Video Transcript
The spongy mesophyll cells in the leaf are separated by many air spaces. How does this aid key biological processes in the plant? (A) It allows substances like water and minerals to be transported around the leaf to
be used in respiration and photosynthesis. (B) It allows sunlight to penetrate down to the palisade layer to increase the rate
of photosynthesis. (C) It allows easy diffusion of gases for respiration and photosynthesis. Or (D) it increases the size of the leaf, which allows the maximum amount of sunlight
to be captured for photosynthesis.
This question is asking about the role of a specific layer in the leaf, the spongy
mesophyll layer. Let’s review the structure of the leaf, focusing on the spongy mesophyll so we can
come up with the correct answer.
The leaf is an organ that is specially adapted for its role in photosynthesis. Photosynthesis is how plants make their own food by using sunlight energy to convert
carbon dioxide and water into glucose, releasing oxygen gas as a waste product. The light is absorbed by a green pigment, called chlorophyll, found in the
chloroplasts. Chloroplasts are the organelles where the reactions of photosynthesis take place.
Most leaves have a large surface area to be exposed to as much sunlight as possible
and are thin so the diffusion distance for carbon dioxide, a substrate of
photosynthesis, is short. Leaf size is primarily affected by temperature, rainfall, and sunlight, not by the
spongy mesophyll. So we can rule out option (D).
Now, let’s look at a cross section through a leaf so we can see how all the layers
work together. At the top is the waxy cuticle. This is to reduce water loss from the leaf. It is secreted by the upper epidermis. Both these layers are transparent so that light can get to the palisade mesophyll
layer beneath. We can therefore rule out option (B) as allowing sunlight to penetrate down to the
palisade layer refers to the waxy cuticle and upper epidermis.
Below the upper epidermis is the palisade mesophyll layer. This is made up of columnar cells, which are tightly packed together at the top of
the leaf to enable as much sunlight as possible to reach the chloroplasts. The cells contain lots of chloroplasts as this is the main layer for
photosynthesis.
Below the palisade mesophyll layer is the spongy mesophyll layer which is made up of
irregularly shaped cells, which also have chloroplasts, just not as many. This is because their main role is not photosynthesis, but gaseous exchange. There are many air spaces around the cells to allow for the exchange of oxygen and
carbon dioxide between the cells and the atmosphere. Oxygen is needed for respiration and carbon dioxide is needed for photosynthesis.
During the day, the carbon dioxide enters the leaf through the stomata, tiny holes,
mainly in the lower epidermis. Stomata is the plural, but if talking about just one it is stoma. At night, these stomata shut to reduce water loss. The opening and closing of the stomata is controlled by the guard cells on either
side.
Between the mesophyll layers are the vascular bundles. These are made up of xylem and phloem vessels. Xylem vessels carry water and minerals from the root to the leaf. Phloem carries nutrients, mainly sucrose and amino acids, to wherever they are needed
by the plant. The water that the xylem brings to the leaf is used for photosynthesis. We can therefore remove option (A) as it is xylem that carries the water and water is
not a substrate for respiration.
Now that we have reviewed the basic structure of the leaf, let’s return to our
question. We can now see that the correct answer is (C). The spongy mesophyll layer in the leaf aids key biological processes in the plant
because it allows easy diffusion of gases for respiration and photosynthesis.
