Table of Contents
Photosynthesis is the central anabolic (building) process by which certain organisms convert light energy into chemical energy stored in organic molecules. In this chapter, the whole process is introduced as a framework; details of chloroplast structure, photosystems, individual reaction steps, and influencing factors are treated in the corresponding subchapters.
What Photosynthesis Achieves – in Organisms and on Earth
Photosynthesis links the energy of sunlight to the metabolism of living beings:
- It captures light energy and converts it into chemical energy.
- It uses that energy to build energy‑rich organic compounds (mainly carbohydrates) from simple, low‑energy inorganic substances.
- It provides the basis for nearly all food chains and for the oxygen content of Earth’s atmosphere.
In chemical shorthand, oxygenic photosynthesis (as in plants, algae, cyanobacteria) is often written as:
$$
6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} \xrightarrow{\text{light}} \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2}
$$
This is an overall equation. It hides many intermediate steps and molecules, which are addressed later. The important points for now are:
- Inorganic starting materials: carbon dioxide ($\mathrm{CO_2}$) and water ($\mathrm{H_2O}$)
- Product: an energy‑rich organic compound (here written as glucose $\mathrm{C_6H_{12}O_6}$ as a model)
- By‑product: molecular oxygen ($\mathrm{O_2}$)
- Energy source: light
Organisms That Perform Photosynthesis
Not all living beings can photosynthesize. Those that do are called photoautotrophs: they use light as an energy source and inorganic carbon (usually $\mathrm{CO_2}$) as a carbon source.
Important groups:
- Plants (land plants and many aquatic plants): chloroplast‑containing eukaryotes
- Algae: diverse mostly aquatic photosynthetic eukaryotes
- Cyanobacteria: photosynthetic prokaryotes (once called “blue‑green algae”)
These organisms form the primary producers in most ecosystems. They supply:
- Organic molecules as food for themselves and for heterotrophic organisms
- Oxygen for aerobic respiration
Some bacteria perform anoxygenic photosynthesis: they use light to build organic substances but do not release oxygen and may use other electron donors (e.g. hydrogen sulfide instead of water). The overall principles are similar, but the details and ecological roles differ.
Two Main Parts of Photosynthesis
Although the process is continuous in the cell, it is useful to separate photosynthesis into two functionally different reaction complexes:
- Light‑dependent reactions
- Require light directly
- Convert light energy into chemical energy (ATP) and a reducing power (NADPH)
- Split water in oxygenic photosynthesis and release $\mathrm{O_2}$
- Light‑independent reactions (also called the dark reactions or Calvin cycle)
- Do not use light directly, but depend on ATP and NADPH produced in the light‑dependent reactions
- Fix inorganic $\mathrm{CO_2}$ into organic molecules
- Build carbohydrates and other carbon skeletons
This division emphasizes a key idea:
- The plant first harvests energy (light‑dependent reactions) and stores it in universal cellular forms (ATP, NADPH).
- It then invests that energy to drive carbon fixation and biosynthesis (light‑independent reactions).
Photosynthesis as an Anabolic Process
Photosynthesis is a classic example of anabolism:
- Anabolic reactions build complex molecules from simple ones.
- They require an input of energy and reducing equivalents.
In photosynthesis:
- The building blocks are $\mathrm{CO_2}$ and water (or other inorganic donors in anoxygenic forms).
- The products are energy‑rich organic compounds (such as sugars) and, in oxygenic photosynthesis, oxygen.
- The energy and reducing power needed to drive these energetically “uphill” syntheses are provided by ATP and NADPH from the light‑dependent reactions.
The carbohydrates produced do not stay in that form only. They can:
- Be converted into starch or other storage molecules
- Serve as starting material for the synthesis of lipids, amino acids, nucleotides, etc.
- Be broken down later in catabolic processes such as cellular respiration and fermentation to release energy for the cell
Thus, photosynthesis and cellular respiration are tightly coupled in global energy and matter cycles: one builds and stores, the other breaks down and releases.
Redox Nature of Photosynthesis
Photosynthesis is fundamentally a series of redox reactions (oxidation–reduction reactions):
- Carbon in $\mathrm{CO_2}$ is in a highly oxidized state.
- In carbohydrates (e.g. glucose), carbon atoms are more reduced.
Therefore, photosynthesis involves:
- Reduction of $\mathrm{CO_2}$ to form organic compounds
- Oxidation of water to $\mathrm{O_2}$ (in oxygenic photosynthesis)
Electrons flow from a donor (water or another substance) to carbon, and this flow is driven “uphill” by light energy absorbed by pigment molecules.
The key consequences:
- Light energy is converted to a proton gradient and high‑energy electrons, then to ATP and NADPH.
- These energetic molecules then power the chemical reduction of $\mathrm{CO_2}$.
Ecological and Global Significance
On a planetary scale, photosynthesis has profound effects:
- Oxygen production: Over geological time, oxygenic photosynthesis by cyanobacteria and later by algae and plants transformed Earth’s atmosphere, enabling aerobic respiration and complex multicellular life.
- Carbon fixation: Photosynthesis captures vast amounts of atmospheric $\mathrm{CO_2}$ and incorporates it into the biomass of producers.
- Base of food chains: Nearly every heterotrophic organism depends directly or indirectly on the organic matter formed by photoautotrophs.
- Global cycles: Photosynthesis is a major driver of the global carbon cycle and influences climate and nutrient cycles together with other metabolic processes.
Overview of Energy and Matter Flow
To place photosynthesis in the context of metabolism:
- Energy flow:
- Sun → light‑harvesting pigments → ATP and NADPH → chemical bonds in organic molecules
- Later, through respiration, those bonds are broken and energy is made available to cells.
- Matter flow:
- Inorganic carbon ($\mathrm{CO_2}$) → organic carbon in carbohydrates and other biomolecules
- Water and mineral nutrients are incorporated into biomass
- Oxygen (in oxygenic photosynthesis) is released, used by aerobic organisms, and returned as $\mathrm{CO_2}$ in respiration.
This coupling between light capture, carbon fixation, and global cycles is what makes photosynthesis a central theme in metabolism and energy conversion.
In the following subchapters, the structural basis in chloroplasts, the detailed roles of photosystems, the stepwise course of the light‑dependent and light‑independent reactions, and environmental influences on photosynthesis are explored in more depth.