Photosynthesis: Light and Dark Reactions

Posted on Jan 16, 2025 in Biology

Fundamentals of Photosynthesis

In photosynthesis, light is captured by pigments. When a photon is absorbed by an electron, a photosynthetic pigment is excited. The pigment electron will be provided again if there is a primary donor of electrons. The liberated electrons pass to a primary electron acceptor and then to an electron transport chain. Reactions that occur in this chain are exothermic. The energy is used to create a proton gradient through the thylakoid membrane. Subsequently, the passage of protons through the ATP synthase is used in the synthesis of ATP (ADP + Pi = ATP, photophosphorylation). There are two types of pigments:

  • Antenna pigments: Capture light energy and transmit it to other pigments.
  • Target pigments: Capture light energy, and the excited electron is transferred to the primary electron acceptor.

Photosynthesis Stages and Location

There are two types of photosynthesis between the photosynthesizing organisms:

  1. Oxygenic: Higher plants and cyanobacteria. The primary electron donor is water (2H2O = 4H+ + 4 electrons + O2).
  2. Anoxygenic: Purple and green sulfur bacteria. SH2 is used as the primary electron donor.

Phases of Oxygenic Photosynthesis

1. Light Phase

This phase occurs in the thylakoid membrane. It involves:

  • Photosynthetic pigments grouped in photosystems.
  • Photosynthetic electron transport chain.
  • ATP synthases.

The purpose of the light phase is the derivation of ATP by photophosphorylation and obtaining reducing power (NADPH + H+). Both products are used in the dark phase.

Details of this phase:

  1. Photosynthetic pigments are attached to thylakoid transmembrane proteins, forming photosystems. Each photosystem is formed by antenna pigments, some proteins, and a reaction center with target pigments. The reaction center is formed by a primary electron donor (water) and a primary electron acceptor. In chloroplasts of higher plants, there are two photosystems: P680 and P700.

  2. Photosynthetic chain is formed by proteins whose prosthetic groups are only transported by electrons (cytochromes) and proteins and lipids carrying H. In the chain, there are two processes of photosynthetic ATP synthesis:

    • Acyclic photophosphorylation: Products obtained are ATP and NADPH + H+.
    • Cyclic photophosphorylation: The product obtained is ATP.

    Which one will occur is determined by the levels of NADPH + H+ in the thylakoid.

2. Dark Phase (Calvin Cycle)

This stage does not need direct light but needs the products obtained in the light phase (ATP and NADPH + H+). The dark phase, or Calvin cycle, takes place in the chloroplast stroma. At this stage, the reduction and assimilation of CO2 occur to transform it into glucose. The global reaction that summarizes the process would be:

6CO2 + (Rubisco) + 12NADPH + 12H+ + 18ATP + 12H2O = C6H12O6 + 12NADP+ + 18ADP + 18Pi

Rubisco (ribulose 1,5-bisphosphate carboxylase) has little affinity for CO2. When the concentration of oxygen is very high, it stops the Rubisco from fixing CO2 and causes a break of ribulose diphosphate (photorespiration):

RuBP = PGA + 1.5 glycolic acid

Glycolic acid goes into peroxisomes, where every two glycolic acid molecules form one molecule of CO2 and one molecule of phosphoglyceric acid. To prevent photorespiration, some plants, such as tropical grasses, fix CO2 by an enzyme different from Rubisco, PEP carboxylase. In this case, the assimilation of CO2 involves two cell types: mesophyll cells and sheath cells.

Factors Influencing Photosynthesis

  1. Temperature: Typical graph of an enzyme.
  2. CO2 concentration: The substrate of photosynthesis is CO2, so its graphic is the same as the Michaelis-Menten.
  3. O2 concentration: Increased O2 reduces the efficiency of Rubisco due to photorespiration.
  4. Light intensity: Each species is adapted to different light intensities.
  5. Water shortage: The stomata close to prevent water loss, which implies a minority entry of CO2 and a decrease in the photosynthetic rate.
  6. Photoperiod (illumination time): Some plants increase their photosynthetic rate with the photoperiod, while others need a few hours of darkness.