Chapter 8: Carbon Reactions Flashcards
how many tons of carbon are converted to biomass via photosynthesis every year?
200 billion
- photosynthesis
- ocean uptake - photosynthetic organisms in the ocean
the carbon reactions are coupled to the light reactions
- light reactions generate NADPH and ATP for carbon reaction to convert CO2 to sugar
- in the stroma of chloroplast via calvin cycle
what are the three components of the calvin cycle?
- carboxylation
- reduction
- regeneration
how can the calvin cycle be regulated?
- compartmentalization
- regulation of key steps
- modulation by feedback inhibition
compartmentalization
- separation either physical or in terms of reactants
- reactants for three steps of calvin cycle are different
regulation of key steps
control whole reactions by coupling equilibrium
reactions with irreversible reactions, and activating/deactivating steps near the beginning and
end of the pathway
- Rubisco is activated by specific enzyme (rubisco reductase) which itself requires ATP for
activation
- Rubisco is activated by light- causes stromal pH and high Mg2+ (Mg2+ is released from
the lumen to the stroma to balance influx of protons; both activate Rubisco
modulation of feedback inhibition
driving forces reduced by increasing concentration of products
- Rubisco is deactivated by high sugar concentration
photorespiration
- C2 oxidative photosynthetic cycle
- occurs because rubisco is both a carboxylase and an oxygenase
- arose because chloroplast evolution happened in a time when O2 concentration in the atmosphere was low and CO2concentration was much higher
starch
- storage
- polymer of glucose - crystal that is a single molecule
- can get bigger and bigger without becoming more than 1 molecule
- cannot be transported because it is highly immobile and water soluble
- stored in chloroplast of mesophyll cells
sucrose
- transport
- small and very mobile
- synthesis is in the cytosol
- super high energy but inert - does not react
- during the day starch is formed in the chloroplast but at night it is broken down into sucrose and is transported to the rest of the plant where needed through the phloem
- amount of sucrose is highly regulated because it can change water potential
- disaccharide comprised of fructose and glucose
what are the four things that you need to know about the carbon reactions?
- all carbon reactions use it - reductive pentose phosphate cycle
- conceptual design of the metabolic pathway and how it is regulated
- photorespiration
- starch and sucrose synthesis
carboxylation
- CO2 and H2O react with one molecule of ribulose 1,5-biphospahte to create 3-phosphoglycerate (3PGA)
- this reaction is catalyzed by rubisco
- each ribulose 1, 5 biphosphate (RUBP) generates 6 unstable carbohydrate intermediates that break down into 3 PGA (3-phosphoglycerate) molecules
- 5 carbons in each RUBP needs 15 carbons =+ 3 CO2 = 18 carbons which generate 6 three carbon sugars
rubisco
- most abundant protein on earth - 50-60%
- more active at higher pHs
- can act as a carboxylase or an oxygenase
reduction
- two reaction reduce the 3PGA from carboxylation
- ATP phosphorylates 3PGA to make 1,3-biphosphoglycerate, which is catalyzed by and 3-phosphoglycerate kinase
- next NADPH reduces 1,3-biphosphoglycerate to glyceraldehyde-3-phosphate (G3P), catalyzed by NADP-glyceraldehyde-3-phosphate dehydrogenase
regeneration
- at the end of the reaction, 3 molecules of ribulose 1,5-biphosphate are generate from 5 molecules of G3P.
- the sixth molecule is used in starch/sucrose synthesis
how is the calvin cycle regulated?
- enzymes are sensitive to light in the carbon reactions and turn on and of when light is or is not available
- NADPH and ATP are light driven as well
induction period
- in the morning when light appears, it takes a moment for products to build up and processes to start
regulation of rubisco
- CO2 functions as an activator and as a substrate in carboxylation
- rubsico activase requires ATP from light reactions to turn on rubisco which explains why it is sensitive to light
- rubisco activase also requires CO2
- ferredoxin-thioredoxin system
ferredoxin-thioredoxin system
- ferredoxin-thioredoxin reductase regulates rubisco and other reactions for the synthesis of NADPH
- regulates through electron cycling in the cytochrome
- regulates the generation of enzemyes 3, 6, 9, and 12 (NADP-glyceraldehyde-3-phosphate dehydrogenase, fructose 1,6-biphosphatase, sedoheptulose 1,7-biphosphatase, and phosphoribulokinase)
- to work, light transfers electrons from water to reduce ferredoxin, ferredoxin reductase reduces the disulphate bond in thioredoxin which cleaves disulphate bonds to activate those enzymes
carboxylation in photorespiration
- rubisco binds to RUBP which launches a proton from the acceptor and forms an unstable intermediate
- CO2 and O2 both attack this intermediate
- if O2 attacks first, a 5 carbon intermediate is formed
- hydration fo the 5 carbon intermediate for 2-phosphoglycerate and 3-phosphoglycerate. 3 PGA is used in photosynthesis, the other is not
- if CO2 attacks first - a 6 carbon intermediate forms
- the six carbon is hydrated and gets cleaved to form 2 3 PGA molecules and normal calvin cycle continues
why do plants photorespirate?
- no net gain or loss of carbon
- get rid of toxic peroxides
what organelles are involved with photorespiration?
- chloroplast: 2 carbon intermediate produced the 2 PGA
- peroxisome: used to processes toxic peroxides
- mitochondrion: linked to electron transport chain
what are the 5 pivotal enzymes of the light-dark transition?
- Rubisco
- Fructose 1,6-biphosphate
- Sedoheptulose 1,7-biphosphate
- Phosphoribulokinase
- NADP-glyceraldehyde-3-phosphate dehydrogenase
starch and sucrose usage
- during the day, sucrose is accumulated, which drives the synthesis of starch
- at night, starch is broken down into sugars in the chloroplasts to be exported out
- activated and deactivated separately - starch activator is ATP sucrose activator is UTP
