From Air and Water to Plant: Tracing Matter - 5th Grade Science

SECTION 1

The Phenomenon: The Mystery of the Growing Tree

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Anchoring Phenomenon

Think about that: the tree gained over 164 pounds of material, but the soil barely changed. Where did all that new matter come from? The tree grew thick bark, strong branches, and thousands of leaves — but if it didn't come from the soil, what was the source?

Van Helmont's willow tree experiment — the tree gained 164 lbs but the soil barely changed.

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Thinking Questions

If the soil barely lost any weight, what materials might the tree have taken in to grow?
What enters a plant from the air and water? Could those substances actually become part of the tree?
How could you design a model to trace where the matter in a plant actually comes from?

SECTION 2

What Scientists Know: Where Plant Matter Comes From

Van Helmont's experiment puzzled scientists for centuries. Today, we know the answer — and it's pretty amazing. Most of the matter (the actual "stuff") that makes up a plant does not come from the soil. Instead, it comes from two sources that might surprise you: air and water.

Plants take in carbon dioxide (CO₂) from the air and water (H₂O) from the soil. Using energy from sunlight, plants rearrange the atoms in these molecules to build a sugar called glucose (C₆H₁₂O₆). This process is called photosynthesis. The glucose is then used to build every part of the plant — the cellulose in cell walls, the starch stored in roots, the proteins in leaves, and more.

Carbon Dioxide from the Air

Plants absorb carbon dioxide (CO₂) through tiny pores on their leaves called stomata. Carbon dioxide is a gas made of one carbon atom and two oxygen atoms. The carbon atoms from CO₂ become the backbone of the sugars, starches, and wood that make up the plant's body.

Water from the Soil

Roots absorb water (H₂O) from the soil and transport it up through the plant. Water provides hydrogen atoms that are also built into glucose. The oxygen atoms from water are mostly released back into the air as oxygen gas (O₂) — the very gas we breathe!

Sunlight Provides Energy, Not Matter

Here's a key distinction: sunlight provides the energy to power the reaction, but sunlight is not matter. It doesn't have mass. The mass of the plant comes from the atoms in air and water that are rearranged during photosynthesis, not from the light itself.

Minerals from the Soil Are Only a Tiny Part

Plants do take in minerals (like nitrogen and phosphorus) from the soil, but these make up only a small fraction of a plant's weight — often less than 5%. That's why van Helmont's soil barely lost any weight. The overwhelming majority of a plant's mass comes from air and water.

SECTION 3

Let's Investigate: Modeling How Matter Moves into Plants

Scientists use models to trace the path of matter through processes that are too small or too slow to observe directly. We can't see individual CO₂ molecules entering a leaf, but we can build a model that tracks where the atoms go. The Science and Engineering Practice we're using here is Developing and Using Models — a core skill that scientists use every day.

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Investigation Spotlight: Model It — Tracing Atoms Through Photosynthesis

What scientists do: Scientists create diagrams and equations that track individual atoms from their starting molecules (reactants) to their ending molecules (products). In photosynthesis, the simplified equation is:

Photosynthesis Equation

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

What to notice: Count the atoms on each side. On the left: 6 carbon, 18 oxygen, and 12 hydrogen atoms. On the right: the same totals — 6 carbon, 18 oxygen, and 12 hydrogen. Matter is conserved. No atoms appear from nowhere or vanish. They simply rearrange.

Materials for a classroom model: Colored beads or blocks (red = oxygen, black = carbon, white = hydrogen), pipe cleaners to link them, labels for "air," "water," "glucose," and "oxygen gas."

A model showing how matter flows from the environment into and out of a plant during photosynthesis.

Look at the diagram above. Notice how carbon dioxide enters through the leaves, and water enters through the roots. Inside the leaf, these molecules are rearranged with the help of sunlight energy. The result? Glucose — a sugar whose atoms came directly from the air and water — and oxygen gas, which is released back into the environment. Every atom is accounted for.

