Conversion of light energy to chemical energy. Reactions of photosynthesis, where they take place, and their ecological importance.


Have you hugged a tree lately? If not, you might want to give it some thought. You, along with the rest of the human population, owe your existence to plants and other organisms that capture light. In fact, most life on Earth is possible because the sun provides a continuous supply of energy to ecosystems.
All organisms, including humans, need energy to fuel the metabolic reactions of growth, development, and reproduction. But organisms can't use light energy directly for their metabolic needs. Instead, it must first be converted into chemical energy through the process of photosynthesis.

What is photosynthesis?

Photosynthesis is the process in which light energy is converted to chemical energy in the form of sugars. In a process driven by light energy, glucose molecules (or other sugars) are constructed from water and carbon dioxide, and oxygen is released as a byproduct. The glucose molecules provide organisms with two crucial resources: energy and fixed—organic—carbon.
  • Energy. The glucose molecules serve as fuel for cells: their chemical energy can be harvested through processes like cellular respiration and fermentation, which generate adenosine triphosphate—ATP\text{ATP}, a small, energy-carrying molecule—for the cell’s immediate energy needs.
  • Fixed carbon. Carbon from carbon dioxide—inorganic carbon—can be incorporated into organic molecules; this process is called carbon fixation, and the carbon in organic molecules is also known as fixed carbon. The carbon that's fixed and incorporated into sugars during photosynthesis can be used to build other types of organic molecules needed by cells.

The ecological importance of photosynthesis

Photosynthetic organisms, including plants, algae, and some bacteria, play a key ecological role. They introduce chemical energy and fixed carbon into ecosystems by using light to synthesize sugars. Since these organisms produce their own food—that is, fix their own carbon—using light energy, they are called photoautotrophs (literally, self-feeders that use light).
Humans, and other organisms that can’t convert carbon dioxide to organic compounds themselves, are called heterotrophs, meaning different-feeders. Heterotrophs must get fixed carbon by eating other organisms or their by-products. Animals, fungi, and many prokaryotes and protists are heterotrophs.
Besides introducing fixed carbon and energy into ecosystems, photosynthesis also affects the makeup of Earth’s atmosphere. Most photosynthetic organisms generate oxygen gas as a byproduct, and the advent of photosynthesis—over 33 billion years ago, in bacteria resembling modern cyanobacteria—forever changed life on Earth1^1. These bacteria gradually released oxygen into Earth’s oxygen-poor atmosphere, and the increase in oxygen concentration is thought to have influenced the evolution of aerobic life forms—organisms that use oxygen for cellular respiration. If it hadn’t been for those ancient photosynthesizers, we, like many other species, wouldn't be here today!
Photosynthetic organisms also remove large quantities of carbon dioxide from the atmosphere and use the carbon atoms to build organic molecules. Without Earth’s abundance of plants and algae to continually suck up carbon dioxide, the gas would build up in the atmosphere. Although photosynthetic organisms remove some of the carbon dioxide produced by human activities, rising atmospheric levels are trapping heat and causing the climate to change. Many scientists believe that preserving forests and other expanses of vegetation is increasingly important to combat this rise in carbon dioxide levels.

Leaves are sites of photosynthesis

Plants are the most common autotrophs in terrestrial—land—ecosystems. All green plant tissues can photosynthesize, but in most plants, but the majority of photosynthesis usually takes place in the leaves. The cells in a middle layer of leaf tissue called the mesophyll are the primary site of photosynthesis.
Small pores called stomata—singular, stoma—are found on the surface of leaves in most plants, and they let carbon dioxide diffuse into the mesophyll layer and oxygen diffuse out.
Each mesophyll cell contains organelles called chloroplasts, which are specialized to carry out the reactions of photosynthesis. Within each chloroplast, disc-like structures called thylakoids are arranged in piles like stacks of pancakes that are known as grana—singular, granum. The membrane of each thylakoid contains green-colored pigments called chlorophylls that absorb light. The fluid-filled space around the grana is called the stroma, and the space inside the thylakoid discs is known as the thylakoid space. Different chemical reactions occur in the different parts of the chloroplast.

The light-dependent reactions and the Calvin cycle

Photosynthesis in the leaves of plants involves many steps, but it can be divided into two stages: the light-dependent reactions and the Calvin cycle.
  • The light-dependent reactions take place in the thylakoid membrane and require a continuous supply of light energy. Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds, ATP\text{ATP}—an energy storage molecule—and NADPH\text{NADPH}—a reduced (electron-bearing) electron carrier. In this process, water molecules are also converted to oxygen gas—the oxygen we breathe!
  • The Calvin cycle, also called the light-independent reactions, takes place in the stroma and does not directly require light. Instead, the Calvin cycle uses ATP\text{ATP} and NADPH\text{NADPH} from the light-dependent reactions to fix carbon dioxide and produce three-carbon sugars—glyceraldehyde-3-phosphate, or G3P, molecules—which join up to form glucose.
Overall, the light-dependent reactions capture light energy and store it temporarily in the chemical forms of ATP\text{ATP} and NADPH\text{NADPH}. There, ATP\text{ATP} is broken down to release energy, and NADPH\text{NADPH} donates its electrons to convert carbon dioxide molecules into sugars. In the end, the energy that started out as light winds up trapped in the bonds of the sugars.

Photosynthesis vs. cellular respiration

At the level of the overall reactions, photosynthesis and cellular respiration are near-opposite processes. They differ only in the form of energy absorbed or released, as shown in the diagram below.
At the level of individual steps, photosynthesis isn't just cellular respiration run in reverse. Instead, as we'll see the rest of this section, photosynthesis takes place in its own unique series of steps. However, there are some notable similarities between photosynthesis and cellular respiration.
For instance, photosynthesis and cellular respiration both involve a series of redox reactions (reactions involving electron transfers). In cellular respiration, electrons flow from glucose to oxygen, forming water and releasing energy. In photosynthesis, they go in the opposite direction, starting in water and winding up in glucose—an energy-requiring process powered by light. Like cellular respiration, photosynthesis also uses an electron transport chain to make a H+\text{H}^+ concentration gradient, which drives ATP\text{ATP} synthesis by chemiosmosis.
If those things don't sound familiar, though, don't worry! You don't need to know cellular respiration to understand photosynthesis. Just keep reading and watching, and you'll learn all the ins and outs of this life-sustaining process.