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C3.2 · Explain matter and energy changes in photosynthesis

Learn to explain matter and energy changes in photosynthesis through clear examples and targeted practice.

Ontario Grade 12 Biology

Metabolic Processes

Ontario Grade 12 Biology — study topic C3.2

A plant uses materials from its surroundings to make sugar. The sugar contains matter, including carbon that came into the plant as carbon dioxide. Photosynthesis also changes energy: light energy is captured and some is stored as chemical energy in sugar. To explain these changes, we will track matter and energy separately. Matter is made of atoms. In a chemical reaction, atoms are rearranged into different substances; they are not created or destroyed.

What you will learn

  • Describe the overall matter changes in photosynthesis.
  • Explain how light energy is transferred into chemical energy stored in sugar.
  • Read and balance the overall photosynthesis equation by counting atoms.
  • Explain what the overall equation shows and what it leaves out.

1. SBI3U bridge: cells, materials, and reactions

Cells are the basic units of living things. In plant cells, photosynthesis mainly takes place in structures called chloroplasts. Chloroplasts contain chlorophyll, a green pigment that absorbs some light energy.
Plants obtain carbon dioxide from the air and water from their surroundings. These substances are used in photosynthesis. Oxygen and sugar are products of the overall process. Sugar is a molecule made of carbon, hydrogen, and oxygen atoms.
A chemical reaction changes which atoms are grouped together. The atoms do not vanish when a reaction occurs. This gives us a simple way to check an equation: count each kind of atom before and after.
  • Chloroplasts are the main sites of photosynthesis in plant cells.
  • Carbon dioxide and water are reactants; sugar and oxygen are products.
  • Chemical reactions rearrange atoms.

2. The overall model: matter and energy

An overall equation summarizes the main inputs and outputs of photosynthesis. The substances on the left are the reactants. The substances on the right are the products. The number in front of a formula is a coefficient. It tells how many molecules are represented.
The balanced equation shows equal numbers of carbon, hydrogen, and oxygen atoms on both sides. It accounts for matter in the overall reaction. Light energy is also shown as an input, but it is not a substance made of atoms, so it is not part of the atom count.
Energy changes form during photosynthesis. Chlorophyll absorbs some light energy. In the chloroplast, energy is transferred into chemical forms that help build sugar. Some of the energy is stored as chemical energy in the sugar. Photosynthesis does not create energy; it transfers energy from light into a form stored in a substance.
A broad model separates photosynthesis into two connected parts. The light-dependent reactions capture light energy, use water, release oxygen, and form energy carriers. Energy carriers are molecules that transfer usable chemical energy. The Calvin cycle uses carbon dioxide and energy carriers from the light-dependent reactions to help build sugar. It does not capture light directly.
6CO2+6H2O+light energy→C6H12O6+6O26\mathrm{CO_2}+6\mathrm{H_2O}+\text{light energy}\rightarrow\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}
  • The overall equation summarizes matter inputs and outputs.
  • Light energy is transferred into chemical energy stored in sugar.
  • The light-dependent reactions and Calvin cycle are connected parts of photosynthesis.

3. Read the equation carefully

Read an equation from left to right. The arrow separates reactants from products. A coefficient multiplies every atom in the formula that follows it. For example, six carbon dioxide molecules contain six carbon atoms and twelve oxygen atoms.
On the reactant side, six water molecules contain twelve hydrogen atoms and six oxygen atoms. Together with the carbon dioxide, the reactants contain six carbon, twelve hydrogen, and eighteen oxygen atoms. On the product side, one sugar molecule contains six carbon, twelve hydrogen, and six oxygen atoms. Six oxygen molecules contain twelve more oxygen atoms. The totals match.
This overall balance supports the idea that matter is rearranged. It does not show every step in the chloroplast or trace each individual atom through all those steps. The equation also does not tell us how fast photosynthesis happens or exactly how much sugar a particular plant makes.
  • A coefficient applies to every atom in a formula.
  • Balanced equations account for the same number of each type of atom on both sides.
  • An overall equation is a summary, not a step-by-step account.

4. Build a complete explanation

A clear explanation of photosynthesis includes both matter and energy. First name the reactants and products. Then explain that their atoms are rearranged, as shown by the balanced equation. Finally describe the energy change: chlorophyll absorbs light energy, and some energy is transferred into chemical energy stored in sugar.
Keep the limits of the model in mind. The overall equation is useful for tracking matter and showing that light energy is required. The two-part model gives a broad account of how light capture connects to sugar building. Neither representation lists every reaction that occurs.
  • Matter is rearranged while energy is transferred.
  • Use the overall equation for the net change, not as a complete pathway.

Worked example

Check matter in the overall equation

Count carbon, hydrogen, and oxygen atoms on both sides of the balanced equation. What does the match show?
  1. Count the reactants
    Six carbon dioxide molecules contain six carbon atoms and twelve oxygen atoms. Six water molecules contain twelve hydrogen atoms and six oxygen atoms. Together, the reactants contain six carbon, twelve hydrogen, and eighteen oxygen atoms.
    (6C,12H,18O)(6\mathrm{C},12\mathrm{H},18\mathrm{O})
  2. Count the products
    One sugar molecule contains six carbon, twelve hydrogen, and six oxygen atoms. Six oxygen molecules contain twelve oxygen atoms. The products therefore contain six carbon, twelve hydrogen, and eighteen oxygen atoms.
    (6C,12H,18O)(6\mathrm{C},12\mathrm{H},18\mathrm{O})
  3. Interpret the match
    Each type of atom has the same total on both sides. The equation accounts for matter by showing an overall rearrangement of atoms.
Answer: Both sides contain six carbon, twelve hydrogen, and eighteen oxygen atoms. The equation is balanced for matter.
Check: Count oxygen from both reactants: twelve atoms in carbon dioxide plus six in water equals eighteen.

