes facilitated diffusion is glucose. Glucose, like water, relies on a special carrier protein. This protein is called a GluT transporter. (There are other transporter proteins that move other materials.) The GluT transporter protein changes shape to move the sugar molecules through the cell membrane at a specific point.

    These are only a few examples; other proteins assist in the process of passive transport.

    • Each protein handles a specific set of ions and molecules.
    • The protein may vary in size to facilitate transport.
    • The cell never exerts energy.
    • The material can only move from areas of high concentration to low.

    What Else Is Moved Via Passive Transport?

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    Active Transport

    Active transport is a method of cell transport that requires energy. Energy is expended because the cell has to move materials from an area of low concentration to an area of high concentration.

    Marshmallow Stash Imagine you have 5 large marshmallows in your mouth. Adding another marshmallow would be like a cell trying to complete an active transport. Not an easy task.

    Sodium and potassium both travel by active transport. They rely on a special protein to make it through the cell membrane. This protein, called a pump, is fueled by the cell's chemical energy. Fuel is brought to the pump by a helpful nucleotide called adenosine triphosphate (ATP). (You might have heard of ATP. It's an important building block in DNA and RNA!)

    Once it has a good store of chemical energy the pump goes to work. It picks up sodium from inside the cell and moves it outside. Then, on does the same for potassium in reverse. The protein pump opens and closes, moving sodium and potassium until it runs out of energy. Then it waits for another charge from a passing ATP.

    Up until now all of the methods of cell transport have moved molecules through the lipid bilayer a few at a time. The next two types of active transport help the cell get rid of (or gain) large amounts of a particular substance such as enzymes and hormones.

    This bulk transport may be called either exocytosis or endocytosis, depending on the direction of movement.

    During exocytosis, materials are moved out of the cell. Molecules are surrounded by a substance similar to the cell membrane forming a package, called a vesicle. The vesicle fuses with the cell membrane. And the wall reshapes to incorporate the extra material and the contents of the package are released outside of the cell. Endocytosis occurs in reverse, with the vesicle forming outside of the cell, and materials being released into the cytoplasm.

    Endocytosis of solid materials, such as bacteria, is called phagocytosis. White blood cells often perform phagocytosis to bring bacteria inside of the cell membrane. Of course, these cells don't dissolve the vesicle; they use it as a contained environment for dissolving harmful elements. Just one of the many ways cell transport keeps you active.

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    Vocabulary

    • Active transport
    • Aquaporins
    • Channel
    • Diffusion
    • Endocytosis
    • Exocytosis
    • GluT transporter
    • Lipid bilayer
    • Osmosis
    • Passive transport
    • Pump
    • Selective permeability
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    Study Tip

    Having trouble keeping exocytosis, endocytosis and phagocytosis straight?

    Sound it out.

    • EXit the cell by EXocytosis
    • ENter the cell by ENdocytosis
    • and Fight the Flu with PHagocytosis
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    Resources

    More About Cell Transport

    • Biology 4 Kids: Looking At Cell Functions (http://www.biology4kids.com/files/cell2_main.html)
    • Transport In And Out Of Cells (http://www2.estrellamountain.edu/faculty/farabee/biobk/biobooktransp.html)
    • Bio Review: Cell Transport (http://library.thinkquest.org/28751/review/cells/5.html)
    • YouTube: Cell Membrane Passive Transport (http://www.youtube.com/watch?v=JShwXBWGMyY)

    More About Cells:

    • National Center for Biotechnology Information : What Is A Cell? (http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html)
    • Livestrong: How Is Hydrogen Used In The Body (http://www.livestrong.com/article/28599-hydrogen-used-body/)

    Photo Credit:

    • "Marshmallow Stash" by Pretty In Print. Creative Commons/Flickr