How Does Photosynthesis Work?

The photosynthesis process, in which plants and trees turn light from the sun into nutritional energy, may at first seem like magic, but directly and indirectly, this process sustains the entire world. As green plants reach for the light, their leaves capture the sun's energy by using light-absorbing chemicals or special pigments to make food from carbon dioxide and water pulled from the atmosphere. This process releases oxygen as a by-product back into the atmosphere, a component in air required for all breathing organisms.

Green, growing things are necessary for all life on the planet, not just as food for herbivores and omnivores, but for oxygen to breathe. The photosynthesis process is the primary way oxygen enters the atmosphere. It is the only biological means on the planet that captures the sun's light energy, changing it into sugars and carbohydrates that provides nutrients to plants while releasing oxygen.

Think about it: Plants and trees can essentially pull energy that starts in the outer reaches of space, in the form of sunlight, turn it into food, and in the process, release the needed air that organisms require to thrive. You could say that all oxygen-producing plants and trees have a symbiotic relationship with all oxygen-breathing organisms. Humans and animals provide carbon dioxide to plants, and they deliver oxygen in return. Biologists call this a mutualistic symbiotic relationship because all parties in the relationship benefit.

In the Linnaean classification system, the categorization and ranking of all living things, plants, algae and a type of bacteria called cyanobacteria are the only living entities that produce food from sunlight. The argument for cutting down forests and removing plants for the sake of development seems counterproductive if there are no humans left to live in those developments because there are no plants and trees left to make oxygen.

Plants and trees are autotrophs, living organisms that make their own food. Because they do this using the light energy from the sun, biologists call them photoautotrophs. Most plants and trees on the planet are photoautotrophs.

The conversion of sunlight into food takes place at a cellular level within the leaves of plants in an organelle found in plant cells, a structure called a chloroplast. While leaves consist of several layers, photosynthesis happens in the mesophyll, the middle layer. Small micro openings on the underside of leaves called stomata control the flow of carbon dioxide and oxygen to and from the plant, controlling the plant's gas exchange and the plant's water balance.

Stomata exist on the bottom of leaves, facing away from the sun, to minimize water loss. Little guard cells surrounding the stomata control the opening and closing of these mouth-like openings by swelling or shrinking in response to the amount of water in the atmosphere. When the stomata close, photosynthesis cannot occur, as the plant cannot take in carbon dioxide. This causes carbon dioxide levels in the plant to drop. When the daylight hours become too hot and dry, the stroma closes to conserve moisture.

As an organelle or structure at a cellular level in the plant leaves, chloroplasts have an outer and inner membrane that surrounds them. Inside these membranes are platter-shaped structures called thylakoids. The thylakoid membrane is where the plant and trees store chlorophyll, the green pigment responsible for absorbing the light energy from the sun. This is where the initial light-dependent reactions take place in which numerous proteins make up the transport chain to carry energy pulled from the sun to where it needs to go within the plant.

The photosynthesis process is a two-stage, multi-step process. The first stage of photosynthesis begins with the Light Reactions, also known as the Light Dependent Process and requires light energy from the sun. The second stage, the Dark Reaction stage, also called the Calvin Cycle, is the process by which the plant makes sugar with the help of NADPH and ATP from the light reaction stage.

The Light Reaction phase of photosynthesis involves the following steps:

• Gathering carbon dioxide and water from the atmosphere through the
plant or tree's leaves.
• Light-absorbing green pigments in plants or trees convert the sunlight
into stored chemical energy.
• Activated by light, plant enzymes transport the energy where needed
before releasing it to begin anew.

All this takes place at a cellular level inside the plant's thylakoids, individual flattened sacs, arranged in grana or stacks inside the chloroplasts of the plant or tree cells.

The Calvin Cycle, named for Berkeley biochemist Melvin Calvin (1911-1997), the recipient of the 1961 Nobel Prize in Chemistry for discovering the Dark Reaction stage, is the process by which the plant makes sugar with the help of NADPH and ATP from the light reaction stage. During the Calvin Cycle, the following steps take place:

• Carbon fixation in which plants connect the carbon to plant chemicals
(RuBP) for photosynthesis.
• Reduction phase whereby plant and energy chemicals react to create
plant sugars.
• The formation of carbohydrates as a plant nutrient.
• Regeneration phase where sugar and energy cooperate to form a RuBP
molecule, which allows the cycle to start again.

Embedded within the thylakoid membrane are two light-capturing systems: photosystem I and photosystem II comprised of multiple antenna-like proteins which is where the plant's leaves change light energy into chemical energy. Photosystem I provides a supply of low-energy electron carriers while the other delivers the energized molecules where they need to go.

