Photosynthesis is the fundamental biological process that allows plants and certain other organisms to convert light energy into chemical energy, effectively creating their own food. This mechanism acts as the engine for nearly all life on Earth, supplying the energy that fuels entire ecosystems and forming the base of every food web. The process introduces chemical energy and fixed carbon into the biosphere, which is then passed up the food chain. Without this conversion, the planet would lack food and oxygen, making complex life impossible.
Setting the Stage: Inputs and Internal Machinery
For this energy conversion to occur, plants require three primary inputs: sunlight, water, and carbon dioxide. Water is absorbed from the soil by the roots and transported upward. Carbon dioxide gas enters the leaves from the atmosphere through pores called stomata. Sunlight, the energy source, is captured within specialized organelles inside the plant cells known as chloroplasts.
Chloroplasts are contained within a double-membrane envelope and filled with a fluid called the stroma. Floating inside the stroma are stacks of flattened, disc-like sacs called thylakoids, where the initial light-capturing reactions take place. The thylakoid membranes contain the pigment chlorophyll, the molecule that absorbs light energy. Chlorophyll appears green because it efficiently absorbs light from the red and blue parts of the spectrum, reflecting the green light we see.
Phase One: The Light-Dependent Reactions
The first stage begins when chlorophyll molecules within the thylakoid membranes absorb light energy. This absorbed energy excites electrons within the pigment molecules to a higher energy state, initiating a flow through the electron transport chain. The movement of these high-energy electrons converts the light energy into chemical energy.
To replace the lost electrons, a water molecule is split in a process called photolysis. This splitting releases oxygen gas as a byproduct, which diffuses out of the plant and into the atmosphere. It also releases hydrogen ions (protons) into the thylakoid space, creating a concentration gradient across the thylakoid membrane.
The energy stored in this proton gradient is harnessed by an enzyme called ATP synthase. As protons flow through this enzyme, the movement drives the phosphorylation of ADP to form ATP, an energy-carrying molecule. Simultaneously, the energized electrons combine with NADP+ and a hydrogen ion to form NADPH, a molecule that carries reducing power. ATP and NADPH represent the chemical energy produced from light, which powers the next stage of food production.
Phase Two: Building Sugar in the Calvin Cycle
The second stage, known as the Calvin cycle or light-independent reactions, takes place in the stroma outside the thylakoids. This process does not directly require light but relies on the ATP and NADPH generated during the first stage to convert carbon dioxide into sugar. The cycle begins with carbon fixation, where the enzyme RuBisCO attaches carbon dioxide to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP).
This initial six-carbon molecule immediately splits into two molecules of 3-phosphoglycerate (3-PGA). The energy from ATP and NADPH is then invested to convert these 3-PGA molecules into glyceraldehyde-3-phosphate (G3P). G3P is the actual sugar product of the Calvin cycle, and for every six molecules produced, only one leaves the cycle to be used by the plant.
The remaining five G3P molecules are recycled, using additional ATP, to regenerate the original RuBP acceptor molecule. This regeneration allows the cycle to continue fixing more carbon dioxide. The single G3P molecule that exits is the building block for glucose, meaning it takes two turns of the Calvin cycle to produce one six-carbon glucose molecule.
What Happens to the Plant’s New Food
The glucose synthesized from the Calvin cycle serves several purposes for the plant’s survival and growth. A portion of the glucose is immediately used as fuel in the plant’s own cellular respiration, the process that converts stored chemical energy into usable energy for cellular work. Plants require this energy to power various functions, including nutrient uptake, transport, and the synthesis of new cellular components.
Excess glucose is converted into more complex molecules for storage or structure. For long-term energy storage, glucose is linked together to form starch, an insoluble carbohydrate stored in roots, seeds, and other plant parts. Glucose is also polymerized to create cellulose, the tough, fibrous material that forms the primary component of plant cell walls, providing structural support.