Trace matter and energy through organisms using evidence - Middle School Life Science

SECTION 1

Historical Context & Motivation

How Did Scientists Learn What Happens to Food?

Have you ever wondered where your lunch actually goes? For centuries, people did not know how food gives the body energy. Early scientists thought food just disappeared inside us. It took hundreds of years of experiments to figure out that matter (the stuff things are made of) and energy (the ability to do work) follow specific paths inside living things.

This is our anchoring phenomenon: A student eats a sandwich at lunch. By gym class, she has enough energy to run a mile. Where did the sandwich go, and how did it become running energy? Let's trace the evidence scientists collected over time.

1648 • Van Helmont's Willow Tree

Jan Baptist van Helmont grew a willow tree in a pot for five years. The tree gained about 75 kilograms, but the soil barely changed. He wondered: where did the new matter come from?

1770s • Lavoisier Measures Respiration

Antoine Lavoisier showed that animals breathe in oxygen and breathe out carbon dioxide. He proved that the body uses chemical reactions, not magic, to release energy from food.

1840s • Julius von Mayer Links Food and Energy

Mayer proposed that the energy animals use comes directly from the chemical energy stored in food. Energy is not created from nothing — it is transformed.

1937 • Hans Krebs Maps Cellular Respiration

Krebs described the step-by-step chemical reactions cells use to break down food molecules. This cycle showed exactly how matter and energy move inside a cell.

1961 • Peter Mitchell Explains ATP Production

Mitchell discovered how cells use a flow of particles to make ATP, the molecule that powers almost everything a cell does. This completed our picture of energy flow in organisms.

Each discovery above added a piece to a big puzzle. The key question scientists kept asking was: What happens to matter and energy when an organism eats, grows, and moves? Today we can trace both using solid evidence.

SECTION 2

Core Principles & Definitions

The Big Ideas About Matter and Energy in Organisms

Before we trace matter and energy, we need to understand a few core ideas. These ideas connect to the crosscutting concept of Energy and Matter: Flows, Cycles, and Conservation. This means matter is never created or destroyed — it just changes form. Energy also changes form but is never created from nothing.

Matter Is Rearranged, Not Destroyed

Atoms in food molecules get rearranged during chemical reactions. The same atoms that were in your sandwich end up in your body or leave as carbon dioxide and water. No atoms vanish.

Energy Is Transformed, Not Created

Chemical energy stored in food molecules is transformed into motion energy, heat energy, and the energy used to build new molecules. Energy changes form but the total amount stays the same.

Photosynthesis Captures Energy

Photosynthesis is the process plants use to capture light energy and store it as chemical energy in sugar (glucose). Plants take in carbon dioxide and water, and produce glucose and oxygen.

Cellular Respiration Releases Energy

Cellular respiration is the process all organisms use to break down glucose. It releases the stored chemical energy so cells can use it. The waste products are carbon dioxide and water.

Evidence Shows the Path

Scientists use evidence like measuring gases, tracking atoms with special labels, and comparing inputs and outputs. This is the science and engineering practice of constructing explanations from evidence.

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KEY TAKEAWAY

Think of matter like LEGO bricks. When you eat food, your body takes apart the LEGO structures (food molecules) and rebuilds them into new structures (body parts, energy carriers). You never gain or lose bricks — you just rearrange them. Energy is like the instruction manual that makes the building happen. It changes form (from the box to your hands to the new creation), but it doesn't appear from nowhere.

SECTION 3

Visual Explanation: Matter and Energy Flow

Tracing Matter and Energy From the Sun to You

The diagram below shows how matter and energy move from sunlight all the way to your muscles. Notice that photosynthesis and cellular respiration are connected. The outputs of one process become the inputs of the other. This is a great example of the crosscutting concept Systems and System Models — we can model how parts of a system interact.

This diagram shows how matter (atoms) cycles between photosynthesis and cellular respiration, while energy flows from the sun through organisms and is eventually released as heat. The same carbon atoms appear in CO₂, then glucose, then CO₂ again.

