Membrane Transport: Study with Video Lessons, Practice Problems & Examples

Membrane transport is essential for cells to exchange substances with the extracellular fluid, allowing the intake of nutrients and expulsion of waste. It is categorized into passive transport, which requires no energy and moves substances down the concentration gradient, and active transport, which requires energy to move substances against the gradient. Passive transport includes simple diffusion for nonpolar molecules and facilitated diffusion for polar molecules and ions, utilizing protein channels. Active transport, such as the Sodium-Potassium pump, is highly selective and energy-dependent, crucial for maintaining cellular homeostasis.

Membrane Transport Concept 1

Now, when it comes to membrane transport, we're going to say that to perform its functions, a cell continuously needs to exchange substances with the extracellular fluid, so basically outside of itself. We're going to say, membrane transport allows cells to take in essential substances and expel waste products. Now here when it comes to membrane transport mechanisms, they can be broadly categorized into 2 types. We have our passive transport and we have our active transport. We're going to say here with passive transport, this does not require energy and substances move down the concentration gradient. But with active transport, we're going to say it requires energy and substances move against the concentration gradient. So here we have our membrane transport which is broken down into passive transport and active transport. Active requires energy. And with passive transport, it can be further broken down into simple diffusion and facilitated diffusion where we need some assistance for it to occur. Now, here remember a concentration gradient, this is the difference in concentration of a substance over a distance. And we'll see how to distinguish between active and passive transport.

Membrane Transport Example 1

Here it says, in the electron transport chain or ETC, complexes 1, 3 and 4 pump H⁺ ions from the mitochondrial matrix which has a low hydrogen ion concentration to the intermembrane space where we have a high H⁺ ion concentration. What type of membrane transport is this? Alright. So, what's happening here? Well, we're taking H⁺ from an area where the concentration of H⁺ is low, and we're pumping it to where it is high. This is the opposite of what we should expect because typically, we would have the movement of ions from an area of high concentration to an area of low concentration. This is what passive transport would be. Here we're doing the exact opposite. We're taking where H⁺ ions are low and pumping them somewhere where it's already high. There's gonna be a resistance there, which means we're gonna have to supply energy in order to do this. Because of that, this represents active transport. We're working against the concentration gradient. Again, normally we want to go from high concentration to low concentration, but here the opposite is occurring, which means energy will be needed. So again, option A, active transport, would be the best answer.

During respiration, oxygen gas diffuses into cells spontaneously. Which type of transport is this?

• A. Active transport
• B. Passive transport
• C. Both
• D. None of the above

Membrane Transport Concept 2

In this video, we'll take a look at different types of membrane transport mechanisms. Now, here we're going to discuss how non-polar molecules, polar molecules, and ions move across the cell membranes differently. Because of this, we'll need different types of mechanisms.

The first one is simple diffusion. Here, this is the movement driven by concentration gradients. We're going to say that small, non-polar molecules and water are involved. Examples would be oxygen, CO₂, or steroids. If we take a look here, we're going to say this is the outside of the cell and this is the inside of the cell, we can see that these particles are just kind of going through the lipid bilayer, going from the outside, through the bilayer, and winding up on the inside of the cell. This represents simple diffusion.

Next, we have facilitated diffusion. This is gradient-driven and it's through a protein channel. Now, here we're going to discuss dealing with polar molecules and ions. Examples would be glucose, water, chloride ions, and bicarbonate. If we take a look here, we have our integral protein, which, remember, goes through the entire thickness of our lipid bilayer. We're going to say that these particles are going through the protein channel, which goes on both sides of the lipid bilayer. So they go through here, which is on the outside, and exit out here onto the inside. This would be facilitated diffusion, where we are using the protein channel to help move the particles from the outside to the inside of the cell.

Then finally, we have active transport. In this, we're going to say protein channels or pumps use energy to move substances against a concentration gradient. We're going to say this is highly selective and regulated, with different pumps for different substances. Some examples are our Sodium Potassium Pumps, which move sodium out and then potassium inside the cell. So, if we take a look here, energy is involved, and that's what makes this active transport. It's not just simply having an integral protein; we also have the inclusion of energy in order for our particles to go in and out of the cell.

Alright. Just remember, we have different types of membrane transport mechanisms, and they deal with the transporting of materials inside and out of the cell. This involves non-polar molecules, polar molecules, ions, and when it comes to active transport, the inclusion of energy.

