Photosynthesis

Photosynthesis is the way plants, algae, and certain bacteria transform light energy into chemical energy. The absorbing of light waves gives plants energy, which they store in carbohydrate molecules, so they can later release it and use it for their growth. 

Common photosynthesis maintains life on Earth with the oxygen it provides for the atmosphere, while giving plants nutrients.

The type you can explore while observing plants, algae and cyanobacteria, it supplies the air with oxygen and is most common in nature. It functions as a counterbalance to the carbon dioxide produced by all breathing organisms. 

In oxygenic photosynthesis, energy that comes from light transfers electrons from water to carbon dioxide, resulting in carbohydrates. During this process, water becomes “oxidized” by losing an electron, so that oxygen is produced along with carbs and emitted as a waste product.

Do you wonder why there are so many different colored flowers and vegetables? The answer is simple – pigments. Pigments are molecules that trap sunlight and give plants their color. The 3 main pigment varieties are chlorophylls, carotenoids, and phycobilins.  

Chlorophylls absorb blue and red but reflect green light and are the reason we instinctively connect Flora with the color green. Carotenoids get along well with green light but reflect red to yellow-colored pigments. Phycobilins predominantly reflect red or blue light.

Pigments are the molecules responsible for color in nature. They trap sunlight and reflect certain wavelengths. The color spectrum of the waves that are not absorbed by the plant is observed externally as color.

The green pigments in the chloroplasts of algae and plants is known as chlorophyll. This biomolecule is crucial for absorbing light energy and is the reason we see so many green plants in nature.

Carotenoid pigments absorb bluish-green light and give plants a red, orange, or yellowish dye. They are the magic ingredient in carrots and pumpkins. 

Phycobilins can be seen in red algae or in cyanobacteria. They reflect red or blue light.

Have you noticed that usually a plant’s leaves and stem are green, its flower buds and fruits are dyed by nature in a really catchy color, and its root is either colorless or really pale? Well, for that we should blame plant’s plastids. 

Plastids are organelles that store vital chemical compounds used by the plant cell. The pigments that they contain can determine the cell’s color. Different plastids have different roles in the photosynthetic process and can transform into one another in certain circumstances.  

Chloroplasts contain chlorophyll, which makes them green. Although present in all cells of a plant’s green parts, they are predominantly concentrated in the leaves or stems. 

Leaves are specially adapted structures for catching light, however cacti possess a different leaf anatomy and many conduct photosynthesis through their stems. 

Chromoplasts synthesize and store pigment. You can find this plastid type in fruits, flowers, or roots as well as leaves, but mainly in autumn when they start to age and lose their green color. Chromoplasts accumulate a lot of carotenoid pigments and are quite orange in hue.  

Unlike other plastids, leucoplasts are non-pigmented and lack photosynthetic pigments. 

Located in roots, bulbs, and seeds, these plastids have the genetic potential of developing photosynthesis because if exposed to sunlight they will be transformed into chloroplasts or chromoplasts. 

Light-dependent (or simply “light”) reactions require, as the name suggests, a source of light. During the reaction 1 molecule of the pigment chlorophyll takes in 1 photon, while losing 1 electron. Freed electrons travel through a special electron transport chain. 

The “electron hole” in the chlorophyll molecule regains an electron by a process called photolysis. Light reactions give the plant a lot of energy and are vital for the development and metabolism of its cells. 

During photosynthesis in light-dependent reactions, by adopting 1 photon, the molecule of chlorophyll loses 1 electron and becomes oxidized. Water brings balance into the equation by giving away electrons and producing oxygen.   

The process of light absorption happens within 2 specialized units called photosystem I and II. Each photosystem has an “antennae” that capture light energy in the form of photons, a reaction center that converts light into chemical energy as well as some other components. 

The missing electron from photosystem II is replaced by an electron from photosystem I, which is similarly freed from the reaction center of photosystem I after a photon adoption. The lack of an electron in photosystem I is compensated by an electron that comes from photolysis. 

The cyclic reaction happens only at photosystem I. The freed electron from the chlorophyll molecule is passed down the electron acceptor molecules and eventually returns back to the same photosystem, unlike the more adventurous scenario observed in the non-cyclic reaction. 

After light, there’s darkness, or at least that’s the scenario with the photosynthetic process, performed in plants. These so called “dark” reactions use the energy stored during the previous (light) phase of photosynthesis in order to synthesize carbohydrates. 

They begin with a carbon fixation, which brings CO2 to the table in a succession of chemical reactions in which carbon dioxide is transformed into sugars such as sucrose and starch.

This is the natural process of assimilating carbon from carbon dioxide in the atmosphere. Photosynthesis produces simple carbon sugars, which are then used in the creation of other organic compounds, or as a fuel in cellular respiration.    

Carbon fixation is just the first step of a cyclic process called Calvin’s cycle. What happens is a chain of biochemical reactions performed by plant’s chloroplasts. The Calvin-Benson cycle synthesizes sugars using carbon dioxide.

Early photosynthetic systems were apparently anoxygenic, using various other molecules as electron donors rather than water. Fossils that may have contained photosynthetic organisms have been dated at more than 3 billion years old. Today’s photosynthesis is mostly oxygenic. 

That is good news for us, since we are breathing the fine air of Earth’s atmosphere precisely because oxygenic photosynthesis is all over the place. Or, should we say, all over the planet!

This event, known by many names, such as Oxygen Catastrophe, or The Great Oxidation, is an evolutionary cornerstone for all life forms. Dioxygen (O2) appears in Earth’s atmosphere, as a result of biological evolution, that is, life forms created oxygen. 

Some scientists believe that chloroplasts are photosynthetic bacteria that adapted to life during symbiotic relationships with plants. Chloroplasts have their own DNA, different than the nuclear DNA of the plant’s cells. 

The genes in chloroplasts’ DNA are similar to those found in cyanobacteria.

It is not scientifically proven when oxygenic photosynthesis evolved. Nevertheless, we can “blame” a common ancestor of still present cyanobacteria for developing the biological capacity of using water as the source for electrons in photosynthesis. 

It is unlikely though that the first photosynthetic cyanobacteria generated oxygen because the atmosphere back then contained almost no O2.

A pigment called xanthopterin is used by the Oriental hornet (Vespa orientalis) for converting sunlight into electrical power. This is an important evolutionary step by members of the animal kingdom towards engaging in photosynthesis.

The term sums up all the possible schemes for capturing and accumulating energy from sunlight in the chemical bonds of a fuel, hence the name “solar fuel.” 

Artificial photosynthesis is a chemical process that replicates the natural one, converting light, water, and CO2 into carbs.