Photosynthesis: Carbon Cycle & Climate

A simple explanation of how plants photosynthesize. We also answer questions like: Why is Photosynthesis Important? What is the formula? How does photosynthesis connect the carbon cycle? How is it related to climate change?
Photosynthesis Easy Guide

In this article, we explain all about photosynthesis – arguably the single most important process on Earth. We explain its formula, how it works and why it’s essential to life. We also explain how photosynthesis generates precious oxygen, connects the carbon cycle, and helps to combat climate change by absorbing carbon dioxide from the atmosphere.

What is Photosynthesis?

Photosynthesis is a biochemical process which allows plants to convert sunlight into energy (food) and then store it. 1

This plant-produced energy is then available to all other creatures throughout the food web on land and in the ocean. Herbivores acquire this energy by eating plants, and carnivores get it by eating herbivores.

During photosynthesis, green plants make use of water (from their roots), carbon dioxide (from the air), the green pigment called chlorophyll (in their leaves), and light energy (from the sun) to create glucose and other sugars. It’s not just land-based plants that photosynthesize.

Ocean organisms known as phytoplankton (mostly green algae) also convert sunlight into energy. Phytoplankton are the foundation of the marine food web, feeding everything in the ocean – from microscopic zooplankton all the way up to huge whales.

Organisms, like plants, that can make their own energy using light and carbon dioxide via the process of photosynthesis, are called photoautotrophs. Organisms, like animals or humans, who are unable to produce their own food and who must rely on photoautotrophs to do it for them, are called heterotrophs.

How Photosynthesis Works
Photosynthesis is one of the most important biological processes on Earth. It generates energy for living creatures; it provides most of the oxygen that humans and animals breathe; it recycles carbon dioxide for the benefit of our climate system, as well as the carbon cycle. Image: At09kg/CC BY-SA 3.0

Why Is Photosynthesis Important?

Photosynthesis is not merely a means of turning sunlight into usable energy – after all, sunlight disappears at night. Photosynthesis is also a way of storing sunlight for use when there’s no sunshine. Plants turn light energy into chemical energy and then store it in their leaves, for use at night or whenever there is no sunlight. In any event, photosynthesis powers 99 percent of Earth’s ecosystems. 2

Without photosynthesis, plants and phytoplankton could not survive, and without these primary producers – the only living things on Earth that can convert the sun’s energy into food – the Earth’s biological food chain would collapse. (For details of the process on which plants expend most of their energy, see: What is Transpiration?)

What Is The Formula For Photosynthesis?

The basic formula for photosynthesis is:

Carbon dioxide + water + sunlight -> carbohydrate + oxygen

To be specific, photosynthesis combines light energy with six molecules of carbon dioxide and six molecules of water to produce six molecules of oxygen and one molecule of sugar. As illustrated in this chemical formula:

How Does Photosynthesis Work Exactly?

Photosynthesis requires carbon dioxide (CO2), water and sunlight. Plus, it requires chlorophyll, the green pigment in the leaves of green plants. Carbon dioxide is ingested into the plant through small, regulated openings called stomata, located on the underside of leaves. Each stoma is regulated by cells on either side of it that swell or shrink in response to osmotic changes. Stomata also act as the exit for oxygen produced during the photosynthetic process.

In plants, photosynthesis takes place inside leaves. Leaves contain several layers of cells. Photosynthesis occurs in the middle layer (the mesophyll), inside a specialist cell compartment (the chloroplast). Chloroplasts are surrounded by a double membrane. Inside the chloroplast, surrounded by an area of fluid (the stroma), there are stacks of disc-shaped compartments (the thylakoids), inside which, is a green pigment (chlorophyll). Chlorophyll is a large molecule with a special structure that allows it to capture light energy and convert it to high energy electrons, which are used to produce the sugar or glucose.

Photosynthesis takes place in two sequential stages: (1) Light-dependent reactions. (2) Light-independent reactions. This second stage is called the Calvin Cycle because it was discovered by the American biochemist and Nobel Prize winner Melvin Calvin (1911-97).

