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Biochem 385 Unit 2 > Lecture 14: Carbon Fixation: The C3 and C4 Pathway > Flashcards

Lecture 14: Carbon Fixation: The C3 and C4 Pathway Flashcards

1
Q

C3 Pathway

A

converts CO2 and RuBP to 3phosphoglycerate
first step of Calvin cycle
cataluzed by Rubisco

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2
Q

How many turns of Calvin cycle to get GAP?

A

three turns

need 3 CO2s

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3
Q

What regulates Calvin Cycle enzymes

A

Light

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4
Q

ways light regulates Calvin cycle enzymes

A

1) Inc Rubisco activity in response to elevated pH and Mg2 (during the day) in stroma
2) Thioredoxin-mediated reduction of disulfide bonds

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5
Q

Photorespiration

A

wasteful side reaction of Rubisco
uses O2, makes 2-phosphoglycolate instead of CO2 to make GAP.
C4 and CAM pathways help to minimize its effects by inc concs of CO2

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6
Q

Whats the product of carbon fixation?

A

GAP
its a 3 carbon sugar
we later make it a hexose nrg so chem nrg can be used at night

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7
Q

What 2 pathways use GAP as a metabolic intermediate

A

glycolysis and gluconeogenesis

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8
Q

What does the Calvin Cycle generate?

A

triose phosphates (GAP!)

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9
Q

types of hexose sugars we can make using GAP

A

1) sucrose (transport to other plant tissues)
2) starch (nrg stores in cells)
3) cellulose (cell wall synth)
4) pentose phosphates (5 carbon sugars for metabolic intermediates)

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10
Q

What we get out of the light dependent reactions of photosynth

A

ADP and NADP+

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11
Q

Calvin Cycle Stage 1

A

Rubisco combines 3 RuBPs and 3 CO2s to make 6 3-PGA

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12
Q

Calvin Cycle Stage 2: Reduction Stage

A

reduce 6 3-PGA to make 6 GAP

one GAP used by gluconeogensis to make sugar

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13
Q

How many carbon to make 1 GAP?

A

3

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14
Q

Whats the source of carbon in the Calvin Cycle

A

CO2

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15
Q

Calvin cycle Stage 3

A

5 GAPS used to replinish 3 RuBP

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16
Q

“Dark Reactions”

A

another name for Calvin Cycle

BUT CALVIN CYCLE IS MOST ACTIVE DURING THE DAY (b/c there is lots of NADPH and ATP in the day b/c photosynth makes them)

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17
Q

So what property of Calvin Cycle reactions gave them the name “Dark Reactions”

A

these reactions don’t “need” light directly to run

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18
Q

Stage 1: CO2 fixation to RuBP to make TWO 3-PGA

A

catalyzed by rubisco
4 steps
very favorable
ADOL CLEAVAGE STEP: major contributor to the favorable free nrg change

once CO2 added at a time

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19
Q

Rubisco

A

very SLOW
(only 3 molecs of CO2 fixed per second
most abundant enzyme (in plants)

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20
Q

WHY is there so much rubisco?

A

because it is really freaking slow

21
Q

Stage 2: Reduction of 3phosphoglyceradte to form glyceraldehyde

A

3-PGA converted to GAP
6ATP and 6NADPH needed in stage 2 for EVERY THREE CO2 converted to ONE GAP (2 molecs of 3PGA, 3 turns of Calvin Cyclel; 2x3=6)

lots of nrg used

(for every 3 CO2 fixed by carboylation of 3 RuBP molecs, 3 3phosphoglyceradte are made in stage 1)

22
Q

How many ATP and NADPH for each GAP that leaves the cycle?

A

6ATP
6NADPH
(Stage 2!!! 3 turns of cycle)

(3 more ATP used in stage 3 to regner 3 RuBP molecs)

23
Q

What happens to glyceraldehyde 3-phosphate? DIfferent fates of GAP

A

1) converted to DHAP
2) Combined with DHAP to make starch
3) converted to DHAP for export from chloroplasts
4) cystolic DHAP used for glycolysis or sucrose

can leae or stay in chloroplast, depends on concentrations

see slide 10

24
Q

Stage 3

A

enzyme rxns convert FIVE C3 molecs (GAP or DHAP) into THREE C5 molecs (RuBP) to replinish CO2 supplies

REQUIRES THREE MORE ATP

enzymes: transketose, transaldose

by carbon shuffle rxns

25
Q

WHY carbon shuffle reactions?

A

so we don’t lose and waste any carbon

SEE SLIDE 11!!!

