There are two main kinds of cellular transport: passive transport and active transport.
When passive transport like diffusion occurs, molecules move from a concentrated area to a less concentrated area through channel proteins. This doesn’t require any energy; the pressure in the concentrated area will naturally push molecules to the area of lower pressure or it is simply the difference in its concentration on the two sides of the membrane—its concentration gradient—that drives passive transport and determines its direction.
If the solute carries a net charge, however, both its concentration gradient and the electrical potential difference across the membrane, the membrane potential, influence its transport. The concentration gradient and the electrical gradient can be combined to calculate a net driving force, the electrochemical gradient, for each charged solute. In fact, almost all plasma membranes have an electrical potential difference (voltage gradient) across them, with the inside usually negative with respect to the outside. This potential difference favors the entry of positively charged ions into the cell but opposes the entry of negatively charged ions.
The opposite of the passive transport is called the active transport, where the molecules move from a less concentrated area to a more concentrated area which is mediated by carriers, which are also called pumps. In active transport, the pumping activity of the carrier protein is directional because it is tightly coupled to a source of metabolic energy, such as ATP hydrolysis or an ion gradient
Thus, transport by carriers can be either active or passive, whereas transport by channel proteins is always passive.
- Passive transport: Simple diffusion, Osmosis and the Facilitated diffusion
Diffusion is the movement of particles down their gradient. A gradient is any imbalance in concentration, and moving down a gradient just means that the particle is trying to be evenly distributed everywhere, like dropping food coloring in water. We call this evening-out moving “downhill”, and it doesn’t require energy. The molecule most likely to be involved in simple diffusion is water – it can easily pass through cell membranes. When water undergoes simple diffusion, it is known as osmosis.
Simple diffusion is pretty much exactly what it sounds like – molecules move down their gradients through the membrane. Molecules that practice simple diffusion must be small and nonpolar, in order to pass through the membrane. Simple diffusion can be disrupted if the diffusion distance is increased.
Osmosis is a special term used for the diffusion of water through cell membranes. Water passes by diffusion from a region of higher to a region of lower concentration. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms. Water is never transported actively; that is, it never moves against its concentration gradient. However, the concentration of water can be altered by the active transport of solutes, in a way by which the movement of water in and out of the cell can be controlled.
To explain the behavior of a cell in a solution, we must consider both solute concentration and membrane permeability. Both factors are taken into account in the concept of tonicity, the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (non-penetrating solutes) relative to that inside the cell. If there is a higher concentration of non-penetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa.
If a cell without a wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means “same”), there will be no net movement of water across the plasma membrane. Water diffuses across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable.
When the cell is transferred to a solution that is hypertonic to the cell (hyper means “more,” in this case referring to non-penetrating solutes). The cell will lose water, shrivel, and probably die.
And when the cell is transferred to a solution that is hypotonic to the cell (hypo means “less”), water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.
The passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins. Each aquaporin allows entry of up to 3 billion (3× 109) water molecules per second, passing single file through its central channel, which fits ten at a time. Without aquaporins, only a tiny fraction of these water molecules would pass through the same area of the cell membrane in a second, so the channel protein brings about a tremendous increase in rate.
Facilitated diffusion is diffusion that is helped along (facilitated by) a membrane transport channel. These channels are glycoproteins (proteins with carbohydrates attached) that allow molecules to pass through the membrane. These channels are almost always specific for either a certain molecule or a certain type of molecule (i.e. an ion channel), and so they are tightly linked to certain physiologic functions. For example, one such transporter channel, GLUT4, is incredibly important in diabetes. GLUT4 is a glucose transporter found in fat and skeletal muscle. Insulin triggers GLUT4 to insert into the membranes of these cells so that glucose can be taken in from the blood. Since this is a passive mechanism, the amount of sugar entering our cells is proportional to how much sugar we consume, up to the point that all our channels are being used (saturation). In type II diabetes mellitus, cells do not respond as well to the presence of insulin, and so do not insert GLUT4 into their membranes. This can lead to soaring blood glucose levels which can cause heart disease, stroke, and kidney failure.
- Active Transport
Certain times the body needs to move molecules against their gradient. This is known as moving “uphill”, and requires energy from the cell which is mediated by carriers, which are also called pumps. In active transport, the pumping activity of the carrier protein is directional because it is tightly coupled to a source of metabolic energy, such as ATP hydrolysis or an ion gradient.
This is most obvious in the sodium-potassium pump (Na+/K+ ATPase) that helps maintain resting potential in the cell. This protein uses the energy released from hydrolysis of ATP (adenosine triphosphate) to pump three sodium ions out of and two potassium ions into the cell. ATP is an energy molecule, and when hydrolysis happens, it gets broken down to release the energy that was stored in its chemical bonds. Transport that directly uses ATP for energy is considered primary active transport. Export of sodium from the cell also provides the driving force for several secondary active transporters membrane transport proteins, which import glucose, amino acids, and other nutrients into the cell by use of the sodium gradient. In most animal cells, the Na+/K+ ATPase is responsible for about 1/5 of the cell’s energy expenditure. For neurons, the Na+/K+ ATPase can be responsible for up to 2/3 of the cell’s energy expenditure.
One other location for such an ATP pump is the proton/potassium exchanger (H+/K+ ATPase) found in the stomach. These proton pumps are responsible for creating the acidic environment of the stomach, and can cause acid reflux. Proton pump inhibitors like omeprazole are prescribed to patients with ulcers or acid reflux to help reduce the acidity of their gut.
Secondary active transport moves multiple molecules across the membrane, powering the uphill movement of one molecule(s) (A) with the downhill movement of the other(s) (B). For example, SGLT2 is a glucose transporter that allows glucose (Molecule A) into our cells (against its gradient) by bringing in a sodium molecule (Molecule B) as well. The energy released by sodium traveling down its gradient is enough to power glucose into the cell. Since both molecules moved in the same direction, this molecule is known as a symporter.
Proteins that allow molecules to go in opposite directions are antiporters – one great example of this is the sodium/calcium exchanger used to restore cardiomyocyte (heart cell) calcium concentrations after an action potential. An influx of calcium causes the heart to contract, and the antiporter pushes calcium (Molecula A) out against its gradient, while bringing in a sodium ion (Molecule B) to let the heart relax.