Video: Transport across the cell membrane

This box needs to be delivered by tonight. How can we get it there? It's too big to fit in the car, so that's not an option. Hmmm… let's see if there's a train station. Nope. The closest one is way too far. Maybe we can fly it there -- great. The contents are in the list of restricted items. All right, I've had enough. It's going to be expensive but… open the teleportation chamber!

Sometimes, shipping and distribution can be a real challenge… even for our cells. Let's talk about transport across the cell membrane.

Cell membranes are selectively permeable, meaning that they control which substances can get across and how. To function properly, cells need to bring in substances like ions, oxygen, and glucose. At the same time, cells expel waste products like carbon dioxide, and often release substances like neurotransmitters and hormones.

Cells can move substances across their membrane in many ways: directly through the phospholipid bilayer by using membrane proteins, or by using vesicles.

Transports can be active or passive, depending on whether they require energy or not. Active transport requires molecules like adenosine triphosphate, or ATP, to move substances across the cell membrane. On the other hand, passive transport does not need ATP. Let's start with passive transport.

If cells don't actively expend energy to move substances across their membranes, what determines how nutrients and waste products flow in and out of the cell? Enter the concept of diffusion.

Let's imagine we have a container filled with water and we add some glucose. We have created a solution, where glucose is the solute because it is dissolved in water, which is the solvent.

As soon as we sprinkle the glucose in water, the glucose concentration is high in one area of the container but low everywhere else. This difference creates a gradient that makes the molecules move from high concentration areas to low concentration areas, until they are evenly distributed in the solution, reaching equilibrium. This process, where molecules passively move down their gradient, is known as diffusion.

Several factors influence diffusion. For instance, solute molecules move faster when the solution has a higher temperature, the solute molecules are smaller, and when the concentration gradient is larger.

Going back to our cells, passive transport is driven by different types of gradients. Different concentration of substances in the extracellular fluid and cytosol creates a chemical gradient. For example, if the concentration of sodium is higher in the extracellular fluid, ions will naturally tend to enter the cell by diffusion to distribute uniformly and reach equilibrium.

Electrical potentials also create gradients. Ions and many other molecules carry positive or negative charges. Substances with the same charge repel each other, and thus ions also tend to diffuse down their electrical gradient so that charges are evenly spread.

The combination of chemical and electrical gradients creates an electrochemical gradient, which is the driving force for the diffusion of charged molecules like ions. To learn more about electrochemical gradients and how they work in neurons, check out our introduction to neuron electrophysiology video!

Now let's look at another process relevant for passive transport. In this case, we have a container filled with water. A membrane separates the left and right sides. We sprinkle a bit of glucose into one half of the container and a bit more into the other. If the membrane becomes permeable to glucose but not water, meaning that glucose can freely cross it, the chemical gradient will make the glucose diffuse and distribute evenly across the two compartments.

But what if the membrane blocked glucose molecules and was permeable to water instead? In this case, since glucose cannot cross the barrier, the water molecules are the ones that move from the area with less glucose to the area with more glucose until the amount of water for each glucose molecule is similar between the left and right sides, so the concentrations are equal.

The movement of water from an area of lower solute concentration to higher solute concentration is called osmosis. Osmosis regulates the amount of fluid inside cells. The ability of a solution to change the volume of water in a cell is known as tonicity. Let's see how this works.

For simplicity, let's pretend that tonicity is determined by a single impermeable solute present in both the cytosol and in the extracellular fluid. If the solute concentration is equal inside and outside the cell, the extracellular fluid is isotonic to the cytosol. There is no net movement of water across the cell membrane.

If the solute is more concentrated outside the cell than inside, the extracellular fluid is hypertonic to the cytosol. Since movement of the solute is blocked by the cell membrane, water leaves the cell by osmosis to equalize the concentrations of solute inside and outside the cell. This makes the cell shrink.

The opposite occurs in hypotonic solutions, where the concentration of solutes is lower in the extracellular fluid than in the cytosol. In this case, water flows into the cell by osmosis and the cell swells. If the chemical gradient of the solute is large enough, a cell may take up so much water that it eventually bursts.

Till now, we've learned that passive transport occurs by diffusion via chemical gradients, electrical gradients, and osmosis. Depending on which cell membrane component lets substances through, we can further classify passive transport into simple diffusion and facilitated diffusion.

Simple diffusion is the movement of substances down their gradient through the phospholipid bilayer of the cell membrane. Yep, straight through it. No doors needed.

Substances that can freely cross the phospholipid bilayer include small non-polar molecules like gases, small polar molecules like water, and hydrophobic molecules like steroid hormones. Simple diffusion can move substances into or out of the cell.

But what about substances like ions and large molecules, which are blocked by the phospholipid bilayer? Some of them diffuse down their concentration gradient using proteins like channels and carriers. This process is known as facilitated diffusion.

Channel proteins are water-filled pores that connect the cytosol and the extracellular fluid. They are generally selective for a single substance, depending on the size and electrical charge of their pore. Ions often diffuse through channels. Potassium ions, for example, are present in high concentration inside the cell and diffuse into the extracellular fluid through ion channels down their electrochemical gradient.

But wait! Cells often need to control the concentration of some molecules in their cytosol. That is why different types of channels exist.

