Concentration Gradients and Diffusion - Online Tutor, Practice Problems & Exam Prep

A concentration gradient is the difference in the concentration of a substance between two areas. Molecules move down their concentration gradient from high to low concentration without energy, while moving against it requires energy. Diffusion is the net movement of substances from high to low concentration until equilibrium is reached. For example, when dye is added to water, it diffuses until evenly distributed. Understanding these concepts is crucial for grasping processes like osmosis and active transport in cellular biology.

Concentration Gradients and Diffusion

In this video, we're going to begin our lesson on concentration gradients and diffusion. A concentration gradient is defined as a difference in the concentration of a substance between two different areas. If we're comparing two areas and one area has a higher or lower concentration than another area, then a concentration gradient exists because there is a difference in the concentration between those two areas. However, if we're comparing two areas, the difference in the concentration of a substance between them is what defines a concentration gradient. Now a molecule will be moving with or down its concentration gradient when that molecule is going from an area of high concentration down to an area of low concentration. On the other hand, a molecule will be moving against or up its concentration gradient when that molecule is going from an area of low concentration to an area of high concentration.

So, let’s observe the image below to clarify this idea. Notice that the image highlights concentration gradients and is divided into two halves. On the left-hand side, we see a high concentration of pink molecules, whereas on the right-hand side, there is a significantly lower concentration of these pink molecules. If one of these pink molecules is moving from an area of high concentration towards an area of low concentration, represented by the biker here, then the molecule will be moving down or with its concentration gradient. Just like it doesn't take much energy for a biker to cruise down a hill, it also does not require any energy for a molecule to move down or with its concentration gradient from areas of high concentration toward areas of low concentration.

On the right-hand side, we have an image that shows an area of low concentration of pink molecules on the left and an area of much higher concentration of the pink molecules on the right. So, if a molecule is trying to move from an area of low concentration towards an area of high concentration and the molecule's movement is represented by this biker going uphill, then the molecule would be moving up or against its concentration gradient. Similarly, just as it takes energy for a biker to bike up a hill, it takes energy for a molecule to move up or against its concentration gradient from an area of low concentration to an area of high concentration. Notice here we are pointing out that energy is required.

This introduction to the concentration gradient and molecules moving with or down and against or up the concentration gradient wraps up our lesson. We'll have the chance to practice applying these concepts as we move forward in our course. In the next lesson video, we'll discuss more about diffusion. I'll see you guys in the next video.

Diffusion

In this video, we're going to talk about diffusion. Diffusion is the net movement of a substance from an area of high concentration down to an area of low concentration. Molecules have a natural tendency to diffuse with or down their concentration gradients from an area of high concentration to an area of low concentration. Let's take a look at this example image below at the diffusion of dye in water. Notice that in this image, the blue circles represent water molecules, whereas the red circles represent dye molecules. Over here on the far left, notice that this person is adding a drop of dye to this beaker of water. Initially, when the person first adds the drop of dye, there's quite a high dye concentration in this one initial area. But in other areas of the beaker like here and over here and right here, you can see that there's quite a low dye concentration. There's really no dye at all. What starts to happen is that diffusion is going to occur, and the dye will start to diffuse to areas of low concentration, as we see here, from an area of high concentration down to an area of low concentration. That's what's starting to happen over here in this beaker. It's starting to diffuse and it will continue to diffuse from areas of high to areas of low concentration until equilibrium has been reached. Equilibrium is just when the entire beaker has equal concentrations of the dye, and so the dye is evenly distributed throughout the beaker. This is the idea of diffusion. This concludes our introduction to diffusion and we'll be able to get some practice applying these concepts as we move forward throughout our course. I'll see you all in our next video.

Which of the following statements about diffusion is true?

• A. It's a process where water moves across a semi-permeable membrane to a region of high solute concentration.
• B. It requires an expenditure of energy by the cell.
• C. It's a process where molecules move from a region of lower concentration to a region of higher concentration.
• D. It's a process where molecules move from a region of higher concentration to a region of lower concentration.

Here’s what students ask on this topic:

What is a concentration gradient and how does it affect molecular movement?

A concentration gradient is the difference in the concentration of a substance between two areas. It affects molecular movement by driving molecules to move from an area of high concentration to an area of low concentration, a process that does not require energy and is known as moving down the concentration gradient. Conversely, moving molecules from an area of low concentration to an area of high concentration requires energy and is known as moving against the concentration gradient. This concept is fundamental in understanding processes like diffusion, osmosis, and active transport in cellular biology.

What is diffusion and how does it relate to concentration gradients?

Diffusion is the net movement of a substance from an area of high concentration to an area of low concentration. It is directly related to concentration gradients because molecules naturally move down their concentration gradients during diffusion. For example, when dye is added to water, the dye molecules will spread out from the area of high concentration (where the dye was added) to areas of low concentration until equilibrium is reached, meaning the dye is evenly distributed throughout the water. This process does not require energy and is a key concept in understanding how substances move within biological systems.

What is the difference between moving down a concentration gradient and moving against a concentration gradient?

Moving down a concentration gradient refers to the movement of molecules from an area of high concentration to an area of low concentration. This process does not require energy and occurs naturally. On the other hand, moving against a concentration gradient involves moving molecules from an area of low concentration to an area of high concentration, which requires energy. This energy is often provided by ATP in biological systems. Understanding these differences is crucial for grasping cellular processes like passive and active transport.

How does the concept of equilibrium relate to diffusion?

Equilibrium in the context of diffusion refers to the point at which the concentration of a substance is equal throughout a given space. During diffusion, molecules move from areas of high concentration to areas of low concentration. This movement continues until the concentration of the molecules is uniform, meaning equilibrium has been reached. At equilibrium, there is no net movement of molecules in any particular direction, although individual molecules continue to move randomly. This concept is essential for understanding how substances distribute themselves in biological systems.

Why does moving against a concentration gradient require energy?

Moving against a concentration gradient requires energy because it involves transporting molecules from an area of low concentration to an area of high concentration, which is against their natural tendency. This process is akin to pushing a boulder uphill; it requires an input of energy to overcome the natural gradient. In biological systems, this energy is often supplied by ATP (adenosine triphosphate). Processes like active transport in cells rely on this energy to move essential molecules and ions against their concentration gradients, maintaining cellular function and homeostasis.