Chlorophyll is the light-absorbing pigment, inside the leaves of plants and trees, that begins the photosynthesis process. As an organic pigment within the chloroplast thylakoid, chlorophyll only absorbs energy within a narrow band of the electromagnetic spectrum produced by the sun within the wavelength range of 700 nanometers (nm) to 400 nm. Called the photosynthetically active radiation band, green sits in the middle of the visible light spectrum separating the lower energy, but longer wavelength reds, yellows and oranges from the high energy, shorter wavelength, blues, indigoes and violets.

As chlorophylls absorb a single photon or distinct packet of light energy, it causes these molecules to become excited. Once the plant molecule becomes excited, the rest of the steps in the process involve getting that excited molecule into the energy transport system via the energy carrier called nicotinamide adenine dinucleotide phosphate or NADPH, for delivery to the second stage of photosynthesis, the Dark Reaction phase or the Calvin Cycle.

After entering the electron transport chain, the process extracts hydrogen ions from the water taken in and delivers it to the inside of the thylakoid, where these hydrogen ions build up. The ions pass across a semi-porous membrane from the stromal side to the thylakoid lumen, losing some of the energy in the process, as they move through the proteins existing between the two photosystems. The hydrogen ions gather in the thylakoid lumen where they wait for re-energization before participating in the process that makes Adenosine triphosphate or ATP, the energy currency of the cell.

The antenna proteins in photosystem 1 absorb another photon, relaying it to the PS1 reaction center called P700. An oxidized center, P700 sends out a high-energy electron to nicotin-amide adenine dinucleotide phosphate or NADP+ and reduces it to form NADPH and ATP. This is where the plant cell converts light energy into chemical energy.

The chloroplast coordinates the two stages of photosynthesis to use light energy to make sugar. The thylakoids inside the chloroplast represent the sites of the light reactions, while the Calvin Cycle occurs in the stroma.

Cellular respiration, tied to the photosynthesis process, occurs within the plant cell as it takes in light energy, changes it to chemical energy and releases oxygen back into the atmosphere. Respiration occurs within the plant cell happens when the sugars produced during the photosynthetic process combines with oxygen to make energy for the cell, forming carbon dioxide and water as byproducts of respiration. A simple equation for respiration is opposite that of photosynthesis: glucose + oxygen = energy + carbon dioxide + light energy.

Cellular respiration occurs in all the plant's living cells, not only in the leaves, but also in the roots of the plant or tree. Since cellular respiration does not need light energy to occur, it can occur in either the day or night. But overwatering plants in soils with poor drainage causes a problem for cellular respiration, as inundated plants cannot take in enough oxygen through their roots and transform glucose to uphold the cell's metabolic processes. If the plant receives too much water for too long, its roots can be deprived of oxygen, which can essentially stop cellular respiration and kill the plant.

University of California Merced Professor Elliott Campbell and his team of researchers noted in an April 2017 article in "Nature," an international journal of science, that the photosynthesis process increased dramatically during the 20th century. The research team discovered a global record of the photosynthetic process straddling two hundred years.

This led them to conclude that the total of all plant photosynthesis on the planet grew by 30 percent during the years they researched. While the research did not specifically identify the cause of an uptick in the photosynthesis process globally, the team's computer models suggest several processes, when combined, that could result in such a large increase in global plant growth.

The models showed that the leading causes of increased photosynthesis includes increased carbon dioxide emissions in the atmosphere (primarily due to human activities), longer growing seasons because of global warming due to these emissions and increased nitrogen pollution caused by mass agriculture and fossil fuel combustion. Human activities that led to these results have both positive and negative effects on the planet.

Professor Campbell noted that while increased carbon dioxide emissions stimulate crop output, it also stimulates the growth of unwanted weeds and invasive species. He noted that increased carbon dioxide emissions directly cause climate change leading to more flooding along coastal areas, extreme weather conditions and an increase in ocean acidification, all of which have compounding effects globally.

While photosynthesis did increase during the 20th century, it also caused plants to store more carbon in ecosystems around the world, resulting in them becoming carbon sources instead of carbon sinks. Even with the increase in photosynthesis, the increase cannot compensate for fossil fuel combustion, as more carbon dioxide emissions from fossil fuel combustion tend to overwhelm a plant's ability to uptake CO2.

The researchers analyzed Antarctic snow data collected by the National Oceanic and Atmospheric Administration to develop their findings. By studying the gas stored in the ice samples, the researchers reviewed the global atmospheres of the past.