The great majority of plants are autotrophic: they feed themselves from the atmosphere’s carbon dioxide and from water and minerals in the ground, which they transform into living matter. Photosynthesis is the mechanism that enables the plant to use luminous energy to change simple molecules into organic matter. It takes place in specific cellular structures called chloroplasts. Photosynthesis, like respiration, is part of the metabolism of plants.
Photosynthesis generally takes place in leaves. Leaf cells contain large quantities of chloroplasts (up to 50 per cell). These organelles contain a green pigment, called chlorophyll, which is capable of absorbing luminous energy. This energy is used to make sugar molecules from the water drawn from the ground by the roots and carbon dioxide from the atmosphere. The sugars produced by photosynthesis are then distributed throughout the plant.
The first phase of photosynthesis requires that light fall on the leaf. The chlorophyll that is embedded in the membranes of the thylakoids absorbs luminous energy. This absorption of energy is accompanied by the degradation of water molecules and the release of oxygen. In a second, light-independent phase, the energy absorbed by the chlorophyll is used to transform carbon dioxide molecules into sugar molecules.
The sugars produced by photosynthesis are stored in the cell as starch, transformed into other organic molecules such as proteins, or degraded in the mitochondria during respiration to provide the energy that the plant needs.
Metabolism is the group of chemical reactions that enables a life form to live and develop. There are reactions that build living matter (anabolism) as well as degradation reactions (catabolism). These reactions take place within each cell of the organism.
Photosynthesis, which results in the manufacture of sugars, allows the plant to grow. It consumes luminous energy, water and carbon dioxide. Respiration, on the other hand, consists of the degradation of sugars. It generates the energy needed for the plant’s different activities, such as growth and reproduction. For the plant to grow, the quantity of organic matter produced by photosynthesis must exceed losses due to respiration.
Gas exchanges between the plant and the atmosphere occur through tiny pores called stomata, most of which are located on the leaves. Stomata are made of two cells surrounding an opening, the ostiole. Depending on the plant’s needs, the stomata open or close. During photosynthesis, plants consume carbon dioxide and release oxygen. The opposite occurs when plants respire. Respiration and photosynthesis thus play a major role in maintaining the balance of gases in the atmosphere. Plants also transpire: water vapor is released from the leaves via the stomata. Foliar transpiration allows the plant to maintain an optimal water content.