Worked example

Improve an energy explanation

A student says, “Plants turn light into sugar.” Rewrite the statement to explain the energy change and name where it occurs.
  1. Name the energy input
    Light supplies energy to photosynthesis. Chlorophyll in chloroplasts absorbs some of this light energy.
  2. Describe the transfer
    The captured energy is transferred into chemical forms that help build sugar. Some is stored as chemical energy in the sugar. light energy\rightarrowchemical energy in sugar
  3. State the full idea
    A more complete statement connects energy change to matter change: chloroplasts use captured light energy as carbon dioxide and water are rearranged into sugar and oxygen.
Answer: Chlorophyll in chloroplasts absorbs light energy. Photosynthesis transfers some of that energy into chemical energy stored in sugar as carbon dioxide and water are rearranged into sugar and oxygen.
Check: The explanation describes energy being transferred, not created.

Worked example

Connect the two parts

A learner says that the Calvin cycle directly captures light and releases oxygen. Correct the statement using the broad model of photosynthesis.
  1. Locate light capture and oxygen release
    The light-dependent reactions capture light energy, use water, and release oxygen. The learner has assigned these events to the wrong part.
  2. Describe the Calvin cycle
    The Calvin cycle uses carbon dioxide and energy carriers from the light-dependent reactions to help build sugar. It does not capture light directly.
  3. Explain the connection
    The parts work as a connected model: the light-dependent reactions provide energy carriers, and the Calvin cycle uses them while building sugar.
    light-dependent reactions→energy carriers→Calvin cycle\text{light-dependent reactions}\rightarrow\text{energy carriers}\rightarrow\text{Calvin cycle}
Answer: The light-dependent reactions capture light and release oxygen. The Calvin cycle uses carbon dioxide and energy carriers from those reactions to help build sugar.
Check: The Calvin cycle depends on products of the light-dependent reactions, but does not directly capture light.

Common mistakes and how to avoid them

Saying that soil supplies the carbon atoms in sugar.
Correction: The carbon in sugar comes from carbon dioxide used in photosynthesis.
Treating light as a substance in the atom count or saying energy is created.
Correction: Light is an energy input, not a substance made of atoms. Photosynthesis transfers energy into chemical energy stored in sugar.
Treating the overall equation as a list of every reaction step.
Correction: The equation summarizes the net matter change. The light-dependent reactions and Calvin cycle provide a broad model of connected parts.

Lesson summary

  • Photosynthesis uses carbon dioxide and water to make sugar and oxygen.
  • The balanced equation shows that atoms are rearranged and accounted for.
  • Chlorophyll absorbs light energy, and some is transferred into chemical energy stored in sugar.
  • The light-dependent reactions capture light and release oxygen; the Calvin cycle uses carbon dioxide and energy carriers to help build sugar.

Check your understanding

Question 1

What is the main energy change in photosynthesis?
  1. Chemical energy in sugar changes into light energy.
  2. Light energy is transferred into chemical energy stored in sugar.
  3. Matter becomes energy and atoms disappear.
  4. Heat is the only energy input.
Show answer and explanation
Light energy is transferred into chemical energy stored in sugar.
Photosynthesis captures light energy and stores some of it as chemical energy in sugar.

Question 2

Which substance supplies the carbon atoms used to make sugar?
  1. Water
  2. Oxygen gas
  3. Carbon dioxide
  4. Light
Show answer and explanation
Carbon dioxide
Carbon dioxide is a reactant, and its carbon atoms are used in the overall formation of sugar.

Question 3

Which statement best describes the relation between the two broad parts of photosynthesis?
  1. The Calvin cycle captures light and releases oxygen; the light-dependent reactions build sugar from carbon dioxide.
  2. The light-dependent reactions capture light and release oxygen; the Calvin cycle uses carbon dioxide and energy carriers to help build sugar.
  3. Both parts only release oxygen and do not contribute to sugar production.
  4. The two parts are unrelated processes.
Show answer and explanation
The light-dependent reactions capture light and release oxygen; the Calvin cycle uses carbon dioxide and energy carriers to help build sugar.
The light-dependent reactions provide energy carriers, and the Calvin cycle uses them while helping build sugar from carbon dioxide.

Key terms

Chloroplast
A structure in plant cells where photosynthesis mainly occurs.
Chlorophyll
A green pigment in chloroplasts that absorbs some light energy.
Reactant
A substance used in a chemical reaction.
Product
A substance made in a chemical reaction.
Chemical energy
Energy stored in substances, including sugar.
Light-dependent reactions
The part of photosynthesis that captures light energy, uses water, releases oxygen, and forms energy carriers.
Calvin cycle
The part of photosynthesis that uses carbon dioxide and energy carriers to help build sugar.
Energy carrier
A molecule that transfers usable chemical energy from one part of a process to another.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation C3.2. 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.

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