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.
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.
Passive Transport: Osmosis
The last type of passive transport involves water. No matter what your house is made of, water seems to find a way in during a big rainstorm. Water is a powerful substance. Even though your cell is surrounded by a hydrophobic region created by the phospholipid bilayer, water can still make it into the cell, too. It can pass through your cell membrane in part because it's very small, but it also gets a little help from other molecules and membrane proteins, too. Even with this help, however, this type of transport does not require energy. You might remember that we call this osmosis, or passive transport of water across a membrane. Water is an excellent way to even out solute concentration without having to move solutes themselves.
More water and less solutes inside the cell than outside creates a hypotonic environment.
In the cell above, there are more solutes outside the cell and less solutes inside the cell. Therefore, you might remember that we'd call the inside of the cell a hypotonic solution, having less solutes, more water. Think of the 'O' in hypotonic as a hippo swelling with water by the river, as a reminder that the hypotonic solution is whichever solution has more water in it. We'd refer to the outside of the cell as a hypertonic solution, having more solutes, less water.
In osmosis, water will move to equalize solute concentration. Therefore, water will move from a hypotonic environment to a hypertonic environment. In this case, water would be transported out of the cell, causing the cell to shrink. This would continue to happen until the two sides were isotonic, or equal concentrations of solutes.
What would happen if a cell was placed in a hypotonic solution, with more water and less solutes outside the cell? A cell in this situation would have water move into the cell, causing the cell to swell. If it grows too much, it might burst, like a balloon filled with too much air!
In this lesson, we've learned several different methods of passive transport, or transport across the cell membrane that does not require energy. Passive transport moves across a concentration gradient, or a gradual difference in solute concentration between two areas.
Simple diffusion is the diffusion of small, uncharged, or hydrophobic molecules from an area of high concentration to an area of low concentration across the cell membrane. Facilitated diffusion is diffusion using carrier or channel proteins in the cell membrane that assist in the movement of molecules across a concentration gradient. The third type of movement is known as osmosis, or the movement of water to equalize solute concentration.
Following this lesson, you'll be able to:
- Define 'passive transport' and 'concentration gradient'
- Describe three methods of passive transport across a cell: simple diffusion, facilitated diffusion, and osmosis
- Understand which types of molecules use which type of passive transport