Photosynthesis and Chemosynthesis Processes

Posted on Dec 2, 2024 in Biology

– Phase Cyclic Light:

Intervenes only in PSI, resulting in the synthesis of ATP. Because PSII is not involved, nor is photolysis, consequently, there is no oxygen production or clear reduction of NADP+. The aim of the cyclic phase is to solve the ATP deficit found in the acyclic phase to carry out the subsequent dark phase. The cyclic phase occurs when illuminated with light of a wavelength greater than 680nm, called far-red.

– Phase Dark Biosynthetic

It consists of the addition of CO2 to a sugar called ribulose. It is a purely biochemical process, which does not need the presence of light or even chlorophyll. It usually occurs during the day because it requires ATP and NADPH produced in the light phase and takes place in the stroma of chloroplasts.

Synthesis of Carbon Compounds

Is carried out by means of a process that was discovered by biochemist Melvin Calvin, so-called Calvin cycle. There are 2 processes:

  • – Determination of CO2: Atmospheric CO2 enters the chloroplast stroma and binds to the pentose ribulose-1,5-diphosphate, thanks to the action of the enzyme ribulose-diphosphate carboxylase-oxidase, giving rise to a volatile 6-carbon compound, which dissociates into two 3-phosphoglyceric acid molecules. These are molecules with three carbon atoms, so plants that follow this metabolic pathway are called C3 plants.
  • – Determination of Fixed CO2: For half the consumption of ATP and NADPH that were obtained in the light phase, the 3-phosphoglyceric acid is reduced to glyceraldehyde 3-phosphate. It can follow three paths:
    1. Regeneration of ribulose-1,5-diphosphate. Takes place inside the chloroplasts by means of a process involving compounds with 4.5 to 7 carbons, similar to a process called the pentose phosphate cycle.
    2. Synthesis of starch, fatty acids, and amino acids, takes place within the chloroplasts.
    3. Synthesis of glucose and fructose. Takes place outside the chloroplast, in the cytosol, a process similar to glycolysis but in reverse.

Outcome of Oxygenic Photosynthesis

In the light phase, ATP and NADPH necessary to reduce CO2 in the dark phase to organic matter are produced.
6 CO2 + 12 H2O + light energy → C6H12O6 + 6 O2 + 6 H2O
In the Calvin cycle, for each CO2, 12 H2O are spent and 16 ATP are produced; the remaining 3 ATP are from the light phase.

2.5. Photosynthesis of Nitrogen Compounds

Photosynthesis of nitrogen compounds is carried out from nitrate ions dissolved in the soil. It is done in three stages and thanks to ATP obtained in the light phase:

a) Nitrate ions are reduced to ammonia in the chloroplasts by the enzyme nitrate reductase, with an expenditure of NADH.

b) Nitrite ions are reduced to ammonia in the chloroplast by the nitrite reductase enzyme, with electrons provided by ferredoxin.

c) The ammonia found is rapidly captured by alpha-ketoglutaric acid and gives rise to glutamic acid.

2.6. Photosynthesis of Organic Compounds with Sulfur

Since the ATP and NADPH from the light phase reduce sulfite to sulfate and then with an electron contributed by ferredoxin to hydrogen sulfide.

2.7. Photorespiration

Is a process that occurs when the atmosphere is warm and dry, and the stomata of the leaves are closed to prevent water loss. Under these conditions, the enzyme ribulose-diphosphate carboxylase-oxidase acts as an oxidase and destroys the ribulose-1,5-diphosphate that is required to capture CO2.

2.8. Factors that Affect Photosynthesis

  • Temperature: Each species is adapted to living in a range of temperatures.
  • CO2 Concentration: If light intensity is constant, process performance increases in photosynthetic relation directly with the concentration of CO2 in the air, reaching a specific value at which performance stabilizes.
  • O2 Concentration: As there is a higher concentration of oxygen in the air, there is lower photosynthetic performance, due to photorespiration processes.
  • Light Intensity: Each species is adapted to live within a range of light intensity.
  • Water Scarcity: The scarcity of groundwater and water vapor in the air decreases photosynthetic efficiency.

3) Chemosynthesis

It consists of the synthesis of ATP from the energy that arises in chemical reactions. Living organisms that carry out these processes are the chemolithotrophs.

3.1. Phases of Chemosynthesis

There are two phases:

  • In the first phase, the oxidation reaction of inorganic substances is the energy source for the phosphorylation of ADP in the respiratory chain, a process called oxidative phosphorylation. Some of this ATP is used to cause a reverse electron transport in the same respiratory chain for NADH.
  • In the second phase, the metabolic pathways followed coincide with the dark phase of photosynthesis.

3.2. Chemosynthetic Bacteria

Based on the substrate used, they are classified:

  • Colorless sulfur bacteria: Oxidize sulfur or sulfur compounds. They are aerobic bacteria that need oxygen for oxidation. They are responsible for the transformation of H2S from the decomposition of organic matter.
  • Nitrogen-oxidizing bacteria: Reduce nitrogen compounds. Responsible for oxidizing ammonia and converting it into nitrates. There are two types:
    1. Nitrosifying bacteria: Transform ammonia to nitrite.
    2. Nitrifying bacteria: Convert nitrites into nitrates.
  • Iron-oxidizing bacteria: Oxidize iron compounds.
  • Hydrogen bacteria: Can use molecular hydrogen.

4) Nitrogen-Fixing Organisms

Autotrophic organisms capture nitrogen from dissolved nitrate, while heterotrophic bacteria do so from organic food. There are groups of bacteria and cyanobacteria capable of fixing atmospheric nitrogen. This is made possible by a complex enzyme called nitrogenase.

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TEST UNIT 3:

  • What wavelength does chlorophyll alpha and beta absorb a greater proportion of?
    The corresponding red and blue light.
  • What causes molecular proton pumping in photosynthesis?
    Plastoquinone.
  • What is the purpose of water photolysis?
    To replace the electrons lost in PSII.
  • Into which molecules do excited electrons from chlorophyll P680 pass?
    Plastoquinone.
  • What happens to molecular electro-excited chlorophyll P700?
    Ferredoxin.
  • ATP synthesis in the light phase is due to:
    The passage of protons through the enzyme ATP-synthase from outside to inside of thylakoids.
  • Where is energy stored in the light phase of photosynthesis?
    In the molecules of ATP and NADPH.