Lecture 14: Carbon Fixation: The C3 and C4 Pathway

Question 1

C3 Pathway

Answer 1

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

Question 2

How many turns of Calvin cycle to get GAP?

Answer 2

three turns

need 3 CO2s

Question 3

What regulates Calvin Cycle enzymes

Answer 3

Light

Question 4

ways light regulates Calvin cycle enzymes

Answer 4

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

Question 5

Photorespiration

Answer 5

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

Question 6

Whats the product of carbon fixation?

Answer 6

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

Question 7

What 2 pathways use GAP as a metabolic intermediate

Answer 7

glycolysis and gluconeogenesis

Question 8

What does the Calvin Cycle generate?

Answer 8

triose phosphates (GAP!)

Question 9

types of hexose sugars we can make using GAP

Answer 9

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)

Question 10

What we get out of the light dependent reactions of photosynth

Answer 10

ADP and NADP+

Question 11

Calvin Cycle Stage 1

Answer 11

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

Question 12

Calvin Cycle Stage 2: Reduction Stage

Answer 12

reduce 6 3-PGA to make 6 GAP

one GAP used by gluconeogensis to make sugar

Question 13

How many carbon to make 1 GAP?

Answer 13

3

Question 14

Whats the source of carbon in the Calvin Cycle

Answer 14

CO2

Question 15

Calvin cycle Stage 3

Answer 15

5 GAPS used to replinish 3 RuBP

Question 16

“Dark Reactions”

Answer 16

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)

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

once CO2 added at a time

Question 19

Rubisco

Answer 19

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

Question 20

WHY is there so much rubisco?

Answer 20

because it is really freaking slow

Question 21

Stage 2: Reduction of 3phosphoglyceradte to form glyceraldehyde

Answer 21

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)

Question 22

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

Answer 22

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

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

Question 23

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

Answer 23

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

Question 24

Stage 3

Answer 24

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

Question 25

WHY carbon shuffle reactions?

Answer 25

so we don't lose and waste any carbon | SEE SLIDE 11!!!

Question 26

Transketolase enzymes

Answer 26

transfer 2 carbon fragments

Question 27

Transaldolase enzymes

Answer 27

transfer 3 carbon fragments

Question 28

Carbon Shuffle Reactions

Answer 28

we don't want to lose any carbons we need to create 5 carbon molecules (RuBP) see slide 12!

Question 29

Common irreversible reactions in carbon shuffle rxns

Answer 29

when inorganic phosphate is released | when ATP is used

Question 30

Light Regulation of the Calvin Cycle reactions

Answer 30

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

Question 31

CALVIN CYCLE ONLY ACTIVE IN LIGHT

Answer 31

when we have NADPH and ATP

Question 32

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

Answer 32

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

Question 33

Two light regulations of Calvin Cycle: pH and MG2+

Answer 33

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

Question 34

Two light regulations of Calvin Cycle: Thioredoxin

Answer 34

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

Question 35

Wasteful Photorespiration: Rubsico and Oxygenase

Answer 35

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

Question 36

More on Photorespiration

Answer 36

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

Question 37

So why is rubisco so dang slow?

Answer 37

there were lower O2 concs in the atmosphere long ago | slow reaction may favor CO2 over oxygen

Question 38

Evolutions fix for Photorespiration

Answer 38

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

Question 39

fix for Photorespiration

Answer 39

Hatch-Slack patwhay: | put CO2 on PEP (a 3 carbon molec) to make store on OAA (a 4 carbon molec)

Question 40

OAA is a

Answer 40

transiet CO2 carrier molec | 4 carbon molec

Question 41

Two ways of dealing with Photorespiration

Answer 41

C4 Pathway in tropical plants (SEPARATE CELL TYPES) | CAM Pathway in succulents (SEPARATE TIMES)

Question 42

C4 Pathway

Answer 42

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

Question 43

Mesophyll Cells

Answer 43

OAA enters, converts to malate | malate goes to bundle sheath cell

Question 44

Bundle Sheath Cells

Answer 44

Calvin cycle enzymes are only in these cells | malate comes in and is decarboxylated and the CO2 can be used by Calvin Cycle.

Question 45

C4 Pathway and separation in Space

Answer 45

eliminates oxygenase reaction in rubisco, thus blocks photorespiration REVIEW SLIDE 20

Question 46

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

Answer 46

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

Question 47

C4 high temp advantage

Answer 47

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

Question 48

CAM pathway

Answer 48

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