Photosynthesis

Today on learnalittle, learn how your food feeds itself with a look at photosynthesis.

Photosynthesis is a chemical process that uses sunlight to convert carbon dioxide and water into sugars, usually releasing oxygen as a byproduct. Land plants are the most widely known photoautotrophs (they produce energy from sunlight), along with algae and certain bacteria. Though only a handful of organisms use photosynthesis, all life on earth depends on this process. Cyanobacteria first appeared 3,000,000,000 years ago, and gradually oxygenated the planet, allowing complex aerobic (oxygen-breathing) organisms to evolve. Likewise, photoautotrophs serve as the basic terrestrial food source.

Whereas bacteria absorb sunlight through protein systems in their cell membranes, plants and algae have a more complex arrangement. In plants, photosynthesis takes place within organelles (specialized cellular subunits) called chloroplasts, inside stacks of thylakoid discs. The thylakoids contain chlorophyll, the green pigment utilized to capture photons – individual particles of light. Although chlorophyll is ubiquitous in photosynthesizing organisms, some use a range of non-green light-absorbing pigments to catch additional photons. In both plants and algae, these pigments are imbedded in light-harvesting complexes, specialized proteins arranged to collect as much light as possible.

In plants, photosynthesis is divided into two major steps. In the light reactions, chlorophyll in Photosystem II harvests light energy used to break water (H₂0) into oxygen, electrons (negatively charged subatomic particles) and protons (positively charged subatomic particles) in a process called oxygen evolution. This highly energetic electron then passes through a series of proteins in the electron transport chain, creating more protons as it does. At the end of this chain, the electron is excited (charged with energy) once more from the light collected at photosystem I, then passes through another protein chain to create a molecule of NADPH (a molecular energy packet). Likewise, the protons created during the light reactions interact with the ATP synthase enzyme to make ATP, (another molecular energy packet).

Next, the Calvin cycle continues the process with carbon fixation; the enzyme RuBisCO affixes carbon dioxide to RuBP, an organic compound found in plants. The initial result is a very unstable form of sugar; the rest of the Calvin cycle utilizes energy from the ATP and NADPH created in the light reactions to craft stable molecules of sugar that the plant can use later for energy.

It took a lot to explain this process, but photosynthesis takes place in less than a second, tens of thousands of times in each leaf of every plant in the world. Yet despite the prevalence and importance of photosynthesis, scientists are somewhat stumped at how the process even begins. Oxygen evolution is a very energy intensive process; it’s not easy to pull an electron away from its nucleus. Botanists can’t quite figure how photoautotrophs perform this feat so routinely. These light-absorbers not only make earth our habitable home, they also remind us that even the most basic and vital parts of life can present the biggest mysteries.

Source: 1, 2, 3, 4