Photosynthesis transforms light energy into chemical energy, primarily glucose. This process, carried out by plants, algae, and some bacteria, produces nourishment and releases oxygen into the atmosphere, a gas vital for many life forms. It relies on raw materials absorbed from the environment.
Light Energy
Light serves as the initial energy source for photosynthesis. Plants capture this energy using pigments, predominantly chlorophyll. Chlorophyll molecules are housed within organelles called chloroplasts. Within chloroplasts, light energy is absorbed by chlorophyll and other accessory pigments.
Light absorption excites electrons within pigment molecules, raising them to a higher energy state. This captured energy transfers to reaction centers, driving the synthesis of energy-carrying molecules like ATP and NADPH. Plants utilize visible light for photosynthesis, specifically wavelengths between 400 and 700 nanometers, known as Photosynthetically Active Radiation (PAR). Chlorophyll a and chlorophyll b efficiently absorb light in the blue (around 425-450 nm) and red (around 600-700 nm) regions, reflecting green light.
Carbon Dioxide
Carbon dioxide is a raw material for building sugar molecules. This gas is absorbed from the atmosphere, where its concentration is approximately 0.04% (400 parts per million). Plants obtain carbon dioxide through pores on their leaf surfaces called stomata, which are regulated by guard cells. When stomata open, carbon dioxide enters the leaf’s internal structure.
Once inside the leaf, carbon dioxide molecules are incorporated into organic compounds during the Calvin cycle. Enzymes facilitate the binding of carbon atoms from CO2 to other molecules, leading to glucose synthesis. This conversion of inorganic carbon dioxide into organic sugar molecules is termed carbon fixation. Glucose is used for energy and growth, while oxygen is released back into the atmosphere through the stomata.
Water
Water plays a dual role in photosynthesis, acting as a direct reactant and a transport medium within the plant. Plants absorb water from the soil through their roots, via root hairs, which greatly increase the surface area for absorption. This absorption occurs through osmosis, where water moves from an area of higher concentration in the soil to an area of lower concentration within root cells.
Once absorbed, water is transported upwards from the roots to the leaves through vascular tissues called xylem. This upward movement is driven by transpiration, the evaporation of water vapor from the leaves, which creates a pulling force. In the light-dependent reactions, water molecules are split (photolysis). This splitting provides electrons and protons necessary to convert light energy into chemical energy (ATP and NADPH).