Membrane Transport
Now that you know about the general function of the cell membrane and the specific structures that give it that function, it’s time to get into a little more detail about the specifics of membrane transport.
You’ve already seen the big picture of the functions of the cell membrane. Here, we’re mainly just putting names to ideas. For example, you already know that:
That is really all we’ll be talking about in this lesson! You will learn that these processes are called simple diffusion, facilitated diffusion, and active transport, respectively. Take a minute to see if these names make sense.
You’ve already seen the big picture of the functions of the cell membrane. Here, we’re mainly just putting names to ideas. For example, you already know that:
- Small, nonpolar molecules pass through the phospholipid bilayer in order to diffuse down their concentration gradients.
- Proteins in the cell membrane can allow specific large or polar molecules to pass through either down their concentration gradients, without using energy, or
- Proteins in the cell membrane can force molecules to go against their concentration gradients.
That is really all we’ll be talking about in this lesson! You will learn that these processes are called simple diffusion, facilitated diffusion, and active transport, respectively. Take a minute to see if these names make sense.
Review of Diffusion and Osmosis
A lot of this lesson focuses on the basic chemistry principles of diffusion and osmosis. If you don’t remember these topics well, it may be wise to go back and review diffusion and osmosis before you continue with this lesson. These are covered in Lesson 8: Solubility and Diffusion. You’ll want to especially remind yourself of sublesson 3, Diffusion Across a Semipermeable Membrane.
As a brief reminder, any molecule that is able to diffuse across a semipermeable membrane will do so until its concentration is equal on both sides of the membrane. If a molecule is not able to cross a semipermeable membrane, water will diffuse by osmosis until the concentrations of molecules are equal on both sides.
Be sure to always associate diffusion with particles and osmosis with water. In diffusion, particles move from high concentration to low concentration: The particles want to spread out from each other. In osmosis, if the particles can’t move for some reason, water moves from where particles are at a low concentration to where particles are at a high concentration. You can also think of this as water moving from where water is at a higher “concentration”—there are more water molecules per total molecules—to where water is at a lower “concentration”.
As a brief reminder, any molecule that is able to diffuse across a semipermeable membrane will do so until its concentration is equal on both sides of the membrane. If a molecule is not able to cross a semipermeable membrane, water will diffuse by osmosis until the concentrations of molecules are equal on both sides.
Be sure to always associate diffusion with particles and osmosis with water. In diffusion, particles move from high concentration to low concentration: The particles want to spread out from each other. In osmosis, if the particles can’t move for some reason, water moves from where particles are at a low concentration to where particles are at a high concentration. You can also think of this as water moving from where water is at a higher “concentration”—there are more water molecules per total molecules—to where water is at a lower “concentration”.
Diffusion and osmosis are really just variations of the same idea: Whenever any molecule (a particle or water) can move, it will move until it is as uncrowded as possible, like by moving to the other side of a semipermeable membrane until the concentrations on both sides are equal.
Diffusion and Osmosis in Cells
The basic principles behind diffusion and osmosis don’t change just because we’re now talking about a living cell. Particles still “want” to move to where they’re less crowded and will do so as long as they can get across the membrane. This is called moving down the concentration gradient. But, as part of a living cell that uses energy, membrane proteins can also force something up or against its concentration gradient (to where it is more crowded) by using up energy.
Let’s talk a little more about the specific types of transport that happens in cells.
Let’s talk a little more about the specific types of transport that happens in cells.
Simple Diffusion
Simple diffusion is diffusion of small, nonpolar molecules across the phospholipid bilayer. These molecules can get through any part of the membrane, so they will, without restriction, until their concentration on the outside of the cell is equal to their concentration on the inside of the cell.
Think for a second: If a molecule is more concentrated on the inside of the cell, which way will it move?
It will move down its concentration gradient to get to the outside of the cell! This gets it to the area where it is less crowded.
Similarly, if a molecule is more concentrated on the outside of the cell, it will move down its concentration gradient to get to the inside of the cell.
Now think: is it possible to force a molecule against its concentration gradient by simple diffusion?
No! The cell has no control over simple diffusion. It just happens. Only proteins can be controlled by energy switches (ATP). So, molecules that cross by simple diffusion will always be found in exactly the same amounts on the inside of the cell as they are in the immediate external environment.
The only thing that’s really unique about simple diffusion in a cell compared to simple diffusion problems that we did with U-tubes in the Chemistry unit is what is able to get through this type of semipermeable membrane. In chemistry, we usually talked about semipermeable membranes that let water across, but not ions. The phospholipid bilayer lets across only small, nonpolar compounds. This does not include water.
