Photosynthesis can defensibly be labeled the most important reaction in all of biology. Examine any food web or energy-flow system in the world, and you'll find that it ultimately relies on energy from the sun for the substances that sustain the organisms therein. Animals rely on both the the carbon-based nutrients (carbohydrates) and the oxygen that photosynthesis generates, because even animals that get all of their nourishment by preying on other animals wind up eating organisms that themselves live mostly or exclusively on plants.

From photosynthesis thus flows all of the other processes of energy exchange observed in nature. Like glycolysis and the reactions of cellular respiration, photosynthesis has a bevy of steps, enzymes and unique aspects to consider, and understanding the roles that the specific catalysts of photosynthesis play in what amounts to the conversion of light and gas to food is critical to mastering basic biochemistry.

Photosynthesis had something to do with the production of the last thing you ate, whatever that was. If it was plant-based, the claim is straightforward. If it was a hamburger, the meat almost certainly came from an animal that itself subsisted almost entirely on plants. Looked at somewhat differently, if the sun were to shut itself off today without causing the world to cool off, which would lead to making plants scarce, the world's food supply would soon vanish; plants, which are clearly not predators, are at the very bottom of any food chain.

Photosynthesis is traditionally divided into the light reactions and the dark reactions. Both reactions in photosynthesis play critical roles; the former rely on the presence of sunlight or other light energy, while the latter do not but depend on the products of the light reaction to have substrate to work with. In the light reactions, the energy molecules that the plant needs to assemble carbohydrate are made, while carbohydrate synthesis itself occurs the dark reactions. This is similar in some ways to aerobic respiration, where the Krebs cycle, though not a major direct source of ATP (adenosine triphosphate, the "energy currency" of all cells), generates a great deal of intermediate molecules that drive the creation of a great deal of ATP in the subsequent electron transport chain reactions.

The critical element in plants that allows them to conduct photosynthesis is chlorophyll, a substance that is found in unique structures called chloroplasts.

The net reaction of photosynthesis is actually very simple. It states that carbon dioxide and water, in the presence of light energy, are converted to glucose and oxygen during the process.

6 CO₂ + light + 6 H₂O → C₆H₁₂O₆ + 6 O₂

The overall reaction is a sum of the light reactions and the dark reactions of photosynthesis:

Light reactions: 12 H₂O + light → O₂ + 24 H⁺ + 24e⁻

Dark reactions: 6CO₂ + 24 H⁺ + 24 e⁻ → C₆H₁₂O₆ + 6 H₂O

In short, the light reactions use sunlight to scare up electrons that the plant then channels into making food (glucose). How this occurs in practice has been well studied and is a testament to billions of years of biological evolution.

A common misconception among people studying the life sciences is that photosynthesis is simply cellular respiration in reverse. This is understandable, given that the net reaction of photosynthesis looks just like cellular respiration – starting with glycolysis and ending with the aerobic processes (Krebs cycle and electron transport chain) in mitochondria – run precisely in reverse.

The reactions that transform carbon dioxide to glucose in photosynthesis are far different, however, than those that are used to reduce glucose back down to carbon dioxide in cellular respiration. Plants, keep in mind, also make use of cellular respiration. Chloroplasts are not "the mitochondria of plants"; plants have mitochondria, too.

Think of photosynthesis as something that happens mainly because plants don't have mouths, yet still rely on burning glucose as a nutrient to make their own fuel. If plants cannot ingest glucose yet still require a steady supply of it, then they have to do the seemingly impossible and make it themselves. How do plants make food? They use external light to drive tiny power plants inside them to do it. That they can do so depends to a large extent on how they are actually structured.

Structures that have a lot of surface area in relation to their mass are well positioned to capture a great deal of the sunlight passing their way. This is why plants have leaves. The fact that leaves tend to be the greenest part of plants is the result of the density of chlorophyll in leaves, as this is where the work of photosynthesis is done.

Leaves have evolved pores in their surfaces called stomata (singular: stoma). These apertures are the means by which the leaf can control the entry and exit of CO₂, which is needed for photosynthesis, and O₂, which is a waste product of the process. (It is counterintuitive to think of oxygen as waste, but in this setting, strictly speaking, that's what it is.)

These stomata also help the leaf regulate its water content. When water is plentiful, the leaves are more rigid and "inflated" and the stomata are inclined to remain closed. Conversely, when water is scarce, the stomata open in an effort to help the leaf nourish itself.

