DoAssignment study guide

C3.4 · Compare matter and energy transformations in photosynthesis and respiration

Learn to compare matter and energy transformations in photosynthesis and respiration through clear examples and targeted practice.

Ontario Grade 12 Biology

Metabolic Processes

How two connected processes transform atoms and transfer energy

A plant uses light to build sugar, while plant and animal cells can break down sugar and transfer some of its energy to ATP, a molecule cells use to support their activities. These processes are often compared because the main reactants and products of one are the products and reactants of the other. They are not simply the same event running backward: each involves a series of reactions in living cells. In this lesson, matter means the atoms and molecules that make up substances. Energy means the capacity to cause change or support activity. We will track both, rather than treating matter and energy as if they were interchangeable.

What you will learn

Compare how photosynthesis and cellular respiration transform matter.
Describe how energy is transferred and transformed in each process.
Use balanced equations and atom counts to check a comparison.
Explain what a simplified chemical equation shows and what it leaves out.

1. Bridge: what moves through an ecosystem?

In Grade 11 biology, you learned that organisms interact with one another and with their environment. A plant can take in carbon dioxide from the air and water from its surroundings. It can use light energy to make sugar. An animal can obtain sugar by eating plants or other organisms. In both cases, atoms in food become part of organisms or their surroundings.

This gives us two useful questions. First, where do the atoms go? Second, what happens to the energy? Matter is rearranged into new substances; its atoms do not vanish. Energy changes form or moves between parts of a system. A system is the part we choose to study, such as a plant cell or a leaf.

Matter is made of atoms and molecules; matter is rearranged during chemical reactions.
Energy can be transferred or transformed, for example from light to stored chemical energy.

2. Photosynthesis: building sugar

Photosynthesis is the process in which plants, algae, and some other organisms use light energy to make energy-rich sugars from carbon dioxide and water. In plants, much of this process occurs in chloroplasts, structures in cells that capture light for photosynthesis.

A simplified equation shows the main substances involved. Carbon dioxide and water are the reactants: the starting substances in a reaction. Glucose and oxygen are the products: the substances formed. The equation is balanced, meaning the number of each kind of atom is the same on both sides. The coefficients show how many molecules are represented.

The atoms from carbon dioxide and water are rearranged into glucose and oxygen. The glucose contains carbon, hydrogen, and oxygen atoms. Light supplies energy; it is not matter and is not an atom added to glucose. Some of the incoming light energy is stored as chemical energy in glucose. The equation summarizes the overall change, not every reaction step.

6\mathrm{CO_2}+6\mathrm{H_2O}+\text{light energy}\rightarrow\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}

Photosynthesis transforms carbon dioxide and water into glucose and oxygen.
Light energy is transferred into chemical energy stored in the products, especially glucose.
A balanced equation represents conservation of atoms, not conservation of energy as a matter-like substance.

3. Cellular respiration: transferring energy from glucose

Cellular respiration is a set of reactions in which cells break down glucose and transfer some of its chemical energy to ATP. ATP stands for adenosine triphosphate. It is a molecule that can transfer energy to support cell activities. In many cells, most of cellular respiration takes place in mitochondria, although some steps occur elsewhere in the cell.

The simplified overall equation shows glucose and oxygen as reactants, and carbon dioxide and water as products. The atoms are rearranged: carbon atoms from glucose end up in carbon dioxide, while hydrogen and oxygen atoms are found in the products. The equation does not show each step or every substance involved along the way.

Cellular respiration does not turn all the energy in glucose into ATP. Some energy is transferred to ATP, and some is released to the surroundings as heat. Energy is therefore transferred and dispersed in different forms; it is not a material ingredient that disappears. Both plants and animals carry out cellular respiration because both need to transfer energy from food molecules to support cell activity.

\mathrm{C_6H_{12}O_6}

+6O_2\rightarrow6CO_2+6H_2O+energy transferred to ATP and surroundings

Cellular respiration transforms glucose and oxygen into carbon dioxide and water.
Energy from glucose is transferred partly to ATP and partly to the surroundings as heat.
Photosynthesis and respiration have related overall reactants and products, but they are distinct processes.

4. Compare the transformations and use evidence carefully

A direct comparison helps separate matter from energy. In photosynthesis, carbon dioxide and water supply atoms used to form glucose and oxygen. Light energy is transferred into chemical energy stored in products. In cellular respiration, glucose and oxygen are rearranged into carbon dioxide and water. Chemical energy in glucose is transferred to ATP and the surroundings.

A balanced equation is a model, or a simplified representation of a process. Atom counts provide a check on whether its matter accounting is consistent. For example, photosynthesis has six carbon atoms on each side, twelve hydrogen atoms on each side, and eighteen oxygen atoms on each side. This supports the equation's atom balance; it does not, by itself, prove every step of the process or describe how quickly a real organism carries it out.

In experiments, changes in substances such as oxygen or carbon dioxide can provide evidence related to photosynthesis or respiration. Such measurements must be interpreted in context: they are observations of particular conditions, not a complete view of every reaction or energy transfer in a cell. A simplified equation is useful for comparing overall changes, but it leaves out reaction steps, cell details, and how much energy is transferred in a particular situation.

The main matter products of photosynthesis are the main matter reactants of respiration, and vice versa.
Track atoms to compare matter; track energy transfers separately.
Equations summarize overall changes and do not show the full pathway or exact energy amounts.

Worked example

Check the atom balance in photosynthesis

Use the simplified photosynthesis equation to count carbon, hydrogen, and oxygen atoms on each side. Does the equation conserve each kind of atom?

