Photosynthesis
Learn how plants use light energy to build sugar from carbon dioxide and water, name the inputs and outputs, and see why a plant's mass comes from air, not soil.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Photosynthesis, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Imagine a tiny acorn growing into an oak tree that weighs several tons. Where did all that wood come from? Most people guess the soil — and most people are wrong. The wood came mostly out of thin air, plus water pulled up through the roots.
That surprising fact is the heart of photosynthesis, the process plants use to capture light energy and rearrange the atoms in carbon dioxide and water into sugar. In this lesson you will learn what goes into photosynthesis, what comes out, where in the plant it happens, and how to explain the flow of matter in a way that actually accounts for a plant's growing mass. Getting this right sets you up for the rest of the unit, where you will trace that same sugar as it moves through living things.
That surprising fact is the heart of photosynthesis, the process plants use to capture light energy and rearrange the atoms in carbon dioxide and water into sugar. In this lesson you will learn what goes into photosynthesis, what comes out, where in the plant it happens, and how to explain the flow of matter in a way that actually accounts for a plant's growing mass. Getting this right sets you up for the rest of the unit, where you will trace that same sugar as it moves through living things.
What Photosynthesis Is
Photosynthesis is the process in which a plant uses light energy to build sugar molecules out of carbon dioxide and water. Two things are happening at the same time, and it helps to keep them separate in your head.
First, there is an energy change. Light energy from the Sun is absorbed and stored as chemical energy in the bonds of the sugar molecule. The energy does not disappear and it is not created — it changes form, from light to chemical energy.
Second, there is a matter change. The atoms in carbon dioxide and water get taken apart and rearranged into new molecules: sugar and oxygen. No atoms are created here either. Every carbon atom in a plant's sugar came from a carbon dioxide molecule in the air; every oxygen molecule released came from atoms the plant already had.
This happens inside chloroplasts, small structures found mostly in leaf cells. Chloroplasts contain chlorophyll, the green pigment that absorbs light energy — mainly red and blue light — and reflects green light, which is why most leaves look green to your eyes.
A common mix-up is saying that plants "make energy" or "make food out of sunlight." Plants do not make energy, and sunlight is not an ingredient the way carbon dioxide and water are. Sunlight is the energy source that powers the rearranging. The ingredients — the actual matter — are carbon dioxide and water. A precise sentence sounds like this: the plant uses light energy to build sugar from carbon dioxide and water.
First, there is an energy change. Light energy from the Sun is absorbed and stored as chemical energy in the bonds of the sugar molecule. The energy does not disappear and it is not created — it changes form, from light to chemical energy.
Second, there is a matter change. The atoms in carbon dioxide and water get taken apart and rearranged into new molecules: sugar and oxygen. No atoms are created here either. Every carbon atom in a plant's sugar came from a carbon dioxide molecule in the air; every oxygen molecule released came from atoms the plant already had.
This happens inside chloroplasts, small structures found mostly in leaf cells. Chloroplasts contain chlorophyll, the green pigment that absorbs light energy — mainly red and blue light — and reflects green light, which is why most leaves look green to your eyes.
A common mix-up is saying that plants "make energy" or "make food out of sunlight." Plants do not make energy, and sunlight is not an ingredient the way carbon dioxide and water are. Sunlight is the energy source that powers the rearranging. The ingredients — the actual matter — are carbon dioxide and water. A precise sentence sounds like this: the plant uses light energy to build sugar from carbon dioxide and water.
Inputs and Outputs in Words
Scientists summarize photosynthesis with a word equation:
carbon dioxide + water + light energy sugar + oxygen
In chemical symbols, the same statement is , with light energy supplied from outside. You are mainly responsible for the word version, but notice that the number of each kind of atom is the same on both sides — matter is conserved.
