Photosystem I and II — Definition

Definition

Imagine tiny solar panels embedded within the specialized membranes inside plant cells, specifically in structures called chloroplasts. These 'solar panels' are what we call photosystems, and there are two main types: Photosystem I (PSI) and Photosystem II (PSII). They are absolutely crucial for photosynthesis, the process by which plants convert light energy into chemical energy to make their food.

Each photosystem isn't just one molecule; it's a complex assembly of many pigment molecules, primarily chlorophylls and carotenoids, along with proteins. Think of it like a funnel. The outer part of this funnel, called the 'antenna complex' or 'light-harvesting complex,' contains hundreds of pigment molecules.

Their job is to efficiently capture light energy from the sun. When a photon of light strikes one of these pigment molecules, the energy is absorbed, exciting an electron to a higher energy level. This excitation energy isn't lost; instead, it's passed from one pigment molecule to another, like a relay race, until it reaches a special pair of chlorophyll 'a' molecules located at the very center of the funnel.

This central part is known as the 'reaction center.

Now, let's differentiate between PSI and PSII. They are named I and II based on the order of their discovery, not their functional sequence in photosynthesis. In the actual process, Photosystem II acts first.

Its reaction center chlorophyll 'a' is specialized to absorb light most effectively at a wavelength of 680 nanometers, hence it's called P680. When P680 absorbs energy, it becomes excited and donates an electron to a primary electron acceptor.

To replenish this lost electron, PSII has a unique ability: it splits water molecules ($H_2O$) in a process called photolysis. This splitting releases electrons (which replace those lost by P680), protons ($H^+$), and molecular oxygen ($O_2$) as a byproduct – the oxygen we breathe!

The electrons then move through an electron transport chain.

Photosystem I, on the other hand, comes second in the non-cyclic electron flow. Its reaction center chlorophyll 'a' is called P700 because it absorbs light maximally at 700 nanometers. P700 also gets excited by light energy (either directly or via its antenna complex) and donates an electron to its own primary electron acceptor.

The electrons that P700 loses are replaced by the electrons coming from Photosystem II via the electron transport chain. The electrons from PSI are then used to reduce NADP+ to NADPH, another crucial energy-carrying molecule for the synthesis of sugars.

Both photosystems work in concert, absorbing light and driving electron flow, ultimately leading to the production of ATP and NADPH, which are the 'energy currency' and 'reducing power' needed for the subsequent sugar-making reactions.