Transport across membranes

The flow of materials in and out of the cell is primarily controlled by the selective permeability of its membrane.

• Small, non-polar substances such as hydrophobic molecules, and cross it easily.
• Small, uncharged polar substances like water and glucose cross the membrane slowly.
• Large and/or charged molecules, such as ions, proteins and complex carbohydrates cannot cross the membrane at all. Instead, such substances are transported via channel proteins.

Channel proteins form a hydrophilic channel which allows polar molecules to pass through the membrane. They do not undergo a conformational change. Some only allow molecules of a certain radius and electric charge (e.g. ion channels). Others will allow all molecules with certain characteristics (e.g. general cation channels).

An aquaporin is a channel protein which allows the transport of water.

Channel proteins can be gated. One of the best-known examples of gated channels are voltage-gated ion channels in neurons, which will only open once the cell membrane reaches a specific voltage.

Carrier proteins can change shape while shuttling a substance across the membrane. This is known as a conformational change. They are more selective than channel proteins. For example, glucose transporters will not transport fructose, a structural isomer of glucose.

Whether a particular molecule is transported in or out of the cell depends largely on available energy resources.

• Passive transport is a mode of membrane traffic that requires no energy input.
• Active transport uses energy.

The three modes of passive, that is, energy-independent transport, are: diffusion, facilitated diffusion and osmosis.

Diffusion occurs down that substance’s concentration gradient. Molecules are transferred directly through the membrane. For example, diffuses into the lungs and diffuses out of the lungs, both down their respective concentration gradients.

Facilitated diffusion is diffusion that is enabled, or facilitated by the presence of carrier proteins in a membrane.

Osmosis is the diffusion of water through a partially permeable membrane. Water moves from a region of high water concentration, or low solute concentration, to a region of low water concentration, or high solute concentration.

• An isotonic solution has a solute concentration equal to that in a cell. No net water movement will be observed between the solution and the cell.

• A hypertonic solution has a solute concentration that is greater than that of a cell. There is a net movement of water out of the cell and into the solution.

• A hypotonic solution has a solute concentration that is less than that of a cell. There is a net movement of water from the solution into the cell.

Active transport is the movement of a substance against its concentration gradient. This process requires energy.

There are three ways that active transport occurs: via proton pumps, cotransport or bulk transport.

Protein pumps are used in active transport. They use energy in the form of ATP.

Hydrolysis of ATP releases phosphate groups which can bind to protein pumps and induce a change in conformation. The energy released is harnessed to carry out the conformational change.

A well-known example is the sodium-potassium pump (also called ), a pump that transports ions out of the cell and ions into the cell in a single cycle.

In normal conditions, intracellular concentration is much higher than extracellular; and intracellular concentration is much lower than extracellular. The sodium-potassium pump works continuously to maintain this arrangement. These differences in and concentration are extremely important for normal functioning of neuron and muscle cells.

Cotransport uses the energy released when a substance diffuses down its electrochemical gradient to move another substance against its concentration gradient.

For example, plant cells use sucrose-proton cotransporters to accumulate sugars in their cells. In a normal cell, proton concentration is higher outside the cell than inside; this is continuously maintained by the ATP-dependent proton pump.

As protons diffuse back into the cell, they must pass through the sucrose-proton cotransporter. This cotransporter will not do anything unless both a proton and a sucrose molecule are bound. Since protons are actively trying to diffuse back into the cell, this ensures that any returning protons also bring a sucrose molecule along.

In this way, plants transfer sucrose produced by photosynthesis into their vascular tissue, which then distributes sucrose around the plant.

Bulk transport is used for larger molecules, such as proteins and lipids, that do not fit through membranes and transport proteins. Instead, they travel through vesicles. This uses ATP.

There are two types of bulk transport: exocytosis and endocytosis.

Exocytosis is the bulk export of substances from a cell. Such substances include hormones and neurotransmitters, cellulose and proteins.

Endocytosis is the bulk import of substances into a cell. The plasma membrane folds inward to form a pocket, which gradually enlarges, capturing some of the extracellular material surrounding the cell. When the pocket gets big enough, it pinches off, forming a vesicle inside the cell. The three forms of endocytosis are:

• Phagocytosis – endocytosis of a smaller organism or piece of organic matter (“cell eating”). A food vacuole forms. Materials inside are digested and become nutrients for the cell.
• Pinocytosis – endocytosis of the extracellular fluid alone (“cell drinking”). This process is continuous and non-specific. It allows the cell to continuously monitor and clean its external environment.
• Receptor-mediated endocytosis – endocytosis that is triggered by binding of a specific substance, for example, cholesterol, to its receptor. This process allows the cell to increase its rate of endocytosis when a desired molecule is nearby.