How do Plants Release Co2?

Plants release carbon dioxide (CO₂) primarily through a process called cellular respiration. This happens all the time, both day and night, because every living plant cell needs energy to survive. While plants are famous for taking in CO₂ during photosynthesis, they also constantly breathe out CO₂ just like animals do. This article explains exactly how and when plants release CO₂, and why it matters.

What is the main way plants release CO₂?

The main way plants release CO₂ is through cellular respiration. In simple terms, respiration is how cells break down sugars to get energy. When a plant makes sugar during photosynthesis, it stores that sugar for later use. But cells need energy around the clock to grow, repair damage, and move nutrients. To get that energy, the cells combine oxygen with the sugar (or other organic molecules) and release CO₂ and water as waste products.

This process happens in every living part of the plant—roots, stems, leaves, and even flowers. The CO₂ produced inside the cells diffuses out through tiny pores called stomata on leaves and stems. It can also escape through cracks or openings in the bark or root surfaces.

Do plants release CO₂ at night?

Yes, plants release CO₂ at night, and it is typically more noticeable because photosynthesis stops as soon as the sun goes down. During the day, plants are busy taking in CO₂ for photosynthesis, which masks some of the CO₂ they are also releasing through respiration. At night, there is no photosynthesis pulling CO₂ in, so the respiration output becomes the only gas exchange happening.

Many people worry that sleeping near a houseplant might be unhealthy because of this nighttime CO₂ release. However, the amount is very small, far less than the CO₂ produced by a sleeping pet or a person. A single plant's respiration at night raises indoor CO₂ levels by a negligible amount.

How does plant respiration compare to photosynthesis?

Think of photosynthesis as the "making" process and respiration as the "using" process. During photosynthesis, plants use sunlight, water, and CO₂ to make sugar and oxygen. That happens only in the daytime and only in green parts of the plant (mainly leaves). During respiration, plants use oxygen and sugar to release energy, CO₂, and water. Respiration happens all the time in every living cell.

• Photosynthesis: Requires sunlight → produces sugar and O₂ → uses CO₂
• Respiration: Runs 24/7 → produces CO₂ and water → uses O₂ and sugar

Because photosynthesis uses CO₂ and respiration releases it, a plant’s net CO₂ exchange depends on the balance between these two processes. On a sunny day, photosynthesis dominates, so the plant is a net absorber of CO₂. On a dark night, only respiration occurs, so it becomes a net releaser.

What about during the day?

During the day, plants both release and absorb CO₂ at the same time. The stomata stay open (unless it is very dry) to let CO₂ in for photosynthesis, but CO₂ from respiration also exits through those same pores. Because photosynthesis usually happens much faster than respiration when it is sunny, the net movement of CO₂ is into the plant. In fact, a healthy plant on a bright day can take in several times more CO₂ than it releases.

Here is a simple way to picture it: imagine a bucket with a small hole in the bottom. Respiration is the water trickling out of the hole. Photosynthesis is the big hose filling the bucket. On a sunny day, the hose fills the bucket much faster than the leak empties it. At night, the hose is turned off, and only the leak continues.

What factors affect how much CO₂ a plant releases?

Several things influence the rate of respiration and therefore how much CO₂ a plant releases:

• Temperature: Respiration speeds up when it is warmer (within the plant’s survival range). High temperatures cause more CO₂ output.
• Growth stage: Young, actively growing tissues respire more because they are building new cells.
• Time of day: During the day, respiration still happens, but it is often masked by photosynthesis. At night, output becomes the only measurable gas exchange.
• Plant size and health: Larger plants with more living cells release more CO₂ overall. Stressed or injured plants may respire differently.
• Light level: In complete darkness, respiration is the only process. In very low light, photosynthesis may be too weak to offset respiration, so the plant might still be a net CO₂ releaser.

Do all plants release CO₂ the same way?

All plants respire and release CO₂, but the rate varies. For example, a cactus that lives in a hot desert has a special type of photosynthesis called CAM that lets it keep stomata closed during the day to save water. It opens stomata at night to take in CO₂. That means the cactus actually releases most of its respiratory CO₂ during the day when its stomata are partly open for other reasons. Meanwhile, a typical tree like an oak releases CO₂ mostly at night from its leaves and all the time from its roots and trunk.

Roots also release CO₂. Soil contains tons of root respiration, plus microbes that eat root exudates and respire as well. In fact, a large portion of the CO₂ coming from a forest or field actually originates from underground respiration.

Can plants release more CO₂ than they absorb?

