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
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.