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Photosynthesis
Biology H
The chloroplast
Photosynthesis is powered by visible light
Pigments embedded in the thylakoid membrane absorb the light energy
Chlorophylls absorb R, B, V
Carotenoids absorb G, B, V
Membranes are arranged into stacks called grana and are surrounded by fluid called stroma
Act 1: The Light Reactions
Converts light energy to ATP and NADPH
The goal: to use the energy of electrons escaping from chlorophyll to make high energy molecules (ATP
and NADPH) in electron transport chains
Water is split into hydrogen ions, oxygen gas and electrons (electrons returned to chlorophyll)
Occurs in the thylakoid membranes; light energy is absorbed by the pigments in the photosystems
Light energy is absorbed which excites electrons in photosystem (PS) II
The electrons travel through the electron transport chain, to PS I releasing energy which is used to make
ATP from ADP + P
From photosystem I, the electron goes through another ETC to make NADPH from NADP+
Water is split to replace the electrons lost from chlorophyll in PS II
Chemiosmosis
Powers ATP synthesis
Energy from the electrons being passes along the etc is used to pump H+ ions across the thylakoid
membrane creating an imbalance (concentration gradient)
H+ ions diffuse back across the membrane through ATP synthase which releases energy for
phosphorylation of ADP into ATP
End result of the Light Reactions
18 molecules of ATP
12 molecules of NADPH
Oxygen gas
ATP and NADPH go to the next set of reactions, the Calvin Cycle
Oxygen is released to the atmosphere through the stomates
Act 2: The Calvin Cycle
Uses CO2 and energy to make sugar
The goal: using the energy made in the light reactions, combine C, H and O’s to make sugars
It is a cycle—it begins and ends with the same starting compound that is regenerated over and over; it
has to go around twice to make 1 glucose molecule
It occurs in the stroma
Phases of the Calvin cycle
1. Carbon fixation:
5-C molecules (RuBP) are already in the cycle—they are the starting point; carbon dioxide enters the
cycle and are attached to them
An enzyme is used to combine 3 CO2 molecules with 3, 5 carbon molecules to create 6, 3 Carbon
molecules (PGA)
Calvin cycle part 2
2. Sugar formation (reduction): 6 ATP and 6 NADPH are used to convert 6 PGA into 6 PGAL (3 C, also
called G3P) through a series of reactions
1 PGAL is released to the cell; 5 continue on in the cycle
PGAL is the actual product of photosynthesis; can be used as a building block for carbohydrates or other
biological molecules
It takes 2 turns of the cycle to create 2 PGAL which can make 1 glucose molecule
This regenerates ADP, P and NADP+ to be recharged in the light reactions
Calvin cycle part 3
3. Regeneration of RuBP: a series of reactions uses the energy in 3 ATP’s to rearrange the atoms of 5
PGAL to form 3 RuBP which starts the cycle all over again
This regenerates ADP and P to be recharged in the light reactions
Overall, it takes 18 ATP and 12 NADPH to produce 1 glucose molecule