Photosynthesis is the biological process that captures sunlight and converts it into chemical compounds, mainly carbohydrates, that power the metabolism of nearly all organisms. It is the only process that brings the sun's energy into the living world and it also releases oxygen as a byproduct. Only photoautotrophs — plants, algae, and cyanobacteria — carry it out, and they all depend on chlorophyll to absorb light. The OpenStax Biology 2e chapter summary describes photosynthesis as the transformation that enabled the biodiversity seen on Earth today.
Without photosynthesis, the energy from sunlight would not be available to living things in a usable form. The process converts light energy into chemical bonds in carbohydrates, which then fuel the metabolism of the plant itself and of the animals that eat it. It also supplies the oxygen that most organisms need for respiration. According to OpenStax Biology 2e, photosynthesis is the only biological process that captures sunlight energy and turns it into compounds that power every organism's metabolism.
Photoautotrophs are organisms that make their own food using light, and they include plants, algae, and cyanobacteria. These organisms share the ability to perform photosynthesis because they all contain chlorophyll, the pigment that absorbs certain wavelengths of visible light. Not all organisms can do this; animals, fungi, and most bacteria rely on the carbohydrates produced by photoautotrophs. OpenStax Biology 2e notes that only these three groups carry out photosynthesis.
Chlorophyll is a pigment that absorbs specific wavelengths of visible light, mostly in the blue and red parts of the spectrum, while reflecting green light. That is why leaves appear green to us. The absorbed light energy excites electrons in the chlorophyll molecule, starting the flow of energy that eventually becomes chemical bonds. OpenStax Biology 2e identifies chlorophyll as a requirement for photosynthesis in plants, algae, and cyanobacteria.
In plants, photosynthesis takes place mainly in the mesophyll, the tissue inside the leaf. Gas exchange — carbon dioxide in and oxygen out — happens through small openings called stomata. Each stoma is regulated by two guard cells that can open or close it, balancing the need for carbon dioxide with the risk of water loss. OpenStax Biology 2e describes this arrangement as the entry point for the raw materials of photosynthesis.
The chloroplast is the organelle where photosynthesis occurs, and it has a double-membrane envelope. Inside, there are stacks of flattened sacs called thylakoids; a single stack is called a granum. The fluid surrounding the thylakoids is the stroma. OpenStax Biology 2e explains that chloroplasts are ancestrally derived from ancient free-living cyanobacteria, which is why they have their own DNA and membranes.
Photosynthesis is divided into two stages: the light-dependent reactions and the light-independent reactions. The light-dependent reactions occur in the thylakoid membranes and require direct light to produce ATP and NADPH. The light-independent reactions, also called the Calvin cycle, use those products to build carbohydrates from carbon dioxide. OpenStax Biology 2e presents these two stages as the core framework of the chapter.
The light-dependent reactions begin when a photon strikes photosystem II (PSII). The energy travels to the reaction center, known as P680, and then to the electron transport chain. This chain pumps hydrogen ions into the thylakoid interior, creating a gradient. OpenStax Biology 2e describes this as the first major phase of photosynthesis.
When a photon hits photosystem II, it excites an electron in P680, which is passed to the electron transport chain. As electrons move through the chain, energy is used to pump hydrogen ions from the stroma into the thylakoid space. This creates a high concentration of hydrogen ions inside the thylakoid. OpenStax Biology 2e explains that this gradient is later used to make ATP.
The hydrogen ions stored inside the thylakoid flow back out through a protein called ATP synthase. This flow, called chemiosmosis, drives the enzyme to attach a phosphate group to ADP, forming ATP. ATP is the main energy currency of the cell. OpenStax Biology 2e presents chemiosmosis as the mechanism that converts the ion gradient into usable chemical energy.
After the electron transport chain, the electron reaches photosystem I (PSI). A second photon is absorbed there, re-energizing the electron. The electron is then transferred to NADP+ along with a hydrogen ion to form NADPH. OpenStax Biology 2e notes that NADPH is another energy carrier used in the next stage of photosynthesis.
The electrons that replace those lost by P680 come from water. When water is split, oxygen is released as a byproduct. This is the source of the oxygen we breathe. OpenStax Biology 2e states that water is split in the light-dependent reactions, releasing oxygen.
The light-independent reactions, or Calvin cycle, do not need light directly. They use ATP and NADPH from the light-dependent reactions to convert carbon dioxide into carbohydrates. The cycle is named after Melvin Calvin, who helped discover it. OpenStax Biology 2e describes the Calvin cycle as the stage where carbon fixation occurs.
In the Calvin cycle, the enzyme RuBisCO catalyzes a reaction between carbon dioxide and a five-carbon sugar called RuBP. This step fixes inorganic carbon into an organic molecule. RuBisCO is thought to be one of the most abundant enzymes on Earth. OpenStax Biology 2e identifies RuBisCO as the key enzyme for carbon fixation.
After three cycles of the Calvin cycle, a three-carbon molecule called G3P leaves the cycle. This molecule becomes part of a carbohydrate, such as glucose. The cycle must turn three times to produce one net G3P. OpenStax Biology 2e explains that G3P is the direct product that exits to build sugars.
ATP and NADPH produced in the light-dependent reactions are consumed in the Calvin cycle. ATP provides the energy, and NADPH provides the reducing power to convert carbon dioxide into G3P. Without these carriers, the Calvin cycle would stop. OpenStax Biology 2e makes clear that the two stages are interdependent.
The light-dependent reactions supply ATP and NADPH to the Calvin cycle. The Calvin cycle, in turn, produces ADP and NADP+ that return to the light-dependent reactions. This exchange creates a continuous cycle of energy and matter. OpenStax Biology 2e describes this interdependence as essential for sustained photosynthesis.
Plants contain both chloroplasts and mitochondria, so they perform both photosynthesis and respiration. Photosynthesis stores energy in carbohydrates, while respiration releases energy from them. This allows plants to function in both light and dark conditions. According to OpenStax Biology 2e, plants rely on both processes to interconvert essential metabolites.
Photosynthesis transformed life on Earth by making the sun's energy available to living organisms. It enabled the rise of complex, oxygen-breathing life by releasing oxygen into the atmosphere. Today, it continues to be the foundation of nearly all food chains. OpenStax Biology 2e notes that photosynthesis is the source of the oxygen and the organic compounds that sustain ecosystems.
Photosynthesis has two main stages: light-dependent reactions in the thylakoid membranes and the Calvin cycle in the stroma. The light-dependent reactions use water, light, and photosystems II and I to produce ATP and NADPH, releasing oxygen. The Calvin cycle uses ATP and NADPH to fix carbon dioxide into G3P, a carbohydrate precursor. OpenStax Biology 2e presents these concepts across sections 8.1, 8.2, and 8.3, with the final sentence noting that plants rely on both photosynthesis and respiration to function.
