Principles of Transmembrane Transport - Online Tutor, Practice Problems & Exam Prep

Transmembrane transport involves the movement of molecules across cell membranes, which are semipermeable, allowing selective passage. Molecules can cross via passive transport, moving from high to low concentration without energy, or active transport, requiring ATP to move against a gradient. Key transport proteins include channels, transporters, and ATP-powered pumps. Understanding concentration gradients, membrane potential, and electrochemical potential is crucial for cellular function, as they regulate the internal environment and maintain homeostasis.

Membranes and Gradients

So in this video, we are going to be talking about the principles of transmembrane transport. The first thing we're going to talk about is mainly just membrane gradients, and what property of the membrane allows them to develop gradients. Cells must be able to communicate across their membrane barriers in order to exchange materials with their environment. The internal part of the cell has to expel or take in things found in the extracellular environment, which means that membranes are semipermeable. They only allow certain molecules to cross. You can't have everything in the extracellular environment entering into cells. You only want certain amounts of things. And so that makes the membrane semipermeable. Small nonpolar molecules like oxygen or carbon dioxide can rapidly cross the membrane. Uncharged polar molecules can cross if they are small, but the large ones are pretty much excluded, and charged molecules and ions cannot pass the membrane at all. The semipermeability allows the cell to regulate what things are getting in and what things are not.

Because there are differences between the internal environment and the external environment, this is a really important concept. The intracellular concentration of things varies from the external concentration. This results in four terms that I really want to make sure we understand and that are really important for this chapter. The first is concentration gradients, and that's when the concentrations of molecules differ on either side of the membrane. Then you have the electrical potentials, which is when there is a charge difference; the intracellular environment is more positive than the extracellular environment. The electrochemical potential describes the combination of the concentration gradient and the electrical potential. This sums up the concentration difference and the electrical difference into one driving force, the electrochemical potential. Finally, you have membrane potential, which is actually the difference between the concentration and electric potentials.

Generally, the overall net charge or the overall difference must be balanced, or the cell will just kind of explode. Here we have a membrane, and across the membrane, you have an electrical difference where there are more positive charges on one side and more negative charges on the other. This combination of differences refers to the electrochemical potential, and the difference between them refers to the membrane potential. Now, let's move on.

Passive and Active Transport

So now, let's talk about the 2 different types of transport across membranes, which are passive and active. Molecules cross the membrane barrier using these two methods: passive transport and active transport. We've discussed these concepts before in some introductory biology courses, but let's review them again. Passive transport moves molecules through a gradient, usually from an area of high concentration to an area of low concentration. This can happen through simple diffusion, which means the molecule does not need any assistance in crossing and passing through the membrane, or through facilitated diffusion, which requires some type of assistance for the molecule to cross the membrane. Generally, this passive way of transporting or passive diffusion requires no energy input. It will make it across whether it does it by itself or whether it is facilitated by something. On the other hand, active transport involves a molecule moving against its gradient from low concentration to high concentration, and this process requires energy, usually derived from ATP.

When comparing simple and facilitated diffusion, in passive diffusion, the molecules just pass through the membrane without any assistance. In contrast, facilitated diffusion involves molecules getting through the membrane with the help of some type of protein. However, this entire process requires no energy because it is passive. There are three classes of transmembrane proteins that transport molecules across the membrane. The first are channels, which provide a portal for a molecule to pass but are specific, allowing only molecules of a specific size or charge to pass through the membrane. Then, there are transporters, which are highly selective in allowing molecules to pass, usually permitting only a specific molecule that can bind to a specific binding site. Lastly, there are ATP-powered pumps that facilitate crossing by using energy from ATP.

In summary, we have channel proteins that allow similar molecules or those with a similar charge or shape to get through. We have transporters that only allow specific molecules, for example, glucose and sodium, to get through. Then, we have ATP-powered pumps, which utilize ATP to transport various substances across the membrane. With these three mechanisms in focus, let's now move on.

Which of the following gradients describes an electrical charge difference across a membrane?

• A. Concentration gradients
• B. Electrical gradients
• C. Electrochemical gradients
• D. Membrane potential

Facilitated diffusion differs from simple diffusion because why?

• A. Facilitated diffusion requires energy to move substances across the membrane
• B. Facilitated diffusion requires a protein to move substances across the membrane
• C. Simple diffusion requires energy to move substances across a membrane
• D. . Simple diffusion requires a protein to move substances across the membrane

Which is an example of facilitated diffusion?

• A. Fat soluble molecules passing through the plasma membrane with the concentration gradient
• B. Water traveling across the plasma membrane with the help of channel proteins called aquaporins
• C. K+ being moved across the plasma membrane against its concentration gradient

Here’s what students ask on this topic:

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

Passive transport moves molecules from an area of high concentration to an area of low concentration without the need for energy. It includes simple diffusion, where molecules pass directly through the membrane, and facilitated diffusion, where molecules require assistance from transport proteins. Active transport, on the other hand, moves molecules against their concentration gradient, from low to high concentration, and requires energy, usually in the form of ATP. This process often involves ATP-powered pumps that use energy to transport molecules across the membrane.

How do concentration gradients and electrochemical potentials influence transmembrane transport?

Concentration gradients refer to the difference in the concentration of molecules on either side of the membrane. Molecules tend to move from areas of high concentration to low concentration, driving passive transport. Electrochemical potentials combine the concentration gradient and the electrical potential (charge difference) across the membrane. This combined force influences the movement of ions and charged molecules. The membrane potential, which is the difference between the concentration gradient and the electrical potential, also plays a crucial role in determining the direction and rate of transmembrane transport.

What are the main types of transmembrane proteins involved in transport?

There are three main types of transmembrane proteins involved in transport: channels, transporters, and ATP-powered pumps. Channels provide a portal for specific molecules or ions to pass through the membrane based on size or charge. Transporters are highly selective and only allow specific molecules that can bind to their binding sites to pass through. ATP-powered pumps use energy from ATP to transport molecules against their concentration gradient, enabling active transport.

Why are cell membranes considered semipermeable, and what factors determine their permeability?

Cell membranes are considered semipermeable because they allow certain molecules to pass while restricting others. Factors determining permeability include the size, charge, and polarity of the molecules. Small nonpolar molecules like oxygen and carbon dioxide can rapidly cross the membrane. Small uncharged polar molecules can also pass, but larger molecules and charged ions are generally excluded. This selective permeability is crucial for maintaining the cell's internal environment and regulating the exchange of materials with the extracellular environment.

What roles do ATP-powered pumps play in transmembrane transport?

ATP-powered pumps play a critical role in active transport by moving molecules against their concentration gradient, from low to high concentration. These pumps use energy derived from ATP to perform this function. They are essential for maintaining concentration gradients of ions and other molecules across the membrane, which is vital for various cellular processes, including nutrient uptake, waste removal, and maintaining the cell's electrochemical balance.