SECTION 4

What We Discovered: Tracing Each Atom

Let's trace the journey of matter step by step. When we use a model to follow the atoms, something remarkable becomes clear: no matter is created or destroyed during photosynthesis. Every atom that enters the plant as CO₂ or H₂O can be found in the products — glucose and oxygen. This is the principle of the conservation of matter.

Step-by-Step: The Journey of Carbon

A molecule of carbon dioxide floats through the air. It enters a leaf through a tiny opening called a stoma (plural: stomata). Inside the leaf's cells, the carbon atom from that CO₂ is pulled away from its two oxygen atoms. That carbon atom is then bonded to hydrogen and oxygen atoms (from water) to build part of a glucose molecule. Over time, the plant uses that glucose to make cellulose (for cell walls), starch (for energy storage), and other organic molecules. That single carbon atom — once floating invisibly in the air — is now part of a solid leaf, stem, or root.

Step-by-Step: The Journey of Water

A water molecule is absorbed by a root hair deep in the soil. It travels up through tiny tubes in the stem called xylem until it reaches the leaf. Inside the leaf cell, the water molecule is split apart. Its hydrogen atoms are attached to carbon atoms to help form glucose. Most of its oxygen atoms are released into the air as O₂ gas. So the hydrogen from water becomes part of the plant, while the oxygen goes back into the atmosphere for animals (and us!) to breathe.

Substance	Where It Starts	How It Enters the Plant	Where Its Atoms End Up
Carbon dioxide (CO₂)	Air (atmosphere)	Through stomata in leaves	Carbon → glucose → plant structures; Oxygen → released as O₂
Water (H₂O)	Soil (ground water)	Through roots → up xylem	Hydrogen → glucose → plant structures; Oxygen → released as O₂
Minerals (N, P, K, etc.)	Soil	Dissolved in water through roots	Small amounts used for proteins, DNA, etc. (< 5% of plant mass)
Sunlight	Sun	Absorbed by chlorophyll in leaves	Provides energy — NOT matter (has no mass)

Tracing atoms through photosynthesis — inputs and outputs match, confirming conservation of matter.

The atom-tracking diagram above is a powerful model. Count the atoms going in to photosynthesis and the atoms coming out. They match exactly. This is evidence that matter is conserved. The atoms aren't created or destroyed — they are simply rearranged into new molecules. The carbon and hydrogen atoms that were once part of air and water molecules are now part of the plant's solid body.

SECTION 5

Patterns and Connections: Energy and Matter

The crosscutting concept at the heart of this lesson is Energy and Matter: Flows, Cycles, and Conservation. This is a big idea that appears across all areas of science, not just biology. The core principle is that matter is conserved — it is neither created nor destroyed, only rearranged — and that energy drives these rearrangements.

Let's see how this same pattern shows up in different areas of science:

Science Area	Example	Matter In	Matter Out	Energy Source
Life Science (Plants)	A tree growing	CO₂ from air, H₂O from soil	Glucose → plant structures, O₂ released	Sunlight
Life Science (Animals)	A puppy growing	Food (originally from plants), O₂ from air	Body growth, CO₂ exhaled, waste	Chemical energy in food
Earth Science	Water cycle	Water evaporates from oceans	Water falls as rain, flows to rivers → back to ocean	Sun's heat
Physical Science	Burning a candle	Wax (C, H) + O₂ from air	CO₂ + H₂O released as gas	Chemical energy in wax

Do you see the pattern? In every example, matter enters a system, gets rearranged, and exits in a new form — but the total amount of matter stays the same. Energy is what drives the rearrangement. In photosynthesis, light energy rearranges CO₂ and H₂O into glucose. In a burning candle, chemical energy rearranges wax and oxygen into CO₂ and water vapor. The pattern is the same.

SECTION 6

Real-World Connections: Why This Matters

Understanding where plant matter comes from isn't just an interesting science fact — it connects to some of the most important environmental issues facing our planet.

