Biol 212- Unit 5: Alternatives To C3 Carbon Fixation:

Front 1

In high heat and light is C3 carbon fixation still efficient?

Back 1

In C3, rubisco fixes CO2 to RuBP (5C) to generate
two PGA molecules. But in high heat and high light, rubisco can fix oxygen instead of carbon dioxide to the RuBP molecule.

Front 2

What happens to the RuBP + O2?

Back 2

forms PGA + 2PG..
- PGA is converted to GAP and used by ox. resp.

- 2PG goes through a salvage pathway that recycles the 2PG (is able to recover 1 carbon but uses lots of ATP)

Front 3

Photorespiration:

Back 3

a process in plant metabolism by which RuBP (a sugar) has oxygen added to it by the enzyme (rubisco), instead of carbon dioxide during normal photosynthesis. This process reduces efficiency of photosynthesis in C3 plants. CO2 is regenerated, hence its name.

Front 4

In photorespiration, 2 x 2-PG becomes what, and what is lost?

Back 4

1 x 3PGA, and one CO2 is lost. therefore 1 of 10 carbons is lost overall.

Front 5

Photorespiration has an effect on plants. Does warm weather or tropical weather affect the efficiency of photosynthesis in plants?

Back 5

yes by about 50 to 90%

Front 6

Why does photorespiration exist?

Back 6

because under high oxygen, oxygen becomes a effective competitive inhibitor of rubisco binding carbon dioxide.

Front 7

Is there any possible ways plants minimize oxygen binding of rubisco?

Back 7

yes, it has to do with leaf anatomy combined with physiology... [evolution and adaptation]

Front 8

C3 leaf anatomy:

Back 8

(outside in)
1. outside: waterproof and nearly gas proof, waxy coating called a cuticle

2. 1st cellular layer: epidermis (physical barrier).
- generally a single layer of closely interlocking cells
- do not allow passage of water or gas between cells.

Front 9

Stomata of C3 leaf:

Back 9

located in lower epidermis.
- tiny holes to interior of leaf
- are flanked by 2 guard cells that can open or close to allow gas or water vapor exchange with atmosphere
- generally found only on the lower epidermis to min. water loss

Front 10

Mesophyll of C3 anatomy:

Back 10

- is sandwhiched between the upper and lower epidermis.

- composed of photosynthetic parenchyma cells

- (upper half) palisade parenchyma which carry out light rxn's.

- (lower half) where C/B cyc. is carried out. generally in spongy arrangement with airspaces that are cont. with stomata... helps to scatter light that reaches through as well as promotes better gas exchange.
C/B mostly occurs in the mesophyll parenchyma

Front 11

Vascular tissue in the mesophyll:

Back 11

- continuous with vasc. tissue of the stem.

- repeatedly subdivides from the central vein into smaller and smaller veins and ramify through out the leaf.

- increased surface area increases gas/nutrient exchange between cells and vessels.

- gives structural support to the leaf

- found within the spongy mesophyll.

Front 12

summary of C3 leaf anatomy:

Back 12

Front 13

When does the problem with rubisco binding O2 instead of CO2 occur?

Back 13

- in hot weather when stomata closes cutting off CO2 supply from outside.

- light reaction still occurs, leading to a build up of O2 in the leaf

- Ox. resp. wont use up all the O2, nor generate enough CO2 to compensate for the high O2 concentration through out the whole day.

- leads to a favor of binding O2 over CO2 with rubisco.

Front 14

How do you increase the CO2 concentration with the stomata are closed or nearly closed?

Back 14

C4 anatomy and metabolism

Front 15

C4 leaf anatomy:

Back 15

Very similar to C3 leaves both have:
-cuticle
-epidermis
-stomata
- palisades
-spongy mesophyll
-veins

Front 16

What's the difference between C4 and C3 leaves:

Back 16

- mesophyll is arranged differently

- there are photosynthetic bundle sheath cells.

Front 17

What are some physiological tricks carried out to concentrate the carbon dioxide in C4?

