Photosynthesis and Respiration Quiz

Study Notes

### Photosynthesis and Respiration

Photosynthetic organisms, such as plants, algae, and some bacteria, use photosynthesis to capture light energy from the sun and convert it into chemical energy, which is stored in the form of sugars.
This process uses carbon dioxide from the atmosphere, water, and light energy to produce glucose and oxygen.
Respiration is the process by which organisms break down sugars to release energy in the form of ATP, which is used for various cellular processes.
In the light, photosynthetic organisms can also use oxygen and other factors for respiration in addition to the process of photosynthesis.

### Oxygenation of RuBP

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is a bi-functional enzyme that plays a crucial role in carbon fixation during photosynthesis.
It can catalyze both the carboxylation and oxygenation of RuBP (ribulose-1,5-bisphosphate).
During carboxylation, Rubisco adds carbon dioxide to RuBP, producing two molecules of 3-PGA (3-phosphoglycerate), which then enter the Calvin-Benson cycle to regenerate RuBP.
During oxygenation, Rubisco adds oxygen to RuBP, producing one molecule of 3-PGA and one molecule of 2-PG (2-phosphoglycolate).
2-PG is a toxic by-product and must be recycled through a process called photorespiration.

Photorespiration is a metabolic pathway that consumes oxygen and releases carbon dioxide.
It is an energetically costly process that is thought to be a consequence of Rubisco's ability to bind oxygen.
Photorespiration results in loss of fixed carbon, reducing the efficiency of photosynthesis.
This process also helps in the recovery of carbon and the turnover of reducing power and ATP, reducing photoinhibition, which is the inhibition of photosynthesis by excessive light energy.
However, photorespiration can be mitigated by using C4 photosynthesis, which is an efficient method that minimizes photorespiration and increases photosynthetic efficiency.

Rubisco is the most abundant enzyme on Earth, representing 50% of soluble leaf protein and 75% of chloroplast stromal protein.
It has a slow reaction rate compared to other enzymes, with a turnover number (kcat) of only 3 sec-1.
Rubisco is activated by CO2 and Mg2+ ions, which bind to the active site and promote the formation of an enediolate intermediate required for catalysis.
Rubisco is regulated by Rubisco activase (RCA), an ATP-dependent enzyme that removes inhibitors like RuBP and Ca1P from the active site, allowing the enzyme to function properly.
The assembly of Rubisco requires the coordinated action of molecular chaperones, which help fold and assemble the subunits of the enzyme.

The activity of Rubisco is affected by various factors, including atmospheric concentrations of CO2 and O2, temperature, and the availability of Mg2+ ions.
Higher temperatures favor the oxygenation reaction of Rubisco, resulting in increased photorespiration.
The specificity factor of Rubisco is a measure of oxygenation over carboxylation, and it tends to be higher for Rubisco from C3 plants, implying that evolution has improved photosynthetic activity and the affinity of Rubisco for CO2.
However, attempts to improve Rubisco by using Mn2+ instead of Mg2+ have led to increased oxygenation, indicating that this approach is not very effective.
Several mechanisms have evolved to regulate Rubisco activity. These mechanisms include the activation of Rubisco by CO2 and Mg2+, the removal of inhibitors like RuBP and Ca1P by RCA, and the regulation of Rubisco content by varying the expression levels of its subunits.