Stage 1. Light-Dependent Reactions

In this part of the photosynthesis process, energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy, in the form of two molecules – ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These molecules carry the energy that will be used in stage 2. All this happens inside a multi-protein complex called a photosystem, located inside the thylakoid compartments within the chloroplast.

Stage 2. Light-Independent Reactions

This stage takes place (day or night) outside the thylakoid compartments, in the stroma of the chloroplasts. During this series of reactions (known as carbon fixation) energy carried by the ATP and NADPH molecules, drives a chemical process that utilizes the carbon in carbon dioxide (from the atmosphere) to build the three-carbon sugar G3P (glyceraldehyde-3-phosphate). Six molecules of carbon dioxide are used in the process, which generates one molecule of sugar.

The G3P is then shared with other cells outside the chloroplast, who use it to create a variety of other sugars, like glucose. These products are then transported to other parts of the cell, including the mitochondria, where they are broken down into energy that can be used for repair and growth. During the daytime, when photosynthesis is taking place, plants typically produce more glucose than they can consume. This extra energy is stored by synthesizing glucose (a 6-carbon sugar) to make starch. Plants rely primarily on starch for energy during the night. 3

The oxygen that is produced is released from the same tiny holes through which the carbon dioxide entered.

Photosynthesis In The Ocean 

The way land plants photosynthesize is different from the way ocean plants, like phytoplankton, do it. This is because it is more difficult for sunlight to penetrate water than air. Specifically, the yellow and red portions of light only penetrate to a depth of 100 metres (330 feet), less than half the depth of the blue and green portions of light. Unfortunately, the chlorophyll in phytoplankton absorbs mostly red, orange and blue light. Happily, nature has evolved some helpful chemical molecules called phycobiliproteins, which are able to convert the colours of light available in the oceans to a colour that chlorophyll molecules can use.

These phycobiliproteins are found inside the cell membranes of tiny marine organisms called cyanobacteria (also known as blue-green algae). These ancient microbes have been around for billions of years, and have long since adapted to dim light conditions. 4

Phycobiliproteins help by converting green light to red light, which is ideal for chlorophyll. 5 It takes a lot of work by three different phycobiliprotein molecules to complete the colour conversion, but it’s proving very effective.

Scientists believe that phytoplankton contribute between 50 to 85 percent of the oxygen in Earth’s atmosphere. 6

Other important marine plants whose photosynthesis has a major impact, include mangroves (mangals), seagrasses and kelp forests, all of which are important reservoirs of “blue carbon” – so-called because the carbon is stored underwater.

Other Benefits Of Photosynthesis

The photosynthesizing performed by plants and marine organisms provides three other major benefits for the planet. It produces oxygen, it recycles carbon dioxide (CO2) from the atmosphere into living creatures on land and sea, and it helps to mitigate global warming.

Photosynthesis is the No 1. Source of Oxygen

Photosynthesis is the main source for Earth’s oxygen, which is essential for breathing and cellular respiration. Without photosynthesis, which allows plants and phytoplankton to produce oxygen as a byproduct, there would be no aerobic organisms like humans or animals. Also, there would be no ozone layer to protect us from damaging UV radiation.

As we have seen, plants and algae are vital energy-providers and they’re also essential for the health of our climate system, due to their photosynthetic uptake of carbon dioxide, but their role in the oxygen cycle is equally critical.

Photosynthesis Connects the Carbon Cycle

Photosynthesis is essential to the fast carbon cycle, for the simple reason that there is no other way to circulate carbon dioxide from the atmosphere to living creatures on Earth. Photosynthetic plants and trees capture carbon (CO2) from the air and pass it up the food chain to a variety of organisms. (See also: Tree-Planting: Is It the Answer to Global Warming?)

