Membrane Transport: Passive Transport

So, we’ve just described the cell’s Plasma Membrane (PM) and how it separates the water inside a cell from the water outside a cell. But no man cell is an island. Except pancreatic islet cells, which is a joke we won’t cover until A&P 2.

Obviously we’re going to need to import nutrients and export waste products. How do we get things into and out of cells?

Methods of Transport:

  • Passive Transport
    • Diffusion
    • Facilitated Diffusion
  • Active Transport
    • Pumps
    • Endocytosis
    • Exocytosis
    • Phagocytosis
  • Secondary Active Transport
    • Symport/Cotransport
    • Antiport

What separates passive from active transport is energy. Passive transport does not require energy, while active transport does require energy. Energy is provided by a molecule called ATP. We’ll get more into what ATP is in a later post, but for now, just know that it provides the energy for active transport.

To understand passive transport, you have to understand Diffusion. Basically, if things are highly concentrated in one area, they will tend to spread out to areas where they are less concentrated.

The example I always use is a shoe box with a bunch of marbles on one side. If you gently shake the box, the marbles will bounce into each other and spread out to fill the rest of the box. When the marbles are equally spread out, we would say they are at equilibrium.

Molecules are no different. When they’re highly concentrated in one area, they keep bouncing off of each other until they spread into areas with fewer of that molecule in it. We can see that happening here:

As you can see, the food dye is highly concentrated around the skittles. It dissolves in the water and spreads into new areas, where the concentration of dye is lower. Molecules will always move along their concentration gradient, which again, means from areas of high concentration to areas of low concentration.

Now that we understand diffusion, let’s jump in with Passive Transport methods! Passive transport doesn’t require energy because molecules are following their concentration gradient. Some molecules are able to freely cross the Plasma Membrane. We would say these molecules are membrane permeable. Membrane permeable molecules must be small and have no charge. They must be lipophilic, meaning they love the lipid interior of the Plasma Membrane. Examples include O2, CO2, ethanol (drinking alcohol), and steroid hormones. These molecules just follow their concentration gradient and diffuse into or out of the cell.

Most molecules are not membrane permeable. In fact, we would say they are membrane impermeable. Molecules that are large, charged, or both cannot cross the Plasma Membrane. We still need to get some of these molecules into the cell. To do this, we can insert a channel in the membrane, which lets them through. When we use a channel to allow substances to cross the Plasma Membrane, we call it Facilitated Diffusion because the diffusion was facilitated by a protein channel.

For example, remember that water has a partial charge. Therefore, water cannot cross the plasma membrane. We can let water into or out of a cell using protein channels called aquaporins. In some cells (ex: certain cells in the Renal Tubules) we don’t water to cross the plasma membrane, so we can remove the aquaporins in the membrane.

Other examples of molecules that require facilitated diffusion include ions (Sodium, Potassium, Chloride, Calcium) and even glucose. When our blood-sugar levels rise, we release the hormone insulin. Insulin causes glucose channels to be inserted in the Plasma Membrane, allowing glucose to diffuse into the cell. Type II Diabetes is an insensitivity to insulin — the cells don’t respond to insulin, so the channels don’t get inserted into the membrane and glucose can’t cross the PM. The blood-sugar stays high while the cells starve and cannibalize themselves.

As you can see, passive transport relies heavily on diffusion. Next time, we’ll pick it up with Active Transport, which will require energy (and more proteins!)

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