What is Photosynthesis?
Photosynthesis is one of the most important biological processes on Earth. It is carried out by plants, algae, and cyanobacteria (blue-green bacteria), and it forms the foundation of nearly all food webs.
Photosynthesis: The process by which light energy is converted into chemical energy, using carbon dioxide and water to produce glucose and oxygen.
Organisms that can make their own organic molecules from inorganic raw materials using an external energy source are called autotrophs. Photosynthesis is the mechanism of autotrophic nutrition in plants, algae, and cyanobacteria. In contrast, heterotrophs (animals, fungi, most bacteria) must obtain organic molecules by consuming other organisms , ultimately depending on autotrophs as the source of chemical energy in ecosystems.
The overall equation for photosynthesis is:
In plain English: carbon dioxide and water are combined using light energy to make glucose (a sugar used for energy and building materials) and oxygen (released as a by-product).
This equation is a simplified summary of what is actually a complex series of reactions involving two main stages, many intermediate steps, enzymes, and molecules. The two stages are the light-dependent reactions and the light-independent reactions (Calvin cycle). Don't be misled by how tidy the summary equation looks!
The glucose produced stores chemical energy in its bonds. This energy can later be released through cellular respiration to power the organism's activities. The oxygen released is what most living things , including us , breathe.
Chloroplast Structure and the Two Stages of Photosynthesis
Photosynthesis in plants and algae takes place inside chloroplasts. The structure of the chloroplast is directly linked to the two stages of photosynthesis.
Chloroplast: A double-membrane-bound organelle found in plant and algal cells that is the site of photosynthesis. It contains an internal membrane system (thylakoids) and a fluid-filled matrix (stroma).
Key structural features:
- Thylakoid membranes , flattened, interconnected membrane sacs arranged in stacks called grana (singular: granum). Photosynthetic pigments are embedded here. This is where the light-dependent reactions occur.
- Stroma , the fluid-filled space surrounding the thylakoids. This is where the light-independent reactions (Calvin cycle) occur.
- Double membrane (envelope) , the outer and inner membranes that enclose the chloroplast.
The two stages , overview:
| Feature | Light-dependent reactions | Calvin cycle (light-independent) |
|---|---|---|
| Location | Thylakoid membranes | Stroma |
| Requires light directly? | Yes | No (but depends on products of light stage) |
| Key inputs | Water, light energy, , | , ATP, reduced NADP |
| Key outputs | ATP, reduced NADP, | Glucose (via G3P), , |
The light-independent reactions are sometimes misleadingly called the "dark reactions" , but they do not require darkness. They simply do not directly use light. They run continuously as long as ATP and reduced NADP are being supplied by the light-dependent reactions.
