Lecture 3: CO2 acquisition by plants and photorespiration

Question 1

who were Calvin + Benson?

Answer 1

they illuminated micro algae (Chlorella, Scenedesmus) with 14CO2 for short periods and allayed the first products of CO2 fixation by paper chromatography and autoradiography

Question 2

rubisco =

Answer 2

Ribulose
1,5‐bisphosphate
carboxylase‐oxygenase

Question 3

what is the worlds most abundant protein and what is it the first step of

Answer 3

RUBISCO

first step of CO2 assimilation

Question 4

The Benson-Calvin Cycle =

Answer 4

–> 3RuBP (3X5C)
–> [carboxylation by rubisco]
–> + 3CO2
–> 6 glycerine 3-P (6X3C)
–> + 6ATP + 6NADPH
–> [Reduction]
–> 6 triode-P (6X3C)
=1 x 3C product
–> [regeneration] (5X3C)
–> + 2ATP = REPEAT

Question 5

Triode phosphate is used to make

Answer 5

carbohydrates
- left over triose‐P is either used to make starch inside the chloroplast or is exported from the chloroplast via the phosphate translocator , to make sucrose

Question 6

Atmospheric CO2 over the last 3500 years

Answer 6

dropped from 50,000 ppm to today, v low.

across rubisco time, O2-evolving photosynthesis + land plants

Question 7

Atmospheric O2 over the last 3500 years

Answer 7

Increased over time slowly

across rubisco time, O2-evolving photosynthesis + land plants

Question 8

RuBP + CO2 + H2O (5C+C) —CARBOXYLASE–>

Answer 8

2 ( glycerine 3-P)

2X3C

Question 9

RuBP + O2 (5C) —OXYGENASE–>

Answer 9

glycerine 3-P + glycol late 2-P

3C + 2C

Question 10

peroxisome

Answer 10

a small organelle present in the cytoplasm of many cells, containing the reducing enzyme catalase and usually some oxidases

Question 11

photorespiratory pathway equation =

Answer 11

2 X 2C –> 1 X 3C + CO2

Question 12

photorespiratory pathway

Answer 12

In chloroplast: 
Rubisco -->
2 glycollate-2-P -->
In cytoplasm (peroxisome): 
2 glycollate -->
2 glycine (2C) --> 
In mitochondrion:
[glycin carboxylase]
- loss of CO2, NH3, NADH -->
In cytoplasm (peroxisome): Serine (3C) -->
In chloroplast glycerate-3-P

Question 13

photorespiration explanation =

Answer 13

a respiratory process in many higher plant where they take up oxygen in the light and release CO2, contrary to the general process of photosynthesis

Question 14

photorespiration definition =

Answer 14

the light‐ dependent loss of CO2 from leaves

Question 15

why does photorespiration occur?

Answer 15

• Glycollate cannot be further metabolised by the Benson-Calvin cycle
• Glycollate metabolism results in the loss of CO2 & NH3 and the return of only 3/4 of the carbon originally in glycol late to the chloroplast
• photorespiration salvages glycollate

Question 16

increasing CO2 ____ photorespiration

Answer 16

decreases

Question 17

increasing temperature ___ photorespiration

Answer 17

increases

Question 18

photorespiration is a major drain on a plants carbon how can this be improved?

Answer 18

• improve Rubisco
• • evolution
• • genetic engineering
• Evolved CO2-concentrating mechanisms
• -C4 photosynthesis
• -Active uptake of inorganic carbon by aquatic organisms (algae, cyanobacteria)
• Improve CO2 fixation by genetic engineering

Question 19

In C4 plants what is the first product of CO2 fixation

Answer 19

a C4 acid compared to glycerine 3-P (3C) in C3 plants

Question 20

what is carbon fixation

Answer 20

the conversion process of inorganic carbon (carbon dioxide) to organic compounds by living organisms e.g. photosynthesis

Question 21

example of types of C4 plants

Answer 21

many are tropical grasses, including the some of world’s most important crop species (maize, sugar cane, millet)

Question 22

% of C4 plants in flowering plant species and grasses

Answer 22

about 3% of total flowering plant species, about 50% of all grasses are C4

Question 23

majority of grasses in savannas (about 15% of Earth’s vegetated surface) are C3/C4

Answer 23

C4

Question 24

C4 plants are responsible for %% of global photosynthesis

Answer 24

about 20%

Question 25

C4 plants are responsible for __% of the worlds grain harvest

Answer 25

40%

Question 26

4 examples of C4 environments

Answer 26

• savanna grasslands
• tropical wetlands
• salt marshes
• hot deserts and semi-deserts

Question 27

typical C3 crop plants

Answer 27

• potatoes
• rice
• wheat

Question 28

typical C4 crop plants

Answer 28

• sugar cane

- maize

Question 29

C4 anatomy in a maize leaf

Answer 29

• wreath (Kranz) of bundle-sheath cells
• mesophyll cells surrounding
• CO2 is concentrated in the bundle-sheath & photorespiration suppressed
• PEP carboxylase & Rubisco are compartmented between the mesophyll + bundle-sheath

Question 30

C4 photosynthesis

Answer 30

• CO2 enters from atmosphere via stomata
MESOPHYLL:
• HCO3- is fixed by PEP carboxylase into C4 acids (malate and aspartate)
• C4 acids are transported to bundle-sheath
BUNDLE-SHEATH
-C4 acids are decarboxylated
-CO2 is fixed by Rubisco & the Benson-Calvin cycle
• C3 portion returns to mesophyll