🌲 Forests as Carbon Storage

Because trees build their trunks, branches, and roots from carbon atoms pulled out of CO₂ in the air, forests act as enormous carbon sinks. A mature tree can store hundreds of pounds of carbon in its wood. When large areas of forest are cut down and burned, all that stored carbon is released back into the atmosphere as CO₂, contributing to climate change. Understanding photosynthesis helps us understand why protecting forests is so important.

🌾 Farming and Food Production

Every bite of food you eat was originally built from air and water through photosynthesis — either directly (when you eat plants) or indirectly (when you eat animals that ate plants). Farmers work to give plants the best conditions — enough water, sunlight, and soil nutrients — so that photosynthesis can produce as much food as possible. Agricultural engineers design greenhouses that can control CO₂ levels and light to maximize plant growth.

🔧 Engineering Connection: Carbon Capture

Engineers today are trying to build machines that do what plants do naturally: pull CO₂ out of the air. These "carbon capture" systems are inspired by photosynthesis. Some engineers are even designing artificial leaves — devices that use sunlight to split CO₂ and water, just like a real leaf does. Understanding the science of how matter moves from the environment into plants is the foundation for these engineering solutions.

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🛠 Design Challenge: Think Like an Engineer

A source of CO₂ (where would it come from on a space station?)
A source of water
A light source to replace sunlight
Any nutrients or minerals the plant might need

NASA engineers have actually solved this problem! They grow plants on the International Space Station using LED lights, recycled water, and the CO₂ that astronauts exhale. The plants, in turn, produce oxygen for the astronauts to breathe. It's a cycle of matter!

SECTION 7

Key Vocabulary Review

Photosynthesis — The process by which plants use sunlight energy to convert carbon dioxide and water into glucose (sugar) and oxygen. This is how plants make their own food and build their body.

Carbon dioxide (CO₂) — A gas found in the air made of one carbon atom and two oxygen atoms. Plants absorb it through their leaves and use its carbon atoms to build glucose.

Glucose (C₆H₁₂O₆) — A simple sugar molecule that plants produce during photosynthesis. It is made of 6 carbon, 12 hydrogen, and 6 oxygen atoms. Plants use glucose as food and as a building block for their structures.

Stomata — Tiny pores (openings) on the surface of leaves that allow carbon dioxide to enter the plant and oxygen to exit.

Conservation of matter — The scientific principle that matter cannot be created or destroyed. In any process, the total number of atoms stays the same — they are simply rearranged into new molecules.

Model — A representation (such as a diagram, equation, or physical replica) used to describe, explain, or predict a natural process. Scientists use models to trace how matter moves through systems.

Matter — Anything that has mass and takes up space. Atoms and molecules are matter. Light and heat are not matter — they are forms of energy.

Xylem — Tubes inside a plant's stem and roots that transport water from the roots up to the leaves.

SECTION 8

Practice: Test Your Understanding

Question 1: Foundational

A student plants a tiny sunflower seed in a pot. After several months, the sunflower grows to be over five feet tall and weighs much more than the original seed. Where does most of the new material that makes up the sunflower come from?

A. Most of the new material comes from the soil the plant is growing in.
B. Most of the new material comes from air and water that the plant takes in.
C. Most of the new material comes from sunlight energy absorbed by the leaves.
D. Most of the new material comes from the nutrients in the fertilizer added to the pot.

Show answer

The correct answer is B. Plants use carbon dioxide from the air and water from the soil as the raw materials to build new plant matter through photosynthesis. Most of the mass of a plant comes from these two sources. Choice A is incorrect because although plants absorb some minerals from soil, the soil itself does not account for most of the plant's mass. Choice C is incorrect because sunlight provides energy to drive photosynthesis, but light is not matter and does not become part of the plant's physical material. Choice D is incorrect because fertilizer provides small amounts of nutrients, but these make up only a tiny fraction of the plant's total mass.

Question 2: Foundational

A class is making a model to show how a tomato plant gets the matter it needs to grow fruit. Which model best traces matter from the environment into the plant?

