Photosynthesis

Question 1

which is the ‘normal’ photosynthesis

Answer 1

C3
light reaction in chloroplast thylakoid membrane.
Dark in chloroplast stroma - C fixing, used RUBisCO

Question 2

reactions involved in C3 photosynthesis

Answer 2

2H20 + light -> O2 +2H +2e-
2NADP + 2H+ -> 2NADPH
ADP + H+ -> ATP

Question 3

what is rubisco?

Answer 3

enzyme crucial for C fixation in plants.
Ribulose 1, 5 bisphosphate carboxylase/oxygenase
acts as either carboxylase or oxygenase.
rubisco activity underlies all plants productivity

Question 4

which is a wasteful reaction of rubisco

Answer 4

acts as an oxygenase.
produces 3PGA and 2-PG, recycling 2-PG is costly and releases fixed O2.
Requires photorespiration

Question 5

why does rubisco have 2 functions?

Answer 5

possibly evolved when there was little O2 in the atmosphere.
active site doesn’t discriminate well between oxygen and co2.
at low temp - rubisco higher affinity for CO2
High temp - O2 preferred.

Question 6

what is photoinhibition

Answer 6

suppression of photorespiration.

Question 7

describe the process of photorespiration

Answer 7

multiorganellar process.
chloroplast - 2PG is dephosphorylated
Peroxisome - Glycolate oxidised to form glyoxylate, H2O2 produced.
Mit - glyoxylate is transaminated to form glycine.
overall: 2(2-PG) -> 3PGA.

Question 8

why did C4 photosynthesis evolve?

Answer 8

solves the rubisco problem, no need for photorespiration.

Question 9

in what environment do C3 plants have an advantage

Answer 9

in cooler environments,as rubisco prefers CO2.

in hot tropical environments there is more photorespiration.

Question 10

what is an example of convergent evolution at a molecular level?

Answer 10

C4 ph evolved over 60x independently.

eg in maize, sorghum, sugar cane, teff, millet.

Question 11

hoe does C4 differ from C3?

Answer 11

C4 separates Carbon assimilation (mesophyll cell, exposed to atmosphere. CO2 enters and forms bicarbonate, PEP carboxylase carboxylates PEP to OAA) and fixation (in BS cell, OAA is decarboxylated releasing CO2 which is used by rubisco).
3C compound returns to mesophyll cell.

In C3, all of ph occurs in the chloroplast.

Question 12

what is required for C4 to work?

Answer 12

• special anatomy of mesophyll cells next to BS cell, and vascular tissue and specific enzymes.
• C pump moves OAA from meso to BS so OAA accumulates at a high conc.
• requires additional energy

Question 13

why is C4 beneficial in hot environments over cold.

Answer 13

• requires additional energy, advantage at higher temps over C3 as in C3 there will be more photorespiration.
• not limited by photoinhibition at higher temps.

Question 14

why is there massive efforts to make C4 rice?

why could it be possible?

Answer 14

C4 rice project
increase in yield and WUE
possible as has evolved many times in grasses, we just need to reprogram it in a shorter time.
requires special anatomy and altered patterns of gene expression and biochemistry.
required enzymes already exist in rice, and vascular bundles in C3 plants already have the biochemical characteristics of C4.

Question 15

how can ph in rice be studied to support the C4 rice project?

Answer 15

radioactively label bicarbonate and x rays show where it is fixed into biochemicals. mainly fixed around vascular tissue, suggesting a C4 like process.

Question 16

two grass families which show an intermediate of C3 and C4

Answer 16

Flaveria and Cleome
include both C3 and C4 species and intermediates
compared the genes of C3 cleome spinosa and C4 Cleome gynadra. found 600 genes different, and many enzymes, and transcription factors.
perhaps couldchange gene expression to alter C3 to C4.

Question 17

what is a major limiting step in engineering rice to C4?

Answer 17

engineering the Kranz anatomy

more info is needed in vein development in C4.

Question 18

CAM

egs

Answer 18

Crassulacean Acid metabolism

eg Prickly pear - economically imp, agave, cacti?

Question 19

what adaptations do plants in hot dry environments have?

