1. Summarize the purpose of the light dependent reactions
2. What molecule is used, and what is released?
• The second stage of photosynthesis uses energy from the first stage to make sugars
• Also called light-independent reactions.
– They happen both when light is present and when light is absent.
• The dark reactions take place in the stroma.
• Carbon dioxide diffuses into the stroma through pores in a plant’s leaves (stomates) and is used to make glucose in a process known as carbon fixation.
– Uses the ATP and NADPH from light dependant reactions
The Pores in a plant’s leaf are called stomates (not to be confused with stroma)
Stomate Up Close
Carbon Dioxide Becomes Part of the
Plant After Entering the Stomates.
Carbon Fixation and the Calvin Cycle.
• Calvin Cycle: A series of enzyme-controlled reactions in which carbon fixation occurs.
– Named after Melvin Calvin, an American Scientist who won the Nobel prize in 1961 for his discovery of the cycle.
– Carbon Fixation: The process in which carbon dioxide is incorporated (fixed) into organic compounds by the processes of photosynthesis and chemosynthesis.
• Also called Carboxylation.
Calvin holding a model of sucrose.
He had his own stamp!
• The Calvin cycle starts and ends with RuBP, which is a five carbon sugar.
• RuBP stands for ribulose biphosphate.
• Carbon dioxide reacts with RuBP, creating two molecules of PGA (phosphoglyceric acid).
– This is the carbon fixation step.
1. 6 CO
2 enter the stroma
2. 6 CO
+ 6 RuBP 12 PGA (3-C)
– Catalyzed by rubisco
3. 12 PGA + ATP + NADPH 12 PGAL
4. 2 PGAL are removed and used to form glucose
5. 10 PGAL remaining are rearranged back into
6 RuBP and cycle start over with new CO
(The Dark Reaction)
• The Calvin cycle is part of the dark reactions.
• The Calvin cycle needs chemical energy produced in the light reactions (ATP and
NADPH)and carbon dioxide from the atmosphere to work.
• Two PGAL molecules form one glucose molecule.
– Need 6 CO
2 to make 1 glucose
• The Calvin cycle is a cycle because it starts and ends with RuBP.
• Where does the O
2 from? (CO
2 or H
that plants release come
• Where does CO
• Why do you think 10 PGAL molecules must be used to make 6 RuBP molecules to restore the
• How does the Calvin cycle build sugar molecules?
The light reactions drive the dark reactions.
The Chemistry of the Chloroplast
The Light Reactions are also called the “Light-Dependent Reactions.”
The Dark Reactions are also called the “Light-Independent Reactions.”
• Without photosynthetic organisms, the food we eat and the air we breath would not exist. As a result, life on earth as we know it depends on the autotrophs.
• “What drives life is thus a little electric current, set up by the sunshine.”
– American biochemist and Nobel laureate Albert Szent-
• Can you name them?
• 1. Light Intensity.
• 2. Temperature.
• 3. Water Availability.
• 4. Mineral Availability.
• The Carbon Cycle: the pathways by which carbon is recirculated through the biosphere.
• *Not to be confused with the Calvin Cycle, which is part of a broader carbon cycle.
• Can you think of important ways human beings have altered or disrupted the carbon cycle? Think carefully and write them down.