Passive Transport

Written by tutor Megan E.

In order to survive, a cell must maintain a careful balance of nutrients, electrolytes, water, and metabolites in its cytoplasm at all times. Many of these required nutrients are supplied to cells by the bloodstream (multicellular organisms) or extracellular fluid (single-celled organisms), and thus the cell must have ways to get these substances into its cytoplasm through its membrane. Also, cells must be able to excrete waste products into the extracellular space so the cytoplasm does not accumulate toxic and harmful products. In addition, an exact gradient of electrolytes must be maintained across the cell membrane in order for cells to perform normal functions like movement, signaling, and both anabolic and catabolic activities. Thus the cell has complex systems of transport to move these substances both in and out of the cell.

Passive transport is the name for a class of transport processes that do not require energy from the cell for the transport to take place. In the terms of thermodynamics, these processes of transport occur spontaneously or have a net negative Gibbs free energy (exergonic reaction). Active transport is the other class of transport systems that do require energy from the cell, usually in the form of ATP, do not occur spontaneously (endergonic reaction). Diffusion, osmosis, and facilitated diffusion are all types of passive transport.

A molecule will undergo passive transport when it flows down its electrochemical gradient, a combination of its chemical and electrical gradients. When the concentration of a solute is higher on one side of the membrane, this is called a chemical gradient. Molecules flow randomly and spontaneously towards areas of lower concentration of that molecule, a process called diffusion (“to spread out”).

When one side of the membrane is positively or negatively charged compared to the other side, this is called an electrical gradient or potential. Positive ions will be attracted to negative ions and thus they will spontaneously move toward each other until neutrality across the membrane is reached. Basically, it is thermodynamically favorable for a molecule to move to the side of the membrane where the electrochemical gradient is lowered, or where there are less molecules like itself. Active transport occurs when a solute moves “uphill” or against its electrochemical gradient because energy is required to concentrate a molecule in an area.

Generally, small nonpolar molecules, such as cholesterol, CO2 and O2, diffuse rapidly across the lipid bilayer because they are hydrophobic and dissolve readily in the membrane. Some small polar molecules such as water or urea can also diffuse directly through the plasma membrane but the process is slow for these molecules. The plasma membrane is impermeable to large or charged molecules. Diffusion is the simplest type of passive transport and requires no proteins to occur. The diffusion of water across a semi-permeable membrane, of which the lipid bilayer is an example, is called osmosis, another type of passive transport. Water diffuses toward the side of the membrane that has more solutes so it can effectively lower their concentration and minimize the electrochemical gradient. Maintaining the correct electrochemical gradient requires osmosis, and if the normal electrochemical gradient is disturbed, osmosis can also cause the cell to shrivel or burst.

Because diffusion is a slow process for many important molecules, a process called facilitated diffusion is used by the cells to rapidly transport molecules across the membrane. Facilitated diffusion occurs when proteins, specific for the transported molecule, are used to move the molecule across the membrane. Channel proteins transport molecules down their electrochemical gradient and can be thought of as a tunnel through the plasma membrane that is specific for a certain type of molecule.

Channels bind weakly to the molecule they transport and are either simply closed or open to movement. Aquaporins are channels specific for water that allow diffusion of water to occur at a much higher rate than diffusion directly through the plasma membrane.

Transporter or carrier proteins are also specific for the molecule they transport, but now the carrier protein actually binds the molecule to be transported and changes conformations in order to move the molecule into or out of the cell. Rather than being simply open or closed, transporters change shape (conformations) when the solute binds and change shape again when the solute is released to the other side of the membrane. This process is relatively fast compared to diffusion, but slow compared to the movement of molecules through channels. Transporter proteins used in passive transport are also called permeases because they make the membrane permeable to a certain substance. Transporter proteins that are used for active transport are often called pumps because work is done and energy expended by the cell to move the molecule from one place to another. More information can be found in our lesson on facilitated diffusion.

It should be noted that transport occurs across all the lipid bilayers in eukaryotic cells, like those around other cellular organelles, in addition to the plasma membrane. The abundance and the specific types of transporter proteins vary in each membrane as each compartment of the cell maintains a particular milieu of substances.

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