If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Photosynthesis review

ENE‑1 (EU)
ENE‑1.I (LO)
ENE‑1.I.1 (EK)
ENE‑1.I.1.i (EK)
ENE‑1.I.2 (EK)
ENE‑1.J (LO)
ENE‑1.J.5 (EK)

Key Terms

PhotosynthesisThe process by which plants, algae, and some bacteria convert light energy to chemical energy in the form of sugars
PhotoautotrophAn organism that produces its own food using light energy (like plants)
ATPAdenosine triphosphate, the primary energy carrier in living things
ChloroplastThe plant cell structure where photosynthesis occurs
ThylakoidsDisc-like structures within a chloroplast that help absorb light
GranaStacks of thylakoids in a chloroplast
ChlorophyllA pigment found in the thylakoid that absorbs light energy and uses it to produce carbohydrates
StromaFluid-filled space surrounding the grana

Photosynthesis reaction

During photosynthesis, photoautotrophs use energy from the sun, along with carbon dioxide and water, to form glucose and oxygen.
The overall equation for photosynthesis is:
In photosynthesis, solar energy is harvested and converted to chemical energy in the form of glucose using water and carbon dioxide. Oxygen is released as a byproduct.

The stages of photosynthesis

There are two main stages of photosynthesis: the light-dependent reactions and the Calvin cycle.
StageLocationEventsRequires sunlight?
Light-dependent reactionsThylakoid membraneLight energy is captured by chloroplasts and stored as ATPYes
Calvin cycleStromaATP is used to create sugars that the plant will use to grow and liveNo
Schematic of the light-dependent reactions and Calvin cycle and how they're connected.
The light-dependent reactions take place in the thylakoid membrane. They require light, and their net effect is to convert water molecules into oxygen, while producing ATP molecules—from ADP and Pi—and NADPH molecules—via reduction of NADP+.
ATP and NADPH are produced on the stroma side of the thylakoid membrane, where they can be used by the Calvin cycle.
The Calvin cycle takes place in the stroma and uses the ATP and NADPH from the light-dependent reactions to fix carbon dioxide, producing three-carbon sugars—glyceraldehyde-3-phosphate, or G3P, molecules.
The Calvin cycle converts ATP to ADP and Pi, and it converts NADPH to NADP+. The ADP, Pi, and NADP+ can be reused as substrates in the light reactions.
Image credit: modified from "Overview of photosynthesis: Figure 6" by OpenStax College, Biology, CC BY 3.0

Common mistakes and misconceptions

  • Photosynthesis and cellular respiration are almost opposite processes. Looking at their equations, they differ only in the form of energy that is being absorbed or released. However, they are not simply the reversal of each other, as each one takes place in its own particular series of steps.
On a simplified level, photosynthesis and cellular respiration are opposite reactions of each other. In photosynthesis, solar energy is harvested as chemical energy in a process that converts water and carbon dioxide to glucose. Oxygen is released as a byproduct. In cellular respiration, oxygen is used to break down glucose, releasing chemical energy and heat in the process. Carbon dioxide and water are products of this reaction.
  • Plants are green because chlorophyll reflects green light. Many people think that chlorophyll is green because it wants to absorb and use green light. However, this is not true. The color we see is actually the color of light that is being reflected. Therefore, chlorophyll reflects green light, while absorbing red and blue light.

Want to join the conversation?