In humans, carbon dioxide produced
by respiration is considered a waste product. Why is the same not completely true
for plants? (A) This is incorrect, as carbon
dioxide is always a waste product. (B) Plants can store carbon dioxide
in their vacuoles to provide structural support. (C) Plants can use carbon dioxide
as a reactant in photosynthesis to produce glucose. Or (D) plants use carbon dioxide as
a source of energy for life processes.
Let’s remove the answer options for
now and remind ourselves about respiration in humans and plants. Animals and plants both need energy
for their life processes. And they get this energy by
carrying out cellular respiration to break down sugars. If we look at the equation for
cellular respiration, we can see that it’s the chemical reaction in which glucose
and oxygen are converted into carbon dioxide and water, and energy is released.
Firstly, where do these organisms
get the glucose and oxygen from? Animals, like us humans, consume
carbohydrates in our food, which are then broken down into glucose by enzymes in our
digestive system. To get the oxygen we need, we
breathe air containing around 21 percent oxygen into our lungs. Plants, on the other hand, cannot
eat or breathe, but they do carry out another chemical reaction which humans
cannot. This is called photosynthesis. And it’s where water and carbon
dioxide are converted into glucose and oxygen using light energy. Glucose and oxygen produced by
photosynthesis can then be used for cellular respiration in plants.
Secondly, what do humans and plants
do with the products of cellular respiration? While the water and energy are
useful to animal cells, they have no need for carbon dioxide. So it is breathed out of the lungs
to remove it from the body. This is why carbon dioxide is
considered a waste product in humans. However, as we’ve seen, plants can
use some of the carbon dioxide produced during cellular respiration as a reactant
for photosynthesis. So carbon dioxide cannot be
considered a waste product in plants.
We have therefore determined that
the correct answer is (C). It is not completely true that
carbon dioxide is a waste product in plants because plants can use carbon dioxide as
a reactant in photosynthesis to produce glucose.