Life Science · Unit 3: Energy for life

Photosynthesis Feeds the World

A giant tree starts as a tiny seed, so where does all that wood come from? You change the light, carbon dioxide and temperature around a plant in a virtual chamber, then read a 1600s willow experiment and real data from blueberry leaves. Afterwards you can explain how producers use photosynthesis to make sugar (glucose), and what speeds it up or holds it back.

Where this lesson sits

  • Virginia SOL LS.4 a
  • Virginia SOL LS.4 b
  • Practices LS.1 b, LS.1 c, LS.1 d, LS.1 e

Essential question: A giant tree starts as a tiny seed, so where does all that wood come from?

  1. Life Science (grade 7) LS.4 a, LS.4 b You are here

    This lesson

  2. Biology

    Preview it at high school level

    In Biology you will write photosynthesis with chemical formulas and balance it, so the same atoms appear on both sides. You will track where each kind of atom goes. One surprise: the oxygen a plant gives off comes from the water it took in, not from the carbon dioxide. You will follow the plant’s sugar as it is linked into long chains called starch and cellulose. You will also study light, carbon dioxide and temperature as limiting factors, using real survey data on underwater grasses in the Chesapeake Bay and how much light reaches them.

  3. Biology II Biology II: Advanced Survey of Biology Topics, BIOII.3 b; Ecology, EC.3 a

    Preview where it goes next

    In Biology II: Advanced Survey of Biology Topics (BIOII.3 b), the single equation splits into two linked stages. In the first, light is captured, water is split and energy is held in carrier molecules. In the second, a cycle of reactions uses that energy to build sugar from carbon dioxide. The Ecology course (EC.3 a) scales up to whole ecosystems: how much new plant material the producers in a forest or a bay make each year, and how much of that energy reaches consumers.

  4. College

Watch first Wood Is Built Mostly From Air and Water · 2:35

Supports

Supports change how much help you get, not what you learn.

On average, a tulip poplar seed weighs less than a tenth of a gram. In a Virginia forest, one tulip poplar grew so big that scientists estimated it held about 10,000 kilograms of dry wood. Where did all that wood come from? Did the soil have to lose as much mass as the tree gained? Decide what you think now.

What do you think? Decide before you read.

  1. Plants get their mass from the soil. That’s what the roots are for.

    Scientists disagree: When scientists dry and weigh soil before and after a plant grows, the soil hardly changes while the plant gains a lot. Roots take in water and small amounts of minerals. Most of the solid material in a plant is built from carbon dioxide taken from the air.

    What scientists think

    Scientists disagree: When scientists dry and weigh soil before and after a plant grows, the soil hardly changes while the plant gains a lot. Roots take in water and small amounts of minerals. Most of the solid material in a plant is built from carbon dioxide taken from the air.

  2. Plants get their food from the soil, so fertilizer is plant food.

    Scientists disagree: Plants make their own food, a sugar called glucose, in their leaves. Fertilizer adds minerals such as nitrogen and potassium. Plants need these minerals to stay healthy, but minerals do not give the plant energy.

    What scientists think

    Scientists disagree: Plants make their own food, a sugar called glucose, in their leaves. Fertilizer adds minerals such as nitrogen and potassium. Plants need these minerals to stay healthy, but minerals do not give the plant energy.

  3. Sunlight is the plant’s food. The light turns into the plant.

    Scientists disagree: Sunlight is energy, not matter, so it cannot become leaves or wood. The plant uses the energy of light to join carbon dioxide and water into sugar. The light supplies the energy and the air and water supply the matter.

    What scientists think

    Scientists disagree: Sunlight is energy, not matter, so it cannot become leaves or wood. The plant uses the energy of light to join carbon dioxide and water into sugar. The light supplies the energy and the air and water supply the matter.

Explore: grow a plant in a test chamber

In this virtual lab, a plant sits in a clear growth chamber. You can turn three dials: how bright the light is, how much carbon dioxide is in the air, and the temperature. A counter shows oxygen bubbles, and a bar shows how much sugar (glucose) the plant makes each hour. A small graph traces the rate. The simulation uses smooth made-up curves, so look at the shape of each graph, not the exact numbers.

Grow a plant in a test chamber⚙ original · interactive
temperature: warm clear growth chamber oxygen bubbles light carbon dioxide pot stem leaf
Oxygen bubbles counted 0 out of 10 (relative units)
Sugar (glucose) made per hour
0 out of 100 (relative units)
Graph the rate against:

The line shows the rate as that one dial moves and the other two stay where you set them. The orange dot is your setting.

