Video Transcript
Palisade cells near the top of the
leaf contain many chloroplasts. Which of the following best
explains why? (A) Palisade cells contain many
chloroplasts to capture the sunlight needed for respiration. (B) Palisade cells contain many
chloroplasts to allow the maximum diffusion of gases. (C) Palisade cells contain many
chloroplasts to expand the cell and increase the surface area available for water
uptake. (D) Palisade cells contain many
chloroplasts to capture the sunlight needed for photosynthesis.
Since the question uses the term
leaf, we know that we’re dealing with a plant. And it mentions a type of cell
that’s found in the leaf. So we’ll need to know about some
plant cells as well. Another key term in the question is
chloroplasts. Chloroplasts are organelles, and
organelles are structures and cells that are responsible for specific functions. Let’s take a look at the terms and
the solution options to see what else we need to know about these plant cells.
Option (A) contains the term
respiration, and respiration occurs in an organelle called the mitochondrion. Option (B) mentions the diffusion
of gases between the interior of a leaf and the environment, which occurs through a
structure called a stoma that’s formed by two guard cells. Option (C) contains the term water
uptake, but that actually occurs in the roots, especially through some tiny
structures called root hairs that increase the surface area where water uptake
occurs. Option (D) mentions the process of
photosynthesis. And photosynthesis can be
summarized by the equation below, where carbon dioxide and water react using the
energy in sunlight to form sugar and oxygen.
So let’s make some connections
between these structures on our diagram. First, water is taken up by the
root hairs. It moves through a tissue called
the xylem, where it finally gets to the leaf. As long as there’s enough water in
the guard cells of the stoma, they’ll stay expanded, which keeps the stoma open for
gas exchange, including the diffusion of CO2 from the atmosphere down its
concentration gradient into the area of the leaf known as the spongy mesophyll. Plants use the carbon dioxide and
water in the process of photosynthesis to make sugar and oxygen. The sugar can be used by the
mitochondria in the process of cellular respiration. The oxygen can be used by the
mitochondria in cellular respiration, or it can exit into the atmosphere by
diffusing down its concentration gradient.
So option (A) must be incorrect
because cellular respiration occurs in the mitochondria and not in the
chloroplasts. Option (B) is incorrect because the
diffusion of gases takes place in the plants’ stomata located on the underside of
their leaves. And option (C) is incorrect because
water uptake occurs in the root hairs, not in the chloroplasts. Option (D) says that chloroplasts
capture sunlight that’s needed for the process of photosynthesis. And that is indeed true. That’s what happens in the
chloroplast.
It’s interesting to note though
that we didn’t even have to know what a palisade cell was to be able to answer this
question correctly. So if you don’t know a term in a
question, don’t let it scare you away. Look at the rest of the question
and try to figure out what you can. On the other hand, it’s even better
to know what a palisade cell is. Palisade cells make up the palisade
mesophyll tissue, which is directly beneath the epidermis and cuticle on the top of
the leaf. And they contain many
chloroplasts. And the tissue beneath the palisade
mesophyll is called the spongy mesophyll. It’s called spongy because there’s
a lot of air pockets, and that allows the gases to diffuse through.
So the answer to the question
“Palisade cells near the top of the leaf contain many chloroplasts. Which of the following best
explains why?” is option (D). Palisade cells contain many
chloroplasts to capture the sunlight needed for photosynthesis.
