Photosynthesis II

Photosynthesis II

Reading:

pp. 62-66 in Biology

Objectives:	At the end of this lecture, you should be able to:
1.	define the terms listed below.
2.	Understand the role of RUBISCO in the C3-Pathway of photosynthesis.
3.	Understand what photorespiration is and how RUBISCO is involved in this process.
4.	Describe the C4-Pathway and understand how it leads to a reduction in photorespiration.
5.	Understand how the CAM pathway can allow photosynthesis to occur with a reduction in water loss in extreme hot and dry climates.
6.	Describe the similarities and differences between the C4 and CAM pathways for photosynthetic carbon fixation.

Terms:
Calvin Cycle	phosphoglyceraldehyde	Mesophyll Cells
C3-Pathway	Photorespiration	Bundle-Sheath Cells
RUBISCO 1,5-Bisphosphate	phosphoglycolate	Kranz Anatomy
Ribulose	C4-Pathway	Crassulacean Acid Metabolism (CAM)
phosphoglycerate	PEP Carboxylase	ATP synthase

Lecture Outline:

I. In the chloroplast, CO₂-Fixation Reactions occur in the stroma of the chloroplast and require the ATP and NADPH generated by the light reactions occurring at the thylakoids.

II. The central metabolic pathway for CO₂-fixation is called the "Calvin Cycle" or "C3-Pathway".

A. Elucidated by Melvin Calvin's research group using 14CO₂.

B. This pathway is called the C3-Pathway since the first measurable product of CO₂ fixation is the 3-carbon molecule, phosphoglycerate.

C. The first step of the C3-Pathway is catalyzed by the enzyme called Ribulose Bisphosphate Carboxylase/Oxygenase or "RUBISCO".

D. In this first step, CO₂ is added to the 5-carbon sugar molecule, Ribulose 1,5-Bisphosphate, which then cleaves to yield two 3-carbon phosphoglycerate molecules.

E. ATP and NADPH are then utilized in the reduction of phosphoglycerate to phosphoglyceraldehyde.

F. While some sugar intermediates are used to regenerate the Ribulose 1,5-Bisphosphate molecules involved in the capture of CO₂, others are exported from the chloroplast for metabolism in the cytoplasm.

G. Sugar intermediates exported from the chloroplast can be utilized by respiration to generate energy in the cell where photosynthesis is taking place or these intermediates can be metabolized into sucrose for export to other regions of the plant.

III. RUBISCO also has the capability of adding an oxygen molecule to Ribulose 1,5 Bisphosphate and this forms the basis for photorespiration.

A. Oxygen addition to Ribulose 1,5-Bisphosphate followed by cleavage of the molecule results in the generation of one molecule of phosphoglycerate and one two-carbon phosphoglycolate molecule.

B. The two-carbon phosphoglycolate molecule is not immediately useful in C3 cycle reactions and it is believed to represent a "wasteful" process.

C. The subsequent metabolism of phosphoglycolate results in the production of CO₂.

D. This overall process is called "photorespiration" because what is observed is a light-dependent oxygen consumption and CO₂ production. However, it is not true respiration (i.e. with energy recovery).

IV. Some plants, especially grasses, can have an additional metabolic pathway for CO₂ fixation that "feeds" CO₂ into the C3-Pathway at a high level.

A. This mode of initial CO₂ fixation is called the "C4-Pathway" since the first measurable product is a four-carbon acid molecule such as oxaloacetic acid.

B. Plants which utilize the C4 path often have a different leaf anatomy which includes large "bundle-shealth" cells that surround the vascular bundle. This is called "Kranz Anatomy".

C. The first reaction of this pathway is mediated by the enzyme PEP carboxylase which adds a CO₂ molecule to the three-carbon molecule, phosphoenolpyruvate (PEP).

D. The initial CO₂ fixation reactions of the C4 pathway take place in leaf mesophyll cells.

E. The four carbon molecule produced by addition of CO₂ to PEP is then transported to the bundle-sheath cells where it is cleaved to release CO₂ to the C3-Pathway.

F. After release of CO₂, the three-carbon molecule is cycled back to the mesophyll cells to again participate in CO₂ fixation.

G. As PEP carboxylase does not catalyze reactions involving oxygen, the effect of this "CO₂ pumping system" is to eliminate photorespiration and promote high photosynthetic rates (especially at high light).

V. To conserve water in extremely dry environments, some plants such as cacti have an alternative photosynthetic path where initial CO₂ fixation occurs at night!

A. With stomates open during the day in extremely hot and dry climates, water loss can become problematic. However, stomates need to be open for CO₂ to enter.

B. In certain plants such as cacti, the stomates only open at night and CO₂ is fixed by PEP carboxylase into 4-carbon acids that are stored in the large central vacuole.

C. During the day, the stomates close and CO₂ is released from the 4-carbon acids to the C3-Pathway which utilizes ATP and NADPH from the light reactions to generate sugar molecules as usual.

I. In the chloroplast, CO2-Fixation Reactions occur in the stroma of the chloroplast and require the ATP and NADPH generated by the light reactions occurring at the thylakoids.

II. The central metabolic pathway for CO2-fixation is called the "Calvin Cycle" or "C3-Pathway".

III. RUBISCO also has the capability of adding an oxygen molecule to Ribulose 1,5 Bisphosphate and this forms the basis for photorespiration.

IV. Some plants, especially grasses, can have an additional metabolic pathway for CO2 fixation that "feeds" CO2 into the C3-Pathway at a high level.

V. To conserve water in extremely dry environments, some plants such as cacti have an alternative photosynthetic path where initial CO2 fixation occurs at night!

For Further Exploration:

I. In the chloroplast, CO₂-Fixation Reactions occur in the stroma of the chloroplast and require the ATP and NADPH generated by the light reactions occurring at the thylakoids.

II. The central metabolic pathway for CO₂-fixation is called the "Calvin Cycle" or "C3-Pathway".

IV. Some plants, especially grasses, can have an additional metabolic pathway for CO₂ fixation that "feeds" CO₂ into the C3-Pathway at a high level.

V. To conserve water in extremely dry environments, some plants such as cacti have an alternative photosynthetic path where initial CO₂ fixation occurs at night!