Photosynthesis: An Overview of Autotrophic Nutrition

Every gram of food a human eats, every breath of oxygen taken, and almost every joule of energy that flows through the living world can be traced back to one process: photosynthesis. Green plants, algae and cyanobacteria are described as autotrophs (Greek: "self" + "nourishing") precisely because, unlike animals and fungi, they do not need to consume other organisms to obtain organic food. Instead, they manufacture their own organic molecules from the simplest possible inorganic raw materials -- carbon dioxide from the air and water from the soil -- using light energy captured by the green pigment chlorophyll. Photosynthesis is therefore the defining biochemical process of autotrophic nutrition, and it is not an exaggeration to call it the single most important biochemical process on the planet.

The overall transformation can be summarised in one balanced equation, even though, as later sections show, it actually proceeds through dozens of individual enzyme-catalysed steps:

6CO₂ + 12H₂O → (light, chlorophyll) C₆H₁₂O₆ + 6O₂ + 6H₂O

Read carefully, this equation already hints at several of the chapter's central themes. Twelve molecules of water are consumed, but only six are regenerated, meaning six water molecules are genuinely used up (split) to release oxygen -- a fact whose significance is explored in the section on early experiments and again when the light reaction's photolysis step is described. Sunlight and chlorophyll appear above the arrow because they are the essential energy source and catalytic apparatus, not raw materials consumed in stoichiometric amounts. And glucose, though it is the product usually shown for simplicity, is in reality more often assembled into starch for storage or sucrose for transport within the plant.

The importance of this single process for the rest of the living world is difficult to overstate. Photosynthesis is the ultimate source of the food energy that flows through almost every food chain on Earth, whether the herbivore eating a leaf, the carnivore eating the herbivore, or the human eating bread made from wheat grain. It is also the source of virtually all the molecular oxygen (O2) in the atmosphere that aerobic organisms, including humans, depend on for respiration -- a byproduct so significant that Earth's atmosphere would have an entirely different, oxygen-poor composition without billions of years of continuous photosynthetic activity. And by removing carbon dioxide from the atmosphere and locking it into organic carbon compounds (and, over geological time, into fossil fuels and carbonate rocks), photosynthesis has also been a central regulator of Earth's climate and carbon cycle across its history.

Physiologists traditionally divide the overall process into two broad, sequential phases, a division first clearly articulated by F.F. Blackman in the early twentieth century and explored throughout this chapter. The first, the photochemical (light) phase, takes place on the thylakoid membranes inside the chloroplast, strictly requires light, and converts light energy into the chemical energy of ATP and NADPH while splitting water and releasing oxygen. The second, the biosynthetic (dark) phase, takes place in the fluid stroma surrounding the thylakoids, does not itself require light directly (though it depends entirely on the ATP and NADPH the light phase supplies, and several of its enzymes are light-activated), and uses that chemical energy to fix atmospheric CO2 into carbohydrate through the Calvin cycle, or, in many warm-climate and arid-climate plants, through the specialised C4 or CAM variants of that same underlying biochemistry. The chapters that follow trace this whole story in order: the historical experiments that first revealed each of these facts, the chloroplast's structure as the physical site where they occur, the pigments that capture light, the light reaction itself, the Calvin cycle and its C4 and CAM variants, the wasteful side reaction of photorespiration, and finally the external factors -- light, CO2, temperature and water -- that together set the overall rate at which a real leaf, in a real field, actually photosynthesises.