Stomata: The microscopic lungs of the planet

If you’ve ever looked closely at a leaf, you might have noticed its smooth, green surface. However, you have probably never seen the microscopic pores that control many of the processes essential to a plant's survival. Though invisible to the naked eye, these pores, known as stomata, are fundamentally important to sustaining life on Earth.

 

What are stomata?

Stomata (singular: stoma) are tiny pores found on the surfaces of leaves and some stems. They’re surrounded by specialised guard cells that control when the pores open and close. Depending on the plant species, stomata may be found on the upper surface (epistomatous), lower surface (hypostomatous), or both sides of the leaf (amphistomatous).

The shape of the guard cells differ too. They are kidney-shaped in dicotyledonous plants and dumbbell-shaped in grasses and other monocotyledonous plants. Despite these variations, their purpose is the same: to regulate the exchange of gases like carbon dioxide and oxygen, and to control water loss from the plant.

 

Stomata regulate gaseous exchange and transpiration in plants. 

How stomata open and close

The opening and closing of stomata is a finely tuned process controlled by the turgor pressure within the guard cells, essentially, how swollen or firm those cells are with water. When the guard cells take in potassium ions (K⁺) through active transport (a process that uses energy in the form of ATP), water follows by osmosis. The guard cells swell, bend outward, and the pore opens.

 

The opening and closing of stomata is regulated via potassium (K+) ions. The turgidity of the guard cells controls the aperture of the stoma.

 

When conditions change, for example, at night or during drought, potassium ions leave the cells, water follows, and the cells become flaccid, closing the pore. This mechanism helps plants balance their need to absorb carbon dioxide for photosynthesis with their need to prevent excess water loss.

Light, water, and the environment

Stomata are sensitive to environmental cues. Light triggers them to open, with a pigment called zeaxanthin acting as a blue-light sensor. Carbon dioxide levels inside the leaf also matter; low CO₂ encourages stomata to open, allowing more gas exchange. Water stress causes the opposite reaction. When soil dries out, plants produce the hormone abscisic acid (ABA), signalling the guard cells to close the pores and conserve moisture. Heat, wind, and low humidity also push stomata to close, preventing rapid dehydration.

It’s a constant balancing act between photosynthesis and transpiration.

 

Image taken from Dang et al., 2004. Starch metabolism in guard cells: At the intersection of environmental stimuli and stomatal movement.

The great balancing act

Plants live in a world of trade-offs. They must strike a balance between photosynthesis and transpiration. They must open their stomata to take in carbon dioxide for photosynthesis, the process that creates the sugars they need to grow. This same process also allows plants to release oxygen. The evaporative loss of water from stomata drives the transpiration stream, moving water and minerals from the roots to the shoots and leaves. This evaporation of water also helps cool the plant, protecting the efficiency of the chemical reactions occuring within the leaf. However, when stomata are open, plants are losing precious water. This tug-of-war has led plants to evolve different strategies for managing stomatal behaviour, depending on their environment. Scientists group them into three broad photosynthetic types: C₃, C₄, and CAM plants.

 

Aspect	C₃ Plants	C₄ Plants	CAM Plants
Main habitat	Cool, wet environments	Hot, dry environments	Very dry/arid environments
Carbon fixation	Direct via Rubisco	CO₂ fixed in mesophyll cells, Calvin cycle in bundle sheath cells	CO₂ fixed at night, Calvin cycle during the day
Photorespiration risk	High	Low	Very low
Stomata behaviour	Open during day	Open during day (moderate)	Open at night, closed during day

Stomata and climate change

Over millions of years, the number and size of stomata on leaves have changed in response to atmospheric CO₂ levels. Fossilised leaves show that when CO₂ levels were high, plants tended to have fewer stomata, since they could absorb enough gas without needing many pores. Conversely, during periods of low CO₂, plants evolved to have more stomata to capture as much as possible. In today’s world, as CO₂ levels rise due to human activity, many modern plants are responding in similar ways, by reducing stomatal density.

Every microscopic pore on a leaf is part of a vast, planet-wide feedback loop between the biosphere and the atmosphere. Understanding how stomata respond to changing CO₂ helps scientists predict how plants and the ecosystems they support will cope with the challenges of global climate change.

Try it yourself: stomata up close

Want to see these tiny 'lungs' in action? You can make your own leaf impression using a simple classroom technique using Ivy leaves.

1.       Paint a thin, even layer of clear nail polish over a small (2×2 cm) area of the lower leaf surface a leaf.

2.       Let it dry completely (about 10 minutes).

3.       Place a piece of clear tape over the dried polish and press gently.

4.       Peel the tape off carefully. The nail polish layer should lift off with it.

5.       Stick the tape (sticky side down) onto a clean microscope slide and trim any excess.

6.       Label your slides. You have just captured a microscopic view of a plant’s breathing pores!

Epidermal impressions of the lower leaf surface of Ivy (Hedera helix) viewed under a microscope at 400x

Small openings, big impact

It’s easy to overlook something as small as stomata, yet these pores play a crucial role in sustaining life on Earth. So next time you walk past a patch of greenery, remember, every leaf is quietly breathing, thanks to the microscopic lungs of the planet.