Plants do not only carry out photosynthesis. Like animals, they also carry out respiration and release water from their leaves through transpiration.
Basic Knowledge
Respiration (respiration) is a reaction that breaks down organic matter to obtain energy:Organic matter + oxygen → carbon dioxide + water + energy
Plants carry out respiration both day and night. It occurs in every cell of the roots, stems, flowers, and fruits, not only in leaves.
During the day, because photosynthesis > respiration, CO₂ is absorbed overall; at night, only respiration occurs, so CO₂ is released.
Transpiration (transpiration) is the process by which water inside a plant becomes water vapor and is released through stomata. Transpiration is significant because it:
① Promotes water uptake from the roots (transpiration stream)
② Lowers the plant’s temperature (heat of vaporization)
③ Provides the force that supplies nutrients carried to the leaves throughout the plant
Transpiration becomes active when the temperature is high, humidity is low, there is wind, and light is present.
Respiration (respiration)（A reaction that obtains CO₂, water, and energy from organic matter and oxygen）
Transpiration (transpiration)（The process by which water is released as water vapor through leaf stomata）
Transpiration stream（The flow of water from roots to leaves caused by transpiration）
Cobalt chloride paper（Paper that changes from blue → red (pink) when it contacts water. Used to detect transpiration）
Petroleum jelly（A waxy substance used in experiments that block stomata）
Heat of vaporization（Heat taken from the surroundings when water evaporates. The same principle as sprinkling water）
Further Study (Background and Significance)
A typical experiment for investigating the amount of transpiration:
① Compare A, with petroleum jelly applied only to the upper surface of the leaves; B, with petroleum jelly applied only to the lower surface; C, with none applied; and D, with all leaves removed.
② Measuring the decrease in water generally gives C > A > B > D. This supports the idea that there are more stomata on the underside.
③ The amount of transpiration from the underside of the leaves is calculated as (C - A), and the amount from the upper surface as (C - B).
A large tree is known to transpire several hundred liters of water per day. Forests are important components of the water cycle because they lower the temperature of the ground and cause rain to fall. Without transpiration, water in the soil could not reach the leaves from the roots, so the plant itself could not survive.
Respiration occurs in the mitochondria, which are different organelles from the chloroplasts used for photosynthesis. These two organelles coexist in plant cells.
- Respiration: organic matter + O₂ → CO₂ + water + energy
- Respiration occurs day and night and in all organs
- Transpiration: water vapor is released through stomata
- Significance of transpiration: water uptake, cooling, and nutrient supply
- There are more stomata on the underside → greater transpiration from the underside
- Transpiration is active in high temperature, low humidity, wind, and sunlight
- Respiration occurs in mitochondria; photosynthesis occurs in chloroplasts
Points to Note (Easily Confused)
① Respiration (oxygen consumption and CO₂ release) and photosynthesis (CO₂ absorption and oxygen release) are reverse reactions. ② Respiration (gas exchange) and transpiration (water vapor release) are different phenomena. Both occur through stomata. ③ In a transpiration experiment, the surface coated with petroleum jelly is the surface where transpiration does not occur. ④ Plants do not stop respiring. It is easy to mistakenly think that they only carry out photosynthesis.
Practice
- Do plants respire during the day, at night, or both?
- Name one significance of transpiration.
- What does the difference in transpiration between a plant coated with petroleum jelly only on the upper surface of its leaves and an uncoated plant represent?