Photosynthesis and Cellular Respiration

Grade 9 · Science · Study Guide

Understand how cells capture, convert, and release energy through the complementary processes of photosynthesis and cellular respiration.

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It occurs primarily in the chloroplasts and is summarized as: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. The process has two stages: the light-dependent reactions (in the thylakoid membranes) capture sunlight and produce ATP and NADPH while splitting water and releasing oxygen, and the Calvin cycle (in the stroma) uses that ATP and NADPH to fix carbon dioxide into glucose.

Cellular respiration is the reverse process — cells break down glucose to release the stored chemical energy as usable ATP. The overall equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Aerobic respiration occurs in three stages: glycolysis (in the cytoplasm) splits glucose into two pyruvate molecules producing a small amount of ATP; the Krebs cycle (in the mitochondrial matrix) extracts electrons from carbon compounds releasing CO₂; and the electron transport chain (on the inner mitochondrial membrane) uses those electrons to produce the majority of ATP (~34 molecules per glucose) via chemiosmosis.

These two processes are complementary: the products of photosynthesis (glucose and oxygen) are the reactants of cellular respiration, and vice versa, forming the basis of energy flow through ecosystems.

Key terms

Photosynthesis
The process by which chlorophyll-containing organisms convert CO₂, water, and light into glucose and oxygen.
Chloroplast
The organelle in plant and algal cells where photosynthesis occurs; contains thylakoids and stroma.
Cellular Respiration
The metabolic process of breaking down glucose using oxygen to produce ATP, CO₂, and water.
ATP (Adenosine Triphosphate)
The primary energy currency of cells; energy is released when its phosphate bond is broken.
Calvin Cycle
The light-independent (dark) reactions of photosynthesis that use ATP and NADPH to fix CO₂ into glucose in the stroma.
Electron Transport Chain
A series of protein complexes on the inner mitochondrial membrane that use electrons to pump protons and generate large amounts of ATP.

Flashcards

Thylakoid
Flattened membrane sacs inside the chloroplast where light-dependent reactions occur and chlorophyll is embedded.
Stroma
The fluid-filled space inside the chloroplast surrounding the thylakoids; site of the Calvin cycle.
Glycolysis
The first stage of cellular respiration occurring in the cytoplasm; splits glucose into 2 pyruvate molecules, yielding 2 ATP net.
Krebs Cycle
The second stage of aerobic respiration in the mitochondrial matrix; oxidizes pyruvate derivatives, releasing CO₂ and producing electron carriers.
NADPH
An electron carrier produced in the light-dependent reactions; donates electrons/hydrogen for glucose synthesis in the Calvin cycle.
Chemiosmosis
The process by which proton gradients across a membrane drive ATP synthesis by flowing through ATP synthase.
Chlorophyll
The green pigment in chloroplasts that absorbs light energy (mainly red and blue wavelengths) to power photosynthesis.
Fermentation
An anaerobic pathway that regenerates NAD⁺ from NADH to allow glycolysis to continue when oxygen is absent, producing lactic acid or ethanol.

Questions

  1. Where do the light-dependent reactions of photosynthesis take place?

    1. Stroma of the chloroplast
    2. Thylakoid membranes
    3. Mitochondrial matrix
    4. Cytoplasm
    Show answer

    Answer: Thylakoid membranes

    The light-dependent reactions occur in the thylakoid membranes, where chlorophyll absorbs light to split water, release O₂, and produce ATP and NADPH.

  2. Cellular respiration and photosynthesis have identical overall chemical equations but run in opposite directions.

    Show answer

    Answer: True

    The overall equations are reverse reactions of each other: photosynthesis builds glucose from CO₂ and water using energy, while respiration breaks glucose down to CO₂ and water releasing energy.

  3. The stage of cellular respiration that takes place in the cytoplasm and splits glucose into two pyruvate molecules is called ___.

    Show answer

    Answer: glycolysis

    Glycolysis occurs in the cytoplasm of all cells and is the first stage of cellular respiration, breaking one glucose molecule into two 3-carbon pyruvate molecules.

  4. Which molecule is produced during the light-dependent reactions and used as an energy source in the Calvin cycle?

    1. Glucose
    2. CO₂
    3. ATP
    4. Water
    Show answer

    Answer: ATP

    ATP (and NADPH) are produced in the light-dependent reactions and then used in the Calvin cycle to power the fixation of CO₂ into glucose molecules.

  5. Anaerobic respiration produces more ATP per glucose molecule than aerobic respiration.

    Show answer

    Answer: False

    Aerobic respiration yields approximately 36-38 ATP per glucose. Anaerobic respiration (fermentation) yields only 2 ATP per glucose — far less efficient.

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