Some are open most of the time and are called leak channels. Other channels are gated, meaning that they only let substances through in response to a specific stimulus. Some examples include ligand-gated channels that open in response to chemical substances, voltage-gated channels that respond to membrane potential changes, or mechanically-gated channels that are activated by physical forces like pressure.

Remember how osmosis drives water in and out of the cells? This process is so important that water can be moved in two ways.

Most water molecules move across the cell membrane by facilitated diffusion through channel proteins called aquaporins. But since water molecules are small, they can also squeeze between the phospholipid tails and cross the cell membrane by simple diffusion.

What if a cell needs a molecule that doesn't fit through a channel? Larger substances can diffuse using carrier proteins. Instead of creating a tunnel in the cell membrane, carrier proteins bind with specific molecules and carry them across the cell membrane by changing conformation. Carriers that move one substance at a time are called uniporters.

But what if a cell is already hoarding nutrients, but it wants even more? Let's dive into active protein-mediated transport.

Active protein-mediated transport uses energy and membrane proteins to move substances against their concentration gradient. Primary active transport uses ATP directly. Later, we'll see how secondary active transports use ATP indirectly.

In primary active transport, enzymes known as ATPases split ATP into ADP and a phosphate group, releasing the energy required to activate the transport. If there's one active transporter you need to know about, that's the sodium-potassium pump. Here is how it works.

To start, three sodium ions bind to the sodium-potassium pump in the cytosol. When ATP is hydrolyzed, the phosphate group alters the conformation of the pump, releasing sodium ions into the extracellular fluid. This reveals two sites for potassium ions in the extracellular fluid. When the phosphate group detaches, the pump returns to its original conformation and releases the potassium ions inside the cell.

So the sodium-potassium pump uses ATP to ensure that the cytosol has a high concentration of potassium and a low concentration of sodium.

The concentration gradients created by the sodium-potassium pump are used to transport other substances across the membrane via secondary active transport.

Symport carrier proteins, or cotransporters, move multiple substances in the same direction. Take a look at this sodium-glucose secondary active transporter. Since the concentration of sodium ions in the extracellular fluid is high, the sodium ions want to move into the cell down their chemical gradient.

The extracellular fluid also contains some glucose molecules. Even if the glucose concentration is already high in the cytosol, the cell wants even more, so the glucose must be brought in against its concentration gradient.

Here's the trick: when sodium ions bind to a sodium-glucose secondary active transporter to enter the cell down their concentration gradient, the carrier doesn't let them through immediately. Instead, the transporter reveals a site that attracts a glucose molecule. Only when both sodium and glucose bind to it, the carrier protein changes conformation and releases both into the cytosol.

Antiport carriers, or exchangers, also mediate secondary active transport in a similar but opposite way. They still rely on the movement of ions down a concentration gradient created using active transport, but in this case, the substances move in opposite directions.

For instance, antiport carriers take advantage of the movement of sodium into the cell down its concentration gradient to move hydrogen ions out of the cell against their concentration gradients.

Enough of ions and small molecules, what if the cell needs to take up something much larger? It can be done using active vesicular transport.

Vesicular transport moves substances by enclosing them in vesicles, which are spherical structures made up of a phospholipid bilayer. This is active transport because ATP is used to create and move vesicles.

Vesicular transport is classified into endocytosis, where vesicles detach from the cell membrane to bring in large molecules, and exocytosis, where vesicles fuse with the cell membrane to release substances into the extracellular fluid. Let's take a closer look at the main types of vesicular transport.

Phagocytosis is the endocytosis of large particles. Our white blood cells may even eat other cells like bacteria. When a bacterium binds to a specific membrane receptor, two large tentacles called pseudopods extend from the cell membrane and engulf the particle. The pseudopods swallow the particle into a large vesicle called a phagosome. The phagosome then fuses with a lysosome filled with enzymes that digests the bacterium.

If phagocytosis is basically the cell eating something, pinocytosis is more like taking sips of extracellular fluid. In this process, the cell membrane folds inward, capturing some extracellular fluid into small vesicles. In the cytosol, these vesicles form a pinosome, which fuses with a lysosome to digest the contents.

During pinocytosis, cells drink extracellular fluid without really paying attention to what substances are in it. But what if the cell craves something more specific?

In receptor-mediated endocytosis, the cell uses receptors to take up specific substances. This process occurs at coated pits, regions of the membrane lined with a protein called clathrin. This protein folds the cell membrane inwards to form a vesicle filled with extracellular fluid and the membrane receptors bound to the substance needed. The rest of the pathway is similar to other methods of endocytosis.

Thus, the cell has multiple ways it can take things in. But how does it send stuff out? Sometimes, cells have waste products like whatever residual body the lysosomes leave behind.

Cells also use vesicles to release neurotransmitters or hormones to communicate with other cells. This is when cells use exocytosis, an active transport where vesicles fuse with the cell membrane to release their content into the extracellular fluid. The phospholipids of the vesicles are then recycled into the cell membrane. That's efficiency!

Sometimes, cells can combine endocytosis with exocytosis in a process called transcytosis. This is used, for instance, by epithelial cells to move substances from the intestinal lumen to the extracellular fluid.

That concludes this tutorial on how our cells regulate the transport of substances across their cell membrane, between the extracellular fluid and their cytosol. Looks like our bodies are made up of millions of shipping and distribution specialists!

Continue to explore all the transports across the cell membrane in our study unit at Kenhub. Let's learn together!