lecture: variations in photosynthesis part 3

LECTURE: VARIATIONS IN PHOTOSYNTHESIS PART 3 KIRSTIN BROWN

Lecture: VARIATIONS IN PHOTOSYNTHESIS part 3 Outline: 1. CAM plants o Review of photorespiration o CAM photosynthesis

Leaf Structures Where is photosynthesis occurring? o Stomata allow CO2 to enter and O2 to leave ➢ Also leads to water loss through transpiration ➢ Under hot, dry conditions – stomata close

➢ Prevents water loss, but leads to build up of O2

Photorespiration Why is build up of O2 a problem? o Rubisco, the enzyme that normally fixes CO2,

can also use O2 as a substrate RuBP

CO2 Rubisco

+ RuBP

+

O2

Rubisco

Calvin Cycle

Photorespiration

o Wastes energy and decreases sugar production o Under hot dry conditions, photorespiration

increases o Some plants – C4 and CAM plants – have strategies to minimize this problem

Photorespiration Why is build up of O2 a problem? o Rubisco, the enzyme that normally fixes CO2,

can also use O2 as a substrate 6 O2

Rubisco

6 RuBP

x6 6 phosphoglycolate

6 3-PGA

Photorespiration Why is build up of O2 a problem? o The 3 molecules of phosphoglycolate go

through a salvage pathway – half of the 3-PGA is recovered 6 O2

Rubisco

6 RuBP

x6 6 phosphoglycolate

Not enough C to regenerate 6 RuBP

C3 Plants

6 3-PGA 3 CO2 3 3-PGA

9 G3P

Not G3P exits Calvin cycle to make glucose

CAM Photosynthesis Separate light and dark reactions in time o Stomata opened at night: CO2 enters, is fixed by

PEP carboxylase and converted to malate

o Malate is stored in vacuoles until daytime o Stomata closed during the day: malate broken

down to CO2 which enters the Calvin cycle

Day time – Stomata closed

Night time – Stomata open PEP

CO2 Calvin Cycle

+

oxaloacetate

PEP Carboxylase

ADP Malate

ATP

CO2 Pyruvate

Pyruvate

Malate

CAM Photosynthesis Examples of CAM plants

C3, C4, and CAM plants Summary Type Separation of CO2 fixation

C3 C4 CAM

Stomata open Environment during