Back 17

- under high heat cond. the spongy mesophyll is inhibited from carbon fixation by the limitation of rubisco

- veins are directly surrounded by a layer of bundle sheath cells and then a layer of palisades mesophyll

- since the palisades cells are so tightly packed together, the gases will have a harder time diffusing from the palisades to the bundle sheath cells (incl. CO2 and O2)

Front 18

describe the process of C4 fixation:

Back 18

1. CO2 dissolves in water [CO2 + H2O --> HCO3- + H+]

2. palisades cells have an enzyme called PEP carboxylase (HCO3- is the substrate for PEP carboxylase, not CO2). [PEP CO2ase: HCO3- + PEP --> oxaloacetate (C4). ]

3. PEPCO2ase doesn't confuse CO2 with O2 like rubisco does.
(4) PEP CO2 ase also has a very low Km for HCO3- , so it is a very efficient scavenger of HCO3-

Front 19

Cont. of C4 fixation process:

Back 19

5. Palisades cells fix CO2 into OAA
- then C4 plants use NADP dependent malic enzyme to convert OAA into malate (in mesophyll)
- malate difuses into the bundle sheath cells via the plasmodesmata.

5. bundle sheath cells then deCO2late malate --> Pyr + CO2

6. pyr diffuses back to palisades cells to participate in the cycle again.

7. CO2 has been delivered to bundle sheath cells, deep in the leaf away from O2

8. C/B occurs in the bundle sheath cells.

Front 20

What are the effects of C4 fixation:

Back 20

- by malate dumping off CO2 in the bundle sheath cells, has the effect of concentrating CO2 in the bundle sheath.

- [CO2] is now high enough to be fixed by rubisco in the bundle sheath and the C/B cyc. funcitons in teh bundle sheath only.

- once photosynthate is made, it is loaded into the pholem.

Front 21

What are some C4 plants?

Back 21

tropical grasses like:
-maize
-carbgrass
-sugarcane

Front 22

What happens with rubisco does combine RuBP with O2?

Back 22

photorespiration occurs

(1) O2 + RuBP (5C) -------- 2C (PG) + 3PGA

Front 23

What happens to the 2C compound (2-PG)?

Back 23

hrough a complex pathway that involves rxns in the peroxisome and mito,

(2) the 2C compound is converted into PGA.

(3) This path is one way that the 2C compound is used to make the intermediates mentioned before and one of the two C’s in 2PG is salvaged.

(4) It is most likely that this photoresp. salvage pathway is a recent evolutionary addition and therefore is still quite cumbersome, i.e. it has yet to be streamlined by further evolutionary refinement.

Front 24

the salvage pathway of photorespiration:

Back 24

Front 25

CAM:

Back 25

Crassulacean acid metabolism.

- occur in some plants which deal with high heat and lack of water

- only lose 50 to 100g of water by transpirtion through stomata per g of photosynthase made compared to 250 to 500g in C4 and C3.

- CAM plants have a competitive advantage in scarce water environment.

- IS PRIMARILY ABOUT MINIMIZING WATER LOSS

Front 26

What plants have CAM metabolism:

Back 26

- cacti
- succulents like agave
- pinapple

Front 27

How does CAM metabolism min. daytime water loss, while still allowing photosynthesis to cont. ?

Back 27

1. accomplishes the task of fixing C in desert conditions in which stomata are closed.
- uses temporal separation of cellular activities.

In the night (open stomata)
- biochemical reaciton in CAM are similar to C4.
- in the cytosol, PEP carboxylase meidates [HCO3- + PEP --> OAA]
- OAA is reduced by NADPH to malate and then malate is stored in the vacule.

In the day (closed stomata, no water or gas exchange)
- malate is transported to the chloroplast releasing CO2.
- with increase concentration of CO2, trapped inside the leaf C/B cyc. occurs in the bundle sheath cells.

Front 28

comparison of C4 and CAM:

Back 28

C4: spacial separation, min. photorespiraiton

CAM: temporal separation, min. water loss.

Front 29

How is this wholesale change in CAM metabolic activity regulated?

Back 29

1. the regulation of the CAM day/night cycle is a good example of how organism uses enzymes to regulate physiology.

2. Mechanism's that function to regulate various activities like this one are used in all eukaryotic cells and the principles behind them are all the same.

Front 30

How do plants make sure to make malate during the night and turn off C/B cyc. during the day when there is no CO2 available?

Back 30

1. they control it by regulation of the activity of PEP carboxylase.