A detail students often get backwards: water enters through the roots but carbon dioxide does not. Roots pull in water and dissolved minerals; the carbon comes in as a gas through the leaves. Another frequent slip is listing "soil" or "nutrients" as inputs to photosynthesis. Minerals from soil matter for a plant's health — they help it build proteins and chlorophyll — but they are not reactants in photosynthesis, and they make up only a tiny fraction of a plant's mass.
carbon dioxide + water + light energy sugar + oxygen
In chemical symbols, the same statement is , with light energy supplied from outside. You are mainly responsible for the word version, but notice that the number of each kind of atom is the same on both sides — matter is conserved.
| Input | Where the plant gets it |
|---|---|
| Carbon dioxide | From the air, entering leaves through tiny pores called stomata |
| Water | From the soil, absorbed by roots and carried up to the leaves |
| Light energy | From the Sun, absorbed by chlorophyll in chloroplasts |
| Output | What happens to it |
|---|---|
| Sugar (glucose) | Used for energy, or built into cellulose, starch, and other plant structures |
| Oxygen | Released into the air through the stomata |
Where a Plant's Matter Really Comes From
Here is the idea this lesson is really built around: the matter that makes up a plant comes mostly from the air and from water, not from the soil.
Think about it as atom bookkeeping. Sugar has the formula . The carbon atoms can only have come from carbon dioxide, a gas in the air. The hydrogen atoms can only have come from water. Wood, leaves, stems, and fruit are built largely out of that sugar, linked together into long cellulose chains. So a tree is, in a real sense, built from air and water.
A scientist named Jan Baptist van Helmont tested this about 400 years ago. He planted a small willow in a pot of dried soil, watered it for five years, and weighed everything again. The willow had gained about 74 kilograms. The soil had lost only a few dozen grams. He concluded the plant came from water — half right, because he did not yet know about carbon dioxide.
Why do students go wrong here? Because we water and fertilize plants, and because "plants get their food from the soil" sounds like common sense. But food, for a plant, means sugar, and the plant builds that itself. Soil provides water, mineral elements like nitrogen and phosphorus, and physical support. If soil were the source of a plant's mass, a pot would visibly empty out as the plant grew — and it does not.
When you explain this on a homework question, name the specific molecules: carbon dioxide from the air supplies carbon and oxygen atoms, water supplies hydrogen atoms, and those atoms get rearranged into sugar that becomes plant body.
Think about it as atom bookkeeping. Sugar has the formula . The carbon atoms can only have come from carbon dioxide, a gas in the air. The hydrogen atoms can only have come from water. Wood, leaves, stems, and fruit are built largely out of that sugar, linked together into long cellulose chains. So a tree is, in a real sense, built from air and water.
A scientist named Jan Baptist van Helmont tested this about 400 years ago. He planted a small willow in a pot of dried soil, watered it for five years, and weighed everything again. The willow had gained about 74 kilograms. The soil had lost only a few dozen grams. He concluded the plant came from water — half right, because he did not yet know about carbon dioxide.
Why do students go wrong here? Because we water and fertilize plants, and because "plants get their food from the soil" sounds like common sense. But food, for a plant, means sugar, and the plant builds that itself. Soil provides water, mineral elements like nitrogen and phosphorus, and physical support. If soil were the source of a plant's mass, a pot would visibly empty out as the plant grew — and it does not.
When you explain this on a homework question, name the specific molecules: carbon dioxide from the air supplies carbon and oxygen atoms, water supplies hydrogen atoms, and those atoms get rearranged into sugar that becomes plant body.
When Photosynthesis Speeds Up or Slows Down
Photosynthesis is not a switch that is simply on or off — its rate depends on conditions, and this is where a lot of lab investigations in this unit come from.
A classic classroom investigation uses a water plant like elodea in a beaker: count the oxygen bubbles rising from the leaves per minute with a lamp near the beaker, then farther away. More bubbles means faster photosynthesis, because oxygen is an output. This works as evidence precisely because you know oxygen is a product, not an ingredient.
