Photosynthesis: Energy Input and Factors Affecting It

Posted on Jan 9, 2025 in Biology

Photosynthesis: A Unique Mechanism of Energy Input

Photosynthesis is a unique mechanism for energy input into the biosphere. Light is captured and converted into chemical energy in the form of organic matter, stored as carbohydrates, producing free O2. This process distinguishes the plant kingdom. It is influenced by the following factors: light, temperature, air humidity, water availability, carbon dioxide (CO2), and nutrients. Free oxygen in the atmosphere reaches 21%.

Historical Contributions to Understanding Photosynthesis

  • Aristotle: Noted the intervention of green light.
  • Joseph Priestley (1733-1804): Discovered that plants purify “dephlogisticated air” (air with O2).
  • Jan Ingenhousz (1730-1799): Found that plants vitiate the air both in light and darkness.
  • Theodore Nicholas de Saussure (1767-1845): Showed that sugar production requires water, and photosynthesis exchanges similar volumes of CO2 and O2.
  • Dutrochet (1837): Determined that only cells containing chlorophyll incorporate CO2.
  • Sachs (1859): Proposed the basic equation: 6CO2 + 6H2O + solar energy (6kcal/mol) → C6H12O6 + 6CO2.
  • Arnon (1957): Demonstrated the reaction: NADP+ + H2O + ADP + Pi → NADPH + 1/2O2 + ATP + H+.

Endergonic Processes and the Global Response

Photosynthesis involves endergonic processes, requiring radiant energy to initiate chain reactions leading to the synthesis of organic compounds. The global response of photosynthesis is:

hv

nCO2 + 2nH2O → n(CH2O) + nO2

Enzymes

Blackman (1905) measured the rate of photosynthesis under different conditions (light and temperature) and proposed that the reactions were controlled by enzymes.

CH2O is the empirical formula of carbohydrates synthesized as a result of photosynthesis. Carbon incorporated into the Earth through photosynthesis ranges from 70,000 to 120,000 million tons, equivalent to 170 to 290 million tons of sugar. Half of the plant biomass is synthesized on land, and the other half in the upper layers of lakes and seas.

Chloroplasts: Structure and Function

Chloroplasts originate from protoplastids and divide as the embryo develops. Upper layers have over 100 chloroplasts, each enclosed in a double membrane that controls the passage of molecules. Each chloroplast contains approximately 50 grana.

  • Stroma: An amorphous, gelatinous material rich in enzymes located within the chloroplast. This is where the conversion of CO2 into carbohydrates occurs.
  • Thylakoids: Membranes found in the form of flattened, closed sacs within the stroma. The thylakoid membrane contains pigments involved in light absorption, primarily chlorophyll a and chlorophyll b. Yellow-orange pigments, such as carotene and xanthophylls, are also present.
  • Grana: Stacks of thylakoids connected by lamellae that pass through the stroma. They contain between 2 and 100 thylakoids per granum.

Photosynthetic Pigments

Photosynthetic pigments include chlorophyll, phycocyanin, carotenoids, chlorophyll a and b, other chlorophylls, and phycoerythrin.

Pigments and Light Absorption

Pigments absorb light energy that can be used by living systems. When a pigment absorbs a photon, an electron is released to a higher energy level, becoming “excited.” This state lasts for approximately one millionth of a second and can be dissipated as heat.

Tetrapyrrole

Tetrapyrrole is a basic structure of porphyrins, including chlorophylls, hemoglobin, and cytochromes. The characteristic chromophore of chlorophyll is generated by the binding of pyrrole rings.