Often a large truck might be needed if you are moving large boxes and furniture. Just as a truck is needed to help move these items from one location to another, a cell needs help moving things as well, or facilitating the diffusion process. This would be the job of a special type of protein.
Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. The difference in the concentrations of the molecules in the two areas is called the concentration gradient. Diffusion will continue until this gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes “down the concentration gradient.” The end result of diffusion is an equal concentration, or equilibrium, of molecules on both sides of the membrane.
If a molecule can pass freely through a cell membrane, it will cross the membrane by diffusion (see the Figure below).
What happens if a substance needs assistance to move across or through the cell membrane? Facilitated diffusion is the diffusion of solutes through transport proteins in the cell membrane. Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient (see the Figure below).
Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins. A transport protein completely spans the membrane, and allows certain molecules or ions to diffuse across the membrane. Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.
A channel protein, a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly. Water channel proteins (aquaporins) allow water to diffuse across the membrane at a very fast rate. Ion channel proteins allow ions to diffuse across the membrane.
A gated channel protein is a transport protein that opens a “gate,” allowing a molecule to pass through the membrane. Gated channels have a binding site that is specific for a given molecule or ion. A stimulus causes the “gate” to open or shut. The stimulus may be chemical or electrical signals, temperature, or mechanical force, depending on the type of gated channel. For example, the sodium gated channels of a nerve cell are stimulated by a chemical signal which causes them to open and allow sodium ions into the cell.
A carrier protein is a transport protein that is specific for an ion, molecule, or group of substances. Carrier proteins “carry” the ion or molecule across the membrane by changing shape after the binding of the ion or molecule. Carrier proteins are involved in passive and active transport. Glucose molecules are too big to diffuse through the cell membrane easily, so they are moved across the membrane through carrier proteins. In this way glucose diffuses very quickly across a cell membrane, which is important because many cells depend on glucose for energy.
Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl–), are important for many cell functions. Because they are charged (polar), these ions do not diffuse through the membrane. Instead they move through ion channel proteins where they are protected from the hydrophobic interior of the membrane. Ion channels allow the formation of a concentration gradient between the extracellular fluid and the cytosol. Ion channels are very specific, as they allow only certain ions through the cell membrane. Some ion channels are always open, others are “gated” and can be opened or closed. Gated ion channels can open or close in response to different types of stimuli, such as electrical or chemical signals.
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Which of the statement(s) is true about passive transport?
It involves the movement of molecules along the concentration gradient.
This process does not require energy.
The molecules move from low concentration to high concentration.
Facilitated diffusion is a type of passive transport.
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