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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Day and Night Gas Exchange
Plants, just like us, need to exchange gases with their environment to stay alive. But unlike us, plants don't have lungs! Instead, they have tiny pores called stomata on their leaves that allow gases to move in and out. The way plants exchange gases changes between day and night because of the different processes happening inside plant cells.
Key Definitions:
During the day: Plants carry out both photosynthesis and respiration. Photosynthesis usually happens at a faster rate than respiration when there's enough light.
During the night: Only respiration occurs as photosynthesis requires light. This changes the overall pattern of gas exchange.
Stomata are like the plant's breathing control system. Each stoma (singular of stomata) is made of two guard cells that can change shape to open or close the pore between them. When stomata are open, gases can freely move in and out of the leaf.
During daylight hours, plants are busy photosynthesising. This process dramatically affects the gases moving in and out of the plant.
When the sun is shining, plants use light energy to convert carbon dioxide and water into glucose and oxygen. This means during the day:
The chemical equation for photosynthesis shows this exchange:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
On a sunny day, a single leaf can contain up to 100,000 stomata! The majority of stomata are usually found on the lower surface of the leaf, which helps reduce water loss while still allowing for gas exchange.
When darkness falls, the gas exchange pattern in plants changes significantly because photosynthesis stops but respiration continues.
At night, plants only carry out respiration, which requires oxygen and produces carbon dioxide. This means:
The chemical equation for respiration shows this exchange:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Photosynthesis > Respiration
Net intake of CO2
Net output of O2
Stomata usually open
Only Respiration
Net intake of O2
Net output of CO2
Stomata often closed
Photosynthesis ≈ Respiration
CO2/O2 exchange balanced
Called the "compensation point"
Stomata adjusting
Plants need to balance gas exchange with water conservation. The opening and closing of stomata is a carefully controlled process.
Guard cells control stomatal opening through changes in their shape:
Several factors affect whether stomata open or close:
Environmental conditions have a significant impact on how plants exchange gases.
Plants must adjust their gas exchange based on environmental challenges:
In hot, dry environments, plants face a dilemma: they need CO2 for photosynthesis, but opening stomata leads to water loss. Many desert plants have adaptations like:
Water plants face different challenges for gas exchange:
Crassulacean Acid Metabolism (CAM) plants like cacti and pineapples have evolved a special adaptation for gas exchange in hot, dry environments. They open their stomata at night to collect CO2 and store it as an acid. During the day, they keep stomata closed to prevent water loss and release the stored CO2 internally for photosynthesis. This reversed pattern helps them survive in desert conditions where water conservation is crucial.
Understanding plant gas exchange has important real-world applications:
Plants have evolved sophisticated mechanisms to balance their need for gas exchange with other survival requirements. The key differences between day and night gas exchange are:
During the day, when light is available:
During the night, when no light is available:
This day-night cycle of gas exchange is essential for plant survival and growth, allowing plants to make food while managing water loss and adapting to their environment.
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