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Passive Transport Across the Cell Membrane

Membrane Transport Processes

As you now know, the cell is contained within a membrane. This membrane defines the boundaries of the cell and if something wants to get in or out of the cell, it has to cross the plasma membrane. This can happen in one of a few ways. Let’s take a look

What is membrane transport you ask? Well it is simply the process of a substance  (proteins, ions, lipids etc) moving across the cell’s plasma membrane. These processes can either move across the membrane passively, or they can be actively transported across the membrane. Let’s take a look at passive membrane transport processes.

Passive Processes

There are two types of passive transport: diffusion and filtration. We will talk about filtration when we discuss the capillaries. For now though lets just focus on diffusion. Diffusion is a passive transport process that is driven by concentration gradients. Lets take a look at a simple example.

Say we drop a sugar cube in a beaker filled with distilled water. The concentration of sugar in the sugar cube is very high, while the sugar concentration in the water is very low essentially 0%. Once in solution (water) solute molecules (sugar molecules) are constantly moving and colliding with each other. More collisions will take place in areas where the solute is more densely concentrated, and therefore the solute will bounce and ricochet off eachother pushing themselves into areas of lower concentration. Eventually the solute will diffuse throughout the solution until all regions of water within the beaker contain the some concentration of solute molecules.

This same process can drive transport of certain substances across the membrane by utilizing one of three different types of diffusion

Simple diffusion

Remember that the lipid core of the plasma membrane prevents charged particles from crossing through it passively. This means that only lipids and very small molecules like O2 and CO2 can diffuse directly through the phospholipid bilayer. This process is termed simple diffusion because these substances don’t require the assistance of a protein carrier or channel as do the next two types of diffusion which fall into the category of simple diffusion. Again with all types of diffusion, a concentration gradient is the primary driver of simple diffusion

Facilitated Diffusion

Facilitated diffusion allows for larger charged particles, as well as water (Osmosis) to diffuse across the membrane through either a channel protein, or a carrier protein

  • Channel mediated diffusion: Wow what an intuitive name for this type of passive membrane transport. In channel mediated diffusion a transmembrane integral protein spans the entire plasma membrane and contains a channel. Basically, this protein forms a tunnel that connects the intracellular fluid with the extracellular fluid and allows substances to flow down their concentration gradient either into or out of the cell.  Keep in mind that this is a selective transport process, meaning that there are there are many types of channel proteins that allow only specific substances to pass through.
  • Carrier Mediated diffusion: In carrier mediated diffusion, a protein “carrier” embedded in the membrane binds with certain substances either in the intracellular or extracellular fluid. Once these particle bind to the carrier protein, it undergoes a conformational shape change which transports the substance across the membrane. Again, this is a selective process. Certain carrier proteins are responsible for transporting certain substances.

Osmosis

Osmosis is very similar to channel mediated diffusion, except that in osmosis, solvent is transported across the membrane rather than solute. In the human body this solvent is water, and the channels it travels through are called aquaporins.

Water can be driven into and out of the cell down its concentration gradient just like solute can. Picture this example.

Picture two beakers connected in the middle by a tube. In this tube there is a filter that only allows water through. In one beaker we have salt water and in the other we have pure distilled water which contains no solute. Diffusion of solute cannot occur because the filter only allows water through. So the salt in the salt water cannot diffuse down its concentration gradient into the distilled water as it would if there was no filter. Instead, water will diffuse down its concentration gradient from the pure distilled water into the beaker containing the salt water. This will help equalize the concentration gradients between the beakers.

Make sense? If not, leave me a comment so I can try to explain it again.

Lets take a look at some examples of diffusion of water across the cell membrane in three different examples

A cell in a hypertonic solution: A hypertonic solution is a solution that has a higher concentration of solute dissolved in it than does intracellular fluid. When a cell is placed in a hypertonic solution water diffuses down its concentration gradient out of the cell. This leads to a decrease of intracellular volume which causes the cell to shrink and shrivel.

A cell in a hypotonic solution: A hypotonic solution has less solvent dissolved in it than does the intracellular fluid. Conversely, a hypotonic solution has a higher concentration of water than does the intracellular fluid. This leads to a net diffusion of water down its concentration gradient into the cell. This leads to a net diffusion of water into the cell which increases intracellular volume and can ultimately lead to a rupture of the plasma membrane.

A cell in an isotonic solution: An isotonic solution has the same relative concentrations of solute and solvent as the intracellular fluid. Although water may still diffuse into and out of the cell, the amount of water leaving the cell will be equal to the amount entering the cell, and thus there will be no net change of intracellular volume.

Thats it!! We talked about simple diffusion, facilitated diffusion, osmosis, and three different types of solutions that our cells can be exposed to and what the effects may be. You might have guessed that the fluid surrounding the cells in our body (interstitial fluid) is relatively isotonic. In fact, one of the functions of homeostasis is to maintain the isotonicity of the interstitial fluid.

Good luck guys, as always feel free to ask questions.

Trevor

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