26
Q

Transketolase enzymes

A

transfer 2 carbon fragments

27
Q

Transaldolase enzymes

A

transfer 3 carbon fragments

28
Q

Carbon Shuffle Reactions

A

we don’t want to lose any carbons
we need to create 5 carbon molecules (RuBP)

see slide 12!

29
Q

Common irreversible reactions in carbon shuffle rxns

A

when inorganic phosphate is released

when ATP is used

30
Q

Light Regulation of the Calvin Cycle reactions

A

At NIGHt: plants rely on glycolysis and mitochondrial aerobic respiration to make ATP

31
Q

CALVIN CYCLE ONLY ACTIVE IN LIGHT

A

when we have NADPH and ATP

32
Q

What would happen if glycolysis, pentose phosphate pathway, and the Calvin Cycle were active at the same time?

A

starch degredation and carbohydrate biosynthesis happening at the same time would quickly drain ATP and NADPH pools in stroma, wasting energy

33
Q

Two light regulations of Calvin Cycle: pH and MG2+

A

at night: Calcin cycle has reduced activity b/c flux is very decreased
no pumping, pH in lumen and stroma are both 7

during the day: differnt pH, pH 5 in stroma, 8 in lumen b/c protons pumped into lumen b/c of light activation.
lower pH inside, higher outside
balanced in charge by transport of Mg from lumen into stroma. Thus Rubisco and FBPase acrivities maximal at pH8 and high Mg conc

phopshorylation of lysine reside when the pKa is right due to this process!

slide 14

34
Q

Two light regulations of Calvin Cycle: Thioredoxin

A

Thioredoxin is a redox protein that interconverts disulfide bridges in cystine residues

active in calvin cycle in REDUCED STATE (reduced by photosynthetic ETS)

when no light, spontaneous oxidation occurs, then they’re inactive

slide 15

35
Q

Wasteful Photorespiration: Rubsico and Oxygenase

A

if we use oxygen instead of CO2 (b/c rubisco), we get out 1 molec of 3-phosphoglycerate and 1 of 2-phosphoglycerate, instead of two 3-phosphoglycerate

we’re short one carbon b/c we didn’t use CO2
this is wasteful because we have to use nrg to make another 3-phosphoglycerate

36
Q

More on Photorespiration

A

it is rubisco oxygenastion reaction and glycolate pathway together

O2 consumed and CO2 released

requires NRG input in the form of ATP

see slide 17

37
Q

So why is rubisco so dang slow?

A

there were lower O2 concs in the atmosphere long ago

slow reaction may favor CO2 over oxygen

38
Q

Evolutions fix for Photorespiration

A

High temps: higher O2:CO2 ratio
more O2 dissolved
plants at higher temps have to deal with more photorespiraton. but they have very low levels of photorespiration b/c of some fixes that they have

39
Q

fix for Photorespiration

A

Hatch-Slack patwhay:

put CO2 on PEP (a 3 carbon molec) to make store on OAA (a 4 carbon molec)

40
Q

OAA is a

A

transiet CO2 carrier molec

4 carbon molec

41
Q

Two ways of dealing with Photorespiration

A

C4 Pathway in tropical plants (SEPARATE CELL TYPES)

CAM Pathway in succulents (SEPARATE TIMES)

42
Q

C4 Pathway

A

Mesophil Cells and Bundle Sheath cells
Stomata gaurd cells let O2 and CO2 in
CO2 used to from OAA

43
Q

Mesophyll Cells

A

OAA enters, converts to malate

malate goes to bundle sheath cell

44
Q

Bundle Sheath Cells

A

Calvin cycle enzymes are only in these cells

malate comes in and is decarboxylated and the CO2 can be used by Calvin Cycle.

45
Q

C4 Pathway and separation in Space

A

eliminates oxygenase reaction in rubisco, thus blocks photorespiration

REVIEW SLIDE 20

46
Q

Explain why C4 plants exist if they cost more nrg to run than the C3 pathway?

A

they don’t cost more nrg. they end up costing less nrg b/c they dont have to deal with bad rxns of photorespiration

47
Q

C4 high temp advantage

A

can reduce photorespiration process
don’t have wasteful oxygenase rxn happening
DOES take NRG input to temp store CO2
AT high temps, O2:CO2 increases, we have more dissolved oxygen at high temps

48
Q

CAM pathway

A

separation in time

NIGHT: stomata open when temps cooler, CO2 captured by mesophyll cells and incorporated into OAA by PEP carboxylase. Malate stored in vacule. Calvin cycle inactive

DAY: stomata close, CO2 released, stored malate goes into Calvin cycle, CO2 fixed into carbohydrates/starch

Biochem 385 Unit 2 (13 decks)

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