One thing to keep straight: plants do not stop needing energy at night. They carry out cellular respiration around the clock, breaking sugar back down to release usable energy — which is the next piece of this unit. Photosynthesis happens only in the light, in cells with chloroplasts. Respiration happens all the time, in every living cell of the plant. Saying "plants breathe in carbon dioxide and breathe out oxygen" is too simple; during daylight a healthy plant releases far more oxygen than it uses, but the gases move both directions.
| Condition | Effect on the rate | Why |
|---|---|---|
| Brighter light | Faster, up to a point | More light energy available to power the reaction |
| Darkness | Stops | No energy source to drive the rearranging |
| More carbon dioxide | Faster, up to a point | More raw material for building sugar |
| Too little water | Slows | Stomata close to save water, blocking carbon dioxide |
| Very cold temperature | Slows | The chemical steps run more slowly |
One thing to keep straight: plants do not stop needing energy at night. They carry out cellular respiration around the clock, breaking sugar back down to release usable energy — which is the next piece of this unit. Photosynthesis happens only in the light, in cells with chloroplasts. Respiration happens all the time, in every living cell of the plant. Saying "plants breathe in carbon dioxide and breathe out oxygen" is too simple; during daylight a healthy plant releases far more oxygen than it uses, but the gases move both directions.
Key terms
- Photosynthesis.
- The process in which a plant uses light energy to build sugar from carbon dioxide and water, releasing oxygen.
- Chloroplast.
- The structure inside plant cells, found mostly in leaves, where photosynthesis takes place.
- Chlorophyll.
- The green pigment inside chloroplasts that absorbs light energy and gives leaves their color.
- Glucose.
- The sugar built during photosynthesis, with the formula ; it stores chemical energy and supplies the carbon for plant structures.
- Stomata.
- Tiny pores, mostly on the underside of leaves, that let carbon dioxide in and oxygen and water vapor out.
- Input (reactant).
- A substance that goes into a process and gets rearranged; for photosynthesis, carbon dioxide and water.
- Output (product).
- A substance produced by a process; for photosynthesis, sugar and oxygen.
- Conservation of matter.
- The principle that atoms are not created or destroyed in a chemical change, only rearranged, so the atoms in a plant must have come from somewhere.
Worked example
A student plants a young tomato seedling with a mass of 20 grams in a pot holding 4,000 grams of dry soil. She waters it and sets it in a sunny window. Ten weeks later the plant has a mass of 900 grams. She carefully dries and weighs the soil again and finds 3,988 grams. Explain where the plant's added mass came from, and use the word equation for photosynthesis in your answer.
Step 1: Find how much mass the plant gained. grams gained.
Step 2: Find how much mass the soil lost. grams lost.
Step 3: Compare the two numbers. The soil lost only 12 grams, but the plant gained 880 grams. The soil cannot possibly be the source of most of the new plant material — it is short by 868 grams.
Step 4: Identify the real sources using the word equation. Photosynthesis is carbon dioxide + water + light energy sugar + oxygen. The only matter entering the plant in large amounts is carbon dioxide gas from the air, which enters through the stomata in the leaves, and water absorbed by the roots.
Step 5: Track the atoms. Carbon dioxide supplies carbon and oxygen atoms; water supplies hydrogen atoms. Powered by light energy, the plant rearranges these atoms into glucose, . Glucose molecules are then linked into cellulose and other materials that become stem, leaf, and root tissue.
Step 6: Account for the missing 12 grams of soil. Those are mineral elements such as nitrogen and phosphorus that the roots absorbed. They matter for building proteins and chlorophyll, but they are a tiny fraction of the 880 grams gained.
Complete answer: Almost all of the plant's added mass came from carbon dioxide in the air and from water, rearranged into sugar during photosynthesis. Soil contributed water and a small amount of minerals, not the bulk of the plant's matter.
Step 2: Find how much mass the soil lost. grams lost.
Step 3: Compare the two numbers. The soil lost only 12 grams, but the plant gained 880 grams. The soil cannot possibly be the source of most of the new plant material — it is short by 868 grams.
