Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.1 General equation of photosynthesis

Photosynthesis is the process by which absorbed light energy is transformed into the chemical energy of Organic compounds. Furthermore, photosynthesis not only drives the reduction of СО2, nitrates, and sulfates, but also channels energy into various endergonic processes, most notably the Transport of substances.

The overall equation for photosynthesis can be expressed as follows:

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which captures the fundamental Nature of the phenomenon: in the presence of light, green plants convert highly oxidized compounds—carbon dioxide and Water—into organic matter, releasing molecular oxygen in the process.

Over the course of plant evolution, the leaf emerged as the specialized organ of photosynthesis. On average, leaves absorb 80–85% of the energy of photosynthetically active radiation (PAR), which corresponds to the visible region of the electromagnetic spectrum with a wavelength of 400–700 nm, as well as 25% of the energy of infrared rays, accounting for about 55% of total solar radiation.

Leaves reflect 10% of PAR and 45% of infrared radiation, while transmitting 5% and 30%, respectively. Photosynthesis utilizes only 1.5–2% of the absorbed photosynthetically active radiation; the remaining absorbed energy is largely expended on water evaporation, or Transpiration (95–98%), with the rest dissipated via heat exchange with the atmosphere.

Leaf area can become quite extensive, often exceeding the ground area occupied by the plant. To quantify the size of the photosynthetic apparatus, researchers use the leaf area index (L), defined as the leaf area (m²) per 1 m² of ground area. For temperate agricultural crops, average leaf area indices typically range from 3 to 5, while in humid southern latitudes they can reach 8–10. For instance, the total leaf surface area of 1 hectare of cereal crops during flowering reaches 20,000–40,000 m², whereas in low-growing apple orchards it spans 25,000–30,000 m²/ha during the vegetative season.

Photosynthesis is driven by the selective absorption of light by pigments in the visible region of the solar spectrum. Photosynthetic Pigments account for

10–15% of the dry mass of METABOLISM/14.html">Chloroplasts. Chemically, photosynthetic pigments are divided into two main groups: chlorophylls and carotenoids.