Membrane Transport Example 2

This example question asks, "How would a molecule of stearic acid cross the cell membrane?" Remember, stearic acid represents a saturated fatty acid. It has the shorthand notation of 18:0, meaning it has 18 carbons and 0 pi bonds. Due to this large number of carbons, it is considered a nonpolar molecule. Remember, fatty acids tend to be nonpolar overall because of their long hydrocarbon tail. Since it is a nonpolar molecule, it is most likely going to cross the cell membrane by simple diffusion. Remember, simple diffusion is the transport mechanism of choice when it comes to small nonpolar molecules, as well as water. Therefore, the answer here would be option a.

How would you expect an H⁺ ion to move out of the cell if [H⁺] inside the cell is lower than extracellular fluid?

• A. Simple diffusion
• B. Facilitated diffusion
• C. Active transport
• D. None of these

In oxidative phosphorylation, H⁺ ions from the intermembrane space of mitochondria to the mitochondrial matrix, which type of membrane transport is this?

• A. Simple diffusion
• B. Facilitated diffusion
• C. Active transport
• D. None of these

Here’s what students ask on this topic:

What is the difference between passive and active transport in cell membranes?

Passive transport and active transport are two primary mechanisms for moving substances across cell membranes. Passive transport does not require energy and moves substances down their concentration gradient, from areas of high concentration to low concentration. Examples include simple diffusion, where nonpolar molecules like oxygen and carbon dioxide pass directly through the lipid bilayer, and facilitated diffusion, where polar molecules and ions move through protein channels. Active transport, on the other hand, requires energy, usually in the form of ATP, to move substances against their concentration gradient, from areas of low concentration to high concentration. An example is the Sodium-Potassium pump, which moves sodium ions out of the cell and potassium ions into the cell, crucial for maintaining cellular homeostasis.

How do nonpolar and polar molecules move across the cell membrane?

Nonpolar molecules, such as oxygen and carbon dioxide, move across the cell membrane through simple diffusion. They pass directly through the lipid bilayer without the need for energy or assistance from proteins, driven by their concentration gradient. Polar molecules and ions, however, cannot easily pass through the hydrophobic lipid bilayer. They require facilitated diffusion, which involves protein channels or carriers embedded in the membrane. These proteins help transport substances like glucose, water, and ions (e.g., chloride and bicarbonate) across the membrane, also driven by their concentration gradient but without the need for energy.

What is the role of protein channels in facilitated diffusion?

Protein channels play a crucial role in facilitated diffusion by providing a pathway for polar molecules and ions to move across the cell membrane. These channels are integral membrane proteins that span the lipid bilayer, allowing specific substances to pass through. Facilitated diffusion is driven by the concentration gradient, meaning substances move from areas of high concentration to low concentration without the need for energy. Examples of substances that use protein channels include glucose, water, chloride ions, and bicarbonate ions. These channels ensure that essential molecules and ions can enter or exit the cell efficiently, maintaining cellular function and homeostasis.

What is the Sodium-Potassium pump and why is it important?

The Sodium-Potassium pump is a type of active transport mechanism that moves sodium (Na⁺) ions out of the cell and potassium (K⁺) ions into the cell, against their concentration gradients. This pump requires energy in the form of ATP to function. For every three sodium ions pumped out, two potassium ions are pumped in. This process is crucial for maintaining the electrochemical gradient across the cell membrane, which is essential for various cellular functions, including nerve impulse transmission, muscle contraction, and maintaining cellular osmotic balance. The Sodium-Potassium pump helps keep the internal environment of the cell stable, which is vital for the cell's overall health and function.

What are the different types of passive transport mechanisms?

There are two main types of passive transport mechanisms: simple diffusion and facilitated diffusion. Simple diffusion involves the movement of small, nonpolar molecules, such as oxygen and carbon dioxide, directly through the lipid bilayer, driven by their concentration gradient. Facilitated diffusion, on the other hand, involves the movement of polar molecules and ions through protein channels or carriers embedded in the cell membrane. This type of diffusion also relies on the concentration gradient but requires the assistance of specific proteins to help substances like glucose, water, chloride ions, and bicarbonate ions cross the membrane. Both mechanisms do not require energy input.