Sometimes it’s easy to forget that plants exchange gases too because you can’t see evidence of ventilation. As well as needing oxygen for respiration, plants also need carbon dioxide for photosynthesis. In addition, oxygen is a waste product of photosynthesis and carbon dioxide is a waste product of respiration.
Dicotyledonous Plants
First of all, what do we mean by a dicotyledonous plant? Well, don’t worry too much about it, but they are a group of flowering plants which typically have most of their stomata on the bottom of their leaves. They also tend to have broad leaves, and their vascular bundles (which we will come across when looking at transport in plants) are in a ring through the stem.
You will have probably seen a diagram similar to this one before, showing the different layers of cells in a leaf. For gas exchange, we are interested in the middle and bottom parts of the leaf. The surface of the mesophyll cells is the main gas exchange surface in the leaf. Mesophyll cells create a large surface area for diffusion and the spongy mesophyll cells are loosely packed to enable gases to move around them. The oxygen and carbon dioxide gases move in and out of the leaf through pores in the epidermis called stomata (singular = stoma). Guard cells control whether the stomata are open or closed, and therefore whether gases can diffuse in and out of the leaf. Water can also be lost through the stomata.
Xerophytic Plants
Another word that isn’t very self explanatory! A xerophytic plant is one that is well adapted to survive in very dry conditions. Cacti, succulents, and marram grass are good examples. They need to have some extra adaptations to avoid water loss while the stomata are open for gas exchange because of their dry habitat.
The leaf of a xerophytic plant is often curled so that the surface containing the stomata is protected from wind. This is to maintain a low concentration gradient of water – if windy conditions are constantly removing water from around the outside of the stomata, the concentration gradient of water between the inside and outside of the leaf will be high, and more water will be lost from the leaf. If the leaf surface is protected, water will be trapped near the outside of the stomata and the concentration gradient will be low. We will look at this in more detail when we learn about transpiration.
Summary
There isn’t too much to learn about gas exchange in plants for A Level biology, but make sure you know the key points:
- Gas exchange occurs at the surface of the mesophyll cells in dicotyledonous plants.
- Plants both use and produce oxygen and carbon dioxide.
- Gases move in and out of leaves through stomata, which are open and closed by guard cells.
- Water is also lost through the stomata, so xerophytic plants are adapted to prevent water loss.
As always, if you have any questions please leave them below and I’ll get back to you as soon as I can.
