Photosynthesis is the remarkable process through which plants, algae, and some bacteria convert sunlight into food.
This occurs mainly in the leaves, where a green pigment called chlorophyll captures sunlight. This light energy is essential
for transforming carbon dioxide from the air and water from the soil into glucose, a simple sugar that serves as food for the plant.
As a bonus, oxygen is released into the atmosphere, which is crucial for most life forms on Earth.
The simplified chemical reaction for this process can be written as:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6O2
In this equation, six molecules of carbon dioxide and six molecules of water combine to produce one molecule of glucose and six molecules of oxygen. Photosynthesis is vital not just for plants, but for all living organisms, as it forms the base of the food chain and contributes to the oxygen we breathe. Without photosynthesis, life on Earth as we know it would not be possible.
For detailed process of photosynthesis, please read further about the various photosynthesis reaction and the role of pigment in the process:
Photosynthesis is a fundamental process through which plants convert light energy into chemical energy, which produce food to the plant. This occurs mainly in the chloroplasts of leaves, where chlorophyll absorbs light. Photosynthesis is a fascinating process that allows plants, algae, and some bacteria to produce glucose. The most effective wavelengths for photosynthesis fall within the Photosynthetically Active Radiation (PAR) range, specifically between 400 and 700 nm. Chlorophyll predominantly absorbs blue light (400-500 nm) and red light (600-700 nm) mainly. This process not only fuels plant growth but also produces oxygen, supporting nearly all life on Earth. Understanding photosynthesis involves exploring the roles of pigments, the biochemical reactions that follow light absorption, and the interconnected cycles that govern energy transformation.
Plants contain different pigments that play crucial roles in the process of photosynthesis reaction . At the core of this process, there are specialized pigments, with chlorophyll being the most prominent. Chlorophyll gives plants their green colour and absorbs light from the light source primarily in the blue and red wavelengths. When light strikes a leaf surface, chlorophyll captures specific wavelengths, exciting electrons and initiating the photosynthetic process.
In addition to chlorophyll, plants also contain phytochromes, which are sensitive to red and far-red light. Phytochromes help regulate various growth processes, such as seed germination and flowering, based on light conditions. Carotenoids, another group of pigments, absorb blue and green light, providing additional protection against photo-oxidative damage. Together, these pigments maximize light absorption, ensuring that plants can efficiently harness energy for their metabolic activities.
Moreover, the interplay between these pigments extends beyond mere light absorption; it also influences the plant's overall health and adaptability. For instance, in response to varying light conditions, plants can adjust the concentration of carotenoids and other accessory pigments to optimize their photosynthetic efficiency and protect against excessive light that could lead to photodamage. Additionally, some pigments, like anthocyanins, not only contribute to colour of fruit but also serve protective roles, shielding against UV radiation and attracting pollinators. The synergistic functions of these pigments underscore their vital role in not just energy capture but also in promoting resilience and reproductive success in the plant kingdom.
Once chlorophyll absorbs light energy, it triggers the photosynthetic reactions i.e. light-dependent reactions, which occur in the thylakoid membranes of chloroplasts. These reactions are divided into two phases: one occurring in Photosystem II (PS-II) and other in Photosystem I (PS-I).
In PS-II, absorbed light energy excites electrons within chlorophyll molecules. These high energy electrons are then transferred through a series of proteins known as the electron transport chain. As electrons move along the chain, they help pump protons (H⁺ ions) into the thylakoid lumen, creating a proton gradient. This gradient is crucial for synthesizing ATP (energy) through a process called photophosphorylation, where ATP synthase catalyses the conversion of ADP and inorganic phosphate into ATP.
Simultaneously, PS-I absorbs light energy, further energizing electrons that have passed through the electron transport chain. These high-energy electrons are transferred to the final electron acceptor, NADP⁺, converting it into NADPH. Both ATP and NADPH produced in the light-dependent reactions are essential for the subsequent phase of photosynthesis. The further process of conversion of NAPH into the glucose take place in the Calvin cycle.
The Calvin cycle is a vital process of photosynthesis that takes place in the stroma of chloroplasts, transforming carbon dioxide into glucose using ATP and NADPH produced during the light-dependent reactions. This cycle consists of three main stages: carbon fixation, reduction, and regeneration.
In the first step, carbon fixation, the enzyme RubisCo captures carbon dioxide from the atmosphere and attaches it to a five-carbon sugar called ribulose bisphosphate (RuBP). This reaction produces a six-carbon intermediate that immediately splits into two three-carbon molecules known as 3-phosphoglycerate (3-PGA).
During the reduction phase, ATP and NADPH are employed to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. Some of these G3P molecules are used to synthesize glucose, while others are recycled to regenerate RuBP, enabling the cycle to continue. This ongoing process is essential for providing energy and organic compounds that sustain plant growth and development.
The Krebs cycle, or citric acid cycle, is integral to cellular respiration, occurring in the mitochondria, the powerhouse of cells. While the Calvin cycle produces glucose during photosynthesis, the Krebs cycle converts this glucose into usable energy (ATP) and releases carbon dioxide as a byproduct. This cycle processes pyruvate from glucose breakdown, transferring energy to electron carriers like NADH and FADH₂. These carriers contribute to the electron transport chain, leading to ATP production. Understanding the Krebs cycle underscores the connection between photosynthesis and cellular respiration, illustrating how energy captured during photosynthesis is transformed into a form that fuels plant growth and functions.
Read more about Absorption theory of Plants.