There is no strict cut-off for “how big is too big” to get across the membrane by simple diffusion. Like most things in science, it’s a spectrum. Very small, very nonpolar things, like CO₂ and O₂, will always get across very quickly. Very small, somewhat polar things, like the toxin carbon monoxide (CO), will get across pretty quickly, but just a little bit slower than CO₂ or O₂. Very small, very polar things, like water (H₂O) will only get across pretty slowly and in pretty small amounts. Similarly, medium-sized, very nonpolar things, like steroid hormones, will get across somewhat more slowly than very small, very nonpolar things. The larger or more polar something is, the less likely it is to get across by simple diffusion.
Think for a second: If a molecule is more concentrated on the inside of the cell, which way will it move?
It will move down its concentration gradient to get to the outside of the cell! This gets it to the area where it is less crowded.
Similarly, if a molecule is more concentrated on the outside of the cell, it will move down its concentration gradient to get to the inside of the cell.
Now think: is it possible to force a molecule against its concentration gradient by simple diffusion?
No! The cell has no control over simple diffusion. It just happens. Only proteins can be controlled by energy switches (ATP). So, molecules that cross by simple diffusion will always be found in exactly the same amounts on the inside of the cell as they are in the immediate external environment.
The only thing that’s really unique about simple diffusion in a cell compared to simple diffusion problems that we did with U-tubes in the Chemistry unit is what is able to get through this type of semipermeable membrane. In chemistry, we usually talked about semipermeable membranes that let water across, but not ions. The phospholipid bilayer lets across only small, nonpolar compounds. This does not include water.
There is no strict cut-off for “how big is too big” to get across the membrane by simple diffusion. Like most things in science, it’s a spectrum. Very small, very nonpolar things, like CO₂ and O₂, will always get across very quickly. Very small, somewhat polar things, like the toxin carbon monoxide (CO), will get across pretty quickly, but just a little bit slower than CO₂ or O₂. Very small, very polar things, like water (H₂O) will only get across pretty slowly and in pretty small amounts. Similarly, medium-sized, very nonpolar things, like steroid hormones, will get across somewhat more slowly than very small, very nonpolar things. The larger or more polar something is, the less likely it is to get across by simple diffusion.
Facilitated Diffusion
Facilitated diffusion is also a type of diffusion, meaning that molecules are still going down their concentration gradients, from more concentrated to less concentrated. However, now, instead of just happening at any point along the cell membrane like in simple diffusion, transport is facilitated by membrane proteins. This allows larger and more polar things to cross the membrane.
Remember, each membrane transport protein is responsible for transporting only a specific type of molecule. There are many, many different types of transport proteins. If there is a transport protein for a molecule, then it will be able to cross the membrane. If there isn’t a transport protein for a molecule, then it won’t be able to cross the membrane. One very important type of transport protein is called aquaporin (“water pore”). As you may guess from the name, this is the transporter for water. Water is a polar molecule, so it doesn’t cross the phospholipid bilayer very easily. However, most cells have abundant aquaporins, so osmosis can happen very readily by facilitated diffusion. |
Note that, in order for facilitated diffusion to occur, the molecule must be in higher concentration in the external environment than in the internal environment, because we aren’t spending any energy to push something against a concentration gradient. But, that doesn’t mean that the molecule has to be permanently in higher concentration outside of the cell. For example, you don’t constantly eat food, but nutrients don’t just diffuse away from your cells when you stop eating. Transport can be unidirectional, meaning something is brought into the cell and is stuck there until it is either used or we don’t want it anymore. This is one of the reasons that facilitated diffusion is a little different than just “regular diffusion but with proteins.”
Another important reason that facilitated diffusion is a little different than “regular diffusion but with proteins” (which is mostly true) is that we can control it. We can tell proteins when to turn on and when to turn off, and we can tell our cell to start making more of one protein or less of another. For example, when we want to tell a muscle cell to contract, a neurotransmitter (a chemical signal from a nerve cell) called acetylcholine can tell your cells to open up their ion channels, which causes a whole chain of reactions that makes that muscle cell contract. Ions are still moving down their concentration gradient, but it’s controlled.
Another important reason that facilitated diffusion is a little different than “regular diffusion but with proteins” (which is mostly true) is that we can control it. We can tell proteins when to turn on and when to turn off, and we can tell our cell to start making more of one protein or less of another. For example, when we want to tell a muscle cell to contract, a neurotransmitter (a chemical signal from a nerve cell) called acetylcholine can tell your cells to open up their ion channels, which causes a whole chain of reactions that makes that muscle cell contract. Ions are still moving down their concentration gradient, but it’s controlled.
