It may seem impossible that single-celled organisms are as complex as larger organisms, but even these tiny creatures use many of the same methods of obtaining nutrients and expelling waste products that we do. All cells, whether they are an entire organism by themselves or just one tiny part of a larger organism, need membrane transport mechanisms to maintain solute concentrations, import large molecules, and maintain their water balance. Cells transfer substances across the cell membrane using two basic methods, active transport, which includes energy-driven proteins, endocytosis and exocytosis, and passive transport, which includes diffusion and facilitated diffusion.
This stuff will definitely be on the AP test. So, stick with us as we cover the basics of membrane transport, including the differences between active and passive transport. This video covers section 2.6 of the AP Biology curriculum, Membrane Transport. We'll start by looking at the differences between active and passive transport. Then, we'll take a specific look at both passive transport, including diffusion and facilitated diffusion, and the energy dependent modes of active transport.
After the quiz, we'll take a look at how cells can take in large amounts of material via endocytosis and how cells can export large amounts of material via exocytosis. If you only need to review one of these topics, feel free to skip forward to the times outlined here. Let's jump right in. The difference between active transport and passive transport is simple. Active transport requires energy.
As we will see in a few slides, active transport can get this energy from ATP or it can utilize the potential energy stored in a concentration gradient. Active transport requires energy because it is moving a substance against the concentration gradient. In other words, the molecules are moving from an area of low concentration to an area of high concentration.
By contrast, passive transport does not require energy. No energy is needed because all forms of passive transport are moving molecules from an area of high concentration to an area of low concentration. Passive transport includes simple diffusion through the plasma membrane as well as facilitated diffusion through ion channels and carrier proteins. Let's take a look at each of these modes of transport.
Passive transport does not require energy simply because molecules are moving in the direction they would be moving anyway, from high to low. There are two basic types of passive transport, simple diffusion and facilitated diffusion. Let's take a closer look at simple diffusion. Some molecules, like oxygen, water, and carbon dioxide, are small enough that they can pass right through the plasma membrane. Oxygen and carbon dioxide are nonpolar, uncharged molecules.
This means that the hydrophobic core of the lipid bilayer does not effectively block them from passing through. While water is a polar molecule, it does not carry a charge. So water can slip through the plasma membrane when concentration gradients or pressure changes force it to move. When water moves across the membrane, it is called osmosis, and we will take a closer look at this phenomenon in section 2.8. Now let's take a look at facilitated transport.
Facilitated transport is required for ions and large molecules. Ions cannot pass through the plasma membrane because they carry a charge and are blocked by the hydrophobic core. So, they must pass through hollow proteins known as channel proteins.
Large molecules, such as glucose, are simply too large and polar to pass through the small gaps in the plasma membrane. These molecules are also too large for channel proteins, so they require a special carrier protein. These large molecules enter the carrier protein and bind to the active site which changes the conformation of the protein.
This change causes the protein to open on the other side of the membrane releasing the molecule and resetting the process. We will cover both of these transfer proteins further in section 2.7. Think about this.
Passive transport is sort of like a dam. The substance that wants to move, consider this the lake, is blocked by the plasma membrane, or the dam. The substance wants to move down its concentration gradient, Much like the water in the lake wants to move downstream, but it can only do this through ion channels or carrier proteins, just like the gates of that dam that slowly let water through. However, what if you wanted to move water from the stream below back into the lake above the dam? For that, you would need a pump that uses energy to move water against gravity.
That is the same as active transport, which we'll look at next. Active transport requires energy because it is moving molecules from an area of low concentration to an area of high concentration. Unlike most forms of passive transport, active transport is directional.
That is, it transports a specific substance in only one direction. There are three main types of proteins that engage in active transport. A uniport, or sometimes uniporter, uses energy.
to actively pump one type of substance against its concentration gradient. A symport or symporter moves two substances at the same time in the same direction across the cell membrane. Some symporters are moving both molecules against their gradient while others use the energy from one substance's gradient to power the movement of another molecule against the gradient.
An antiport or antiporter moves two substances across the cell membrane, but in opposite directions. Antiporters can also use one molecule's gradient to power the movement of another molecule against the gradient. There are two types of energy that can be used to power active transport, primary and secondary. Primary active transport requires chemical energy from ATP or other energy transporting molecules. The ATP molecule reacts with the transporter protein, removing a phosphate group and releasing energy into the protein's molecular structure.
