Question

!!! Three Medal Question !!!

- Summarize what happens during the light reactions of photosynthesis.

- What happens during the carbon fixation step of the Calvin cycle?

- During which stage of photosynthesis is glucose made?

Answer 1

Hi Tanya,
Photosynthesis is the awesome process by which plants MAKE sugar using light! Humans can't do this; we have to eat plants to get sugar. Plants ultimately use carbon dioxide and water and light energy to make glucose. That's what the light reactions are overall.

"Carbon fixation" just means converting a carbon molecule we can't use for energy (in this case carbon dioxide or CO2) to a carbon molecule we CAN use (in this case sugar). This happens during the first phase of the Calvin Cycle because that's the phase in which CO2 is converted to something more useful to the plant. 1 CO2 molecule is added to a sugar called RuBP (ribulose bisphosphate) which already has 5 carbons and the resulting molecule is a 6-carbon sugar called 3-phosphoglycerate. In other words, we started with CO2, we added it to RuBP, and we ended up with a sugar! That means we did carbon fixation because we changed the CO2 molecule into something more usable.

To discuss in which stage of photosynthesis glucose is made, we first have to understand what phases of glucose exist. There are 3 phases: 1 is Carbon Fixation which we addressed above, 2 is Reduction where energy carriers like ATP and NADPH2 help convert the 6-carbon precursor sugars to glucose, and 3 is Regeneration where more ATP is used to convert the leftover, non-glucose sugar, back to RuBP which we can use again in Phase 2.

Hope that helps!

Answer 2

yes ! That was really helpful ;) Thank You @ambika.rustagi

Answer 3

I also wanted to know -
What are the stages of photosynthesis? Which stage occurs first?

Answer 4

The key to remembering the stages of photosynthesis is to remember the overall equation for photosynthesis: CO2 + H2O + Light --> Glucose + Oxygen. So we know that when we're done with all the stages of photosynthesis, we have to use up light, water, and carbon dioxide and we have to end up with glucose and oxygen.

The first stage is to make energy carriers using the energy from light.
The energy from sunlight excites the electrons in the water. They go down the electron transport chain to NADP+. Once NADP+ gets an electron, it turns into NADPH. The hydrogen in H20 is used to help convert ADP to ATP. So now we have NADPH and ATP and we've used up the electrons and hydrogen in water, so we're left with oxygen! That means we've used up one of our reactants (H2O) and we've made one of our products (Oxygen)!
This stage is called the Light Dependent Reaction because it needs light.

The second stage is to use the energy carriers and the carbon dioxide to make sugar.
We talked about how carbon dioxide gets changed into glucose in your first question. To change carbon dioxide into glucose, the plant needs energy carriers to help the Calvin Cycle along. These energy carriers are the NADPH and ATP we created in the first stage of photosynthesis! So in this stage, we use up the carbon dioxide, NADPH, and ATP and we end up with our other product, glucose!
This stage is called the Light Independent Reaction because it doesn't require light at all - in fact the plant usually does this step during the evening.

Answer 5

in my first question I wanted to know "what happens during the light reactions of photosynthesis"?

and in the second "What happens during the carbon fixation step of the Calvin cycle"?

you did explain me but didnt tell me "what happens"

@ambika.rustagi