8.3 The Process of Photosynthesis
Describe what happens during the light-dependent reactions.
Describe what happens during the light-independent reactions.
Identify factors that affect the rate at which photosynthesis occurs.
Lesson Summary
The Light-Dependent Reactions: Generating ATP and NADPH
Photosynthesis begins with these reactions, which occur in thylakoid membranes.
Photosystems are clusters of proteins and chlorophyll in thylakoid membranes.
High-energy electrons form when pigments in photosystem II absorb light. The electrons pass through electron transport
chains, a series of electron carrier proteins.
The movement of electrons through an electron transport chain causes a thylakoid to fill up with hydrogen ions
and generates ATP and NADPH.
ATP synthase is a membrane protein through which excess hydrogen ions escape a thylakoid in a process that
makes ATP.
The Light-Independent Reactions: Producing Sugars They occur in the stroma of thylakoids and are commonly called
the Calvin cycle.
Six carbon dioxide molecules from the atmosphere enter the Calvin cycle and combine with 5-carbon compounds already
present. They produce twelve 3-carbon molecules.
Two 3-carbon molecules are removed from the cycle. They are used by the plant to build sugars, lipids, amino acids, and
other compounds.
The remaining ten 3-carbon molecules are converted back to 5-carbon molecules and begin a new cycle.
Factors Affecting Photosynthesis Many factors influence the rate of photosynthesis.
Temperature, light intensity, and availability of water affect photosynthesis.
C4 and CAM plants have a modified type of photosynthesis that enables the plants to conserve water in dry climates.
The Light-Dependent Reactions: Generating ATP and NADPH
For Questions 1–5, write True if the statement is true. If the statement is false, change the underlined word or words to
make the statement true.
True 1. Photosystems are clusters of chlorophyll and proteins.
False- PSII 2. The light-dependent reactions begin when photosystem I absorbs light.
True 3. Electrons from water molecules replace the ones lost by photosystem II.
False-NADPH 4. ATP is the product of photosystem I.
False-electron 5. ATP and NADPH are two types of protein carriers.
6. How does ATP synthase produce ATP? ATP synthase allows H+ ions to pass through the thylakoid membrane,
rotating the enzyme. The rotation creates the energy needed to bind ADP to a phosphate and produces ATP.
