Which form of energy is converted into food? Exploring the science of photosynthesis and chemosynthesis

November 1, 2025 •

3 min read

Over 99% of all energy used by living things in the biosphere comes from the conversion of light energy into chemical energy. Plants, algae, and some bacteria accomplish this via photosynthesis, a process that forms the basis of nearly all food webs on Earth. However, some extremophiles use chemical energy in a process called chemosynthesis to produce their own food.

Quick Summary

This article explores how light energy and chemical energy are converted into food through two primary biological processes, photosynthesis and chemosynthesis. It details the mechanisms used by different organisms to create energy-rich organic compounds, sustaining life on Earth in both sunlit and sunless environments.

Key Points

Light Energy is Converted by Photosynthesis: The most common form of energy converted into food is light energy from the sun, which plants use during photosynthesis to create chemical energy.
Chlorophyll Captures Sunlight: The green pigment chlorophyll absorbs light energy to initiate the light-dependent reactions of photosynthesis, converting solar energy into chemical energy carriers.
The Calvin Cycle Fixes Carbon: During the light-independent reactions (Calvin cycle), the energy captured by chlorophyll is used to fix carbon dioxide and create glucose, the plant's primary food source.
Chemical Energy Fuels Extremophiles: In environments without sunlight, such as deep-sea vents, some bacteria and archaea perform chemosynthesis, using chemical energy from inorganic compounds to produce food.
Energy Storage is Crucial: After production, organisms store excess food energy, typically in the form of complex carbohydrates like starch, to provide energy during periods without an external energy source.
All Life Depends on Primary Producers: Whether through photosynthesis or chemosynthesis, these primary producers form the foundation of food chains, making all higher life forms dependent on their ability to convert energy into food.

Photosynthesis: The Conversion of Light into Food

Photosynthesis is the process that converts light energy, typically from the sun, into chemical energy in the form of organic compounds like glucose. This critical process is carried out by photoautotrophs, such as plants, algae, and certain bacteria, which form the base of almost all food chains. The entire process occurs within specialized cellular organelles known as chloroplasts, which contain a green pigment called chlorophyll.

The Role of Chlorophyll and Light Reactions

Chlorophyll is essential for photosynthesis due to its ability to absorb light energy, primarily red and blue wavelengths, while reflecting green light. This absorbed light initiates the light-dependent reactions within the thylakoid membranes of chloroplasts. Here, water molecules are split (photolysis), releasing oxygen, protons, and electrons. The energy from these electrons is used to create ATP and NADPH, two vital energy-carrying molecules. Oxygen is released as a byproduct.

The Calvin Cycle: The Dark Reactions

ATP and NADPH from the light reactions power the Calvin cycle (light-independent reactions) in the chloroplast's stroma. This cycle, which can occur in light, uses the enzyme RuBisCO to fix atmospheric carbon dioxide into an organic molecule. Using the stored chemical energy, this carbon is converted into glyceraldehyde-3-phosphate (G3P), which is then used to synthesize glucose and other necessary organic compounds like starch and cellulose.

List of Key Photosynthesis Requirements:

Sunlight: Provides initial energy.
Water ($H_2O$): Source of electrons and protons.
Carbon Dioxide ($CO_2$): Carbon source for organic compounds.
Chlorophyll: Light-absorbing pigment.
Chloroplasts: Site of photosynthesis.

Chemosynthesis: An Alternative Energy Source

In ecosystems without sunlight, such as deep-sea hydrothermal vents, chemoautotrophs (bacteria and archaea) produce food through chemosynthesis. These organisms convert chemical energy from inorganic compounds like hydrogen sulfide or methane into organic matter. This process supports unique food webs in sunless environments.

Comparison Table: Photosynthesis vs. Chemosynthesis


Feature	Photosynthesis	Chemosynthesis
Energy Source	Light energy (e.g., from the sun)	Chemical energy from inorganic compounds
Organisms	Photoautotrophs: plants, algae, cyanobacteria	Chemoautotrophs: specialized bacteria, archaea
Location	Sunlit areas (terrestrial and aquatic)	Environments lacking sunlight, e.g., deep-sea vents
Key Reactants	Carbon dioxide ($CO_2$), water ($H_2O$)	Carbon dioxide ($CO_2$), water ($H_2O$), inorganic chemicals ($H_2S$, $CH_4$)
Pigments Required	Yes (e.g., chlorophyll)	No
Byproducts	Oxygen ($O_2$)	Varies; often sulfur or sulfates
Carbon Source	Carbon dioxide ($CO_2$)	Carbon dioxide ($CO_2$) or methane ($CH_4$)

The Storage of Converted Energy

Energy converted into simple sugars through photosynthesis or chemosynthesis is often stored. Plants store excess glucose as starch, a complex carbohydrate, in various plant parts. This stored energy is released through cellular respiration and forms the base of food chains.

Conclusion: The Foundation of Life's Energy

Light energy, through photosynthesis, is the primary form of energy converted into food globally. In sunless environments, chemical energy through chemosynthesis serves the same function. These processes, carried out by autotrophs, create the chemical energy in sugars that sustains life and forms the foundation of Earth's food webs.

Visit the US Department of Energy's Office of Science for more information on the intricate processes of energy conversion.

Frequently Asked Questions

The primary process is photosynthesis, where plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose.

Photosynthesis consists of two main stages: the light-dependent reactions, which capture light energy, and the light-independent reactions (Calvin cycle), which use that energy to produce glucose from carbon dioxide.

Chemosynthesis is the process by which some organisms, mainly bacteria, use chemical energy from inorganic compounds to produce food. It occurs in environments without sunlight, such as deep-sea hydrothermal vents.

Plants convert the glucose they produce into more complex carbohydrates, like starch, for long-term energy storage. This starch can be stored in various parts of the plant, including seeds, roots, and leaves.

Photosynthesis is crucial because it forms the basis of nearly all food webs on Earth, providing energy for living things. It also releases oxygen into the atmosphere as a byproduct, which is essential for most aerobic life.

No, animals cannot produce their own food from light or chemical energy. They are heterotrophs, meaning they must consume other organisms or their products to obtain the chemical energy stored in food.

ATP, or adenosine triphosphate, is a molecule that stores and transports chemical energy within cells. In photosynthesis, it is generated during the light reactions and used to power the conversion of carbon dioxide into glucose during the Calvin cycle.