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PLANTBIO > Carbon Metabolism > Flashcards

Carbon Metabolism Flashcards

1
Q

Calvin Cycle

A

also known as Photosynthetic Carbon Reduction Cycle (Reductive Pentose Phosphate Cycle)
Products: PGA, NADP+, ADP

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2
Q

14CO2

A 
distinguish between photosynthetic CO2 and 			respired CO2 using 4CO2
rationale:  expose plant for varying times and identify labeled compounds
	short time ->early cpds.
	long time -> late cpds.
paper chromatography
autoradiography
algae - chlorella
apparatus
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3
Q

14CO2 experiment

A

Take a tank of algae, bubble them with CO2, mixture goes to plastic tubing which is illuminated, 14CO2 injected, at the bottom of the tubing, they are dropped in the boiling methanol solution -> autoradiography -> paper chromatography – detect which ones are labeled
—–> labeled compound: PGA (also found in chloroplast)

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4
Q

14CO2 results

A 
after 5 sec
		PGA (phosphoglyceric acid) is labeled:
		       3C compound
		       only its carboxyl carbon labeled
after 30-90 sec
		hexose phosphates labeled 
		PGA --  all of its carbons labeled
		    ->What does this suggest?
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5
Q

14CO2 hypothesis:

A

CO2 + 2C —–light——> PGA
(Assume PGA is the first compound made)
Graph:
1) steady state of 14CO2
2) Turn off the light and see what accumulates
- rxn. above should stop
- What accumulates instead?
>PGA still accumulates (increased)
RUBP disappearing (decreased)

Unexpected result:
CO2 + acceptor doesn’t require light
could RuBP be the acceptor?
rxn doesn’t require light since PGA is accumulating

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6
Q

What is CO2 acceptor? (Old + New hypothesis)

A 
Original hypothesis: two carbon compound?
		CO2  +  2 C   ---light--->     PG
     Experiment: steady state with 14CO2
		turn lights OFF
		PGA accumulates RUBP disappearing
     unexpected result
		CO2  +  acceptor doesn't require light
		could RuBP be the acceptor?
                RuBP is 5 carbons

New Hypothesis:
CO2 + RuBP —–light—-> 2PGA

(RuBP = ribulose -1, 5 - bisphosphate)

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7
Q

Confirming the acceptor (New Hypothesis)

A 
CO2  +  RuBP   -----light---->     2PGA
data: steady state with 14CO2
	  remove CO2; keep lights ON
          RuBP increases
          PGA disappears
 Why? because it is a cycle
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8
Q

Calvin Cycle

A

CO2 + RuBP ——> 2PGA
(1C) (5C) (two 3C = 6C)

3 CO2 + 3 RuBP ————> 6 PGA
(3C) (15 C) (18C)
|
3 RuBP net gain triose-P
(15C) (3C)
used again

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9
Q

Calvin Cycle Process

A

RuBP and CO2 react in a carboxylation reaction and produce PGA
1 atp and 1 nadph per each PGA –> reduction reaction (ATP + NADPH = NADP+ +ADP) then occurs – > glyceraldehyde3phosphate -> remaining carbon is regenerated

cycle occurs in the stroma inside the cholorplast
triose-P exported (PGA and PGAL) via the
(phosphate - phosphate ester transporter)
2 NADPH : 3 ATP : 1 CO2

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10
Q

Calvin Cycle efficiency

A

A net gain of 1 hexose requires:
6CO2 + 12 NADPH + 18 ATP

The process is ~90% energy efficient at converting chemical energy into sugars

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11
Q

Autocatalytic

A

Calvin cycle can be autocatalytic ( if one of the reaction products is also a reactant and therefore a catalyst in the same or a coupled reaction)

Autocatalytic –> 15 Rubp + 15co2 -> 30 PGA -> 30 triose-P -> make 18 RuBP  and react again with 15 CO2 and so on – buildup of RuBP OR sugar and starch output
     Starch:	storage product
		        made in chlpts
     sucrose:	storage product
		        made in cytoplasm from PGA
		         carbon translocation
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12
Q

Activation of enzymes

A

Light activation of enzymes
light –> enzymes active
dark –> enzymes inactive

light is not absorbed by enzymes
regulation is indirect
P. E. T. involved

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13
Q

RuBP Carboxylase / Oxygenase (Rubisco)

A 
abundant protein
Molecular Weight:   500,000
8 large subunits, 8 small subunits
	- large subunit
			chlpts genome
			catalytic activity
	- small subunit
			nuclear genome
			regulatory activity

Rubisco is the common link between the two pathways (calvin cycle and photorespiration)

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14
Q

activation of RuBP

A

activity/activation depends on light

	enzyme  +  CO2		(inactive)
		 |
         enzyme--CO2		(inactive)
		 |
      enzyme--CO2--Mg+2	(ACTIVE)
	     -> maximum activity requires Rubisco activase

readily reversible
depends on [Mg+2] and [CO2]

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15
Q

light dependent activation of Rubisco

A

Proton (H+) accumulation inside the lumen, Mg2+ kicked out to maintain neutrality
—- > Mg2+ activates Rubisco enzyme
—-> Carbon dioxide is needed to activate rubisco
stromal [Mg+2] increases
stromal pH increases to optimum for Rubisco

lumen pH = 5.0
stroma pH = 8.0

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16
Q

Rubisco activase function

A 
1) Carboxylation
RuBP  +  CO2-------> 2 PGA
(5 C)        (1 C)		 (6 C)
     slow:     3 CO2 / sec
     large amount of Rubisco

2) Oxygenation
RuBP + O2 —> PGA + Phosphoglycolate
(5 C) (3 C) (2 C)

O2, CO2 compete for Rubisco active site
[O2] > [CO2] atmospheric concentration
KM (O2) > KM (CO2)
KM: measure of affinity of enzyme for substrate
very slow 1/3 O2 / sec
difficult to measure oxygenase reaction

17
Q

Photorespiration

A

(also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted.