Count reactant atoms
The coefficient 6 applies to each carbon dioxide and water molecule. Count each element separately on the left side.
$6\mathrm{CO_2}+6\mathrm{H_2O}$
Count product atoms
Glucose contains six carbon, twelve hydrogen, and six oxygen atoms. The six oxygen molecules add twelve more oxygen atoms.
$\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}$
Compare counts
The counts match for all three elements, so the equation conserves atoms. This checks matter accounting, not the energy amount or reaction steps.
$\mathrm{C}:6=6,\quad\mathrm{H}:12=12,\quad\mathrm{O}:18=18$

Answer: Yes. Both sides contain 6 carbon atoms, 12 hydrogen atoms, and 18 oxygen atoms.

Check: The oxygen count on the product side includes six oxygen atoms in glucose and twelve in the six oxygen molecules.

Worked example

Compare the overall matter changes

A student says, “Photosynthesis and cellular respiration have exactly the same reactants and products.” Use their simplified equations to assess the claim.

Identify photosynthesis reactants and products
Read the equation from left to right. Carbon dioxide and water are used to form glucose and oxygen.
$6\mathrm{CO_2}+6\mathrm{H_2O}\rightarrow\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}$
Identify respiration reactants and products
Cellular respiration uses glucose and oxygen and forms carbon dioxide and water. The main substances are reversed in the overall comparison.
$\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}\rightarrow6\mathrm{CO_2}+6\mathrm{H_2O}$
Evaluate the claim
The claim is inaccurate: the processes do not have the same substances on the same side. Their overall matter changes are related, but each process follows its own series of reactions.
$\text{photosynthesis products} \approx \text{respiration reactants}$

Answer: The student is incorrect. The main reactants and products are reversed in the two simplified equations.

Check: The comparison concerns overall matter changes. It does not mean that one process is simply the other process running backward.

Worked example

Separate matter from energy

A classmate says that light is converted into oxygen during photosynthesis and that respiration destroys the energy in glucose. Correct both statements using the course-level model.

Track matter in photosynthesis
Oxygen is matter, so its atoms come from the reactants, not from light energy. The equation shows atoms being rearranged from carbon dioxide and water into products.
$6\mathrm{CO_2}+6\mathrm{H_2O}\rightarrow\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}$
Track energy in photosynthesis
Light supplies energy. Some of it becomes chemical energy stored in glucose; light itself is not a substance that becomes oxygen. light energy\rightarrowchemical energy in products
Track energy in respiration
Respiration transfers some chemical energy from glucose to ATP and some to the surroundings as heat. Energy changes form or moves; it is not destroyed. chemical energy in glucose\rightarrowenergy transferred to ATP and surroundings

Answer: Oxygen is formed by rearranging atoms from matter in the reactants. Light provides energy that becomes stored chemical energy. Respiration transfers energy from glucose rather than destroying it.

Check: Do not treat energy as an atom or substance in the reaction. Keep the matter account and the energy account distinct.

Common mistakes and how to avoid them

Saying that plants photosynthesize but do not carry out cellular respiration.

Correction: Plants carry out both processes. Photosynthesis stores some light energy in products; respiration transfers energy from glucose to ATP and the surroundings.

Treating light energy as matter or as a source of oxygen atoms.

Correction: Light provides energy. Oxygen atoms in the products come from matter in the reactants.

Assuming that the two processes are one reaction simply run in opposite directions.

Correction: Their overall equations show related reactants and products, but each process involves its own series of reactions in cells.

Assuming all energy in glucose is transferred to ATP.

Correction: Some energy is transferred to ATP, and some is released to the surroundings as heat.

Lesson summary

Photosynthesis uses light energy to transform carbon dioxide and water into glucose and oxygen.
Cellular respiration transforms glucose and oxygen into carbon dioxide and water while transferring energy to ATP and the surroundings.
Matter is tracked by following atoms; energy is tracked by following transfers and transformations.
Balanced equations summarize overall changes but do not show every reaction step or exact energy amount.

Check your understanding

Question 1

Which statement best compares energy in the two processes?

Photosynthesis transfers some light energy into chemical energy in products; respiration transfers some chemical energy from glucose to ATP and the surroundings.
Photosynthesis creates energy from carbon dioxide; respiration destroys all energy in glucose.
Both processes store all incoming energy in glucose.
Energy is matter, so it is counted by balancing oxygen atoms.

Show answer and explanation

Photosynthesis transfers some light energy into chemical energy in products; respiration transfers some chemical energy from glucose to ATP and the surroundings.

The comparison distinguishes energy transfers from matter changes. Photosynthesis stores some light energy as chemical energy, while respiration transfers some energy from glucose to ATP and the surroundings.

Question 2

Which set names the main products in the simplified cellular respiration equation?

Glucose and oxygen
Carbon dioxide and water
Carbon dioxide and glucose
Water and oxygen

Show answer and explanation

Carbon dioxide and water

In the simplified equation, glucose and oxygen are reactants. Carbon dioxide and water are products.

Question 3

What does a balanced photosynthesis equation establish?

That every reaction step has been shown
That the process occurs at the same rate in every plant
That each kind of atom has equal counts on both sides
That all light energy becomes glucose

Show answer and explanation

That each kind of atom has equal counts on both sides

Balancing checks the count of each kind of atom in the overall equation. It does not establish the reaction steps, rate, or exact energy transfer.

Key terms

Matter: The atoms and molecules that make up substances.
Energy: The capacity to cause change or support activity; it can be transferred or transformed.
Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Balanced equation: An equation with equal numbers of each kind of atom on both sides.
ATP: A molecule that can transfer energy to support cell activities.
Chloroplast: A structure in plant and some other cells where photosynthesis takes place.
Mitochondrion: A cell structure where many steps of cellular respiration take place.

Continue through SBI4U

View the complete SBI4U Ontario Grade 12 Biology curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation C3.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question