Photosynthesis in plants: C3 and C4 -this is a presentation I used to for my Plant Physiology course. -it is an introduction to photosynthetic carbon fixation as it occurs in C3 and C4 land plants.

Photosynthesis Chloroplast Stroma Light Reactions CO2 fixation reactions C3 photosynthesis Calvin Cycle Photorespiration C4 photosynthesis

Photosynthesis – using energy from light to make sucrose from CO2

Chloroplast review -outer membrane -inner membrane -stroma -thylakoid CO2 fixation part of photosynthesis, we are interested in the stroma compartment (pl –stromal) stroma –the "soup" of the chloroplast where the enzymes involved in CO2 fixation occur

We recently reviewed the light reactions of photosynthesis that convert light energy into biological energy such as ATP an NADPH. This occurs in the thylakoid membranes that form pancake like stacks called grana Stroma grana membrane stacks

from CO2 molecules....this process is completely catalyzed by enzymes The CO2 fixing or carbon fixing reactions of photosynthesis refers to the synthesis of sugars from CO2 molecules....this process is completely catalyzed by enzymes from the outside air Stroma

C3 photosynthesis and the Calvin cycle When plants first appeared in algal form some 2 billion years ago, the kind of photosynthetic process they "invented" and past on to their land plant descendents was the C3 process of photosynthesis Today, 99.9% of all plant species have C3 photosynthesis. What is C3 photosynthesis? The C3 in C3 photosynthesis refers to the first stable molecule formed from an enzyme catalyzed CO2 fixation event.

CO2 is combined with the 5-carbon molecule ribulose-1,5- bisphosphate (RuBP) by the enzyme Rubisco. This process is termed CO2 fixation……CO2 is being "fixed" onto a carbon skeleton. The 6-carbon intermediate spontaneously breaks down to two molecules of 3-phosphoglycerate (3-PGA)

the Calvin cycle processes the fixed CO2 into sugars for export to the phloem......some of the sugar is held back for resynthesizing RuBP so another round of CO2 fixation can occur Rubisco enzyme catalyzes this step sucrose exported into phloem

Calvin Cycle – Function is to regenerate the molecule RuBP for another round of CO2 fixation

Rubisco and the Calvin cycle evolved in the first plant forms 2 billion years ago when the earths atmosphere was about 10% CO2 and less than 1 % O2. After a billion yrs or so, plants "terraformed" the earth and in the process sucked down atmospheric levels of CO2 to about 0.03% and increased O2 levels to 20% This radical change in atmosphere exposed a design flaw in Rubisco hinted at by its name: Ribulose Bisphosphate Carboxylase Oxygenase = RuBisco The oxygenase indicates that Rubisco sometimes mistakes O2 for CO2 and fixes it into the 5-carbon Calvin cycle "receptor" molecule. O=O O=C=O

Around 30% of the time, Rubisco makes the mistake of fixing O2 instead of CO2. ………….this is very bad for the plant. Why? Rubisco

1. the plant just "ate" O2…….there is no food value to oxygen, no calories!.... the plant invested much energy to synthesize RuBP only to have it wasted in O2 fixation….it started with 5-carbons before and it had 5-carbons after. 2. the molecule that was created from the O2 fixation, phosphoglycolate, cannot be used directly for making sugars……it must be "reprocessed" into a usable molecule such 3. the plant reprocesses phosphoglycolate using a biochemical pathway termed the photorespiration pathway

The photorespiration pathway is a long, tortuous circuit that passes through three organelles. When phosphoglycolate is processed by this pathway, a CO2 is lost from the molecule so that now, not only does the plant NOT fix CO2, but when it fixes O2, it LOOSES an already fixed CO2.

Another view of the photorespiration pathway: Thus, in terms of energy and carbon combined, the net loss to the plant is 40%. The flaw in Rubisco has caused C3 plants to become 40% less productive.

C4 plants out perform C3 plants in open sunlight environments -C4 plants have evolved a means of suppressing photorespiration and the fixation of O2 by Rubisco.

(photosynthesis rate) (light intensity) (peak sunlight)

C4 photosynthesis is named so because the first stable molecule formed from the fixation of CO2 is a 4-carbon organic acid: phosphoenolpyruvate (3 carbons) oxaloacetic acid (4 carbons) PEP carboxylase CO2

Why is there no photorespiration in C4 plants? Since nature could not fix the flaw in Rubisco, the C4 photosynthetic mechanism move Rubisco into a part of the leaf where photorespiration will be supressed Evolutionary changes in the leaf: 1. two kinds of photosynthesis cells: work cooperatively in process 2. photosynthesis enzymes: some occur in only one cell-type, some occur in the other cell-type 3. two CO2 fixation enzymes are used in photosynthesis versus one as in C3 plants;

Corn leaf cross section two kinds of leaf cells involved in photosynthesis: 1. mesophyll cell 2. bundle sheath cell (vascular) bundle sheath with bundle sheath cells vascular bundle (the xylem and phloem pipes)

The C4 pathway is a two-part/two-cell process Part One: occurs in the mesophyll cells ✓ C4 enzymes located in these cells: -PEP carboxylase Part Two: occurs in the bundle sheath cells -Rubisco -the Calvin cycle

CO2 fixation by PEP carboxylase Part One: CO2 fixation by PEP carboxylase PEP PEP carboxylase *CO2 OAA C3 C4 *from the air PEP carboxylase: fixes ONLY CO2, never fixes O2

C4 acid is then transported (diffuses) into the bundle sheath cells via plasmodesmata and broken down into CO2 and a C3 organic acid: suberin layer in cell wall plasmodesmata CO2

Mechanism of C4 Photosynthesis plasmodesmata CO2 suberin layer in cell wall -traps CO2 inside cell.... CO2 builds up to more than 400 times the air level of CO2 .....when CO2 concentration is so high within the cell, Rubisco is swamped with CO2 and never "sees' O2 thus, NO O2 fixation occurs and NO photorespiration