Answer 19

1. phenological - Evade drought, grow and rep after rain, otherwise stay quiescent
2. Morphological - deep roots, tissue storage capacity, tiny/absent leaves, high xylem transport.
3. anatomical - thick cuticle, rolled leaves, stomatal crypts
4. biochemical - stomata open at night, reduce water loss as RH is increased at night.

Question 20

how is CAM different to C3 and C4

Answer 20

temporal separation of the 2 carboxylases.

Rubisco at day, PEPc at night.

Question 21

what occurs in CAM ph in the dy and the night

Answer 21

DAY : stomata closed. OAA which has accumulated overnight in the vacuole is then decarboxylated, forming 3C acid (then forms starch) and CO2. CO2 fixed by rubisco in the calvin benson cycle.
PEPc switched off.

NIGHT : Stomata open. PEPc active. Carbon conc mechanism is active.
CO2 enters, and froms HCO3-.
Starch combines with HCO3- and PEPc. it is carboxylated to form 4C acid (OAA or malate). high concentration.

Question 22

in which plants has CAM evolved?

Answer 22

New world cacti
agave
6% angiosperms - vanilla
pineapple
mainly xerophytes and epipytes.
samphire and other aquatic plants eg Littorella uniflora.

more widespread than C4, and has evolved many times.

Question 23

describe a classical view of4 stages of CAM

Answer 23

phase 1 - night, stom open, PEPc active, CO2 fixed.
phase 2 - dawn, PEPc and rubisco activ. stom open
phase 3 - rubisco active, malate consumed, stom closed
phase 4 - if soil is moist, st open and direct fixation by rubisco of CO2 from atmosphere.

Question 24

however, a more modern view of CAM is

Answer 24

there is a continuum of CAM physiologies. 
eg C3 plants might use CAM at night. 
1. CAM plasicity
2. CAM cycling
3. CAM idling

Question 25

descibe the idea of CAM plasticity

Answer 25

if there is sufficient water, will switch to C3 instead. Facultative CAM.
eg Clusior minor - in experimental removal of water (drought), will switch to CAM.

Question 26

what is CAM cycling?

Answer 26

close stom at night, CO2 uptake in the day and fixes CO2 at night released by respiration into organic acid and can use C the next day.

Question 27

what is cam idling

Answer 27

in extreme drought
tightly closed stomata, no gas exchange.
plants just recycle respired CO2 through CAM pathway.

Question 28

chemical reactions in CAM

Answer 28

nocturnal - CO2 fixation into malate - crucial.
PEP + HCO3- –PEPc–> Pi + OAA
OAA + NADPH —-> NAD+ + malate

Daytime: decarboxylation of stored malate
A) Malate + malic enzyme +NAD(P)+ —> NADPH +CO2 + pyruvate
Pyruvate + pyruvate pi di kinase + (ATP+Pi) —> (AMP+PPi) +PEP.

B) Malate +NAD+ —> NADH + OAA
OAA + ATP –PEPc–> ADP +CO2 + PEP

the CO2 in A and B is fixed by rubisco in the calvin cycle

Question 29

how is CAM regulated?

Answer 29

control of PEPc activity is essential to regulating CAM

• inhibited by malate (malate builds up overnight so in early morning and most of the day PEPc not active)
• if it is phosphorylated, insensitive to malate.

Question 30

study of CAM regulation1

Answer 30

Radiolabelled 32p, incorporated into PEPc.
Dark: more phosphorylated PEPc, insensitive to malate.
increase in PEPcK - PEPc kinase.
PPCK transcript increases at night.
this is a circadian regulated process.
more malate is needed to inhibit PEPc at night.
PEPc levels higher in subjective night and subjective day.

Question 31

why would CAM be engineered in agriculture?

Answer 31

higher WUE, may be easier to engineer than C4.

Question 32

what is needed for CAM

Answer 32

• PEPc to fix C at night
• diurnal carboxylation of malate
• succulence, so can accumulate malate in vacuoles.
• daily patterning of stomatal opening.

Question 33

what further research is needed to install CAM into plants?>

Answer 33

computational biology
CAM germ plasm
CAM biodesign
CAM systems biology