Photosynthesis and Respiration

Grade 6 · Science · Study Guide

Understand how plants make food through photosynthesis and how all living things release energy through cellular respiration.

Photosynthesis is the process by which plants, algae, and some bacteria use light energy to make their own food. It takes place primarily in the chloroplasts of plant cells, which contain a green pigment called chlorophyll. The basic equation for photosynthesis is: carbon dioxide + water + light energy → glucose + oxygen. Plants absorb carbon dioxide from the air through tiny pores called stomata and take up water through their roots.

The glucose produced during photosynthesis is used as fuel for the plant's life processes and as a building block for growth. Oxygen is released as a byproduct — this is the source of the oxygen in Earth's atmosphere that animals need to breathe.

Cellular respiration is essentially the reverse of photosynthesis. All living organisms — plants, animals, and fungi — use cellular respiration to break down glucose and release the stored energy. The equation is: glucose + oxygen → carbon dioxide + water + energy (ATP). This process occurs in the mitochondria of cells.

These two processes are complementary. Plants perform both photosynthesis and respiration, but during daylight they produce far more oxygen than they consume. Animals rely entirely on respiration and depend on plants for both oxygen and food energy. Together, photosynthesis and respiration cycle carbon and oxygen through Earth's ecosystems.

Key terms

Photosynthesis
The process by which plants use light, carbon dioxide, and water to produce glucose and oxygen.
Chlorophyll
The green pigment in chloroplasts that absorbs light energy to power photosynthesis.
Cellular respiration
The process by which cells break down glucose to release energy (ATP), producing carbon dioxide and water.
Chloroplast
The organelle in plant cells where photosynthesis takes place.
Stomata
Tiny pores on the surface of leaves that allow gases (CO2 and O2) to enter and exit the plant.
Glucose
A simple sugar (C6H12O6) produced during photosynthesis that serves as the primary fuel for cellular respiration.

Flashcards

Photosynthesis equation
CO2 + H2O + light energy → glucose (C6H12O6) + O2
Cellular respiration equation
Glucose (C6H12O6) + O2 → CO2 + H2O + energy (ATP)
Chloroplast
Plant cell organelle where photosynthesis occurs; contains chlorophyll.
Mitochondria
Cell organelle where cellular respiration occurs; called the powerhouse of the cell.
Stomata
Tiny pores in plant leaves that allow CO2 in and O2 out during photosynthesis.
ATP
Adenosine triphosphate; the usable energy molecule produced by cellular respiration.
Autotroph
An organism that makes its own food, like plants that perform photosynthesis.
Heterotroph
An organism that cannot make its own food and must consume other organisms for energy.

Questions

  1. What are the raw materials (reactants) used in photosynthesis?

    1. Glucose and oxygen
    2. Carbon dioxide, water, and light energy
    3. Oxygen and glucose
    4. Nitrogen and carbon dioxide
    Show answer

    Answer: Carbon dioxide, water, and light energy

    Photosynthesis requires three inputs: carbon dioxide (from air), water (from soil via roots), and light energy (from the sun). These reactants are converted into glucose and oxygen.

  2. Cellular respiration occurs only in animal cells, not in plant cells.

    Show answer

    Answer: False

    All living cells, including plant cells, perform cellular respiration to release energy from glucose. Plants perform both photosynthesis AND respiration.

  3. The green pigment in plant cells that captures light energy for photosynthesis is called ________.

    Show answer

    Answer: chlorophyll

    Chlorophyll is the green pigment found in chloroplasts that absorbs light energy (mainly red and blue wavelengths) to power the photosynthesis reactions.

  4. Where does cellular respiration take place in the cell?

    1. Chloroplast
    2. Nucleus
    3. Mitochondria
    4. Cell wall
    Show answer

    Answer: Mitochondria

    Cellular respiration takes place in the mitochondria, often called the powerhouse of the cell because it produces ATP — the cell's usable energy currency.

  5. Oxygen is a byproduct of photosynthesis.

    Show answer

    Answer: True

    Oxygen is released as a byproduct of photosynthesis. It comes from the splitting of water molecules during the light-dependent reactions, and it diffuses out through the stomata.

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