Exer 10 Photorespiration

PHOTORESPIRATION Exercise 10 CHUA, Edbelyn ABENIDO, Shannen VALEZA, John Ruben Group 1 WX-5L

Energy-converting Processes • Photosynthesis light energy -> chemical form (organic compounds)

• Respiration • energy released: ATP

• Photorespiration • does not result to ATP intermediate products: glycine and serine

Photorespiration • light + high temperature • IN: high [O2] • OUT: CO2 • Three organelles: chloroplast peroxisome mitochondrion

Figure 10.1. Reactions occurring in photorespiration (Nelson and Cox, 2008).

Table 10.1. Characteristics of plant groups exhibiting photorespiration.

Feature

C3

C4

CAM

leaf structure

bundle sheath cells with no chloroplasts

bundle sheath cells with chloroplasts

mesophyll cells have large vacuoles

enzyme for fixing atmospheric CO2

Rubisco

PEPCase

PEPCase

optimum T°

15°C – 25°C

30° – 40°C

35°C

Feature

C3

C4

most plants efficient at midrange T° where a majority of plants grow

other characteristics

CAM xerophytes

more efficient in not as efficient as hot, dry C3 or C4 but allows environments plants to survive in arid conditions PEPCase reduces photorespiration because it does not bind O2

Temporal mesophyll cells fix mesophyll cells fix separation of CO2 CO2 and produce CO2, bundle fixation and glucose sheath cells glucose produce glucose production (spatial separation)

Table 10.2. Plants exhibiting photorespiration.

C3 rice tobacco soybean

C4 corn sugarcane

CAM pineapple kataka-taka

OBJECTIVES • compare the response of species exhibiting high and low rates of photorespiration and relate them to changes in carbon dioxide/oxygen levels

METHODOLOGY

Figure 10.2. Photorespiration set-up.

RESULTS Table 10.3. Observation of the rice (C3) and corn (C4) plants in two separate set-ups for 5 days. CONTROL PLANT

PLANT INSIDE BELL JAR

DAY

RICE*

CORN+

RICE

CORN

0

high vigor green leaves

high vigor green leaves

green to yellow green leaves decresing vigor

high vigor green leaves

1

high vigor normal leaves

high vigor normal leaves

light yellow leaves decreased vigor

high vigor green leaves

2

normal vigor green leaves

normal vigor green leaves

decreased vigor thinner stem

high vigor green to yellowish leaf parts

* - C3 plant; + - C4 plant

Table 10.3. Observation of the rice (C3) and corn (C4) plants in two separate set-ups for 5 days. (continued) CONTROL PLANT

PLANT INSIDE BELL JAR

DAY RICE*

CORN+

RICE

CORN

3

healthy plant green leaves

constant vigor green leaves

yellow leaves thin stem

green leaves with yellow tips

4

healthy plant green leaves

normal vigor green leaves

yellow leaves thin stem

wilting of some leaves decreased vigor

5

healthy plant green leaves with some wilted leaves

healthy plant green leaves

yellowing of leaves many leaves wilted thin stem

wilting leaves some parts healthy

* - C3 plant; + - C4 plant

RESULTS

Figure 10.3. Rice and corn control set-ups (left) and rice and corn inside bell jar for photorespiration set-up (right) at day 5.

DISCUSSION • increased [O2]; decreased [CO2] • presence of sunlight  Photosynthesis (PS)  O2 produced

• inc [O2]: higher photorespiration rate in C3  C3 plants do not exhibit CO2 conc mechanism

• high [O2]

 fixation by ribulose 1,2-bisphosphate oxygenase in C3

• no ATP = no Energy for growth and maintenance of plants  wilting

DISCUSSION • C4 plants

 spatial separation of enzymes  useful when [CO2] is low • PEPCase in bundle sheath

 fixation of CO2 • PS still possible and continued production of ATP

 source of energy for plant growth and repair

Answers to Study Questions 1. Will [O2] in the set-up increase or decrease? What effect wwould there be on C3 plants for such change in O2 level?  increase, since plant continues to photosynthesize  induce photorespiration since C3 do not exhibit CO2 concentrating mechanism  rubisco will act as oxygenase  O2 fixated, NO ATP = NO Energy  low source of E for growth and maintenance

Answers to Study Questions 2.

Define CO2 compensation point and light compensation point.

 CO2 comp pt: [CO2] in which fixation is ZERO  Light comp pt: reached evident with net CO2 exchange that is ZERO because of rate of PS or CO2 uptake is balanced with Respiration  Additional, CCP is the pt at w/c the light level where CO2 is released during respiration is offset by CO2 fixation during PS.

Answers to Study Questions 3.

With the decline in [CO2] in the set-up, which species would still exhibit net photosynthesis after one (1) week? Explain your answer.  CORN (C4) – low CO2 comp pt  photosynthetic efficiency  PS is reduced: efficient use of CO2 due to spatial separation of enzymes  bundle sheath cells – CO2 fixation  mesophyll cells – PEPCase