Plant cells are eukaryotic cells, meaning that they have both the four structures common to all cells (DNA, a cell membrane, cytoplasm and ribosomes) and a number of specialized organelles. Plant cells, however, unlike animal and other eukaryotic cells, have cell walls, like bacteria do but constructed using different chemicals.

Plant cells also have nuclei, and their organelles include the mitochondria, the endoplasmic reticulum, Golgi bodies, a cytoskeleton and vacuoles. But the critical difference between plant cells and other eukaryotic cells is that plant cells contain chloroplasts.

Within plant cells are organelles called chloroplasts. Like mitochondria, these are believed to have been incorporated into eukaryotic organisms relatively early in the evolution of eukaryotes, with the entity destined to become a chloroplast then existing as a free-standing photosynthesis-performing prokaryote.

The chloroplast, like all organelles, is surrounded by a double plasma membrane. Within this membrane is the stroma, which functions sort of like the cytoplasm of chloroplasts. Also within the chloroplasts are bodies called thylakoid, which are arranged like stacks of coins and enclosed by a membrane of their own.

Chlorophyll is considered "the" pigment of photosynthesis, but there are several different types of chlorophyll, and pigment other than chlorophyll participate in photosynthesis, too. The major pigment used in photosynthesis is chlorophyll A. Some non-chlorophyll pigments that take part in photosynthetic processes are red, brown or blue in color.

The light reactions of photosynthesis use light energy to displace hydrogen atoms from water molecules, with these hydrogen atoms, powered by the flow of electrons ultimately liberated by incoming light, being used to synthesize NADPH and ATP, which are needed for the subsequent dark reactions.

The light reactions occur on the thylakoid membrane, inside the chloroplast, inside the plant cell. They get underway when light strikes a protein-chlorophyll complex called photosystem II (PSII). This enzyme is what liberates the hydrogen atoms from water molecules. The oxygen in the water is then free, and the electrons freed in the process are attached to a molecule called plastoquinol, turning it into plastoquinone. This molecule in turn transfers the electrons to an enzyme complex called cytochrome b6f. This ctyb6f takes the electrons from plastoquinone and moves them to plastocyanin.

At this point, photosystem I (PSI) gets on the job. This enzyme takes the electrons from plastocyanin and attaches them to an iron-containing compound called ferredoxin. Finally, an enzyme called ferredoxin–NADP⁺reductase (FNR) to make NADPH from NADP⁺. You don't need to memorize all of these compounds, but it is important to have a sense of the cascading, "handing-off" nature of the reactions involved.

Also, when PSII is liberating hydrogen from water to power the above reactions, some of that hydrogen tends to want to leave the thylakoid for the stroma, down its concentration gradient. The thylakoid membrane takes advantage of this natural outflow by using it to power an ATP synthase pump in the membrane, which attaches phosphate molecules to ADP (adenosine diphosphate) to make ATP.

The dark reactions of photosynthesis are so named because they do not rely on light. However, they can occur when light is present, so a more accurate, if more cumbersome, name is "light-independent reactions." To clear matters up further, the dark reactions are together also known as the Calvin cycle.

Imagine that, when inhaling air into your lungs, the carbon dioxide in that air could make its way into your cells, which would then use it to make the same substance that results from your body breaking down the food you eat. In fact, because of this, you would never have to eat at all. This is essentially the life of a plant, which uses the CO₂ it gathers from the environment (which is there largely as a result of the metabolic processes of other eukaryotes) to make glucose, which it then either stores or burns for its own needs.

You have already seen that photosynthesis starts by knocking hydrogen atoms free from water and using the energy from those atoms to make some NADPH and some ATP. But so far, there has been no mention of the other input into photosynthesis, CO2. Now you'll see why all of that NADPH and ATP was harvested in the first place.

In the first step of the dark reactions, CO2 is attached to a five-carbon sugar derivative called ribulose 1,5-bisphosphate. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, much more memorably known as Rubisco. This enzyme is believed to be the most abundant protein in the world, given that it is present in all plants that undergo photosynthesis.

This six-carbon intermediate is unstable and splits into a pair of three-carbon molecules called phosphoglycerate. These are then phosphorylated by a kinase enzyme to form 1,3-bisphosphoglycerate. This molecule is then converted to glyceraldehyde-3-phosphate (G3P), liberating phosphate molecules and consuming NAPDH derived from the light reactions.

The G3P created in these reactions can then be put into a number of different pathways, resulting in the formation of glucose, amino acids or lipids, depending on the specific needs of the plant cells. Plants also synthesize polymers of glucose that in the human diet contribute starch and fiber.