Step 4: Identify the real sources using the word equation. Photosynthesis is carbon dioxide + water + light energy sugar + oxygen. The only matter entering the plant in large amounts is carbon dioxide gas from the air, which enters through the stomata in the leaves, and water absorbed by the roots.
Step 5: Track the atoms. Carbon dioxide supplies carbon and oxygen atoms; water supplies hydrogen atoms. Powered by light energy, the plant rearranges these atoms into glucose, . Glucose molecules are then linked into cellulose and other materials that become stem, leaf, and root tissue.
Step 6: Account for the missing 12 grams of soil. Those are mineral elements such as nitrogen and phosphorus that the roots absorbed. They matter for building proteins and chlorophyll, but they are a tiny fraction of the 880 grams gained.
Complete answer: Almost all of the plant's added mass came from carbon dioxide in the air and from water, rearranged into sugar during photosynthesis. Soil contributed water and a small amount of minerals, not the bulk of the plant's matter.
Practice questions
Which list correctly gives the inputs of photosynthesis?
- Sugar and oxygen
- Carbon dioxide, water, and light energy
- Soil, water, and oxygen
- Carbon dioxide, sugar, and minerals from soil
Answer: Carbon dioxide, water, and light energy
Inputs are what goes into the process. Carbon dioxide and water supply the atoms, and light supplies the energy that powers the rearranging. Sugar and oxygen are outputs, so the first choice reverses the equation. Soil is not an input to photosynthesis at all — it holds the water the roots absorb and supplies small amounts of minerals, but the carbon in a plant comes from air, not dirt.
A greenhouse grower notices her lettuce grows faster when she adds extra carbon dioxide to the air inside the greenhouse. Using what you know about photosynthesis, explain why extra carbon dioxide would increase growth.
Answer: Carbon dioxide is a raw material for photosynthesis, so more of it lets plants build sugar faster, and that extra sugar becomes new plant tissue.
A complete answer connects three ideas. First, carbon dioxide is an input: its carbon and oxygen atoms end up in glucose. Second, if more raw material is available and light is plentiful, the rate of photosynthesis increases, so more sugar is produced per day. Third, sugar is not just fuel — it is the building material the plant links into cellulose and other structures, so more sugar means more mass and faster visible growth. Students who only write 'carbon dioxide helps plants' have not shown the mechanism.
A classmate says, 'Plants eat soil to grow. That's why we put them in pots of dirt.' Write a short response that corrects the misconception and gives evidence.
Answer: Plants do not eat soil; their mass comes mostly from carbon dioxide in the air and from water. Evidence: when a plant gains hundreds of grams, the soil in its pot loses only a few grams.
The correction should name the true sources of matter and give a reason. Van Helmont's willow experiment, or any potted-plant mass comparison, shows the gap directly: the plant gains far more mass than the soil loses, so the soil cannot be the source. Add the role soil does play — supplying water, mineral elements, and physical support — so the answer is accurate rather than dismissive.
FAQ
- Do plants do photosynthesis at night?
- No. Photosynthesis requires light energy, so it stops in the dark. But the plant is still alive and still needs usable energy, so it keeps breaking down stored sugar through cellular respiration all night — the same process that happens in your cells.
- Why are most leaves green?
- Chlorophyll, the pigment that absorbs light for photosynthesis, absorbs red and blue light well but reflects green light. The green light bouncing off the leaf is what reaches your eyes.
- If plants get their matter from air, why do we water and fertilize them?
- Water is one of the two inputs that supply atoms for sugar, and it also keeps cells firm and carries minerals through the plant. Fertilizer supplies elements such as nitrogen, phosphorus, and magnesium that plants need to build proteins, DNA, and chlorophyll. Those are essential, but they make up only a small fraction of a plant's total mass.
- Is the oxygen we breathe really from plants?
- Yes — oxygen is an output of photosynthesis. Land plants and, even more so, algae and other photosynthetic organisms in the ocean release oxygen gas as they build sugar from carbon dioxide and water.
Learn this with a teacher, not a page
The Crimsora tutor teaches Photosynthesis live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.