Gas exchange in plants

Kim and Philip are having a picnic in the park. Mmm, the air is so fresh here! And so full of oxygen that all these plants here produce. It’s nice that you’re enjoying the fresh air, Kim. Did you know that plants need oxygen too?

We usually think of plants as oxygen producers. Plants absorb carbon dioxide from the air, water from the soil, and then use them in the process called photosynthesis. What photosynthesis produces is oxygen and glucose. Glucose is plant “food”. Plants use glucose in a process that releases energy and “powers” plant cells.

This process is called cellular respiration. Yet to carry out cellular respiration, plants need oxygen! Cellular respiration also releases two products, carbon dioxide and water; which can then be used for photosynthesis! This seems like a perfect system. What one process needs, the other creates.

But, to carry out photosynthesis, plants need more carbon dioxide than they can produce through cellular respiration. At the same time, cellular respiration uses less oxygen than is produced through photosynthesis. Basically, plants don’t produce enough carbon dioxide, but they produce more oxygen than they need. So plants exchange gases with the air that surrounds them: they absorb carbon dioxide, and release extra oxygen. Let’s take a closer look at how this works!

There are small openings called stomata, on the leaves of plants, and on some stems. You can think of them as tiny mouths that open and close, to let the air in and out. Open stomata allow gases to move in and out of the plant. But if stomata were open all the time, water vapour would move out of the plant when the air is dry. So to prevent the plant from drying out, each opening, or stoma, is surrounded by two specialised, crescent-shaped cells.

These cells control when stomata open and close. They are called guard cells. When humidity in the air is high, the concentration of water in guard cells is lower than in their environment. Because of this, water moves into the guard cells, which causes them to swell. Guard cells have a cell wall that is thicker on the side that faces the stoma, compared to the side that faces the other cells.

So the thinner cell walls further away from the opening, will expand more than the thicker wall closer to the opening. This causes guard cells to curve and the stoma to open. Gases can get through! When conditions change and humidity is low, the amount of water in the guard cells is higher than in their environment, so water moves to surrounding cells. Loss of water from the guard cells causes them to shrink, and the opening to close.

Whether the gases move in or out of the plant depends on environmental conditions too! For example on a sunny day, the rate of photosynthesis is greater than that of respiration. Carbon dioxide is taken in through the stomata, while oxygen is given out. When plants are exposed to dim light, the rate of photosynthesis is equal to the rate of respiration. In that case, neither carbon dioxide nor oxygen are taken in or given out.

When plants aren’t exposed to any light, there is no photosynthesis, but respiration still happens. Because of this, oxygen is taken in, while carbon dioxide is given out. But don’t worry, there is still plenty of oxygen around for all the other organisms even at night! Philip! We should breathe in as much of the fresh air as possible before we go back to the city!