Move a dial and watch the bubbles, the sugar bar and the graph.

In this sketch the curves are smooth and made up, water never runs short, the plant is one block with a leaf, and every number is in relative units. The ends of the temperature graph are made up. Look at the shape of each graph, not the exact numbers.

Rule to keep: more light or more carbon dioxide speeds up photosynthesis only up to a point, then another factor holds it back, and too much heat slows it down.

Try this.

  1. Predict what happens to the bubble count as you turn the light from dim to very bright. Then test it. Does the rate keep rising?
  2. Set the light very bright. Now raise the carbon dioxide. Predict first: will the rate go up again?
  3. Hold light and carbon dioxide steady and move the temperature from cold to warm to very hot. Write your prediction, then run it.

Explain: how a plant makes its own food

  • producer
  • photosynthesis
  • radiant energy
  • chemical energy
  • chloroplast
  • chlorophyll
  • sugar (glucose)
  • rate

A deer cannot make its own food. It has to eat. A plant does not eat, yet it grows. A producer is a living thing that makes its own food. Green plants, algae and phytoplankton are producers. Phytoplankton are tiny producers that drift in water, such as algae and some bacteria.

Plants, algae and phytoplankton make food by photosynthesis, the process that uses the energy of sunlight to turn carbon dioxide and water into sugar and oxygen. The energy in sunlight is radiant energy, energy that travels as light. In words:

carbon dioxide + water + radiant energy → sugar (glucose) + oxygen

The sugar it makes is called glucose. Sugar (glucose) is a small sugar molecule that stores energy. In chemistry this is written 6CO2 + 6H2O → C6H12O6 + 6O2, but you only need the words.

The energy does not disappear and is not made. It changes form. Radiant energy becomes chemical energy, energy stored in the way atoms are joined in a molecule. Sugar holds more chemical energy than the carbon dioxide and water it came from.

AI-generated illustration: a young leafy plant in sunlight with roots showing in the soil. Light rays from the sun reach a large leaf, one arrow carries air into the leaf and another carries air out, a gold arrow runs down the stem from the leaf, and a blue arrow runs up from the roots. Five numbered circles mark the light and the four arrows.
  1. sunlight
  2. carbon dioxide
  3. oxygen
  4. sugar (glucose)
  5. water
  1. sunlight
  5. water

Word bank: carbon dioxide, oxygen, sugar (glucose)

Check your labels
  1. sunlight
  2. carbon dioxide
  3. oxygen
  4. sugar (glucose)
  5. water
Photosynthesis has inputs that enter the plant and outputs that leave the leaf or move through the plant. Name the five numbered things.
Key idea: Producers use radiant energy to turn carbon dioxide and water into sugar (glucose) and oxygen, storing the energy as chemical energy.

Explain: where it happens and where the sugar goes

In plants and algae, photosynthesis happens inside a chloroplast, a green organelle found in some plant cells, mostly in leaves. (Bacteria that photosynthesize have no chloroplasts.) Root cells, which sit in the dark, usually have none. Each chloroplast holds chlorophyll, a green pigment that captures radiant energy. Plant cells and animal cells shows where chloroplasts sit in a cell.

The chloroplasts work as part of a larger system:

  • Roots take in water from the soil.
  • Leaves take in carbon dioxide from the air through tiny openings.
  • Leaves catch the light, and chloroplasts make the sugar.

Carbon dioxide is only a tiny part of air, about 0.04 percent. Even so, the carbon dioxide from the air becomes most of a plant’s solid material. Soil gives water and small amounts of minerals.

Once a plant has sugar (glucose), it can do three things with it:

  • Use it now. The plant’s cells break sugar down to release energy for living. Releasing energy from food shows how.
  • Store it. The plant links sugars into starch and keeps it in roots, seeds and leaves for later.
  • Rearrange it. The plant rebuilds sugar into other compounds, such as the wood of its trunk and the parts of new leaves.
AI-generated illustration: a young tree with gold arrows carrying sugar from one large leaf to a new leaf at a branch tip, into the wood of the trunk, down to the roots where starch is stored, and to a cluster of seeds. An inset zooms into a leaf cell full of green chloroplasts.
Chloroplasts in the leaves make sugar, and the plant stores it as starch or rebuilds it into wood and new leaves. Some sugar is also used right away for energy.