2. PEP carboxylase is the enzyme that converts PEP to OAA. It is probably an allosteric enzyme and it exists in 2 forms.
- can be unphosphorylated (day time) is inactive and is inhibited by malate [ allosteric inhibitor]
- can be phosphorylated (night form) is active and is not inhibited by malate.

Front 31

What happens to PEPCO2ase during CAM metabolism regulation?

Back 31

1. (in the day) enzyme is not phosphorylated, is inhibited by malate and activates C/B cycle by feeding it C as malate from vacuole.

2. (in night): enzyme is phosphorylated, is not inhibited by malate and stores C as malate in vacuole.

Front 32

Is the Km of rubisco for CO2 lower (favors binding of CO2) than for O2, or visa-versa?

Back 32

a. yes, and it functions well under temperate conditions

b. but the difference in Km between CO2 and O2 is not enough to compensate for the high [O2] in high heat and high light conditions

c. the Km changes for the worse (i.e. increases) under high heat and light conditions

Front 33

Maybe the concentration of CO2 is so high in the atm that it effectively out competes O2 for the rubisco.

Back 33

No

Front 34

Gases are dissolved in H2O in the stomata before they are abs. So maybe the different solubilities of CO2 and O2 in H2O will tip the scales in favor of rubisco binding CO2 ?

Back 34

O2 is still much more abundant when the gases are dissolved in H2O, and it the problem gets worse

Front 35

What about when the environmental conditions are so hot and dry that the stomata close to minimize water loss and O2 is then excluded from the chlpts?

Back 35

When the stomata close:
a. CO2 is evolved by ox resp but gets used up more quickly than it is evolved. Therefore CO2 is in short supply.
b. Also O2 is evolved by the light rxns so that the effective [O2] climbs even further.

Front 36

C4 C reduction is an example of...

Back 36

C4 C reduction is a perfect example of how evolution has solved a problem by using the available physiological tools (i.e. available enzymes) and adapting them to a new purpose and also altering anatomy to better make use of those enz. There are 3 different C4 paths (i.e. therefore it evolved 3 separate times)

Front 37

Why haven’t plts evolved a better Rubisco that doesn’t confuse CO2 and O2?

Back 37

Unknown, it might be a relic.
However, this is not the favored explanation as there has been much time to change the enz. through further evolution.
(Unless there is no way for the enz. to retain the ability to bind CO2 without the ability to bind O2)


More likely the 2C compound (2PG) is an intermediate for the biosynthesis of something that can not be synthesized any other way.
(Evidence: plts that have had the photoresp. salvage mech. stopped through genetic engineering are not healthy)

Front 38

Possible evolutionary relic

Back 38

Rubisco. It evolved back in the days when the [O2] in the atm was much lower and therefore not competing with CO2 for rubisco.

Front 39

How can C3 plts can be completely inhibited from fixing CO2 to rubisco?

Back 39

By inhibition of rubisco.
Note C3 plts can be completely inhibited from fixing CO2 by inhibition of rubisco
(1) Under these high light conditions, photoresp. may actually be helpful in C3 plts when the stomata are closed.

(2) In these conditions with the light rxn running hard and the dark rxn inhibited, ATP and NADPH will be accumulating and these are reactive compounds.

(3) Photoresp. uses up ATP and NADPH and thus is a sink for these compounds in these specific conditions and gets rid of the excess.

Front 40

Why hasn't photorespiration been eliminated by evolution?

Back 40

(1) maybe Rubisco cant bind CO2 without sometimes binding O2

(2) maybe photoresp. makes biochemicals that can not be made any other way (evidence: plts altered to eliminate the salvage pathway are “sick”)

(3) Photoresp. uses up ATP and NADPH and thus is a sink for these compounds in these specific conditions and gets rid of the excess.

(4) Photoresp. may be involved in N2 fixation (recent work)

Front 41

C4 reduction of daytime water loss.

Back 41

In C4 metabolism, a high [O2] environment exists inside the leaf.

b) C4, by shuttling C (as OAA) into BS cells buried deep in the leaf, then deCO2ing the OAA, concentrates CO2 deep in the leaf, away from the region of high O2 tension.

c) Then using rubisco to fix CO2 in this compartment, spatial separation of O2 and rubisco allows C fixation without photoresp. (a good example of compartmentalization of cellular activities).