What are the two main components of photosynthesis?
- Light-harvesting reactions 2. The Calvin Cycle
What is the goal of the Calvin-Benson Cycle?
To take NADPH and ATP produced in the light reactions and store their energy by constructing sugars from CO2.
Why must excess ATP be used in the Calvin Cycle?
ATP is unstable, so its energy must be stored in high-energy bonds of sugars.
Where does the Calvin Cycle occur?
In the stroma of the chloroplast.
Where does the CO2 used in the Calvin Cycle come from?
From the atmosphere, entering through stomata.
How is CO2 converted into sugar in the Calvin Cycle?
Through a carbon-reductive process powered by ATP and NADPH.
What can sugars produced in photosynthesis be used for?
- Energy production via respiration 2. Energy storage for later use 3. Growth (e.g., cellulose synthesis)
What is RuBisCO?
An enzyme in the Calvin Cycle that fixes CO2, but can also mistakenly bind O2.
What is photorespiration?
A process where RuBisCO binds O2 instead of CO2, causing RuBP oxidation.
What happens during photorespiration?
O2 + RuBP → 1 PGA + 1 phosphoglycolate.
How do plants deal with phosphoglycolate?
They must spend energy in the peroxisome to convert it into a useful molecule.
Why does photorespiration occur?
When photosynthesis evolved, O2 concentration was low, making RuBisCO’s inefficiency less problematic.
How costly is photorespiration?
Plants may spend up to 40% of their stored sugar energy dealing with RuBisCO fixing O2.
How could plants avoid photorespiration?
By evolving mechanisms to minimize O2 binding in the Calvin Cycle (e.g., C4 and CAM photosynthesis).
What happens if thylakoid membranes are punctured?
There will be much less ATP and NADPH production, but oxygen production will be unaffected.