Yes, under certain conditions. If a plant is not getting enough light, or if the temperature is very high, respiration can outpace photosynthesis. For example, on a cloudy day or in deep shade, a plant might take in less CO₂ through photosynthesis than it releases through respiration, resulting in a net loss of CO₂ to the atmosphere. In a forest that is decaying or being cleared, dead plant matter decomposes and releases huge amounts of CO₂ through respiration from fungi, bacteria, and insects. However, live, healthy plants in good sunlight are almost always net absorbers.

How do plants release CO₂ from their roots?

Root cells respire just like leaf cells do. They use oxygen that diffuses down from the soil pores or floats in water, and they break down sugars that travel from the leaves. The CO₂ produced by roots diffuses out into the surrounding soil. This soil CO₂ can then rise to the surface and escape into the air. In fact, the soil around a plant often has a higher concentration of CO₂ than the air above, which is why you sometimes see little pockets of gas when you dig up wet soil.

Factors that increase root respiration include warmer soil, more moisture (but not waterlogged, because roots need oxygen), and the presence of mycorrhizal fungi that connect with roots. All of these increase the amount of CO₂ released from the root zone.

A simple breakdown of plant CO₂ release

Does placing plants indoors affect indoor CO₂ levels?

Yes, but the effect is very small. A typical houseplant in a normal-sized room releases a tiny amount of CO₂ at night. During the day, if the plant gets enough light, it absorbs CO₂, which can actually lower the room's CO₂ level slightly. For most living spaces, the changes are too minor to matter for human comfort or health. The bigger sources of indoor CO₂ are people breathing and appliances that burn fuel. If you want to monitor CO₂ levels, you can use a simple indoor CO₂ monitor to check if your home needs more ventilation.

How does temperature affect plant CO₂ release?

Temperature is a key driver. For every 10°C (18°F) rise in temperature within the plant's tolerance range, respiration roughly doubles. That means a warm summer night will cause a plant to release significantly more CO₂ than a cool winter night. In some ecosystems, like tropical rainforests, warm temperatures year-round lead to high respiration rates. In contrast, plants in cold climates or during winter have much slower respiration, so their CO₂ release is lower. If a plant gets too hot, its respiration may spike while photosynthesis shuts down, causing a large net release of CO₂.

Do plants release CO₂ when they are damaged or dying?

Yes. When a plant is injured, attacked by pests, or begins to die, cells break down and release CO₂ through a different type of respiration called decomposition. Living cells still respire, but dead tissue is quickly colonized by bacteria and fungi that also respire and produce CO₂. That is why a pile of fallen leaves or a rotting log produces a lot of CO₂ over time. If you are a gardener, you can support the natural cycle by adding composting tools to help manage organic matter decomposition efficiently.

Is plant CO₂ release a problem for the climate?

Plant respiration is a natural part of the carbon cycle. The CO₂ released by plants today often came from the atmosphere just a few years ago, pulled in by the same plants or others nearby. That is completely different from burning fossil fuels, which releases carbon that has been locked underground for millions of years. However, when forests are cut down and the wood decays or burns, that stored carbon is released as CO₂. Deforestation does add CO₂ to the atmosphere. But healthy, growing plants overall absorb more CO₂ than they release.

How can you measure or observe plant CO₂ release?

You can see the effects of plant respiration in a simple experiment. Place a few fresh leaves in a sealed jar in the dark for a few hours. Then add a drop of pH indicator like bromothymol blue or red cabbage juice. The CO₂ released by the leaves will dissolve in any moisture and form carbonic acid, turning the indicator more yellow (if using bromothymol blue) or more pink (if using red cabbage juice). This shows that leaves release CO₂, even without light.

For a more precise measurement, researchers use portable infrared gas analyzers that measure CO₂ concentration changes in a chamber with a plant. But for a home gardener, observing leaf color changes in a sealed bag can be a fun science project.

What about aquatic plants? Do they release CO₂?

Yes, aquatic plants also release CO₂ through respiration, just like land plants. The CO₂ dissolves in the water. Underwater plants often have to use dissolved CO₂ or bicarbonate from the water for photosynthesis. At night, they release CO₂, which can lower the water’s pH slightly. In a fish tank with live plants, this nighttime CO₂ release is usually harmless, but it is one reason why aquarists sometimes use a CO2 drop checker to keep conditions balanced for fish.

Can plants release CO₂ from stored carbohydrates?

Absolutely. Plants store energy as starch, sugar (sucrose), and fats. When they need energy at night or during cloudy periods, they break down these stored carbohydrates. The process is essentially the same as breaking down fresh sugar from photosynthesis. The stored carbohydrates are transported to wherever energy is needed—roots, growing shoots, or flowers—and then respired to release CO₂.

Checklist: Key points about plant CO₂ release

• ☐ Respiration releases CO₂ 24/7 from every living cell.
• ☐ Photosynthesis absorbs CO₂ only in daylight and only in green parts.
• ☐ At night, plants are net

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