Question Video: Describing the Shape of a Red Blood Cell and Recalling Its Significance Biology

The diagram given shows multiple normal red blood cells in a blood vessel. Why does a red blood cell have this shape?


Video Transcript

The diagram given shows multiple normal red blood cells in a blood vessel. Why does a red blood cell have this shape? (A) To increase the available surface area so more oxygen can diffuse in and out of the cell. (B) To provide a large surface area for the active transport of oxygen in and out of the cell. (C) To decrease the surface area and therefore restrict the volume of oxygen lost from the cell. Or (D) to increase the volume of the cell so more genetic material can be carried around the body.

This question is asking about the shape of red blood cells. So let us review some key facts about the structure and function of these specialized cells so we can come up with the correct answer.

Let’s start off with the role of red blood cells in the body. Red blood cells belong to the circulatory system. The blood transports various substances around the body including oxygen nutrients to the cells and wastes, such as carbon dioxide and urea, away from them. The red blood cells make up about 45 percent of the volume of the blood. Their specific role is to carry the oxygen, which is a very important process, since oxygen is a requirement for human body cells. It is used for a specific type of cellular respiration: aerobic respiration.

Cellular respiration is the process by which carbon-containing compounds, usually glucose, are broken down to release energy in the form of ATP. There are two main types of cellular respiration: aerobic, which we have already mentioned, and anaerobic. Aerobic means oxygen, so aerobic respiration requires oxygen. An- means without, so anaerobic occurs without oxygen. Aerobic respiration releases almost 20 times more ATP than anaerobic. This is why it is so important to get enough oxygen to all of our cells.

You might wonder how red blood cells carry their oxygen to body cells. Well, they contain millions of hemoglobin proteins which contain iron and give blood its red color, hem- meaning blood. Mature red blood cells lack a nucleus and most organelles, in order to pack as much hemoglobin into the cell as possible. If they do not have a nucleus, they are not carrying genetic material. And so we can rule out answer option (D).

Next, let’s talk about the shape of the cells. Red blood cells have a biconcave shape, as shown here. We can break down the word “biconcave” to see what it means. The prefix bi- means two, while the root word “concave” refers to curving inward. In the question diagram, the red blood cells appear flat and disc-shaped. However, if you look at the diagram of the section through the cell, you can see that they are thin in the middle and indented on both sides. This is a very important characteristic of red blood cells, because it creates a large surface-area-to-volume ratio. This just means that the red blood cell will have a lot of area to interact with their surroundings.

But why is this important? Well, it allows a large number of oxygen molecules to diffuse into the red blood cells at any one time. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. As the molecules are moving down a concentration gradient, diffusion is a passive process, which means it does not require energy, unlike active transport which does require energy.

In the lungs, oxygen molecules are in a high concentration. So they diffuse into the red blood cells down their concentration gradient. The oxygen then binds to the hemoglobin and is carried in the blood to the body cells, which have a much lower concentration of oxygen molecules. The oxygen molecules therefore diffuse from the red blood cells to the body cells, again down their concentration gradient. This diffusion needs to occur as rapidly as possible to maintain the oxygen supply. So the greater the surface area for the oxygen molecules to diffuse through, the better.

Seeming as this process needs to occur as rapidly as possible, we can rule out option (C), which talks about restricting the volume of oxygen leaving the cell.

Now that we have reviewed the biconcave shape of red blood cells and how this is important for maximizing oxygen diffusion, we are able to take a second look at our question. We are left with options (A) and (B). We now know that oxygen diffuses into and out of the red blood cells. Therefore, the correct answer is option (A). Red blood cells have this biconcave shape to increase the available surface area so more oxygen can diffuse in and out of the cell.

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