Photosynthesis converts light into chemical energy, sustaining most life on Earth. This process is carried out by molecular machinery within the cells of plants, algae, and some bacteria, relying on protein complexes known as photosystems. If Photosystem II, a primary component of this machinery, were suddenly absent, the consequences for photosynthetic organisms would be immediate.
The Role of Photosystem II
Photosystem II (PSII) is the initial protein complex in the light-dependent reactions of oxygenic photosynthesis. Located within the thylakoid membranes of chloroplasts, its primary job is to capture photons of light. This captured light energy is used to energize electrons, setting in motion the entire photosynthetic process. PSII is unique because it replenishes its lost electrons by splitting water molecules, a process called photolysis.
This water-splitting process is the source of nearly all oxygen in Earth’s atmosphere. The reaction produces oxygen gas, protons (hydrogen ions), and high-energy electrons. The protons contribute to a gradient that drives the synthesis of ATP, an energy-carrying molecule, while the energized electrons are the first link in the photosynthetic chain.
These high-energy electrons are transferred to a molecule called plastoquinone, which then shuttles them to the next stage of the process. By capturing light and splitting water, PSII provides the electrons that fuel the subsequent steps of converting light energy into chemical energy.
Immediate Shutdown of the Photosynthetic Process
If Photosystem II were absent, the immediate effect would be the complete stop of linear electron flow. This flow is the sequential movement of electrons from PSII through carrier molecules to Photosystem I (PSI). Without PSII to initiate this process by supplying electrons from water, the entire chain is broken at its first link, leaving the transport machinery idle.
The halt in electron flow directly prevents the production of ATP and NADPH. ATP is generated using a proton gradient built up by the movement of electrons down the transport chain. Without electron flow, this proton gradient cannot be established, and the enzyme ATP synthase would lack the power to produce ATP.
Similarly, the production of NADPH would cease. NADPH is formed when electrons, after being re-energized by Photosystem I, are transferred to NADP+. Without the initial stream of electrons from PSII, there are no electrons to reach PSI and ultimately reduce NADP+ to NADPH, creating an immediate energy crisis within the cell.
The final consequence is the failure of the Calvin cycle, the light-independent stage of photosynthesis. This cycle uses the ATP and NADPH generated during the light-dependent reactions to convert carbon dioxide into glucose. Without the necessary ATP and NADPH, the Calvin cycle cannot proceed, and the organism can no longer fix carbon to produce its own food.
The Isolated Function of Photosystem I
Even without Photosystem II, Photosystem I (PSI) is capable of a separate process known as cyclic photophosphorylation. In this pathway, electrons that are energized by light in PSI are not passed on to create NADPH. Instead, they are rerouted back into the electron transport chain, creating a cyclical flow.
This cyclic electron flow still pumps protons across the thylakoid membrane, establishing a proton gradient. This gradient allows ATP synthase to produce ATP, meaning the organism can still generate some of its required energy currency. This process is a normal part of photosynthesis, often used when the cell’s need for ATP is higher than its need for NADPH.
However, cyclic photophosphorylation alone is insufficient for survival. The issue is that this process does not produce NADPH. Without NADPH, the Calvin cycle cannot reduce carbon dioxide to create sugars. While the organism can produce some ATP, it is unable to manufacture the carbohydrates necessary for building cellular structures and long-term energy storage, leading to starvation.
Consequences for Global Ecosystems
On a global scale, the absence of Photosystem II would cause a collapse of nearly all ecosystems. The primary source of atmospheric oxygen is the water-splitting function of PSII. Without it, the continuous replenishment of oxygen would stop. The existing oxygen would be consumed by the respiration of other organisms, leading to an anoxic atmosphere.
This atmospheric shift would be paralleled by a collapse of the world’s food webs. Photosynthetic organisms, from the smallest algae to the largest trees, form the base of almost every food chain on the planet. Their inability to produce glucose would lead to their widespread death.
This would remove the primary food source for herbivores, which would in turn lead to the starvation of carnivores and omnivores. The intricate web of life would unravel. The outcome would be a mass extinction event, wiping out the vast majority of complex life on Earth, which is dependent on the oxygen and energy that photosynthesis provides.