Look at the top half of the diagram. Photosynthesis takes in CO₂ and H₂O and produces glucose and O₂. Cellular respiration does the reverse — it takes in glucose and O₂ and releases CO₂ and H₂O. Now look at the bottom. Matter (the atoms) cycles back and forth. But energy only flows one way — from the sun to organisms to heat.

SECTION 4

How It Works: The Chemical Reactions

The Two Key Chemical Equations

Scientists use chemical equations (written descriptions of reactions) as evidence for tracing matter. A chemical equation shows us exactly which atoms go in and which come out. If we count the atoms on both sides, the numbers match. This is evidence that matter is conserved — it is not created or destroyed.

PHOTOSYNTHESIS

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

CO₂ = carbon dioxide (from air) • H₂O = water (from soil) • C₆H₁₂O₆ = glucose (sugar) • O₂ = oxygen gas. Light energy is stored as chemical energy in glucose bonds.

CELLULAR RESPIRATION

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + Heat)

Glucose is broken down using oxygen. The atoms rearrange into CO₂ and H₂O. The stored chemical energy is released as ATP (adenosine triphosphate — the cell's energy currency) and heat.

Count the atoms! In the photosynthesis equation, the left side has 6 carbon atoms, 18 oxygen atoms, and 12 hydrogen atoms. The right side has the exact same counts. That is evidence of conservation of matter. The atoms are rearranged, not destroyed.

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NGSS Connection

SEP — Constructing Explanations from Evidence: When you count atoms on both sides of a chemical equation and they match, you are using evidence to explain that matter is conserved. CCC — Energy and Matter: Matter is conserved because atoms are rearranged, not created or destroyed. Energy is transferred between objects or converted in form.

SECTION 5

Types of Evidence Scientists Use

How Do We Know? Gathering Evidence

Scientists don't just guess about where matter and energy go. They gather evidence (observations and data that support a claim). There are several types of evidence used to trace matter and energy through organisms. Each type connects to the science and engineering practice of analyzing and interpreting data.

Scientists use multiple types of evidence to trace matter and energy. Gas measurements (A) track what goes in and out of organisms. Mass measurements (B) show where matter ends up. Isotope tracers (C) follow specific atoms. Calorimetry (D) measures energy transformations.

When scientists combine all four types of evidence, they build a strong case. For example, an experiment might show that a mouse eats 10 grams of food, breathes out 7 grams of CO₂, produces 2 grams of waste, and adds 1 gram to its body. That accounts for all 10 grams — evidence that matter was conserved.

SECTION 6

Worked Example: Tracing a Sandwich

Where Does Your Lunch Go?

Let's go back to our anchoring phenomenon. A student eats a turkey sandwich that weighs about 300 grams. She then runs a mile in gym class. Let's trace the matter and energy using evidence.

Tracing Matter and Energy Through a Turkey Sandwich

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Step 1 — Identify the Matter Inputs

The sandwich contains carbohydrates (bread), proteins (turkey), fats (mayo), and water. These are all made of atoms — mainly carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). These atoms originally came from plants and animals that got them from air, water, and soil.

Inputs: C, H, O, N atoms in food molecules + O₂ from breathing
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Step 2 — Trace Matter Through Digestion

In the stomach and intestines, large food molecules are broken into smaller molecules. Carbohydrates become simple sugars like glucose. Proteins become amino acids. These smaller molecules are absorbed into the blood. No atoms are lost — they are just rearranged into smaller pieces.

Large molecules → small molecules (glucose, amino acids). Same atoms, new arrangement.
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Step 3 — Trace Matter Through Cellular Respiration

Cells use glucose and oxygen in cellular respiration. The carbon and oxygen atoms from glucose combine to form CO₂ (exhaled). The hydrogen and oxygen atoms form H₂O (sweat and urine). Some atoms are used to build new body parts (growth and repair).

Outputs: CO₂ (exhaled) + H₂O (sweat/urine) + new body molecules
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Step 4 — Trace Energy Transformations

The sandwich contained chemical energy stored in the bonds of food molecules. During cellular respiration, this chemical energy is converted to ATP. The student's muscles use ATP to contract, which provides the motion energy for running. Some energy is also released as heat — that's why she feels warm after running.

