Photosynthesis Requires More Than Just Pigments. It Needs Organelles and Enzymes To Control the Reaction
Photosynthesis What: Synthesis of carbohydrates from CO2 and Water using the energy of the sun Why: To provide sugars for the cells to use for energy flow (i.e. cellular respiration) Where: The chloroplast When: Light and dark How: Two separate reactions –Photolysis (light dependent reactions) –Calvin-Benson Cycle (light independent reactions)
Photolysis What: Making ATP and providing hydrogen for The Dark Reactions Where: Thylakoid Membranes of Chloroplast Who: Light, Pigments, Water and NADP (to carry hydrogen)
Photolysis (aka The Light Dependent Reactions) Light strikes accessory pigments in photosystem, exciting their electrons Electrons are passed on to chlorophyll, whose electrons are additionally photoexcited The energy from photoexcitation is used to rebuild ATP from ADP Without photolysis, there would be no ATP for building glucose (i.e. Calvin-Benson Cycle)
Why is water necessary? When chlorophyll is excited, it loses electrons and starts to break down To replace these electrons and preserve chlorophyll, water molecules break apart, providing electrons to “save” chlorophyll Giving up electrons causes water to “split” into oxygen gas (O 2 ) and hydrogen The hydrogen ions are picked up by NADP and carried to the Dark Reactions
If chlorophyll absorbs all colors of spectrum except for green, why do the leaves of deciduous trees change color in the fall?
Chlorophyll & Fall Chlorophyll requires a great deal of ATP and water to synthesize In the fall, there is less water available to make chlorophyll and less available light to use for synthesis As a result, leaves stop producing chlorophyll
Calvin-Benson Cycle What: Synthesis of glucose When: Independent of light Where: Stroma (gel- like interior of chloroplast) Who: CO2, Rubisco, NADPH, ATP
Major Actions of Calvin-Benson Cycle 1) CO2 enters the leaf via the stomata 2) Carbon Dioxide (CO 2 ) is “fixed”/removed from air via the enzyme Rubisco. 3) NADPH gives up its hydrogen to CO 2 in a series of cyclical, enzymatically controlled reactions 4) Glucose is synthesized
Why can’t you do the Calvin Benson Cycle without Photolysis?
Review of Photosynthesis PHOTOLYSIS aka The Light Dependent Reactions Occurs in grana of chloroplast Depend upon light energy to accelerate electrons of pigments ATP is produced to power 2 nd set of reactions Water replaces electrons lost by chlorophyll. This causes water to split, releasing oxygen gas. Leftover hydrogen is used to build glucose THE CALVIN CYCLE aka The Light Independent/Dark Reactions Occurs in stroma Do not require light Using the ATP from the photolysis, the Calvin Cycle “fixes” Carbon Dioxide by combining it with hydrogen to make glucose