Photosynthesis: Light Energy to Chemical Transformation

Posted on Apr 7, 2025 in Biology

Photosynthesis: Transforming Light into Chemical Energy

Photosynthesis, occurring within chloroplasts, is the transformation of light energy into chemical energy. This energy is used to produce organic matter from inorganic substances. It’s essentially the reverse of respiration and is a constructive process.

Solar energy is absorbed by photosynthetic pigments in chloroplasts and used for the synthesis of ATP and glucose from CO2 and H2O. In mitochondria, the released energy is stored in the form of ATP when glucose molecules are oxidized to CO2 and H2O.

Photosynthesis occurs in two phases:

Light-Dependent Reactions (Light Phase)

Pigments in the thylakoid membrane capture light energy. A chain of electron carriers and enzymes produces ATP and NADPH, and O2 is released.

Light-Independent Reactions (Dark Phase or Calvin Cycle)

ATP and NADPH from the light-dependent reactions are used in reactions where inorganic substances are converted into organic compounds. This can occur with or without light, in the chloroplast stroma.

Photosynthetic Pigments

These include:

  • Chlorophylls (main pigments)
  • Carotenoids (accessory pigments)
  • Phycobilins (accessory pigments)

They are all grouped within photosystems in the thylakoid membrane.

  • Photosystem I: traverses the stroma lamellae.
  • Photosystem II: is scarlet.

Synthesis of Glycosides from Fat

  1. Synthesis of Glycerin: glucose > glycerol
  2. Synthesis of Fatty Acids: glucose > (glycolysis) > pyruvic acid > acetyl-CoA > (lipogenesis) > fatty acids
  3. Synthesis of Triacylglycerols: glycerin + 3 fatty acids > (esterification) > triacylglycerols

β-oxidation

The degradation of fatty acids to form molecules of acetyl-CoA in the mitochondrial matrix yields acetyl-CoA, NADH, and FADH2.

Photophosphorylation

  • Non-Cyclic Photophosphorylation: Photosystems I and II are involved. ATP synthase synthesizes ATP and NADPH, and O2 is released.
  • Cyclic Photophosphorylation: Involving Photosystem I, it produces ATP, but O2 is not released.

Factors Influencing Photosynthesis

  • Light Intensity: Photosynthetic activity increases with light intensity up to a certain point.
  • CO2 Concentration: Photosynthetic activity increases with increasing CO2 concentration.
  • Temperature: Photosynthetic intensity increases with temperature up to a maximum.
  • O2 Concentration: High O2 concentration lowers performance.

Calvin Cycle

Occurs in the stroma, consuming ATP and NADPH. RUBISCO catalyzes the incorporation of CO2. With each turn, one carbon atom is incorporated in a CO2 molecule. It takes three turns to generate a triose.

  • Initial Compounds: Ribulose and CO2
  • End Products: Ribulose and Glucose

There are three stages: carboxylation, reduction, and regeneration.

Fermentation

Occurs in the cytosol. It is an anaerobic oxidation process of organic matter, with the purpose of obtaining energy in the form of ATP (from glucose).

Krebs Cycle (Citric Acid Cycle)

A series of reactions in the mitochondrial matrix where carbon atoms of acetyl-CoA are completely oxidized to form CO2, NADH, and FADH2. It is an amphibolic pathway involved in both catabolism and anabolism.

Glycogenesis

Synthesis of glycogen from glucose (anabolic).

Glycogenolysis

Degradation of glycogen to glucose (catabolic).

Gluconeogenesis

Glucose synthesis from non-glycogenic precursors.

Metabolic Routes

  • Reactivation of fatty acids to cytoplasmic.
  • β-oxidation of fatty acids: mitochondria.
  • Krebs cycle: the matrix.
  • Electron transport chain: mitochondrial cristae.
  • Oxidative phosphorylation: mitochondrial.