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The Details of Ps- For H Classes only
Chloroplasts have fluid filled membranes called thylakoids. Stacks of these thylakoids are called grana
Outside the grana is a colloid called the stroma (see diagram)
Light Reactions (part of this happens in what is called Photosystem I and other parts occur in Photosystem II)
The light reactions occur in and around the thylakoids
1) Light is captured by various pigments- the energy is passed to chlorophyll-a
2) electrons in chlorophyll-a become excited (gain energy) and leave the chlorophyll-a molecule.
3) These electrons are passed from molecule to molecule along the Electron Transport Chain
4) Along the way, the energy of these electrons is used to:
a) split water- the oxygen leaves the cell and
b) move the hydrogen part of the water to the inside of the thylakoid. This builds a concentration gradient across the membrane. (lots of H inside, not so much outside)
5) The hydrogen then moves with the gradient and back across the membrane. The energy from the hydrogen as it moves is captured by an enzyme which attaches a third phosphate to ADP to make ATP where the energy is stored. This process is called chemiosmosis.
6) The hydrogen is then attached to NADP to form NADPH. This stores and transfers the hydrogen for the next part of Ps. NADP is a form of the vitamin niacin
Calvin Cycle- Synthetic Phase
This is the part that fixes carbon. It happens in the stroma of the chloroplast.
1) CO2 enters the stroma from the environment
2) It attaches to RuBP (big long name- who cares!!!)
3) The RuBP then breaks into PGA (another big long name)
4) PGA is changed into PGAL using the energy from ATP and the hydrogen stored on NADPH
(the ATP and the NADPH are from the light reactions)
5) Most of the PGAL is used to replace the RuBP but some of it is used to make glucose and other organic molecules.
6) Since PGAL is a three carbon molecule, plants that use this are called C-3 plants
In hot dry climates there are variation of the Calvin Cycle:
C4 plants can operate with lower CO2 levels so they can close the openings into the leaf during the hottest part of the day. They lose less water. These use a 4 carbon molecule instead of PGAL
CAM Pathway plants - the openings into their leaves are open only at night. They bring in CO2 at night and temporarily store it. During the day, the openings are closed and the Calvin Cycle occurs. These plants are slow growers but lose much less water
A summary of both parts of photosynthesis