Since both simple diffusion and facilitated diffusion involve molecules traveling down their concentration gradients, just like regular diffusion that we learned about in chemistry, with no input of energy, they are collectively referred to as passive transport. “Passive” means they don’t require energy.
Active Transport
Active transport forces a molecule up or against its concentration gradient, from an area where it is less concentrated to an area where it is more concentrated. A cell is able to do this by using up cellular energy. (You may remember that ATP is the main source of cellular energy. More on this special molecule in the next lesson.) This type of transport is unique to living cells because it requires that a cell is able to use energy.
Active transport is very important to creating an internal cell environment that is different from the external cell environment. A cell gets to pick and choose what molecules it needs enough or that are rare enough that it is worth spending energy in order to pick them up. (Of course, the cell isn’t really “choosing,” it’s all biochemistry in the end, but go with it). This is kind of how you might choose to spend money on things that you really need, like food, or things that you really like to have but are hard to come by in nature, like cool shoes. The cell is “buying” nutrients from the environment. You are less likely to buy (or need to buy) things that are very abundant in the external environment, like air.
Active transport is also used to force toxins or waste products completely out of the cell. With passive transport, because things just move down their concentration gradient until they are at equal concentrations on the inside and outside of the cell, no molecule that is able to get through the membrane, even in small amounts, will ever reach true zero concentration in the cell. By spending energy, though, the cell is able to force those last few molecules out of the cell. It’s like spending some of your money on home security.
Active transport, like facilitated diffusion, requires membrane proteins. These membrane proteins are often called pumps because of the action that they use to force molecules in or out of the cell.
Because active transport involves proteins and energy, it is easily controlled by the cell. This can either be done using similar mechanisms as the ones that control facilitated diffusion (for example, binding an activator, like insulin, or inhibitor; or changing the amount of protein that is made by the cell), or by cutting off the energy supply.
Focus on the main ideas here: active transport uses energy to force molecules against their concentration gradients, which allows the cell to create an internal environment that is different from the external environment. This video gives a great recap:
Active transport is very important to creating an internal cell environment that is different from the external cell environment. A cell gets to pick and choose what molecules it needs enough or that are rare enough that it is worth spending energy in order to pick them up. (Of course, the cell isn’t really “choosing,” it’s all biochemistry in the end, but go with it). This is kind of how you might choose to spend money on things that you really need, like food, or things that you really like to have but are hard to come by in nature, like cool shoes. The cell is “buying” nutrients from the environment. You are less likely to buy (or need to buy) things that are very abundant in the external environment, like air.
Active transport is also used to force toxins or waste products completely out of the cell. With passive transport, because things just move down their concentration gradient until they are at equal concentrations on the inside and outside of the cell, no molecule that is able to get through the membrane, even in small amounts, will ever reach true zero concentration in the cell. By spending energy, though, the cell is able to force those last few molecules out of the cell. It’s like spending some of your money on home security.
Active transport, like facilitated diffusion, requires membrane proteins. These membrane proteins are often called pumps because of the action that they use to force molecules in or out of the cell.
Because active transport involves proteins and energy, it is easily controlled by the cell. This can either be done using similar mechanisms as the ones that control facilitated diffusion (for example, binding an activator, like insulin, or inhibitor; or changing the amount of protein that is made by the cell), or by cutting off the energy supply.
Focus on the main ideas here: active transport uses energy to force molecules against their concentration gradients, which allows the cell to create an internal environment that is different from the external environment. This video gives a great recap:
Summary
The main things you should be sure to know about each type of transport are:
It is also helpful to review:
This video gives a great review:
- Whether molecules go up or down their concentration gradient
- Whether or not it requires proteins
- Whether or not it requires energy
- Whether or not it can be controlled
- Some examples of which type of molecules can cross in that way
It is also helpful to review:
- What it means to go up or down a concentration gradient
- When osmosis versus diffusion will occur across a semipermeable membrane
- The difference between osmosis and diffusion
- What equilibrium means in the context of osmosis and diffusion
- How to predict which way molecules will go across a membrane that is permeable to that solute and that isn’t (but is permeable to water).
This video gives a great review:
These things are important to understand because they help you to understand how our cells get to be so special and what makes them different from the surrounding environment. It helps you to understand how you get what you need and keep out the things that will hurt you. It gives you an appreciation for how some of the very basic principles that we learned about in chemistry—like the principles of solubility, intermolecular forces, and osmosis and diffusion—relate in very significant ways to your ability to stay alive. Lastly, for fans of applied science, understanding these principles helps us to design drugs/medications, cell dyes, and molecular biology tools that are able to get inside of cells and help save a life or advance the cause of science. Pretty amazing!
Learning Activity
Contributors: Alan Li, Emma Moulton