Photosynthesis feeds almost every food web. A rabbit eats clover. A fox eats the rabbit. Both live on energy the clover captured from sunlight. A few deep-sea communities run on chemicals instead of light, so “almost every” is the honest phrase. You will build full food webs in Food webs and energy.

Key idea: Chloroplasts make sugar (glucose), and the plant uses it now, stores it, or rebuilds it into wood and leaves, which feeds almost all other life.

Explain: what changes the rate

The rate of a process is how fast it happens, such as the amount made per hour. Three conditions change the rate of photosynthesis:

  • Light intensity. More light usually speeds it up, up to a point.
  • Carbon dioxide. More carbon dioxide can speed it up, up to a point.
  • Temperature. Warmth can speed it up, but too much heat slows it down.

Think of a sandwich shop with one worker making bread and one adding fillings. Speeding up the bread maker helps only until the fillings worker falls behind. In a plant, the factors also affect each other, so the comparison is simpler than real life.

Photosynthesis is not the end of the story. Cells release the stored energy in respiration, and the two processes work as partners. Partners in energy puts them together.

Key idea: Light, carbon dioxide and temperature change the rate of photosynthesis, and the rate stops rising when a different factor holds it back.

Elaborate: the willow and the blueberry leaf

First, van Helmont’s willow from the 1600s:

AI-generated illustration: two views of the same pot. At the start, a short bare willow stem is planted in dried soil under a metal plate with small holes, and a watering can adds water. After five years, a tall leafy willow grows from the same pot, with a few fallen leaves on the floor beside it.
Van Helmont’s willow: he weighed the dried soil and the stem, gave only water for five years, then weighed both again. The fallen leaves were never weighed.
MeasurementReported (pounds and ounces)About (metric)
Willow stem at the start5 lb2.3 kg
Willow tree after 5 years169 lb and about 3 oz76.7 kg
Dried soil at the start200 lb90.7 kg
Soil lost after 5 years (dried again)about 2 ozabout 57 g

Source: van Helmont, Oriatrike (English edition, 1662), p. 109; one willow in one pot. The fallen leaves were not weighed. Metric values are modern conversions.

Next, a modern test of light. Scientists clamped blueberry leaves in a gas analyzer at 20 °C and changed the light.

Light (µmol of photons per m² per second)Carbon dioxide taken in (µmol per m² of leaf per second)
0−1.09
250.18
511.21
3016.96
7019.11
1,0009.73
1,5009.95

Source: Jorquera-Fontena, Alberdi and Franck (2014), leaves of four-year-old blueberry plants in an orchard in Chile (normal air, in a study that compared three pruning levels), Figure 4; values read from that figure and posted in a table by Chen and colleagues (2016). The negative value at 0 means the leaf gave off carbon dioxide in the dark, because its cells still release energy from sugar.

  1. How many kilograms did the willow gain? Compare that with the soil lost.
  2. How much did the carbon dioxide taken in go up from 51 to 301 light units? From 1,000 to 1,500?
  3. Where does the rate start to level off? What might be holding it back there?
  • The willow gained about while the soil lost only .
  • When the light went from to , the carbon dioxide taken in .
  • This shows that because .

Claim, evidence, reasoning

Make a claim that answers the essential question: where does a tree’s wood come from, and what limits how fast it is made? Use the willow and blueberry data.

Start with: Most of a tree’s solid mass comes from , and the rate of photosynthesis is limited by .

Start with: The willow gained about kg while the soil lost only g. In the blueberry leaves, uptake rose from to at low light but only from to at high light.

Start with: Photosynthesis builds sugar (glucose) from and using radiant energy, so .

Compare with a model answer

Most of a tree’s solid mass comes from carbon dioxide in the air, and light can limit how fast it is built. Van Helmont’s willow gained about 74 kg while the soil lost only about 57 g, so soil cannot explain the gain. Blueberry leaves took in much more carbon dioxide as light rose from 51 to 301, but almost no more from 1,000 to 1,500. So light limits the rate at first, then something else does. Limits: van Helmont had one tree, never dried it and never measured the air, and the blueberry data come from one study.

Design problem

Plan an experiment to test how light affects the rate of photosynthesis in a water plant such as Elodea from a store. Name your variables, your constants, a control and how many trials you would run.

