Unlike us, plants don't need pantries full of food to stay alive; the Sun is their pantry. But, like us, they require fairly regular sustenance, which they create via photosynthesis. This seemingly magical process occurs through a series of precise steps, each of which we'll cover below. We'll also explain which non-plant organisms use photosynthesis to survive and how unusual plants like cacti turn everyday bits of nature into food.
Photosynthesis: A Chemical and Biological Process
Whether we're standing in a vast forest or our own backyards, plants tend to disguise how they sustain themselves over time. But within every leaf—and sometimes the stem, flower, or fruit—occurs a complex chemical and biological process responsible for turning sunlight, water, and carbon dioxide into chemical energy. This process is what scientists have dubbed photosynthesis.
Photosynthesis starts with roots, stomata, and chloroplasts, biological structures found exclusively in plants. Most of us are familiar with roots. Not only do these keep a plant in place, but they absorb water and other nutrients from the soil. Water is oxidized once it enters a plant, meaning it loses electrons that will become useful for photosynthesis. We'll get back to those electrons in a minute.
Stomata, or plant tissue's equivalent to pores, allow for the exchange of gasses: carbon dioxide in, oxygen and water vapor out. By opening and closing its stomata, a plant can control how much carbon dioxide it consumes and how much of its water content it loses via transpiration. After being absorbed through the stomata, carbon dioxide attaches to Ribulose 1,5-bisphosphate (RuBP), an organic molecule that "kickstarts" a part of photosynthesis called the Calvin cycle.
This cycle can't continue without chloroplasts, a type of organelle found in plant cells. While these resemble the mitochondria found in animal cells, chloroplasts are much larger and contain chlorophyll. Chlorophyll provides the green color most plants are known for while absorbing light energy from the Sun. When the photons from that energy interact with the chlorophyll's electrons—which were obtained through water oxidation—they trigger what's known as a photosynthetic electron-transfer reaction: a chemical reaction that produces a "proton gradient." This gradient acts like a turbine, generating an enzyme called adenosine triphosphate (ATP) synthase and a coenzyme called nicotinamide adenine dinucleotide phosphate (NADPH.)
Remember how carbon dioxide affixed itself to RuBP earlier? That process, called carbon fixation, resulted in the assembly of two three-carbon molecules called 3-phosphoglycerate (3-PGA). The energy stored in ATP and NADPH helps turn these 3-PGA molecules into another three-carbon molecule called glyceraldehyde-3-phosphate, or G3P. While some of these G3P are used to restart the Calvin cycle, others are used to make carbohydrates such as glucose, which are as close to human food as plants get.
Plants can then burn carbohydrates for energy via cellular respiration and store them in long polysaccharide chains known as starch. This is why grains and "starchy vegetables" like corn, potatoes, and winter squash contain more carbohydrates than other plant-based foods: they store more of their energy.
C3 Versus C4 Photosynthesis
Now and then, RuBP will affix itself to oxygen instead of carbon dioxide in a process known as photorespiration. This wasteful pathway results in the consumption—not production—of energy. While this is a fairly common mistake, it occurs more often in hot, dry environments when a plant's stomata close to prevent water loss. This creates an intracellular environment rich in oxygen and relatively low in carbon dioxide.
Some plants, like rice, wheat, and maize, can combat photorespiration through a photosynthesis variation known as C4 photosynthesis. These plants engage an enzyme called phosphoenolpyruvate (PEP) carboxylase near the stomata. Because PEP carboxylase is more attracted to carbon dioxide than oxygen, it ushers carbon dioxide into four-carbon molecules as soon as it enters the stomata. These molecules, called malate, form sheaths around RuBP to prevent photorespiration.
Most plants utilize C3 photosynthesis, though, meaning they don't have built-in safeguards against photorespiration. Some research groups, including those at the University of Illinois and the US Department of Agriculture, are working to "fix" photorespiration by engineering new carbon fixation roadmaps. This would reportedly make C3 plants 40% more efficient at photosynthesis, but in the meantime, most Earthly plants are stuck attaching RuBP to oxygen about 20% of the time.
Photosynthesizing Oddballs
As we hinted at earlier, not every plant uses leaves to photosynthesize, and not everything that photosynthesizes is considered a plant. Cacti and other succulents don't usually have leaves; instead, they feature arms and pads biologically closest to modified stems. These arms and pads contain the stomata and chloroplasts responsible for photosynthesis.
But because succulents tend to live in hot, arid climates, they utilize a third type of photosynthesis known as crassulacean acid metabolism (CAM) photosynthesis. During this process, the plant's stomata open at night when the air is cooler. This allows the plant to capture and store carbon dioxide until the Sun rises again, offering the light necessary to complete photosynthesis.
Algae are widely thought to conduct half of the world's photosynthesis, partly because they're more efficient at turning light into energy and partly because of their sheer quantity. But algae aren't plants, at least not by most standards. Instead, they're considered "protists," a catch-all category that includes almost any multi-celled organism that isn't a plant, animal, fungus, bacterium, or archaeon. Cyanobacteria, a single-celled organism known as "blue-green algae" even though it is not algae, also gets its energy from photosynthesis.
