Lecture 4 Background: Photosynthesis

Light-Independent Reactions

Photosynthesis, the light-independent reactions, are chemical reactions that convert carbon dioxide and other compounds into glucose. They occur in the stroma, the fluid filled area of a chloroplast outside of the thylakoid membranes. These reactions take the products of the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin Cycle: Carbon Fixation, Reduction reactions, and ribulose 1,5-diphosphate (RuDP) regeneration.

The light-independent reactions are sometimes referred to as the dark reactions, though that term may be misleading as they do not actually require darkness to proceed. The term "light-independent" is used to emphasize that the reactions occur regardless of the amount of light present as long as the proper substrate compounds are available. Even this term can be criticized, however, as the availability of substrates in plants depends on photosynthesis, so the reactions cannot be said to be entirely "light-independent." Furthermore, the term "dark reactions" may be more accurate in CAM (Crassulacean acid metabolism) plants, which only take up CO2, which is necessary for the reactions to proceed, at night.

TAKEN FROM WIKIPEDIA AS AN ADDITIONAL RESOURCE - FEEL FREE TO ADD/EDIT

Overview of C4

C4 plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures and nitrogen or carbon dioxide limitation. 97% of the water taken up by C3 plants is lost through transpiration,[2] compared to a much lower[quantify] proportion in C4 plants, demonstrating their advantage in a dry environment.

C4 carbon fixation has evolved on up to 40 independent occasions in different groups of plants, making it an example of convergent evolution.[3] C4 plants arose around 25 to 32 million years ago[3] during the Oligocene (precisely when is difficult to determine) and did not become ecologically significant until around 6 to 7 million years ago, in the Miocene Period.[3] C4 metabolism originated in open habitats, where the high sunlight gave it an advantage over the C3 pathway.[4] Drought was not necessary for its innovation - rather, the increased resistance to water stress was a by-product of the pathway and allowed C4 plants to more readily colonise arid environments.[4]

Today, C4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species.[5] Despite this scarcity, they account for around 30% of terrestrial carbon fixation.[3] Present-day C4 plants are concentrated in the tropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in C3 plants.

Found on Wikipedia and added here as an additional reference.

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Overview of CAM: a two-part cycle

CAM is a mechanism whereby CO2 is concentrated around RuBisCO by day, while the enzyme is operating at peak capacity. This concentration of CO2 increases RuBisCO's efficiency, as it is prone to operate in the "reverse" direction via photorespiration - utilizing oxygen to break down the reaction products the plant would rather it was producing. It differs from C4 metabolism, which spatially concentrates CO2 around RuBisCO.

During the night

CAM plants open their stomata during the cooler and more humid night-time hours, permitting the uptake of carbon dioxide with minimal water loss.

The carbon dioxide is converted to soluble molecules, which can be readily stored by the plant at a sensible concentration.

The chemical pathway involves a three-carbon compound phosphoenolpyruvate (PEP), to which a CO2 molecule is added. This forms a new molecule, oxaloacetate, which in turn forms a malate. Oxaloacetate and malate are built around a skeleton of four carbons, hence the term C4. Malate can be readily stored by the plant in vacuoles within individual cells.

During the day

Malate can be broken down on demand, releasing a molecule of CO2 as it is converted to pyruvate. The pyruvate can be phosphorylated (i.e. have a phosphate group added by the "energy carrier" ATP) to regenerate the PEP with which the plant started, ready to be spurred into action the next night. But it is the release of CO2 that makes the cycle worth the plant's while. It is directed to the stroma of chloroplasts: the sites at which photosynthesis is most active. There, it is provided to RuBisCO in greater concentration, increasing the efficiency of the molecule, and therefore producing more sugars per unit photosynthesis.

The benefits of CAM

A great deal of energy is expended during CAM by the production and subsequent destruction of malate. This is in part countered by the increased efficiency of RuBisCO, but the more important benefit to the plant is the ability to leave most leaf stomata closed during the day.[verification needed] CAM plants are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing CAM plants to grow in environments that would otherwise be far too dry. C3 plants, for example, lose 97% of the water they uptake through the roots to transpiration[2] - a high cost avoided by CAM plants.

Comparison with C4 metabolism

The C4 pathway bears resemblance to CAM; both act to concentrate CO2 around RuBisCO, thereby increasing usefulness. CAM concentrates it in time, providing CO2 during the day, and not at night, when respiration is the dominant reaction. C4 plants, on the contrary, concentrate CO2 spatially, with a RuBisCO reaction centre in a "bundle sheath cell" being inundated with CO2.

TAKEN FROM WIKIPEDIA AS AN ADDITIONAL RESOURCE - FEEL FREE TO ADD/EDIT

Rubisco

This page is just a little more information about Rubisco, in case you're interested. Feel free to add or correct.

Key Points About Rubisco

  • evolved ~3.2 mya
  • very large enzyme (550 000 Kda/mole)
  • very abundant (20 - 30% of all leaf protein)
  • catalyzes the first reaction in the Calvin Benson Cycle:
    • 3 RuPB + 3 C02 _RuBPcarboxylase/oxygenase: RuBisCO___> 6 PGA (Oxygen is incorporated into 1 PGA)
  • a relatively inefficient molecule with a slow turnover rate (1/5 to 1/10 the turn over rate of other enzymes)

The Problem

  • RuBisCO also has affinity for O2 ,and can also catalyse photorespiration:
    • RuBP + O2 _RuBPcarboxylase/oxygenase: RuBisCO___> PGA + PG
    • PG is toxic to plants, so the plant uses ATP or NADPH to metabolize it into PGA, giving up a previously fixed CO2 molecule in the process
    • IOW: If oxygen is present, RuBisCO will also catalyze photorespiration, which uses up energy instead of storing energy
  • RuBisCO's affinity to oxygen increases at higher temperatures
    • for example, at 30C, photorespiration can inhibit photosynthesis by over 25%
  • photorespiration was not a problem when RuBisCO first evolved, because at that time, there was a lot of carbon dioxide in the atmosphere relative to oxygen
  • however, at around 300 mya, atmospheric oxygen rose to its present level in the atmosphere, and photorespiration started to occur

What to do?

Possibilities

  1. Evolve a new RuBisCO. It's possible that RuBisCO has already adapted to the increase in atmospheric oxygen by increasing the relative affinity to carbon dioxide relative to oxygen. The evolutionary limit for adaptation may have already been reached.
  2. Open stomata and let more carbon dioxide in. This wastes water and only brings carbon dioxide to the atmospheric level
  3. Reduce leaf temperature. Waxes and hairs on the leaf surface, but they have limited effectiveness.
  4. Reduce the concentration of O2 around RuBIsCO. This is energetically impossible, I think because the oxygen molecule is so small, it can dissolve fairly effectively. But I could be wrong.
  5. Concentrate carbon dioxide around RuBisCO. This is effective, and done via C4 or CAM photosynthesis in some plants.

C4 and CAM in a nutshell

C4 and CAM photosynthesis concentrate carbon dioxide around RuBisCO by separating the light reaction and the dark reaction by space (as in C4) or time (as in CAM), thus increasing RuBisCO's affinity to carbon dioxide and reducing photorespiration.

Why do the light and dark reactions need to be separated?

  • because the light reactions require the stomata (little holes on the underside of the leaf) to open, which lets in oxygen as well as carbon dioxide
  • because the light reactions produce oxygen as a waste product.
  • the dark reactions (Calvin-Bension cycle) are inhibited by oxygen
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