Imagine you are playing an open-world video game. Your character needs food to keep their energy bar full. If the energy bar runs out, your character slows down and eventually can’t move. Real living things are similar: every plant, animal, and microbe needs a constant supply of energy and matter to stay alive, grow, and reproduce. But where does that energy really come from, and how does the matter that makes up organisms keep cycling around Earth?
The surprising answer: almost all life on Earth depends on a single process happening in plant leaves, algae, and some bacteria—photosynthesis. It is like Earth’s ultimate “charger,” powering food webs and recycling key materials.
Photosynthesis is the process by which certain organisms—mainly plants, algae, and some bacteria—use light energy from the Sun to make their own food. They build energy-rich sugar molecules using simple substances from the environment.
The main ingredients are:
Plants then produce:
Many textbooks show a word equation that describes this overall process (without going into tiny chemical steps):
Carbon dioxide + Water → Sugars + Oxygen (with light energy)
This tells us what goes in and what comes out, without needing the detailed biochemical mechanism.
As shown in [Figure 1], a plant takes in light, carbon dioxide, and water, and releases oxygen while making sugars inside its leaves.
Photosynthesis mostly happens in the leaves of plants. Leaves contain special green structures that capture light. You can think of each leaf as a tiny solar panel that turns sunlight into stored chemical energy.
Some key points about where photosynthesis happens:
Even though these organisms live in very different places, they all play the same basic role: they use light to make food and release oxygen.
Energy cannot be created or destroyed, but it can change form. For life on Earth, the main original source of energy is the Sun. However, most organisms cannot use sunlight directly. They need that energy to be stored in the bonds of food molecules, like sugars and fats.
Photosynthesis is the bridge that moves energy from sunlight into food. Here’s how the flow of energy works:
Because of this, we say that photosynthetic organisms are producers—they produce their own food and bring energy into ecosystems.
To understand how energy moves through living systems, it helps to look at three main roles in an ecosystem:
All these roles are connected through energy flow and matter cycling, and photosynthesis is what starts it all.
A food chain is a simple model that shows how energy flows from one organism to another when one eats another. For example:
Sun → Grass → Grasshopper → Frog → Snake → Hawk
In this chain:
A food web is a bunch of interconnected food chains in one ecosystem. In every case, if you trace the arrows backward, they almost always lead back to producers and photosynthesis.
As shown in [Figure 2], a simple food web includes multiple producers, herbivores, and predators, and all of the arrows start from the producers that perform photosynthesis.
When organisms use food, they break down sugar molecules to release the stored energy. That energy is used for:
Each time energy moves from one level to the next in a food chain, some is released as heat. This energy is not destroyed, but it is no longer in a form that living cells can use for building and powering themselves. Because of this, energy flows one way through an ecosystem: from the Sun → producers → consumers → heat.
This is different from matter, which cycles and is reused over and over. Photosynthesis is the key process that brings usable energy into this one-way flow.
While energy flows in one direction, matter (like atoms of carbon, oxygen, and hydrogen) is constantly being reused. Photosynthesis plays a central role in cycling matter through the environment and living things.
Here’s how the matter cycle works with photosynthesis:
As shown in [Figure 3], photosynthesis draws in carbon dioxide from the air, while respiration and decomposition return it, forming a continuous cycle of matter.
The movement of carbon through the atmosphere, living things, oceans, and soil is called the carbon cycle. Photosynthesis is one of the main processes that removes carbon dioxide from the air and locks that carbon into living organisms.
Key points about the carbon cycle and photosynthesis:
Because photosynthesis moves carbon from the air into living things, it helps control how much carbon dioxide is in the atmosphere. This matters for climate and for the health of ecosystems.
It can be easy to think that plants “eat” soil to grow, but most of a plant’s mass actually comes from air and water, not dirt. The atoms in carbon dioxide and water are rearranged by photosynthesis to form sugars and other molecules.
These molecules are then used to build all the parts of a plant:
When animals eat plants, the matter that was inside the plant becomes part of the animal’s body. So when you grow taller, a lot of the matter that makes up you used to be carbon dioxide, water, and minerals taken up by plants—and the energy that helped build your body came from sunlight!
Every crop—corn, wheat, rice, vegetables, fruits—depends on photosynthesis. Farmers care about anything that affects how much photosynthesis plants can do, such as:
When plants photosynthesize more, they usually grow better and produce more food. This can mean more grain, larger fruits, or more leaves for us or farm animals to eat.
Forests are sometimes called “carbon sinks” because they store large amounts of carbon taken from the atmosphere through photosynthesis. When forests are healthy and growing:
When forests are cut down or burned, much of that stored carbon returns to the atmosphere as carbon dioxide, which can affect Earth’s climate. Understanding photosynthesis helps people make decisions about protecting forests and planting new trees.
In oceans, lakes, and rivers, tiny photosynthetic organisms called phytoplankton and algae perform photosynthesis. They:
Fish and other aquatic animals rely on the oxygen dissolved in water, much of which comes from photosynthesis by these tiny organisms.
People often keep houseplants in their homes, schools, and offices. Besides looking nice, these plants:
This won’t completely “clean” the air in a large room, but it does show the same basic principle: photosynthesis pulls in carbon dioxide and releases oxygen.
Scientists and engineers study photosynthesis to design better solar panels and “artificial leaves” that might one day help produce clean fuels or remove carbon dioxide from the air. These technologies try to copy the way plants capture light and turn it into stored energy, but using man-made materials.
Without going into microscopic details, you can still observe some effects of photosynthesis in everyday life:
Claim:
Claim: Photosynthesis is the process that brings energy into ecosystems and helps cycle matter, such as carbon and oxygen, through living and nonliving parts of the environment.
Evidence:
Reasoning: If producers capture light energy and store it in sugars, then other organisms can obtain that energy by eating the producers or by eating other consumers. Because the atoms in carbon dioxide and water become part of sugar molecules, and those molecules are used to build plant and animal bodies, matter from the air and water becomes part of living organisms. When organisms respire or decompose, carbon dioxide returns to the air, closing the loop. Therefore, photosynthesis both starts the flow of energy into ecosystems and drives the cycling of key matter like carbon and oxygen.
