A brief guide to photosynthesis

Photosynthesis is described as: ‘A process in which electromagnetic energy is converted to chemical energy that can be utilised for various biosynthetic processes.’ Plants require photons, CO₂, and H₂O to produce sugar and carbohydrates (Dawkins, 2010; Capon, 2010). Below is the equation to demonstrate photosynthesis:

6CO₂ + 6H₂O → (Photons & Chlorophyll) C₆H₁₂O₆ + 6O₂

Light-dependent reactions occur in the thylakoid membranes (reaction centres) of the chloroplasts and utilise photons to synthesize ATP and NADPH.

The process commences with photosystem II (250-400 pigment molecules) when a chlorophyll molecule gains sufficient energy from the adjacent pigment, which enables photolysis of H₂O to occur (breaking of hydrogen and oxygen) (Mauseth, 2008). This process produces O₂ which is expelled and electrons and protons (H+) which are utilised. This occurs in the granum of the chloroplast or the ‘stacks of thylakoids’. The H⁺ are then translocated through the cytochrome which energises and excites H⁺ to the second stage of light-dependent reactions which is shown in the following stages:

Photosystem II (P680) → Plastoquinone → Cytochrome b6 → Cytochrome f → Plastocyanin → Photosystem I (P700)

3.3 Photosystem I activates electrons for transfer to the Fd and finally to NADP+, where the protons from water splitting are exhausted to create NADPH⁺ H⁺. To demonstrate the ability to liberate oxygen from water, the following:

The positively charged P800 exerts a strong pull on the H₂0 molecule, splitting it into H⁺ and OH^- ions. It requires CI^- and Mn²⁺ ions to act as a catalysis:

4H₂0 → (Mn²⁺, CI^-) → 4 (OH^-) + 4H⁺

OH donates its electron to oxidise P680:

4(OH^-) → 4e^- → 4OH

OH radical obtained forms H₂0 and liberates oxygen:

4(OH) → H₂0 → 4H⁺ → 4e^- → 0₂

Donation of H⁺ ion from the formation of NADPH is utilised for the Calvin-Benson Cycle. Below is a summary of the differences between the photosystems.

Table 1 Photosystems comparison

The end products are transferred to the light-independent reactions, these reactions occur in the stroma found within the chloroplast between the grana and thylakoid. C0₂ is delivered to the stroma where they are reduced to ATP and combined with H⁺ to power the Calvin-Benson Cycle. This process converts ATP into the carbohydrates needed to power the photoautotrophs. The key enzyme of the cycle is RuBisCO. This enzyme fixes C0₂ to ATP by complex redox reactions to gain the carbohydrates, lipids and proteins which are required for development. The following equation for this process:

3CO₂ + 6NADPH + 5H₂O + 9ATP → glyceraldehyde-3-phosphates (G3P) + 2H⁺ + 6NADP+ + 9ADP + 8Pi

3.5 Plants convert photons into chemical energy with a photosynthetic efficiency between 3-6%, however photosynthesis varies with the frequency of light and intensity temperature and proportions of C0₂, which can vary between 0.1 – 0.8% in the atmosphere. Photosynthetic systems store 469 kilojoules of energy and in some cases up to 502 kilojoules. This process is more productive when both photosystems are balanced in their uptake of light. By comparison, solar panels convert photons into electrical energy at an efficiency of 6 - 20%. This table puts the reactions of photosynthesis into a timescale.