A. Arrows showing sunlight going into the leaves and turning into tomatoes.
B. Arrows showing carbon dioxide entering the leaves and water entering the roots, then combining inside the plant to form new plant material.
C. Arrows showing soil moving up through the roots and into the stems, leaves, and fruit.
D. Arrows showing heat from the sun entering the plant and being stored inside the tomatoes.

Show answer

The correct answer is B. A good model of matter moving into a plant shows carbon dioxide (a gas from the air) entering through the leaves and water entering through the roots. These are the substances that combine during photosynthesis to form new plant material. Choice A is incorrect because sunlight is energy, not matter, so it cannot turn into tomatoes. Choice C is incorrect because soil does not travel up through the plant to become part of it; only water and dissolved minerals are absorbed by roots. Choice D is incorrect because heat is a form of energy, not matter, and cannot be stored as physical material inside fruit.

Question 3: Intermediate

A scientist measures the mass of a sealed container of soil before and after growing a bean plant in it for two months. She finds that the soil lost only a few grams, but the bean plant gained much more mass than the soil lost. What does this evidence tell us about where the plant got most of its matter?

A. The plant created new matter on its own using energy from the sun.
B. The plant got most of its matter from water and a gas in the air, not from the soil.
C. The plant's roots must have reached outside the container to find more soil to absorb.
D. The scale must have been broken because plants can only grow from materials in the soil.

Show answer

The correct answer is B. Since the soil lost only a small amount of mass while the plant gained a lot of mass, most of the plant's new matter must have come from sources other than soil — specifically water and carbon dioxide from the air. Choice A is incorrect because matter cannot be created from nothing; it must come from existing matter in the environment. Choice C is incorrect because the question states the container was sealed, and regardless, plants do not get most of their mass from soil. Choice D is incorrect because it is well established that plants gain much more mass than soil loses, and this observation is consistent across many experiments.

Question 4: Intermediate

Students are building a model to show how matter moves from the environment into a maple tree. They want to show all the inputs of matter the tree needs. Which pair of inputs should they include in their model?

A. Sunlight and oxygen
B. Water and carbon dioxide
C. Oxygen and sugar
D. Sunlight and water

Show answer

The correct answer is B. The two main inputs of matter for a plant are water (absorbed through the roots) and carbon dioxide (absorbed through the leaves). These are the substances the plant uses to build new material. Choice A is incorrect because sunlight is energy, not matter, and oxygen is actually released by plants during photosynthesis rather than being an input. Choice C is incorrect because oxygen and sugar are both outputs of photosynthesis, not inputs. Choice D is incorrect because while water is a correct input of matter, sunlight is energy, not matter, so it should not be listed as a matter input in the model.

Question 5: Advanced

A student sets up two identical pea plants. Plant A is placed in a closed glass box filled with normal air. Plant B is placed in a closed glass box where all the carbon dioxide has been removed from the air. Both plants receive the same amount of water and sunlight. After three weeks, Plant A has grown new leaves and stems, but Plant B has barely grown at all. What does this experiment help demonstrate about how plants get matter?

A. It shows that plants need oxygen from the air to grow new leaves and stems.
B. It shows that carbon dioxide from the air is a necessary source of matter for plant growth.
C. It shows that plants cannot survive without sunlight providing matter for growth.
D. It shows that glass boxes prevent plants from absorbing water through their roots.

Show answer

The correct answer is B. The only difference between the two setups was the presence or absence of carbon dioxide. Since Plant B (without carbon dioxide) barely grew while Plant A (with carbon dioxide) grew normally, this demonstrates that carbon dioxide from the air is a critical source of matter that plants need to build new structures. Choice A is incorrect because the experiment removed carbon dioxide, not oxygen, so it does not test the role of oxygen. Choice C is incorrect because both plants received the same amount of sunlight, so sunlight is not the variable being tested. Additionally, sunlight is energy, not matter. Choice D is incorrect because both plants were in glass boxes and received the same water, so the glass box did not interfere with water absorption.

SECTION 9

What's Next?

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What's Next?