Lecture 14- Photosynthesis: Carbon Reactions

Calvin-Benson Cycle

The carbon reactions that utilise ATP and NADPH in order to carboxylse and reduce molecules into a usable sugar molcule, with the regeneration of precursor molecules necessary to continue the cycle

In total, one cycle that produces one G3P will take one CO2 nine ATP and six NADPH

• So, to make one glucose, it requires two G3P and will take double the amount of substrates

First Stage: Carboxylation & Reduction

The enzyme RuBisCO will initiate the cycle by catalysing the carboxylation of 3 CO2 molecules (one at a time) to 3 RuBP molecules (5C, 15C to start the cycle). This results with three 6C intermediates which quickly clean into six 3C intermediates (18C in total).

These intermediates will then undergo reduction. The first reduction requires an ATP each, using six ATP. The second reduction requires an NADPH each, using six NADPH. This then results with six G3P molecules (sugar precusors).

One G3P molecule will exit the cycle to be used in whatever it is needed for. While the remaining five G3P molecules will go to through regeneration.

Second Stage: Regeneration

The five remaining G3P (15C total) will undergo a series of reactions to get back to three RuBP (15C total). These reactions require three ATP.

RuBisCO

The most abundant protein in the biosphere because it is in all plants. It exists in the stroma and is made up of 16 subunits (8 small from nuclear genome, 8 large from chloroplast genome. For CO2 fixation, it requires Mg²⁺ (also the same of O2 fixation).

When it fixes O2 with RuBP, it makes:

• One 3C intermediate (3-phosphoglycerate) and one toxic 2C intermediate (2-phosphogylcolate)

The ratios of CO2 to O2 and temperatures determine which substrate is used:

• More oxygen and higher temperatures increases the rate of oxygen fixation

RuBisCO is extremely old and is very inefficient and has a lack of specificity

Photorespiration

When the plant consumes O2 and ATP while CO2 is released to get rid of the toxic 2C intermediate by turning it back into the normal 3C intermediate. This process requires ATP, NADPH, and glutamate as well.

It is an incredibly inefficient and wasteful process as CO2 is diverted away from the Calvin-Benson cycle and reduced photosynthetic output

However, it does have roles in promoting nitrate assimilation and carbon-nitrogen balance

• High CO2 promotes sugar production but inhibits nitrate assimilation

• Photorespiration fixes O2 and releases CO2, but promotes nitrate assimilation

Factors Affecting Photosynthetic Output

At high CO2 concentrations, photosynthesis is limited by the regeneration of RuBP (except at higher temperatures)

At low CO2 concentrations, the rate of RuBisCO carboxylation limits photosynthesis

CO2 solubility relative to O2 and Rubisco selectivity both decrease with increasing temperature. Therefore, as CO2 increases photosynthesis becomes less sensitive to inhibition at high temperatures