Membrane Transport

A cell must be able to regulate the movement of substances across the cell membrane. If holes appear through the membrane the cell quickly dies. Only a few substances can easily move across a cell membrane, while many others are blocked:

  • Small molecules like O2, CO2, and water can get across by squeezing between the phospholipids (simple diffusion).

  • Larger molecules like glucose, amino acids, and all ions (e.g. Na+, K+, Ca2+, etc.) will need the assistance of a protein to cross the membrane (facilitated diffusion).

Cells use several mechanism to move material across the membrane, and they can be broadly classified as passive or active.

PASSIVE TRANSPORT

Passive transport does not use any energy, and it is all driven by a concentration gradient. A concentration gradient is a difference in concentration of a substance across an area. In this section we will discuss two forms of passive transport: diffusion and osmosis.

PASSIVE TRANSPORT - DIFFUSION

Diffusion is the natural movement of particles but only in one direction; from a high concentration of a molecule to low concentration of that same molecule.

SIMPLE DIFFUSION

Some molecules use simple diffusion to move into or out of the cell by squeezing between the phospholipids - those molecules are usually small, non-polar molecules such as O2 and CO2. Oxygen is needed by cells to make energy, and the waste product, carbon dioxide, is produced. Diffusion will easily move these two substances to where they need to go.

Image showing the selective permeability of the phospholipid bilayer cell membrane.

FACILITATED DIFFUSION

Muscle and nerves cells (neurons) use ions to signal each other through diffusion, but ions cannot get through cell membranes without help. These cells have special proteins embedded in their membranes called channels that allow ions to get through the membrane. This is still a form of diffusion (passive transport) called facilitated diffusion (because the channels are helping, or facilitating, the diffusion). Ions can still only diffuse from an area of high concentration to an area of low concentration.

Image showing facilitated or passive diffusion.

Cells also need substances that may be too large to get through the cell membrane through ion channels. Common substances would be monosaccharides (glucose) or amino acids. These substances will enter the cell through transport proteins also known as transporters. This is still a form of facilitated diffusion and substances still only diffuse from an area of high concentration to an area of low concentration.

Image demonstrating facilitated diffusion using transport proteins through a cell membrane.

PASSIVE TRANSPORT - OSMOSIS

The movement of water in the body is so important that is gets its own category of passive transport called osmosis. There are two important considerations:

  • Osmosis requires a selectively-permeable membrane to work. The membrane allows water to diffuse but blocks solutes (especially proteins).

  • We determine the direction of water flow by looking at the solute concentration. Water will diffuse from an area of low solute concentration to an area of high solute concentration.

Osmosis sounds like it doesn't have much force, but it is a huge part of our homeostasis. If it is not balanced it can destroy our cells.

Image demonstrating osmosis: water moving through a semipermeable membrane from low solute to high solute concentration.

OSMOSIS and TONICITY

We determine which way osmosis will drive water by looking at the difference in solute concentration between the cell and the surrounding solution, called tonicity. If the cell and the solution have the same solute concentration the solution is isotonic to the cell. This is how red blood cells and plasma (the fluid part of blood) is normally and the cells have the correct shape (the image in the center below with the dimple in the middle of the cell).

  • If the solution has more solute that the cell it is called a hypertonic solution and water will be drawn out of the cell. This can cause red blood cells to crinkle up and not flow properly (the image on the left).

  • If the solution has less solute than the cell it is called a hypotonic solution and water will be pulled into the cell. Red blood cells can swell (again, the wrong shape) and even burst, which will cause rapid death (the image on the right).

Image demonstrating tonicity in cells: hypertonic, isotonic, and hypotonic.

ACTIVE TRANSPORT

If transport uses energy to move material it is called active transport. There are many forms and we will come across a few in BIO 141 and 142. One form of active transport uses membrane proteins called pumps and is essential for the function of muscle cells and neurons as it creates concentration gradients. The pump can move ions (and other molecules) in the opposite direction to diffusion, from a low to a high area of concentration. This takes energy in the form of ATP and it is not "free". We use this form of active transport to set up concentration gradients that membrane channels use to signal muscle cells and neurons.

Image showing active diffusion using the example of the sodium-potassium ion exchange pump.

Active Transport Without Proteins

There are other forms of active transport that move larger amounts of substances and do not require membrane proteins.

Endocytosis is a process of a cell ingesting by enveloping the substance.

  • Phagocytosis is cell eating - think an immune cell eating a bacteria.

  • Pinocytosis is cell drinking.

  • Receptor-mediated phagocytosis is cell eating only very specific substances.

Exocytosis is a process of a cell releasing a substance or substances from the cell.