GED Science Practice Test: Basic Cell Structure and Transport of Materials into and out of the Cell

While cells vary in shape, size, and function, all cells have certain features that support their structure and function. For example, all cells have a plasma, or cell, membrane that separates the internal contents of the cell from the environment outside the cell. The plasma membrane surrounds the cytoplasm, which is a term that collectively refers to everything inside the plasma membrane—the organelles, which are smaller structures within the cell, and the cytosol, which is a gel-like substance that takes up the space in between the organelles. All cells contain some sort of genetic information, which controls the functioning of the cell, in terms of division, reproduction, production of proteins, etc.

Plasma Membrane: While it may seem like the plasma membrane is not particularly important, just separating the cell from the rest of its environment, it actually serves an extremely important purpose.  The plasma membrane regulates what comes into, and goes out of, a cell.  If you doubt how important this role is, imagine what would happen if a poison got into the cell, or waste products were not allowed to leave a cell. Additionally, the larger the cell, the smaller the surface area to volume ratio, and the more difficult that transport of materials becomes.  This is why cells are usually not very large. There are three main processes that describe the movement of materials into or out of a cell:  diffusion, osmosis, and active transport.

Diffusion refers to the movement of substances from areas of high concentration to areas of low concentration. This can occur across a plasma membrane, within the cell, or outside of the cell. One way to make sense of this is in terms of an analogy.  If you’ve ever been relieved to exit a crowded elevator, you’ve experienced a type of “diffusion.”  You move from an area of high concentration of people (in the elevator), to an area of low concentration of people (outside the elevator).   Diffusion allows materials on both sides of a cell membrane to equalize in concentration.

Osmosis is a special type of diffusion, in which water (or other liquids) will move to equalize the concentrations of a substance on both sides of a plasma membrane.   A simple example of osmosis is rehydrating dehydrated fruits or vegetables, such as mushrooms, peppers, etc.  When you soak a dried mushroom, for example, osmosis is occurring as water travels across the cell membranes in the mushrooms to equalize the concentrations of materials on the inside and outside of the mushroom.  The inside of the mushroom cells has high concentrations of nutrients, minerals, etc.  Though you don’t typically refer to water’s concentration, you can also think of osmosis as water moving from an area of high water concentration to an area of low water concentration.  With this thought, you can see how osmosis is a special kind of diffusion.

Both diffusion and osmosis are passive processes; they occur without any energy.  The movement of substances from an area of high concentration to low concentration helps to equalize substances.  These types of processes are said to move substances down a concentration gradient.  A concentration gradient represents the change in concentration from one area to another.  However, sometimes a cell needs to continue to move substances across its plasma membrane up a concentration gradient.  This process requires energy, and is called active transport.

The structure of the plasma membrane allows these processes to occur in different ways.  The plasma membrane is made up of a phospholipid bilayer.  A phospholipid is a lipid (a fat) with a phosphate group attached to it.  The lipid portion is hydrophobic, meaning it does not “like” water.  The phosphate portion is hydrophilic, meaning it “loves” water.  The following is a representation of a phospholipid:

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Because the cytosol inside the cell and the substance outside of the cell are both watery environments, two phospholipids (hence “bilayer”) arrange themselves with their hydrophobic parts away from the water and the hydrophilic parts toward the water as shown below:

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Water, and many small substances can go through directly through this phospholipid bilayer by osmosis or diffusion.  However, larger substances, or substances that need to pass through the membrane by active transport, often pass through proteins that are embedded within the plasma membrane.  You can see a variety of proteins within the plasma membrane pictured below:

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