Chemical energy in food → ATP → motion energy + heat energy
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Step 5 — Check Conservation

If we added up the mass of CO₂ exhaled, H₂O lost, waste produced, and mass added to the body, the total would equal the mass of the sandwich plus the oxygen breathed in. Matter is conserved. The energy from the sandwich equals the motion energy plus the heat released. Energy is also conserved.

Mass in = Mass out. Energy in = Energy out. Conservation confirmed!

SECTION 7

Comparing Photosynthesis and Cellular Respiration

Two Processes, One System

Photosynthesis and cellular respiration are often called complementary processes. This means they work together like two sides of a coin. Comparing them side by side helps us see how matter and energy cycle through living systems.

Feature	Photosynthesis	Cellular Respiration
Who does it?	Plants, algae, some bacteria	All living organisms
Where in the cell?	Chloroplasts	Mitochondria (and cytoplasm)
Matter inputs	CO₂ + H₂O	Glucose (C₆H₁₂O₆) + O₂
Matter outputs	Glucose (C₆H₁₂O₆) + O₂	CO₂ + H₂O
Energy input	Light energy (from the sun)	Chemical energy (in glucose)
Energy output	Chemical energy stored in glucose	ATP + heat
When does it happen?	Only when light is available	All the time (day and night)

Comparison of photosynthesis and cellular respiration

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KEY TAKEAWAY

Think of photosynthesis and cellular respiration like charging and using a rechargeable battery. Photosynthesis is like plugging the battery into a solar charger — it stores energy. Cellular respiration is like using the battery to power a device — it releases the stored energy. The battery (glucose) cycles between being charged and used. The energy flows from the sun to storage to use.

SECTION 8

Connecting to Ecosystems and Advanced Science

From Organisms to Ecosystems

So far we have traced matter and energy through individual organisms. But organisms live in ecosystems (communities of living things and their environment). In an ecosystem, matter and energy pass from one organism to another through food webs. This is a preview of what you will study next.

Concept	What You Learned Today	What Comes Next
Scale	Matter and energy inside one organism	Matter and energy through entire ecosystems (food webs)
Matter	Atoms rearranged during photosynthesis and respiration	Carbon, nitrogen, and water cycle through the whole biosphere
Energy	Chemical energy → ATP → motion + heat	Energy flows from producers → consumers → decomposers, with heat lost at each step
Key Difference	Matter cycles between two processes	Matter cycles through the whole Earth; energy flows one way and does not cycle

From organisms to ecosystems

In high school biology and chemistry, you will study these reactions at the molecular level. You will learn about enzymes, electron transport chains, and the carbon cycle. The crosscutting concept of Scale, Proportion, and Quantity will become important — the same rules about conservation apply whether you look at one cell or the whole planet.

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Fun Fact

Some of the carbon atoms in your body right now were once part of a dinosaur, a tree, or even a volcano. Because matter cycles, atoms get reused over and over for billions of years!

SECTION 9

Practice Problems

Test Your Understanding

Question 1: CONCEPTUAL

A plant takes in carbon dioxide and water, and it produces glucose and oxygen. What happens to the carbon atoms from the carbon dioxide? A) They are destroyed during photosynthesis B) They become part of the glucose molecule C) They are converted into energy D) They leave the plant as oxygen gas

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B) They become part of the glucose molecule. In photosynthesis, carbon atoms from CO₂ are rearranged into glucose (C₆H₁₂O₆). Option A is wrong because atoms cannot be destroyed — matter is conserved. Option C is wrong because atoms are matter, not energy — they do not turn into energy. Option D is wrong because oxygen gas (O₂) contains only oxygen atoms, not carbon atoms.

Question 2: BASIC

A student places a sealed jar with a growing plant in sunlight. After a week, she measures the gases. Which result would be evidence that photosynthesis is occurring? A) Oxygen levels decreased and CO₂ levels increased B) Oxygen levels increased and CO₂ levels decreased C) Both oxygen and CO₂ levels stayed the same D) Both oxygen and CO₂ levels decreased

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B) Oxygen levels increased and CO₂ levels decreased. Photosynthesis uses CO₂ and produces O₂. So if a plant is performing photosynthesis, CO₂ in the jar should go down and O₂ should go up. Option A describes what would happen during cellular respiration only. Option C would mean nothing is happening. Option D makes no scientific sense because gas must go somewhere — it doesn't just vanish.