This allows the protein to grab onto a substrate molecule and move it through the membrane against the concentration gradient. By contrast, secondary active transport does not rely on chemical energy molecules like ATP. Instead, secondary active transport relies on the potential energy stored in a concentration gradient.
For example, this sodium calcium antiporter is using the energy stored in the sodium concentration gradient to move calcium against its concentration gradient. Three sodium molecules move into the antiporter, pushed by the concentration gradient. The antiporter then takes up one calcium ion and the energy from the sodium gradient forces a conformational change, forcing the calcium ion out of the cell against its concentration gradient.
Cells use a wide variety of integral membrane proteins to build up these chemical gradients and use them to power the movement of other substances across their cell membranes. Now that we have covered the differences between active and passive transport, let's see if you can answer some AP-style questions about membrane transporters. Pause the video now and answer this set of questions. You can find the answers to these questions through the quick test prep link in this video's description. Next up, let's look at some forms of membrane transport that are on a much larger scale than individual membrane proteins.
Endocytosis and exocytosis are how the cell can import or export large amounts of material at the same time using large folds of the plasma membrane. The difference is simple to remember if you break down the words. Endo means within or into, whereas cytosis refers to cells.
So endocytosis means into the cell. Cells use endocytosis to take in large molecules, create food vesicles, and even eat smaller cells. By contrast, exo means external or out of, so exocytosis means out of the cell.
Cells use exocytosis to dump entire vesicles into the external environment. Endocytosis and exocytosis are both forms of active transport because it takes a lot of energy to form vesicles and move them around the cell using the cytoskeleton. Let's take a look at the different kinds of endocytosis and exocytosis.
There are three main types of endocytosis that cells use to intake large quantities of material. Phagocytosis, penocytosis, and receptor-mediated endocytosis. Phagocytosis is how cells take in very large macromolecules and even smaller cells.
For instance, entire bacterial cells can be eaten by white blood cells. The cell membrane wraps itself around the large object, then pinches off into a food vacuole. A lysosome will merge with the food vacuole, digesting its contents so the cell can use them.
Similarly, Phenocytosis takes in a large quantity of water and substances by creating an inward fold of the cellular membrane. The folds are generally much smaller than with phagocytosis. In this case, the cell simply sucks in water and the smaller substances that are dissolved in that water.
This is a good way for the cell to take in a large quantity of water and nutrients at the same time. But Cells can use receptor-mediated endocytosis to take in a large quantity of a very specific substance. For instance, this is how your body transfers and recycles molecules like cholesterol, which would otherwise get stuck in the plasma membrane.
Cholesterol is bonded to protein molecules, making lipoproteins. These lipoproteins can bind to specific receptors on the cell's surface. When enough receptors have been activated, The entire portion of the cell membrane undergoes endocytosis.
The vacuole merges with the liposome and is digested completely. The components from the original cholesterol molecules can then be recycled. Phew-wee!
That was a lot, but don't worry, we're almost done here. If you need a quick break, grab some water and hurry back. One more section on exocytosis and we'll be on our way.
For the same reason that cells need to use entire portions of the cell membrane to intake substances, there are many uses for expelling substances with a similar process. This process is known as exocytosis. For instance, this is exactly what happens in your neurons every time they transfer a signal to the next neuron. The nerve impulse comes through the presynaptic neuron ending at the axon terminal.
This causes vesicles full of neurotransmitters to bind with the cell membrane. These neurotransmitters are dumped into the synaptic space via exocytosis. The neurotransmitters quickly reach the next neuron and open ion channels.
This disrupts the electrical balance of the cell membrane, causing a new nervous impulse to travel through the postsynaptic neuron. Now that we have covered endocytosis and exocytosis, let's see if you could pass this section on the AP test. Pause the video now and answer this set of questions.
You can find the answers to these questions through the quick test prep link in this video's description, along with many other AP biology resources that can help you study. Thanks for watching. We hope this video was helpful and informative. Please like this video if it helped you study, leave us any comments or questions you still have about membrane transport, and subscribe to the Biology Dictionary channel to find all of our AP Biology videos and resources. Good luck!