Lecture 3 Flashcards
(24 cards)
What is the main part of the light independent reactions?
The Calvin-Benson-Bassham cycle.
Where do the products for the light independent reactions come from?
The light dependent reactions.
What is the main aim of the Calvin cycle?
Assimilate CO2 into organic molecules to produce 3-carbon sugar phosphate molecules. (Adding CO2 to existing sugars).
Where does the Calvin cycle take place and what does it use?
In the stroma using RuBisCO.
What is RuBisCO?
Ribulose 1,5 bisphosphate carboxylase oxygenase. The worlds most abundant protein which adds CO2 to RuBP to (a 5 carbon molecule) to create two 3 carbon molcules (3-phosphoglycerate).
What is the general order of the Calvin Cycle?
- Fixation of CO2 into an organic molecule.
- Reduction of that organic molecule (twice).
- Regeneration or RuBP.
1st stage of Calvin cycle.
RuBP + CO2 -> 2 x 3-phosphoglycerate.
2nd stage of Calvin cycle.
Reduction of 3-phosphoglycerate into 2 sugars:
one to leave and make sugars
one to regenerate RuBP.
3rd stage of Calvin cycle.
RuBP regenerated using ATP, ready to collect another Carbon molecule.
Why does the Calvin cycle require multiple cycles?
To generate and regenerate its components.
What does the Calvin cycle require overall?
3ATP + 2NADPH.
What is the issue with RuBisCO?
Uses oxygen molecule on CO2 to fix CO2 which can lead to photorespiration.
What is photorespiration?
Fixation of Oxygen instead of CO2. Happens 25% of the time and can be toxic to plants.
How is it a wasteful process?
2-phosphoglycerate can be converted to 3-phosphoglyerate (carbon molecule made during fixation of CO2) but it results in a loss of carbon overall.
How did it evolve?
RuBisCO evolved before there was oxygen in the atmosphere so it wasn’t an issue back then.
Describe a light response curve.
Shows rate of photosynthesis increases with light until other factors become limiting. Photosyn. is most efficient at maximum quantum yield (before light is in excess).
What are some adaptations to a sunny environment?
Smaller, thicker leaves with multiple cell layers that light can penetrate. Paler leaves due to less chlorophyll.
More PS2, less LHCII. Less vulnerable to excess light.
What are some adaptations to a shady environment?
Larger, thinner leaves. Darker due to more chlorophyll. Able to photosynthesise in less light.
Less PS2, more LHCII.
How do plants acclimatise to changing light?
Adjust components of the ETC (phenotype plasticity).
What effect does too much light have on plants?
Sunburn as light energy is harvested by the light harvesting complexes but has nowhere to go. Excited chlorophyll molecules are passed on to O2 creating free radicals within the chloroplast - oxidants that grab electrons from other molecules, damaging proteins, lipids and DNA.
What is a short term (seconds-hours) acclimatory response to excess light?
Non-photochemical quenching (photosynthetic proteins dissipate excess energy as heat).
Stimulate the Xantophyll cycle.
What is the Xantophyll cycle?
Xantophyll are pigments in plants.
When under light stress: Violaxanthin -> zeaxanthin.
When under no stress: zeaxanthin -> violaxanthin.
What is a middle term (hours-days) acclimatory response to excess light?
Changing gene & protein expression to increase production of photosynthetic components and increase photosynthetic capacity.
Ability varies between species.
What is a long term (days-weeks) acclimatory response to excess light?
Phenotype plasticity leads to morphological changes.