The CO2 returns to the atmosphere through respiration (by plants or animals), or enters the soil. Here, it is either recycled back into the food web by decomposers and detritivores, or it enters the slow carbon cycle.

For example, if plants die and are not broken down and recycled by decomposers, their remains may slowly sink into the soil, where – over several million years – they turn into fossil fuels, like coal. (See also: Why is Soil So Important to the Planet?)

Some CO2 ingested by phytoplankton falls into the depths when they, or the creatures that eat them, die, and may only return to the ocean surface thousands of years later after being absorbed and upwelled by deep water thermohaline circulation currents.

A percentage of organic carbon from phytoplankton ultimately ends up in sediments on the ocean floor, and passes into the lithosphere, from where – after millions of years – it is outgassed through volcanic activity.

Photosynthesis and Climate Change

Climate change affects plants in several ways. To begin with, the more CO2 in the air, the more a plant can photosynthesize and the more energy it creates.

Rising temperature also affects photosynthesis, but plants are very good at adapting to hotter conditions provided they get enough water. As the temperature increases, air pressure may reduce, resulting in an increase in the rate of transpiration from tree canopies. However, if the tiny openings (stomata) in the leaves of trees close in order to conserve water, then transpiration rates are likely to diminish.

When temperatures rise, organic matter in the soil tends to be broken down more rapidly by decomposers, making nutrients more readily available to plants. Typically, this increases photosynthetic gain of biomass in nutrient-limited systems.

Combats Global Warming

Photosynthesis helps to limit global warming because of the large quantities of CO2 that plants and trees absorb when photosynthesizing – CO2 that would otherwise contribute to the greenhouse effect in the atmosphere. Statistically, about 25 per cent of carbon emissions from the use of fossil fuels is absorbed and stored by plants. It’s true that plants release some of this CO2 (about half) back into the air during respiration but, overall, vegetation and forests are carbon sinks. In fact, the terrestrial biosphere is one of the largest carbon reservoirs on the planet.

See also: 7 Effects of Climate Change on Plants.

Photosynthesis may help to counter climate change in a second way, although more evidence is needed. Up to now, most climate models have assumed that, as temperature doubles, so does plant respiration. Now, the most comprehensive global study of its kind suggests that this assumption may no longer be true, and that increases in plant respiration may not be as large as previously thought. In other words, plants may become more efficient. They’ll photosynthesize more (absorb more CO2) because there’s more CO2 in the air (due to global warming), but they’ll respire relatively less. 7 See also: Is more CO2 good for plants?


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  2. Overview of Photosynthesis.” Boundless Biology. []
  3. Photosynthetic Cells”. Scitable. []
  4. “The Molecular Biology of Cyanobacteria.” Dordrecht: Springer. Sidler, W. A. 1994. Phycobilisome and phycobiliprotein structure. In: Bryant, D. A., editor. []
  5. Ghosh, T., Paliwal, C., Maurya, R., and Mishra, S. 2015. Microalgal rainbow colours for nutraceutical and pharmaceutical applications. In: Bahadur, B., Venkat Rajam, M., Sahijram, L., and Krishnamurthy, K. V., editors. Plant Biology and Biotechnology: Volume I: Plant Diversity, Organization, Function and Improvement. New Delhi: Springer. p. 777–91. []
  6. “Plankton.” Woods Hole Oceanographic Institute. []
  7. Convergence in the temperature response of leaf respiration across biomes and plant functional types.” Mary A. Heskel, Odhran S. O’Sullivan, Peter B. Reich, Mark G. Tjoelker, Lasantha K. Weerasinghe, Aurore Penillard, John J. G. Egerton, Danielle Creek, Keith J. Bloomfield, Jen Xiang, Felipe Sinca, Zsofia R. Stangl, Alberto Martinez-de la Torre, Kevin L. Griffin, Chris Huntingford, Vaughan Hurry, Patrick Meir, Matthew H. Turnbull, and Owen K. Atkin. PNAS 113 (14) 3832-3837. April 5, 2016. []
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