Photosynthesis: the carbon reactions aka dark reactions Flashcards
(making sugars)
Introduction to the Calvin – Benson Cycle
see: https://www.youtube.com/watch?v=c2ZTumtpHrs
- CO2 to carbohydrates to fuel light reactions and growth
- Reduction reaction
- 3 repeats of the 3 stage cycle to form 1 glucose molecule
see diagrams in notes
The cycle has 3 stage:
-Carboxylation – carbon dioxide fixation captures CO2 from the atmosphere
^ 3RBP (5 carbon) + 3CO2 (1 carbon) -> 6 x (3 carbon) Phosphoglycerate
- Reduction – adds electrons and energy to CO2 molecules using ATP and NADPH
^ 6 Phosphoglycerate -> 6 triose phosphate (3 carbon) one leaves the cycle leaving 5
Regeneration – ATP used to combine 2 G3P to form RBP to start the cycle again
^ Rearrange from 5x3 carbon to 3x5 carbon
The cycle must be repeated 3 times to form one glucose
Step 1: Carboxylation
RuBisCo enzyme required for this reaction
(RuBisCo is the single most common protein in existence)
The Co in RuBisCo is due to its action as a carboxylase and an oxygenase
Step 2: Reduction
ATP broken down to ADP converts 3 6carbon molecules to 6 3 carbon mols
Reduction stage converts to G3P
(see life textbook)
Step 3: Regeneration
regeneration:
6 X 3C —y 1 X 3C (fixed) + 3 X 5C
^1 molecule of Triose phosphate is fixed for every 3 C02 entering the cycle, then the remaining
5 x 3C glyceraldehyde 3-phosphate
is converted to
3 x 5C molecules of ribulose 1,5-bisphosphate
It is a multi-stage process - last stage uses up 3 more ATP
^ 9 ATP and 6 NADPH required for every triose phosphate produced.
- One released as G3P for glucose formation and 5 reused to form RUBP
- So 6 cycles required for one glucose formed – usually starts as sucrose and becomes starch
- Triose = 3 sugar so for example G3P is a triose phosphate
Induction and regulation of the Calvin-Benson cycle
During light-stimulated induction period, biochemical intermediates are built up and enzymes activated. Regulated both by:
a) modification of enzyme levels – production proportional to requirements
– slow process for long-term adaptation
b) their catalytic capacity (changes to structure, including reversible formation of supra-molecular complexes)
-a much quicker process via oxidative/reductive reactions to adjust complex form
Fast acting: Activity of five key enzymes is changed within minutes of transition to light – allows fine tuning to environment.
Regulation of RuBisCo
RuBisCo activity is affected by CO2 availability – in low levels it remains inactive
- It can be switched on and off - modulated by CO2 conc.
- Carbamate has a neg charge due to Mg ion activity
- Light on a chloroplast increases pH and Mg ions which activate enzymes
- when sugar phosphates are bound RuBisCo is prevented from being activated meaning there is already enough sugar – a negative feedback loop to prevent excess sugar production.
- RuBisCo activase enzyme removes sugar phosphate groups to activate RuBisCo
Regulation of other enzymes:
When many electrons are travelling through the chain this activates other enzymes
Electrons are provided by the light reaction
see notes for diagram of ferredoxin-thioredoxins system
Photorespiration: RuBisCo
Results in loss of carbon from the cycle
This can reduce photosynthesis efficiency by 25%
Enzyme is very sensitive to level of CO2 and O2 in the surrounding environment
Affinity for CO2 is 10x higher than for O2
High O2 and high temperature can result in reduced CO2 affinity
High temp results in transpiration – opening of stomata but this risks water loss so in very high temperatures stomata are closed resulting in O2 buildup and CO2 depletion
Resulting in oxygenase behaviour in RuBisCo - using ATP and NADPH without forming glucose
Oxygenase behaviour clears reactive oxygen species buildup when electron transport chains are limited
Phosphoglycolate is converted on to hydrogen peroxide a stress signalling molecule
Photorespiration: Engineering for improved yield:
Phosphoglycolate is converted back to a 3 carbon molecule but this consumes a lot of energy and NADPH
Glycolate shunt is not a natural plant process but can be introduced by engineering plants which as a result will grow faster in unfavourable conditions
Some plants are naturally adapted to avoid photorespiration by C4 or CAM
C4 photosynthesis: effect of CO2
C4 plants are able to grow and photosynthesise even at low CO2 but are less efficient at high CO2 levels (see bottom left graph)
see:
www.steve.gb.com/science/photorespiration.html
http://www.sugarcanetech.org
http://www.agricorner.com/
C4 photosynthesis
PET carboxylase enzyme used instead of RuBisCo functions at higher O2 levels
Able to function even when stomata are closed and CO2 levels are low
CO2+ PEP -> Malate (C4) -> CO2 (to calvin benson) + pyruvate (C3) back to form more malate
Bundle sheath cells occur aroud vascular bundle tissue where phloem can export the products – efficient
Pyruvate -> PEP requires an ATP molecule
Why don’t all plants use C4 photosynthesis?
It requires an extra ATP – so only suitable in high light levels
C3 and C4
(see diagrams of C3 and C4 leaves)
C4 is believed to have evolved multiple times in different plant families and single cell organisms
One example of a C4 photosynthesising plant is Borszczowia aralocaspica a halophyte that grows in extremely dry conditions. Two sets of chloroplasts – interior ones fix in C3 way and outer in C4 way
Advantages of C4 photosynthesis in heat and drought:
High affinity of PEPCase for HCO3- in mesophyll cells means enzyme will be saturated with CO2, even when CO2 levels are low
Carboxylation by PEPCase faces no competition from O2 so stomatal apertures can be reduced without altered gas equilibrium affecting it (so stomata can be kept closed)
High concentration of CO2 in bundle sheath cells reduces C2 oxidative photosynthetic cycle here too.
Effects of living in a high CO2 world
- C3 crops do well – as external CO2 is high
- Rainforest plants suffer
- coccolithophores suffer – as sea water acidifies these calcium coated organisms will be damaged and eventually may become extinct
Crassulacean acid metabolism (CAM)
- Capture CO2 at night and close stomata during the day
- Present in most plants occurring in hot and dry conditions
- Far more efficient in preventing water loss
- CO2 stored in cell vacuoles as malic acid
During the day malic acid is transferred to the chloroplasts as malate sugar (4C) converted by PEP carboxylase to pyruvate (3C) + CO2 (1C) (By NAD-malic enzyme) for use in the Calvin cycle (see diagram in notes)
e.g. The desert plant Opuntia stricta (prickly pear)
e.g. The English Stonecrop – which grow on exposed rock so despite the rainy climate must limit water loss
e.g. Ice plant a halophyte that can use C3 photosynthesis and in low water conditions C4 photosynthesis is induced
