Photosynthesis diagram: illustrative model of photosynthesis showing light capture, electron transport, and proton pumping: 3D computer model.

Photosynthesis diagram showing light capture, electron transport, and proton pumping. Web image measures 600 pixels across, original is 5000 x 5000 pixels.

Photosynthesis is the central biochemical reaction that enables life on Earth. It is a complex and difficult to understand sequence of reactions among various proteins in plant cells and certain bacterial cells. In photosynthesis, energy from the sun is captured and progressively harvested in a ratchet-like mechanism by the movement and energy changes of electrons and by proton pumping. These electrons and protons are derived from hydrogen (a hydrogen atom consists of one proton and one electron). This hydrogen is sourced from water molecules (other sources of hydrogen can be used by certain life forms). When the hydrogen is stripped from water, the remaining oxygen atoms combine to form oxygen molecules (O2), which are released into the atmosphere. The energy trapped by photosynthesis is used to (effectively) build organic molecules from carbon dioxide (CO2) sourced from the atmosphere. In this way, photosynthesis absorbs carbon dioxide and releases oxygen in a neat converse to respiration which burns oxygen to release carbon dioxide.

Photosynthesis illustration: photons of sunlight are shown striking structures inside a plant chloroplast within a plant cell. The main feature is a section through a thylakoid membrane (U-shaped pale brown membrane structure in centre). This bilipid membrane houses various protein complexes that capture the energy in sunlight and drive biosynthesis. Adjacent to the rainbow of sunlight, a blue photon can be seen striking a photosystem II antenna complex, which consists of an array of chlorophyll molecules. Radiative transfer (resonance energy transfer - the electrons themselves do move, they simply pass quanta of energy onto their neighbours) moves the energy to a central chlorophyll molecule, causing an electron to be ejected from it. This drags an electron from a water splitting complex immediately above. The water splitting complex provides a source of new electrons and protons. In the process, oxygen is released (red molecules). Protons (yellow dots) are released into the thylakoid lumen where they help generate a proton concentration gradient (i.e. more concentrated inside the lumen than outside). The electrons released by the splitting of water shuffle into the subjacent chlorophyll molecule and proceed along a chain, giving up their energy progressively as they do so. From photosystem II the electrons are carried by plastoquinol molecules (pale brown elongate molecules that look like mosquito larvae), through the membrane, to a cytochrome molecule (purple lump in the middle of the bottom membrane). The cytochrome molecules can also be seen ejecting protons into the lumen, thereby contributing to the proton gradient. From cytochrome, electrons are ferried by plastocyanin molecules (bluish box-like molecules) to the photosystem I complex (dark greenish lump) situated in the upper membrane. Here, the electrons receive an energy boost courtesy of photons striking the photosystem I complex (red glow). The re-energised electrons are then transported by ferredoxin and other molecules to the little boat shaped molecule in the upper membrane. This is NADP reductase which marries NADP molecules to electrons and protons to create NADPH. NADPH molecules are shown moving upwards where they help drive the Calvin Cycle (not shown). The high concentration of protons in the lumen drives the synthesis of ATP. This happens via the mushroom shaped molecules of ATP synthase seen inserted in the far side of the membrane. Here ADP (adenosine diphosphate) has an additional phosphate pushed onto it to create the highly energetic ATP (adenosine triphosphate). The phosphates are shown as purple balls, dim in ADP and bright in ATP. ATP also helps drive the Calvin cycle.

The Calvin Cycle is the synthesis part of photosynthesis. Here, carbon dioxide molecules enter a biochemical cycle driven by ATP and NADPH resulting in the concatenation of carbon into longer organic molecules.

Cyclic electron transport in shown in the distance at the left edge of the thylakoid section. Here, electrons are transported between a photosystem I complex and cytochrome. The energy captured is recruited to pump protons into the lumen, thereby indirectly driving ATP synthesis.

Thylakoid stacks or grana are shown in the background at the bottom of the illustration. Although generally depicted as discrete flattened sacs that are stacked like coins they actually form a continuous membrane system. The topology is complex and difficult to visualise but effectively ensures that there is one internal compartment (the lumen) and one external compartment.

Why are green plants green? The various pigments used in photosynthesis tend to absorb at the red and blue ends of the spectrum leaving the green light. This is illustrated by the rainbow (spectrum) of light beams, all of which are absorbed except for the green.

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