How do cells move material across their membranes and the differences between the processes?  diffusion, osmosis, facilitated diffusion, primary & secondary active transport, bulk transport,...

How do cells move material across their membranes and the differences between the processes?


diffusion, osmosis, facilitated diffusion, primary & secondary active transport, bulk transport, Na+/K+ pumps, membrane resting potential


Expert Answers
txmedteach eNotes educator| Certified Educator

Most of these processes are actually separated by two things: energy and whether or not a protein is involved.

Simple diffusion is the passing of a substance through a membrane from a region of high concentration to low concentration. This really only happens in cells with small, lipid-soluble molecules like miceles or steroid hormones. Otherwise, most other things are kept out. This is good, though, because we don't want just anything to be able to get in! So, here, we see no energy usage by the cell, nor do we see transport proteins involved.

In facilitated diffusion, we have almost the same deal. Now, however, solutes are passed through channels on the membrane, usually because they can't pass through the membrane on their own. For example, cells are pretty good at keeping Na+ out using the membrane. However, Na+ quickly diffuses into cells that have sodium channels (think nerves). These channels, do not use energy, though.

Osmosis is actually somewhat complicated, and seems counterintuitive sometimes. This is the process by which water actually travels from a region of low solute concentration to high solute concentration. This does make sense though when you consider the water is actually going down its "water concentration" gradient in this process. What really complicates this process is that osmosis actually involves both simple and facilitated diffusion! Some water can actually diffuse into cells through the nonpolar membrane. More water, though, gets into cells through channels called aquaporins (facilitated).

Active transport is separated into two processes: primary and secondary. Primary active transport involves a channel that uses ATP to force the transport of solutes against their concentration gradient. ATP provides energy to power the pump and push these guys in or out! The classic example is the Na+/K= pump where sodium ions are forced outside the cell and potassium ions inside the cell. You need a pump for this because sodium is already much more concentrated outside, and potassium is already much more concentrated inside!

Secondary active transport involves the use of a primary active transport mechansim to bring an unrelated solute into a cell. The easiest example of this is the Na+-Glucose cotransporter. A Na+/K= pump on the other side of the cell forces out sodium to make a low concentration inside the cell. The cotransporter is the only way for sodium to get in, but for sodium to get in, it needs to take glucose with it! So, glucose is actually moved against its concentration gradient because the cell is so sodium deficient as to be willing to take the extra glucose to get more sodium.

Bulk transport can be taken in two ways. The first way is where we consider the mass movement of fluids. For example, blood may be considered to circulate through bulk transfer. However, if you're considering vescicular transfer in the cells, it's different. There are no channels involved here, but there is the synthesis and exocytosis of a chemical involved (lots of energy). For example, if you want to synthsize a hormone and release it on command, you let it stand by in vescicles. When your cell is activated, it then releases all that stuff in on bulk load into the blood stream.

I hope that helps and makes sense! Again, details are left out, but because you could fill books with info about these processes.

txmedteach eNotes educator| Certified Educator

Hey, just noticed you mentioned membrane potentials. Those are more based on facilitated diffusion, where you deal with charged particles and vary the permeability of the membrane to those particles. It's actually very complicated, but it pretty much says there are two forces that move ions across a membrane: concentration gradient and charge. To see more info on this, look at the idea of Nernst Potentional (link below).