Cell transport is a biological process through which materials pass into and out of cells, crossing the membrane or “outer wall” in the process. It happens in almost all organisms, and is an important part of cell health; it allows nutrients to come in and waste to go out, for one thing, and also allows for regulation of things like fluid levels and overall temperature. In general there are two types of transport, namely passive and active. There are some important chemical differences between the two, but on a very basic level passive transport requires no energy output, whereas active transport does require energy expenditure at some level. Membrane crossing isn’t usually something that happens easily, and typically requires the coordination of several different elements. The membrane is what keeps the cell protected and secure, and transport is a unique process that allows for brief penetrations of this otherwise thick boundary.
Passive Transport Generally
Passive transport doesn't require energy from the cell, but rather use utilizes different pressures and fluid concentrations on the outside to trigger penetration and release of the inside. In most of these cases, the cell’s interior has a low concentration of a particular molecule, while the exterior environment has a high concentration of that same component. Transport occurs to balance the concentrations inside and out to reach an equilibrium. There are two main ways through which this sort of transport can happen: simple diffusion or facilitated diffusion.
The Different Types of Diffusion
In simple diffusion, individual molecules travel down what’s known as the “concentration gradient” in the membrane wall from high to low. The concentration gradient works to keep the cell balanced when its internal levels of things like fluid and other nutrients is different from the composition of the outside environment. When conditions are on opposite ends of the spectrum on the outside versus on the inside, the membrane gradient tends to become more porous and small molecules can often pass in and out without much effort in order to restore balance. Osmosis is a type of simple diffusion that specifically describes how water molecules pass through the cell membrane under these conditions; oxygen can also enter in this way. Some viruses can too, though, which means that it isn’t always positive.
Facilitated diffusion, on the other hand, utilizes a protein within the cell membrane to carry molecules into the cell. Glucose is one type of molecule that requires a membrane protein to pass through the cell membrane. This process is still considered passive because it doesn’t really require a true energy output, but the protein molecule acts as sort of a check so that transport isn’t as free as it is in simple diffusion. The cell doesn’t really have to work, in other words, but it still has some control.
Examples of Active Transport
Transport that is “active” is different in that it requires the energy of certain membrane proteins to get across the concentration gradient and either into or out of the cell. It requires more from the cell and it usually costs something in terms of energy and nutrient expenditure, too.
An example of active transport is the sodium-potassium pump in nerve cells. Normally, the concentration of sodium is low in the cell and high outside the cell while potassium concentration is typically high inside the cell and low outside the cell. When a nerve impulse is triggered, sodium and potassium pass through the cell membrane along the concentration gradient. To return the concentration gradient to its initial state, the membrane protein must continually pump sodium out of the cell and bring potassium into the cell. This requires energy from the cell.
Involvement of Vesicles
Other types of cell transport that require energy involve vesicles. Vesicles are small sacs that are produced from the cell's Golgi apparatus. They collect waste products which they expel from the cell by fusing with the cell membrane, though they can also be used to collect molecules that are outside the cell. In a process called endocytosis, the cell membrane surrounds the molecule and incorporates it into a vesicle, which detaches from the membrane and enters the cell. The contents of the vesicle are then digested and released into the cell.