Photosynthesis and Chemosynthesis in Autotrophs

Photosynthesis

All cells can obtain energy from the oxidation of organic molecules (such as glucose). Autotrophic cells must synthesize these molecules before they can catabolize them: autotrophic metabolism. The process of synthesizing biomolecules from CO₂ and H₂O as initial substrates, using sunlight as an energy source, is called photosynthesis.

• Overall reaction: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
• Photosynthetic organisms: Green plants, algae (green, red, brown), cyanobacteria, and some bacteria (green and purple)

Chloroplasts, Pigments, and Photosystems

For light energy to be useful to a living being, it must be captured by molecules capable of absorbing it. These substances are called light-capturing pigments and are found in the thylakoids of chloroplasts. Chromatophores contain a chemical group capable of absorbing light of different wavelengths in the visible spectrum. These pigments can be: chlorophylls (a and b), xanthophylls, carotenoids, etc.

Photosynthesis Stages

• Light reactions (photochemical phase): Occur in the grana of chloroplasts. H₂O molecules are split to release O₂, ATP, and NADPH.
• Dark phase reactions (Calvin cycle): Occur in the stroma. Glucose is created from CO₂ using the ATP generated in the previous phase and the reducing power provided by NADPH.

Nitrate and Sulfate Reduction

Plants can obtain the nitrogen they need from nitrate (NO₃^–), for example. Nitrates are absorbed by the roots and transported by xylem vessels into the chlorophyll parenchyma of the leaf. In nitrate, nitrogen is in a highly oxidized form, whereas in organic compounds, it is in a reduced form. The reduction is performed by NADPH, and the energy needed for the process is provided by ATP. Both products, as we know, are obtained in large quantities during the light phase of photosynthesis. This is why the reduction of nitrogen and its incorporation into organic substances takes place in chloroplasts, although the process does not directly require light.

Note: Nitrates are first reduced to nitrites and then to ammonium ions. The ammonium ion is embedded in a carbon chain to form the amino acid glutamic acid. This amino acid subsequently donates nitrogen to organic molecules that need it.

Finally, sulfur is absorbed by the roots in the form of sulfates (SO₄^2-) or other salts. Once reduced, it is incorporated into other organic substances in a manner similar to what we have seen with nitrogen.

Photosynthesis vs. Chemosynthesis

Photosynthesis: Conducted by photosynthetic autotrophic organisms, i.e., those capable of synthesizing biomolecules using energy from sunlight and CO₂ as a carbon source. Photosynthetic autotrophs include plants, algae, and some bacteria.

Chemosynthesis: This process is performed by chemosynthetic autotrophic organisms, capable of supplying the energy required for biosynthetic processes from the oxidation of inorganic molecules. Chemosynthetic autotrophs are certain bacteria.

Factors Influencing Photosynthesis

1. Light Intensity and Wavelength: Carotenoids and chlorophylls in the photosystems absorb photons of a specific wavelength. If a plant is illuminated with light of an inadequate or insufficient intensity or wavelength, photosynthesis cannot proceed, and the plant will not develop.
2. Temperature: Photosynthesis, like any chemical process, is influenced by temperature. For every 10°C increase in temperature, the rate doubles. However, an excessive increase in temperature denatures the enzymes that catalyze the process, decreasing photosynthetic performance.
3. CO₂ Concentration: If all other factors are held constant, an increase in the amount of CO₂ will increase the yield of photosynthesis up to a maximum value, after which it will stabilize.
4. O₂ Concentration: An increase in O₂ concentration inhibits photosynthesis because oxygen inhibits the enzyme that incorporates CO₂ into Ribulose 1,5-bisphosphate (RuBP).