The presence of solar radiation is important for the process of photosynthesis (photosynthetic assimilation), namely visible light (light of wavelength between 400–800 nm). We can say that this process is a necessary condition for the existence of life on our planet.

In this process, the energy from the Sun is transformed into the energy of chemical bonds in organic molecules. The energy generated is used by plants for incorporation into carbohydrates, amino acids, etc., which are then formed into proteins, fats, polysaccharides, and other substances forming the body of the plant.

As mentioned above, the presence of sunlight, chloroplasts and chlorophyll, carbon dioxide (CO2 ) and water (H2 O) is essential for the photosynthesis process. The product of photosynthesis is carbohydrates. The chemical equation of photosynthesis can be formulated as follows:

Chlorophyll is found in the membrane of chloroplasts stored in the form of large protein complexes. The primary and secondary processes of photosynthesis take place there.

Primary processes

Primary processes are called the light phase (photochemical phase). Chlorophyll A allows the energy of the absorbed photon to be converted into chemical energy. At this stage, ATP (energy source for glucose generation) and coenzyme NADPH (hydrogen source) are formed from the absorbed 2 photons. ATP is formed upon absorption of the first photon, NADPH of the second. First, photosystem II is activated, where a high energy electron is generated by means of light energy. This is then transferred to photosystem I. The released electron from photosystem II is transferred to NADPH and replaced with an electron from water

Secondary processes

Secondary processes are also referred to as the synthetic phase (CO2 fixation phase). These processes, unlike the primary ones, no longer require the presence of light. Therefore, i tis called the dark phase. Carbon dioxide, ATP, NADPH, a substance to which carbon dioxide binds, specific enzymes and coenzymes are required for this reaction. However, this phase is not the same for all plants. Accordingly, we divide plants into two main categories: C3 and C4 plants

The C3 plants: carbon dioxide is attached to an organic compound (ribose 1,5 bisphosphate). These form an intermediate product which breaks down into 2 molecules of acid (3-phospholyceric acid) having 3 carbon atoms. Acid produces glucose with NADPH and ATP consumption. These plants are not beneficial to humans because 50% of the produced carbohydrates consume the plants themselves.

The C4 plants: carbon dioxide is attached to phosphoenolpyruvate, which is then converted to oxaloacetate. Compared to C3 plants, they are more suitable for human nutrition because they form many carbohydrates (e.g. corn) (Križan, 2004).

Factors influencing photosynthesis

The process of photosynthesis is influenced by various factors:

  • Wavelength of light: red (630–750 nm) and blue-violet (420–450 nm) are the most suitable light components. Plants use only 2% of light falling on the plant, the rest is reflected.
  • Carbon dioxide: A large increase or decrease in its concentration in the atmosphere will decrease or fully stop the photosynthesis process.
  • Temperature: the optimal temperature for photosynthesis is dependent on the type of plant. The optimum for plants can be considered a temperature of 0–40 °C.
  • Water: when it is lacking, the plant closes its vents, preventing carbon dioxide uptake and slowing down the photosynthesis.

Heterotrophic organisms

Heterotrophic organisms need to receive organic substances for the carbon and energy source. Unlike autotrophic organisms, they cannot utilize energy from the sun or carbon dioxide from the air. Heterotrophic organisms generally receive complex organic substances from the environment. In their gastrointestinal tract, mechanical and further chemical decomposition of substances occurs (intracellular – especially protozoa; extracellular – takes place in the cavities and parts of the digestive tract, e.g. in the stomach), in the presence of enzymes; so that they can be absorbed into body fluids and delivered to their consumption points, with undigested food left out of the body.

Heterotrophic organisms must also absorb inorganic substances, especially water and biogenic elements (oxygen, carbon, hydrogen, nitrogen). Most multicellular organisms process food in individual parts of the digestive system.

In plants, we also distinguish from where the organism draws organic substances. Accordingly, these may be saprophytic (they take organic matter from dead bodies of plants and animals, e.g. fungi) and parasitic (they take nutrients from a living organism, which we call host, e.g. mistletoe).