A Deeper Look into Photosynthesis Pt 1: Light-Dependent Reaction

Photosynthesis may be the most well-known biological process. Whether or not people have learnt biology before, they can all describe the process of photosynthesis: chlorophyll in leaves takes in sunlight and synthesizes sugar with carbon dioxide and water. But what on earth happens in the leaves to turn inorganic molecules into a complicated organic compound i.e. sugar? In the article, I will dig deeper into one part of photosynthesis: light-dependent reaction.

Fig 1. Photosynthetic Electron Transport Chain (Khanacademy, 2023)

Absorption of Light

From its name, the light-dependent reaction is a series of chemical reactions that happen under sunlight. Sunlight acts as the energy source for the light-dependent reaction to happen. Thus, the first problem plants face is how to absorb light and turn light into chemical energy for reaction. In the chloroplast, there are mainly three types of pigments: chlorophyll a, chlorophyll b, and carotenoids such as lutein. However, only chlorophyll a directly participates in photosynthesis. All the other pigments are accessory pigments. 

Light experiences wave-particle duality, exhibiting the properties of waves and particles. Based on this concept, light absorption occurs when light particles (photons) collide with electrons in chlorophyll a, transmitting energy from themselves to the electrons and activating them. Once energy is transmitted, it changes from luminous energy to chemical energy. The lifespan of an activated electron in chlorophyll a is at most 10⁻⁸ sec. Since activated electrons are extremely unstable, they would quickly release excess energy into the environment in the form of heat or fluorescence, and turn back to the ground state. Excess energy is used for chemical reactions in units called photosystems.

Fig 2. Reflection of the Green Light by Leaves(sc4science, 2024)

Photosystems

Photosystems are the functional units for photosynthesis made of a specific pigment organization and association patterns. There are 200-300 chlorophyll molecules in one photosystem, but only one of them is chlorophyll a, which can transmit the activated electron to a primary electron acceptor. The chlorophyll a molecule, the primary electron acceptor, and several proteins compose the reaction center of a photosystem. All the other pigments outside of the reaction center only pass the luminous energy to one chlorophyll a at the reaction center. These pigments are called antenna pigments.

There are two types of photosystems: photosystem I (PSI or P700) and photosystem II (PSII or P680). The numbers in P700 and P680 represent the absorption peaks of the two photosystems which are respectively 700 nm and 680 nm. On the thylakoid membrane, there are a lot of photosystems that combine together to form electron transport chains.

Fig 3. Structure of Photosystem (Socratic, 2017)

Electron Transport Chain (ETC)

ETC first starts at PSII. PSII loses activated electrons and becomes an oxidizing agent that is oxidative enough to turn H₂O into O₂ + 2H⁺ + 2e⁻. The two electrons generated are reabsorbed by PSII, turning it back to the ground state. The two activated electrons are absorbed by the primary electron acceptor and transmitted to PSI through a chain of electron receptors. ETC is an energy-decreasing chain. Energy gained at the beginning is gradually lost to the environment to form ATP through photophosphorylation while being transmitted among acceptors. This results in a deficiency of energy to reduce NADP⁺, a proton carrier in PSI. Thus, PSI loses two more electrons itself to continue the ETC until there is enough energy to reduce NADP⁺ and H⁺ to NADPH (See Figure 4 below for visual explanation).

Fig 4. Energy Profiling of Photoactivation (Khanacademy, 2023)

The Bigger Picture

Having examined the light-dependent reaction in detail, it’s time to combine all three sections together. To produce one molecule of NADPH, one molecule of H₂O is oxidized. 

H₂O → ½O₂ + 2H⁺ + 2e⁻

NADP⁺ + 2e⁻ + 2H⁺ → NADP + H⁺

The light-dependent reaction can be summarized by the graph below; here, it is shown that light energy and water are used to produce abundant chemical energy and oxygen as a byproduct.

Fig 5. Photolysis (Slideplayer, 2014)

Bibliography

Khanacademy.org. (2023). Khan Academy. [online] Available at: https://www.khanacademy.org/science/ap-biology/cellular-energetics/photosynthesis/a/light-dependent-reactions [Accessed 1 Oct. 2024].

Slideplayer.com. (2014). Photosynthesis!!!! [online] Available at: https://slideplayer.com/slide/4992061/ [Accessed 1 Oct. 2024].

SOUTH CAROLINA GRADE 4 SCIENCE. (2024). 4.P.4A.4 Reflection, Refraction & Absorption. [online] Available at: https://sc4science.weebly.com/4p4a4-reflection-refraction--absorption.html [Accessed 1 Oct. 2024].

What is a photosystem? | Socratic (2017). What is a photosystem? | Socratic. [online] Socratic.org. Available at: https://socratic.org/questions/what-is-a-photosystem [Accessed 1 Oct. 2024].

陈阅增。（2019）。《普通生物学》。第四版。高等教育出版社。