2.2 Photosynthesis

​Captures energy from sunlight to make sugars that store chemical energy.

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​Plants absorb visible light.

​Contain chlorophyll molecules, which absorb some energy from visible light.

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​Two types: Chlorophyll a and b

​They absorb blue and red light

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​Grana: Stacks of

coin-shaped compartments called thylakoids.

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​Stroma: Fluid that surrounds the grana in the chloroplast.

​​Two main parts used for photosynthesis:

​Grana and Stroma

Light-Dependent Reactions

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​Light-Independent Reactions

​Also called "The Calvin Cycle"

Steps 1-7​

Chlorophyll absorbs light energy​

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​Water molecules are broken down into hydrogen ions, electrons, and oxygen.

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​No sugars are formed in this reaction

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​Two groups of molecules capture light at the thylakoids:

​Photosystem II

Photosystem I

Step 1:

​Energy absorbed from sunlight

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Energy transfers to electrons (e-)

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Electrons leave chlorophyll and enter electron transport chain

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​​Electron Transport Chain:

Series of proteins in the membrane of thylakoid

Step 2:

Water molecules split

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Oxygen, hydrogen ions, and electrons separate.

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​Oxygen released as waste

​Electrons replace the electrons that left when light was absorbed.

Step 3:

Hydrogen ions transported

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Movement of electrons down the ETC pumps H⁺ ions across the thylakoid membrane.

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​H⁺ ions build up inside the membrane.

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​Electrons move on to photosystem I.

Step 4:

Energy Absorbed from Sunlight

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Like in photosystem II,

​chlorophyll absorbs light energy and energizes electrons

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​Electrons leave molecules.

Step 5:

NADPH produced

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Energized electrons are added to an NADP+ molecule, to form NADPH.

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​These molecules function similarly to ATP and ADP.

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​NADPH molecules move to the light-independent reactions.

Step 6:

Hydrogen Ion Diffusion

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Hydrogen ions flow through a protein channel in the thylakoid membrane.

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​Move from an area of high concentration of H⁺ ions (inside the thylakoid membrane) to an area of low concentration (outside).

Step 7:

ATP Produced

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Protein channel in step 6 is part of a complex enzyme called

"ATP synthase"

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​As ions flow through the protein, ATP synthase makes ATP by adding phosphates to ADP molecules.

​Also called "The Calvin Cycle"

​Uses CO₂ and energy carried by ATP and NADPH to build sugars.

​Step 1:

​Carbon dioxide added

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​CO₂ added to a

5-carbon molecule, forms 6-carbon molecule.

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​Step 2:

​3-carbon molecules formed

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ATP and NADPH used by enzymes to break

6-carbon molecule down to two 3-carbon molecules.

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​Step 3:

3-carbon molecules exit

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​One 3-carbon molecule exits each cycle.

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​Once two have left the cycle, they bond to form a glucose molecule

(6-carbon sugar)

​Step 4:

3-carbon molecules recycled

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Remaining 3-carbon sugars are converted back to 5-carbon sugars that start the Calvin cycle

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​These are added to CO₂ molecules when they join the cycle