Passive Transport in Cells: Simple and Facilitated Diffusion & Osmosis

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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

A cell membrane is selectively permeable - not permeable to everything. In this lesson, we'll talk about methods of passive transport along a concentration gradient, including simple and facilitated diffusion and osmosis.

Concentration Gradients

Cells have a pretty sophisticated cell membrane, which acts as a barrier to the outside world. We've described this membrane as selectively permeable, meaning not just anything can get through it. The key to this phrase is that the cell membrane is selective, but not impermeable. This is something like how you would keep your home. The walls of your house create a boundary and define the space, but there are still doors and there are windows through which you can let in your friends or some fresh air on a summer's day.

So, what crosses a cell membrane and why? There are several types of ways to transport things across a cell membrane. When and how things travel depends largely on the concentration of solutes in your cells, or the dissolved molecules. In this lesson, we'll discuss methods of transporting solutes across a concentration gradient.

A concentration gradient is a gradual difference in solute concentration between two areas. In this case, it's the difference in solute concentration between the outside of the cell and the inside of the cell. Solutes here would move by diffusion, or movement from a higher concentration of solutes to a lower concentration of solutes in order to equalize solute concentration. This evens out the concentration on both sides.

This is like what happens when you leave the windows of your house open while your neighbor is having a barbecue. The smell might diffuse from next door, where the smell is stronger in the air, into your house, where, unfortunately, there are no hamburgers on the grill. Diffusion occurs until the inside of your house smells like the outside.

Passive Transport: Simple Diffusion

Diffusion across a cell membrane is a type of passive transport, or transport across the cell membrane that does not require energy. Remember that the cell membrane is a phospholipid bilayer. Although the inside and the outside of a cell are both water-based, there is a hydrophobic region in the middle, and this is an important barrier to anything large, charged, or hydrophilic. Molecules that are hydrophobic, just like the hydrophobic region, can pass through the cell membrane by simple diffusion.

Therefore, simple diffusion is the unassisted passage of small, hydrophobic, nonpolar molecules from a higher concentration to a lower concentration. Very small molecules can slip through the cell membrane, too, even if they are hydrophilic - just like a few ants might crawl through a crack in the wall just because they're tiny.

Passive Transport: Facilitated Diffusion

So, how do large, charged, or hydrophilic molecules pass through the cell membrane if they can't simply just diffuse in? Think of how your friends come into your house. Under most circumstances, they'll use a door. A package delivered at your residence would come through your door, too, but it would need someone to carry it in. A fly might come through an open window on its own, while a squirrel could come down your chimney! There are different types of passageways into the cell just like there are different ways to get into your home, depending on who or what is trying to get through. Each method of passage through the cell membrane might be useful to different molecules.

Facilitated diffusion is passive transport that uses integral membrane proteins to help larger, charged, hydrophilic, and polar molecules across a concentration gradient. Remember that integral membrane proteins span the phospholipid bilayer, connecting the inside and the outside of the cell.

There are two types of integral membrane proteins that help transport molecules, like ions and polar molecules, that can't diffuse on their own through the hydrophobic layer. The first are carrier proteins, which are proteins that bind a molecule to facilitate transport through a cell membrane. The second are channel proteins, which are proteins that create a passageway to transport molecules and ions through the cell membrane. This channel protein creates a pore through the hydrophobic region that allows polar molecules to just pass right through.

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