Photosynthesis: the basis of life on Earth

Plant sprouting and growing through photosynthesis

Most living organisms, including humans, need energy and nutrients to live, but where do these nutrients and energy come from? The answer is photosynthesis. Photosynthesis is the ultimate source of nearly all energy used in all living organisms. This process is carried out by plants, algae, and bacteria to transform the energy of sunlight into chemical energy that can be stored and used by them and other organisms. Also, as a byproduct of photosynthesis, oxygen is released into the air for us to take when breathing.

Because of this, without photosynthesis, life on Earth as we know it would not be possible!

If you are new to this topic, it might be interesting for you to know more about it. Here, we describe the basics of this process in plants. Let’s start!

What is photosynthesis?

Photosynthesis is the biological process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of simple sugars such as glucose and sucrose. During photosynthesis, plants take in carbon dioxide (CO2) and water (H2O) from the air and soil. Inside the plant, the water is converted into oxygen and CO2 into glucose by a chemical process. The plant then releases the oxygen into the air, and stores energy in glucose molecules. Later, this simple sugar is converted into more complex molecules like cellulose, lipids, or amino acids, needed for plants to develop and grow.  

As plants can make their own food through photosynthesis, they are called autotrophs or primary producers. They are the basis of ecosystems since other organisms (herbivores) obtain energy by eating them. That includes us as well! Sadly, we cannot use light energy directly to supply our needs. Thus, we depend on plants for oxygen production and food.

So, practically, you and the global population owe your life to photosynthesis!

Why leaves are green?

For photosynthesis to occur, plant cells are equipped with an organelle specialized in absorbing and transforming light energy. That is the chloroplast! Inside it, all the reactions involved in photosynthesis occur. In the chloroplast, there are disc-like structures called thylakoids that contain in their membrane a light-absorbing pigment called chlorophyll. This is organized with other pigments (e.g. carotenoids) and proteins into complexes called photosystems. In plants, there are two photosystems, I and II, which are crucial for capturing of sunlight.

But, did you know that chlorophyll is responsible for the green color of leaves? During photosynthesis, chlorophyll’s job is to absorb energy from sunlight waves. However, it does not absorb the green wavelengths in white light. That particular light wavelength is reflected by the chlorophyll, so it appears that the plant leaves are green!

Aside from chlorophyll, stomata are also crucial for photosynthesis. They are small pores present in the leaves that function as lungs, allowing gas exchange with the environment. However, photosynthesis does not occur only in leaves, although they are the major photosynthetic organs. A particular and fascinating case is cucumber. The fruit of this plant remains green until full maturity and has shown chlorophyll production, stomata presence, and photosynthetic activity!

Zoom-in from a plant leaf to show chloroplast structure. Depicting outer and inner membrane, stroma and disc-like structures called thylakoids.

Photosynthesis process

During photosynthesis, there are two main stages:

  1. Light-dependent reactions: They take place in the thylakoid membranes and convert light energy into chemical energy. Light and water are used to form the energy-carrying molecules ATP and NADPH, which will be needed in the second stage. Also, oxygen is obtained as a byproduct of the light reactions.
  2. Light-independent reactions: Also known as the Calvin cycle, this second stage consists of the fixation of carbon dioxide (CO2) from the air using ATP and NADPH as fuel to obtain simple sugars molecules. Unlike the first stage, it does not occur in the thylakoids but in the inner space of the chloroplast, called stroma.

Light-independent reactions are not really “independent” since they indirectly need the products of the light-dependent reactions.

In case you were wondering: photosynthesis cannot occur at night. Unless you do it in the laboratory with artificial lights! Instead, the dark period is as important as the period during which plants receive light and photosynthesis takes place in both conditions.

Photorespiration, the enemy of photosynthesis?

During the Calvin cycle, the enzyme called Rubisco has a key role in incorporating the CO2 from the air into an organic molecule. In other words, without Rubisco, plants cannot fix the CO2 and produce glucose. However, this enzyme can use either CO2 or oxygen. When Rubisco works with oxygen, a process called photorespiration starts. Although it serves to protect the plant during stress conditions, it implies a loss in terms of carbon fixation, as CO2 can be lost to the environment.

Due to this, plants have developed three photosynthesis pathways: C3, C4, and Crassulacean Acid Metabolism (CAM). Among them, C4 and CAM are adaptations to minimize photorespiration. We will discuss them in our upcoming articles.

DID YOU KNOW?

Rubisco is the most abundant protein in the world!

Importance of photosynthesis

Aside from being the main source of energy, food, and oxygen, photosynthesis has been a trending topic in the last years because:

  • Understanding it will help to improve the use of solar energy as a renewable source of energy.
  • Due to global warming and high CO2 emissions, scientists are developing ways to increase carbon fixation in plants by genetic modification and develop varieties adapted to climate change.
  • As the global population is growing, researchers are trying to increase crop yield for many plants through maximizing photosynthesis.

We hope this article gives you a better view of photosynthesis in plants!

By Valeria Franco Franklin | 20-December-2021

References

  • Becklin, K. M., Ward, J. K. & Way, D. A. (2021). Photosynthesis, Respiration and Climate Change. Advances in Photosynthesis and Respiration (Including Bioenergy and Related Processes). Springer, Cham. https://doi.org/10.1007/978-3-030-64926-5_11
  • Casem, M. L. (2016). Chapter 11 - Cell Metabolism. Case Studies in Cell Biology (In Problem Sets in Biological and Biomedical Sciences), 263-281. https://doi.org/10.1016/B978-0-12-801394-6.00011-7.
  • Hagemann, M., & Bauwe, H. (2017). Photorespiration. Encyclopedia of Applied Plant Sciences (Second Edition), 86-89. https://doi.org/10.1016/B978-0-12-394807-6.00094-0.
  • Pribil, M., & Leister, D. (2017). Photosynthesis. Encyclopedia of Applied Plant Sciences, 90–95. doi:10.1016/b978-0-12-394807-6.00156-8
  • Simkin, A.J., Faralli, M., Ramamoorthy, S. and Lawson, T. (2020), Photosynthesis in non-foliar tissues: implications for yield. Plant J, 101: 1001-1015https://doi.org/10.1111/tpj.14633
  • Yahia, E. M., Carrillo-López, A., Barrera, G. M., Suzán-Azpiri, H., & Bolaños, M. Q. (2019). Photosynthesis. Postharvest Physiology and Biochemistry of Fruits and Vegetables, 47–72. doi:10.1016/b978-0-12-813278-4.00003-8