18
Q

Photorespiration (PR)

A

O2 inhibits photosynthesis – 45% inhibition
lower [O2] – plants grow better
photosynthesis increases
PR - light dependent consumption of
O2 and release of CO2
different from mito respiration

oxygenase is first step in PR
		stimulated by 	
		    high [O2]
		    low [CO2]
		    high T : CO2 comes out of water faster than O2 – high O2, low CO2 ratio 
	             high light

O2 is consumed
CO2 is released
energy is consumed

19
Q

eliminating PR

A

major drain on productivity

2% O2 ——> net photosynthesis 45%
greater than in 21% O2

eliminate PR —-> increase plant productivity

20
Q

Functions of Photorespiration

A

1) glycine / serine production (makes amino acids)
2) recycle carbon
oxygenase is unavoidable
not eliminated by evolution
mutants without PR enzymes
die in air
live in high CO2
3) dissipates excess energy under high light and low [CO2]
way to burn off extra energy

21
Q

Modifying/eliminating PR

A 
a.  chemical inhibition:
 	1970's and 1980's
		inhibit enzymes prior to CO2 release
		  - does it work? NO
         1980's
		selectively inhibit oxygenase (first step,   		
                prevents formation of enzymes)
		can it be done? NO

b. genetic modification – genetically modify Rubisco?
Arabidopsis mutants
mutagenize plant
mutagenize mutants

c. in vitro mutagenesis
clone large subunit
mutagenize; transcribe; translate
look for reduced oxygenase

22
Q

Genetic modification on PR

A
  1. Mutagenize plant to knock out activity of a
    photorespiration enzyme
    PR enzyme deficient —> plant dies in air
  2. mutagenize mutants
    look for plants that live/thrive in air
    hope for reduced oxygenase (Rubisco genetically modified)
    survivors are revertants
    requires large subunit mutation (chlpts)
many genomes/chlpts; several chlpts per cell
little chance of multiple mutations
23
Q

Can reduced oxygenase activity be achieved?

Can the active site of Rubisco be modified to exclude O2 but not CO2?

A

No, not yet

24
Q

Photorespiration facts

A

1) under normal conditions in the field, photorespiration is
not responsible for a significant increase in plant productivity (plant productivity is increased when PR is eliminated)
2) in photorespiration, O2 is consumed and CO2 is released
3) photorespiration occurs within the chloroplast, peroxisome, and mitochondria
4) the first products of the RuBP carboxylation reaction are 2 PGA
5) Products of the RuBP oxygenation reaction are 1 PGA and phosphoglycolate
6) Photorespiration is increased under conditions of high O2, low CO2, high Temp, and high light conditions

25
Q

alternative carbon pathways

A

1) C4 pathway

2) CAM pathway (Crassulacean Acid Metabolism)

26
Q

C4 pathway

A
  • mechanism to reduce Photorespiration
    —> 14CO2 experiment with sugarcane
    4 c-cmpds labeled: malate, aspartate, oxaloacetate labeled (not 3c-cmpd PGA) :
----->low PR; high productivity
			<1% of all plants
				sugarcane
				corn
				crabgrass
2 cell types
		1) bundle sheath cells
			contain Calvin cycle
		2) mesophyll cells
			lack Calvin cycle
  • Process serves to concentrate CO2 in calvin cycle
  • Increase CO2 concentration = more carboxylation, less oxygenation, more Calvin cycle, less photorespiration

CO2 + H2O H+ + HCO3-
PEP + HCO3- -> OAA
present in mesophyll cells (all cells)

high [CO2] in bundle sheath
3C and 4C cpds. move by plasmodesmata
PR CO2 recaptured in mesophyll cell
2 CO2 fixing reactions:  1) PEP carboxylase
			                2) Rubisco uses CO2
27
Q

C4 energetics

A

2 extra ATP / CO2
21% O2 —> C4 plants more productive than C3
2% O2 —> C3 plants more productive than C4
Why? When you are at 2% O2, photorespiration will be
greatly reduced
C4 requires energy

Some C4 enzymes are regulated by the light.
Which enzymes would be good to be light regulated? The ones that use energy
There are 3 different types of C4 metabolism

photosynthesis processes of C4 plants are divided between mesophyll and bundle sheath cells. Two steps of C4 photosynthesis that occur in the mesophyll cells are the light-dependent reactions and a preliminary fixation of CO2 into a molecule called malate.

28
Q

Why does the quantum yield of C3 plants change

with temperature?

A

As temperature goes up, the quantum yield for C3 plants decreases? Because high temperature increases Photorespiration
C3 plants favor lower temperatures
C4 – little photorespiration, won’t be affected as much

29
Q

CAM pathway (Crassulacean Acid Metabolism)

A

adapted to arid environments
similar to C4, but
C4 pathway - spatially separated from C3
CAM - temporally separated from C3
separate based on time
stomata open during night, closed during day
—> a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2).

stomata open during night, closed during day

  • –> more efficient use of CO2 and H2O
  • –> therefore CAM plants have advantage in desert. some plant families use CAM when water is scarce but can transition to C3 when water is abundant.

PEP carboxylase
PEP + HCO3- —> OAA
OAA + NADH —> Malate

malate stored in vacuole at night

malate decarboxylated during the day
CO2 available for Calvin cycle
CO2 doesn’t escape (stomata closed)
high [CO2] reduces PR

regulation of CAM
need separation of carboxylase and
decarboxylase activities

2 forms of PEP carboxylase

	night: insensitive to malate
	day: inhibited by malate

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