Question 3: INTERMEDIATE

A hamster eats 15 grams of food pellets in one day. It produces 3 grams of solid waste and its body mass does not change. Where did the remaining 12 grams of matter go? A) The matter was converted into energy and used up B) The matter was exhaled as CO₂ and lost as H₂O in urine, sweat, and breath C) The matter disappeared during digestion D) The matter is still in the hamster's stomach waiting to be digested

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B) The matter was exhaled as CO₂ and lost as H₂O in urine, sweat, and breath. During cellular respiration, glucose reacts with oxygen to produce CO₂ and H₂O. These gaseous and liquid waste products carry the atoms that were in the food. Option A is wrong because matter cannot turn into energy — atoms are rearranged, not destroyed. Option C violates conservation of matter. Option D is unlikely because the hamster's mass didn't change, so it didn't accumulate extra food inside.

Question 4: APPLIED

A scientist feeds a mouse food that contains specially labeled carbon atoms (isotope tracers). After 24 hours, she detects the labeled carbon in three places: the mouse's exhaled breath, the mouse's muscle tissue, and the mouse's waste. Explain how the labeled carbon ended up in all three locations. A) The carbon atoms were randomly scattered by the mouse's movement B) Some carbon was exhaled as CO₂ during respiration, some was used to build muscle proteins, and some was in undigested food waste C) The carbon was converted to energy in all three places D) The mouse ate the labels separately and they just ended up in different places by accident

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B) Some carbon was exhaled as CO₂ during respiration, some was used to build muscle proteins, and some was in undigested food waste. The food's carbon atoms follow different paths: cellular respiration converts some into CO₂ (exhaled), biosynthesis uses some to build body structures like muscle, and some food passes through without being fully digested. This is strong evidence for tracing matter because the isotope labels let us follow specific atoms. Option A is nonsensical — atoms don't scatter randomly. Option C is wrong because carbon atoms don't become energy. Option D ignores the controlled nature of the experiment.

Question 5: CRITICAL THINKING

A student argues: 'When you exercise hard, you lose weight because you burn up the matter in your body and turn it into energy.' Using what you know about conservation of matter and energy, evaluate this claim. Is it correct? What actually happens to the matter? A) The claim is correct — matter is converted to energy during exercise B) The claim is partially correct — some matter becomes energy, but most is exhaled C) The claim is incorrect — the matter is not 'burned up' but rather exhaled as CO₂ and lost as H₂O through sweat and breathing D) The claim is incorrect — you don't actually lose any matter when you exercise

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C) The claim is incorrect — the matter is not 'burned up' but rather exhaled as CO₂ and lost as H₂O through sweat and breathing. This is a common misconception. During exercise, your body increases cellular respiration to produce more ATP. The glucose molecules are broken apart, and the carbon atoms leave your body as CO₂ when you exhale. The hydrogen atoms leave as H₂O in sweat and breath. You do lose mass, but the matter goes somewhere — it doesn't turn into energy. Matter and energy are different things. Option A and B are wrong because matter cannot convert into energy in biological systems (this only happens in nuclear reactions). Option D is wrong because you do lose mass — it just leaves as gas and water, not as 'burned up' matter.

SUMMARY

Lesson Summary

Summary

In this lesson, you learned to trace matter and energy through organisms using evidence. Photosynthesis captures light energy and stores it as chemical energy in glucose, while cellular respiration breaks down glucose to release energy as ATP and heat. The atoms in food molecules are rearranged, not destroyed — this is conservation of matter. Carbon leaves organisms as CO₂, hydrogen and oxygen leave as H₂O, and some atoms are used to build new body structures.

Scientists gather evidence through gas exchange measurements, mass tracking, isotope tracers, and calorimetry. The key crosscutting concept is Energy and Matter: matter cycles through living systems, while energy flows in one direction — from the sun, to chemical energy, to ATP, and finally to heat. Remember: matter is like LEGO bricks that get rebuilt, and energy is like the power that makes the building happen!