Counting energy, electrons and carbon (free preview) | StemPy Lab
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Counting energy, electrons and carbon
Every molecule of glucose in the Calvin cycle needs 6 carbon dioxide, 18 ATP and 12 NADPH. The 12 NADPH need 24 electrons, which come from 12 split water molecules, and at the least 48 photons. A C4 plant spends 2 more ATP for each carbon dioxide. Cyclic flow makes up the ATP that the non-cyclic path alone cannot give.
This is the first lesson of Photosynthesis in Depth: Counting, Comparing and Reading Graphs, free for everyone to read.
One ledger for the whole chapter
You have met the light reactions and the Calvin cycle as two separate parts. In an exam, a question often joins them. It starts with the sugar and asks how many water molecules, electrons or photons lie behind it. The way to do this is to keep a ledger, and to work backwards from the sugar.
From one glucose back to the light
Start with one glucose.
Carbon. Glucose has 6 carbon atoms, so the Calvin cycle fixes 6 carbon dioxide molecules in 6 turns.
ATP. Each turn spends 3 ATP, so 6×3=18 ATP.
NADPH. Each turn spends 2 NADPH, so 6×2=12 NADPH.
Electrons. Each NADPH takes 2 electrons, so 12×2=24 electrons.
Water. Each split water gives 2 electrons, so 24÷2=12 water molecules. This agrees with the 12 water molecules on the left of the overall equation.
Oxygen. Two water give one oxygen, so 6 oxygen molecules. This agrees with the right side of the equation too.
Photons. Each electron is lifted at PS II and at PS I, so it takes at least 2 photons. For 24 electrons that is 48 photons.
The ledger closes. The carbon, the oxygen and the water all agree with the equation 6CO2+12H2O→C6H12O6+6H2O+6O2. The 48 photons is a minimum. A real leaf loses some light.
Where the ATP comes from
The Calvin cycle needs ATP and NADPH in the ratio 3:2, which is 1.5 ATP for each NADPH. The non-cyclic path gives ATP and NADPH together, but the ATP it makes is not quite enough for this ratio. The shortfall is made up by cyclic photophosphorylation, which gives ATP and no NADPH. This is one reason why a leaf needs both paths.
The cost of the C4 pathway
A C4 plant spends two more ATP for each carbon dioxide, to turn pyruvate back into PEP. Many books count it this way. NCERT says that ATP is needed, but it does not give the number. So a C4 plant spends 3+2=5 ATP for each carbon dioxide, and 6×5=30 ATP for each glucose. The NADPH stays at 12.
Facts that other books add
NCERT does not stress these, but you may meet them in a question.
The water-splitting complex needs manganese, calcium and chloride ions.
In maize, the chloroplasts of the bundle sheath cells have few or no grana, so they make little oxygen. The chloroplasts of the mesophyll cells have grana.
You should find that 2 glucose molecules in a C3 plant need 36 ATP, 24 NADPH and 24 split water molecules. Switch to a C4 plant. The ATP rises to 60, and the NADPH and the water stay the same.
Try it in Python
The program keeps the whole ledger for any number of glucose molecules, for both kinds of plant.
You should see that only the ATP differs. For 2 glucose it is 36 in C3 and 60 in C4. For 5 glucose, the photons are 240 at the least.
Practise with new numbers
Common mistakes
Starting from the light and getting lost. Work backwards from the sugar, one line at a time.
Forgetting that each electron is lifted twice. It takes two photons.
Dividing the electrons by one for the water. Each water gives two electrons.
Counting the water of the equation as the net water. The 12 are the water that is split. 6 new water molecules are made later.
Giving a C4 plant the same ATP as a C3 plant. It spends two more for each carbon dioxide.
Remember these
One glucose: 6 CO2, 18 ATP, 12 NADPH, 24 electrons, 12 water split, 6 O2, at least 48 photons.
The ATP and NADPH needed are in the ratio 3:2. Cyclic flow makes up the ATP.
A C4 plant: 5 ATP for each carbon dioxide, 30 for each glucose.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
How many electrons are needed to make the 12 NADPH for one glucose?
How many water molecules must be split to give them?
What is the least number of photons for one glucose?
Why does a leaf need cyclic flow as well as non-cyclic flow?
How many ATP does a C4 plant spend for one glucose?
Liked this lesson? There is more
Photosynthesis in Depth: Counting, Comparing and Reading Graphs is a Standard course. It adds 2 more lessons, practice and tests on top of the free chapter.
12Problems2Python Labs1Question Set1Mock Test
C3, C4 and photorespiration compared: C3 and C4 plants differ in the first stable product, the first enzyme, the cell that holds the Calvin cycle, photorespiration, the temperature optimum and the response to carbon dioxide. A C4 plant spends more ATP, but it wins in hot, bright places. A C3 plant can do as well in cool, shady places. Photorespiration makes no sugar, no ATP and no NADPH.
Limiting-factor graphs and classic experiments: A graph with two curves shows which factor limits where. Two curves that coincide at low light show light limiting there. Curves that part at high light show a second factor, such as carbon dioxide, limiting. Classic experiments test one factor at a time: a half leaf in potassium hydroxide, a variegated leaf, a covered strip and bubbles counted at different lamp distances.