The Web of Life (53-1) Photosynthesis


The photosynthetic system of plants is Nature’s most elaborate nanoscale biological machine. ~ Indian molecular biologist K.V. Lakshmi

Photosynthesis uses the molecular machinery in plant leaf cells to extract from water and carbon dioxide the electrons and protons needed to produce food and fuel. Like all biochemical reactions, photosynthesis is powered by the actions of electrons.

Plant leaves are green because they are filled with chlorophyll: pigment molecules that selectively absorb light spectra, rejecting (emitting) a certain range seen as green. An incoming photon energizes an electron in the chlorophyll into a mobile state.

Once excited, the electron quickly shuttles from the chlorophyll to a nearby acceptor molecule, setting off a series of electron transfers. Photon capture and conversion into an electron sent on its way happens within 10 to 100 picoseconds. A picosecond is one-trillionth (10–12) of a second.

Moving from one molecule to another within a plant cell’s membrane, a transferred electron ultimately reaches a reaction center, where electrical energy is converted into chemical energy: an ATP molecule.

The initial, near-instantaneous electron transfers are incredibly efficient. Over 95% of light energy hitting a leaf reaches the photosynthesis reaction center.

Succeeding biochemical steps are much less efficient. Nutrients directly affect photosynthetic performance, notably the instant availability of nitrogen and phosphorus. The overall metabolic environment also has an effect.

The superefficient energetics of photosynthetic electron transfer occurs using quantum coherence: photons acting as waves, simultaneously sampling potential pathways and instantaneously selecting the most efficient path. This miracle of productive economy is known as Fermat’s principle.

The structural design for photosynthetic organisms is highly efficient. Light-absorbing chlorophyll molecules in leaves are tightly packed into tiny organelles – chloroplasts – crammed into an arrangement where they come into frequent contact. When excited by photonic energy, chlorophylls no longer act as individual cells, but in concert.

Photosynthesis is a collaborative action. Multicellular cooperation allows plants to absorb energy in a wide spectrum of light. In such a system, other light-absorbing pigment molecules, such as carotenoids, transfer energy efficiently. The individual electrons behave coherently, coordinating their movements as they jostle energy through the system.