One way to do it
  • Question: Does the plant give off oxygen bubbles faster when a lamp is closer?
  • Independent variable: distance from the lamp: 10, 20, 30 and 40 cm.
  • Dependent variable: oxygen bubbles counted per minute.
  • Constants: same length of plant, same amount of water and baking soda, same water temperature, same lamp. An LED lamp gives off little heat, so the water temperature should change very little.
  • Control: one sprig in a tube wrapped in foil, so no light reaches it.
  • Trials: three sprigs at each distance; find the mean bubble count.
  • Safety: keep cords and lamps away from water. Wash your hands. Never pour the plant into a pond or stream, because it can spread.

If bubbles come faster as the lamp gets closer, then level off, light was the limit at first.

Evaluate: check your thinking

The tulip poplar’s wood came mostly from carbon dioxide in the air. Its leaves used radiant energy to join that carbon dioxide with water into sugar, and the tree rebuilt the sugar into wood. The soil gave water and a few minerals, so it did not need to shrink by thousands of kilograms.

Science practices you used: making predictions and testing them, planning a fair test with a control and repeated trials, reading data tables and doing simple math with them, and making a claim backed by evidence and reasoning.

How we measure it

Weighing before and after (with drying)

A balance measures mass at the start and the end of an experiment. Drying a sample first removes water, so the change in mass shows how much solid material was gained or lost.

Gas analyzer on a leaf

A leaf is clamped inside a small clear chamber, and an instrument measures how much carbon dioxide goes in and comes out. The difference shows how fast the leaf takes in carbon dioxide at each light level.

Laser scanning of trees

A scanner sends out laser light and records the shape of a whole tree in three dimensions. Scientists use that shape to estimate how much wood the tree holds without cutting it down.

Finish line

Test yourself

8 questions

Practice and MCAT-style, with explained answers.

Next · Lesson 10

Releasing Energy from Food

Why do sprouting peas in a thermos get warmer than the room, and why does yeast in warm sugar water blow up a balloon?

Continue →

← Lesson 8 · Domains, Kingdoms and DNA

Carbon dioxide, the gas from the air that supplies the carbon in a plant’s sugar
Carbon dioxide, the gas from the air that supplies the carbon in a plant’s sugar ● LIVE · PubChem CID 280
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Jan Baptist van Helmont

Jan Baptist van Helmont

Weighed a willow stem and a pot of dried soil, then weighed both again five years later, and reported that the tree gained far more mass than the soil lost.

Jan Baptist van Helmont was a doctor who lived in the late 1500s and early 1600s. His account of a willow tree was published in 1648 and in English in 1662. He dried 200 pounds of soil in a furnace and put it in a pot. He planted a willow stem that weighed 5 pounds and gave it only water. He covered the pot with a metal plate full of small holes to keep out dust. Five years later the tree weighed about 169 pounds. He dried the soil again, and it had lost only about 2 ounces. He did not weigh the leaves that fell each autumn. He decided the wood came from water alone. That part was wrong, but his careful weighing turned "where does a tree come from?" into a question you can test.

✓ verbatim van Helmont, Ortus medicinae (Amsterdam, 1648), Wellcome Collection record; English edition Oriatrike (London, 1662), p. 109, Internet Archive scan b30327696.
Test yourself
Question 1 of 8 Score 0 / 0

Which list names the three things a plant uses to carry out photosynthesis?
Sources & Integrity

Every structure, slide, simulation, and clip on this page is listed below with the source it came from. None of the 3D structures is drawn by hand or by AI: each one loads by accession ID from RCSB PDB or PubChem. This panel is generated from the lesson's own data file, so what it lists is what the page loads.

Molecular structures live data

  • Carbon dioxide, the gas from the air that supplies the carbon in a plant’s sugar — PubChem CID 280 · PubChem compound record for carbon dioxide, CO2.

Simulations

  • A plant in a growth chamber (simulation) · Original interactive — Biochemistrypedia (Deppmann Lab).

Scientist profiles & quotes

  • Jan Baptist van Helmont — van Helmont, Ortus medicinae (Amsterdam, 1648), Wellcome Collection record; English edition Oriatrike (London, 1662), p. 109, Internet Archive scan b30327696.

Quotes are verbatim from the cited source. Profile text is original to this site and was checked against the sources cited.

Animated explainer

Wood Is Built Mostly From Air and Water · narration: an AI narrator voice (Gemini, Charon) reading narration written from this lesson's text · animation: AI-generated in code (Manim) by Claude, a schematic with no drawn molecular structures, plots computed from their equations · captions from the narration script.

