Cell Membranes and Types of Cell Transport for AP Biology

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By — McGraw-Hill Professional
Updated on Oct 24, 2011

Practice problems for these concepts can be found at:  Cells Review Questions for AP Biology

Cell Membranes: Fluid Mosaid Model

A cell membrane is a selective barrier surrounding a cell that has a phospholipid bilayer as its major structural component. Remember that the outer portion of the bilayer contains the hydrophilic (water-loving) head of the phospholipid, while the inner portion is composed of the hydrophobic (water-fearing) tail of the phospholipid (Figure 6.1).

The fluid mosaic model is the most accepted model for the arrangement of membranes. It states that the membrane consists of a phospholipid bilayer with proteins of various lengths and sizes interspersed with cholesterol among the phospholipids. These proteins perform various functions depending on their location within the membrane.

The fluid mosaic model consists of integral proteins, which are implanted within the bilayer and can extend partway or all the way across the membrane, and peripheral proteins, such as receptor proteins, which are not implanted in the bilayer and are often attached to integral proteins of the membrane. These proteins have various functions in cells. A protein that stretches across the membrane can function as a channel to assist the passage of desired molecules into the cell. Proteins on the exterior of a membrane with binding sites can act as receptors that allow the cell to respond to external signals such as hormones. Proteins embedded in the membrane can also function as enzymes, increasing the rate of cellular reactions.

Cell Membranes: Fluid Mosaic Model

The cell membrane is "selectively" permeable, meaning that it allows some molecules and other substances through, while others are not permitted to pass. The membrane is like a bouncer at a popular nightclub. What determines the selectivity of the membrane? One factor is size and the other is charge of molecules. The bouncer lets small, uncharged polar molecules and hydrophobic molecules such as lipids through the membrane, but larger uncharged polar molecules (such as glucose) and charged ions (such as sodium) cannot pass through. The other factor determining what is allowed to pass through the membrane is the particular arrangement of proteins in the lipid bilayer. Different proteins in different arrangements allow different molecules to pass through.

Types of Cell Transport

There are six basic types of cell transport:

  1. Diffusion, which is the movement of molecules down their concentration gradient without the use of energy. It is a passive process during which molecules move from a region of higher concentration to a region of lower concentration. The rate of diffusion of substances varies from membrane to membrane because of different selective permeabilities.
  2. Osmosis, which is the passive diffusion of water down its concentration gradient across selectively permeable membranes. Water moves from a region of high water concentration to a region of low water concentration. Thinking about osmosis another way, water will flow from a region with a lower solute concentration (hypotonic) to a region with a higher solute concentration (hypertonic). This process does not require the input of energy. For example, visualize two regions—one with 10 particles of sodium per liter of water; the other with 15. Osmosis would drive water from the region with 10 particles of sodium toward the region with 15 particles of sodium.
  3. Facilitated diffusion, that is, the diffusion of particles across a selectively permeable membrane with the assistance of the membrane's transport proteins. These proteins will not bring any old molecule looking for a free pass into the cell; they are specific in what they will carry and have a binding site designed for molecules of interest. Like diffusion and osmosis, this process does not require the input of energy.
  4. Active transport, which is the movement of a particle across a selectively permeable membrane against its concentration gradient (from low concentration to high). This movement requires the input of energy, which is why it is termed "active" transport. As is often the case in cells, adenosine triphosphate (ATP) is called on to provide the energy for this reactive process. These active-transport systems are vital to the ability of cells to maintain particular concentrations of substances despite environmental concentrations.
  5. For example, cells have a very high concentration of potassium and a very low concentration of sodium. Diffusion would like to move sodium in and potassium out to equalize the concentrations. The all-important sodium-potassium pump actively moves potassium into the cell and sodium out of the cell against their respective concentration gradients to maintain appropriate levels inside the cell. This is the major pump in animal cells.

    Types of Cell Transport

  6. Endocytosis, a process in which substances are brought into cells by the enclosure of the substance into a membrane-created vesicle that surrounds the substance and escorts it into the cell (Figure 6.2). This process is used by immune cells called phagocytes to engulf and eliminate foreign invaders.
  7. Exocytosis, a process in which substances are exported out of the cell (the reverse of endocytosis). A vesicle again escorts the substance to the plasma membrane, causes it to fuse with the membrane, and ejects the contents of the substance outside the cell (Figure 6.2). In exocytosis, the vesicle functions like the trash chute of the cell.

Practice problems for these concepts can be found at:  Cells Review Questions for AP Biology

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