Images & figures

3 illustrations in this lesson are AI-generated: drawn by an image model from a written spec, then checked by a separate AI judge against the spec and the lesson text. None shows a chemical structure or data.

  • a young leafy plant in sunlight with roots showing in the soil. Light rays from the sun reach a large leaf, one arrow carries air into the leaf and another carries air out, a gold arrow runs down the stem from the leaf, and a blue arrow runs up from the roots. Five numbered circles mark the light and the four arrows.
  • a young tree with gold arrows carrying sugar from one large leaf to a new leaf at a branch tip, into the wood of the trunk, down to the roots where starch is stored, and to a cluster of seeds. An inset zooms into a leaf cell full of green chloroplasts.
  • two views of the same pot. At the start, a short bare willow stem is planted in dried soil under a metal plate with small holes, and a watering can adds water. After five years, a tall leafy willow grows from the same pot, with a few fallen leaves on the floor beside it.

Every 3D structure is rendered live from an accession ID, and every plot is drawn from its equation in the browser.

integrity Machines check the mechanics. The build confirms that every local file exists, that every question's answer key points at a real choice, and that any AI-generated scientific figure names the person who checked it; a weekly job re-resolves every accession ID against its database. Questions are original; MCAT-style items are written for this site and are not real AAMC questions. Banned sources (the Tymoczko textbook, publisher decks, copyrighted figures) are not used.

Life Science (grade 7)

Producers, such as green plants, algae and phytoplankton, make their own food. In photosynthesis, chloroplasts use radiant energy from sunlight to turn carbon dioxide and water into sugar (glucose) and oxygen. The green pigment chlorophyll captures the light. Most of the solid material in a plant is built from carbon dioxide taken from the air, not from soil. A plant uses its sugar right away, stores it, or rearranges it into wood and leaves. Light, carbon dioxide and temperature change how fast photosynthesis goes. Because animals eat plants or eat animals that ate plants, photosynthesis supports almost every food web.
Biology

In Biology you will write photosynthesis with chemical formulas and balance it, so the same atoms appear on both sides. You will track where each kind of atom goes. One surprise: the oxygen a plant gives off comes from the water it took in, not from the carbon dioxide. You will follow the plant’s sugar as it is linked into long chains called starch and cellulose. You will also study light, carbon dioxide and temperature as limiting factors, using real survey data on underwater grasses in the Chesapeake Bay and how much light reaches them.
Biology II

In Biology II: Advanced Survey of Biology Topics (BIOII.3 b), the single equation splits into two linked stages. In the first, light is captured, water is split and energy is held in carrier molecules. In the second, a cycle of reactions uses that energy to build sugar from carbon dioxide. The Ecology course (EC.3 a) scales up to whole ecosystems: how much new plant material the producers in a forest or a bay make each year, and how much of that energy reaches consumers.
College

A college course treats photosynthesis as two coupled problems. The Light Reactions lesson follows how pigment complexes absorb light, how splitting water feeds electrons into a chain of carriers in the chloroplast’s inner membranes, and how that flow is used to make the energy carriers ATP and NADPH. The Calvin Cycle lesson follows how the enzyme Rubisco attaches carbon dioxide to a five-carbon molecule, how the products become a three-carbon sugar, and why the school equation is only a net summary of many reactions.
Weighing a tree without a scale

You cannot put a big tree on a bathroom scale. At a forest research plot near Front Royal, Virginia, scientists scanned trees with lasers and used each tree’s shape to estimate its wood. For one tulip poplar, the estimate was about 10,079 kilograms of dry wood above the ground. That is an estimate from a model, not a weighing. Most of that dry wood was built from carbon dioxide taken out of the air.

— Stovall, Anderson-Teixeira and colleagues, Data in Brief 19:1560-1569 (2018), Smithsonian ForestGEO plot, Front Royal, Virginia.
Too hot for wheat

Scientists exposed four lines (types) of wheat to 42 °C for four hours and studied the top (flag) leaf. Then they measured photosynthesis with the leaf chamber set to 25 °C. In all four lines, the heated leaves took in less carbon dioxide than leaves that had not been heated. Warmth can speed photosynthesis, but only up to a point. Too much heat damaged the leaves’ ability to make sugar.

— Li and colleagues, PLOS ONE 16(12):e0255896 (2021).

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