Photosynthesis and Cellular Respiration: Definition and Diagram

Photosynthesis and cellular respiration work together to provide energy for sustainability of life on Earth. Except for creatures that depend on sulfur near hydrothermal vents, the bulk of life on Earth relies on glucose. The process of photosynthesis is what is used by the cellular organisms to produce glucose.

The breakdown of glucose and storage of the energy acquired in the molecule ATP are both parts of cellular respiration. Plants generate their energy via photosynthesis and manufacture ATP through cellular respiration. Animals must depend on sugars obtained from plants to provide the substance needed to create ATP in the mitochondria.

Photosynthesis: A Brief Overview

Photosynthesis (from Greek “phos”   meaning light and “synthesis”  meaning putting together) is a biological process used by cellular organisms to use the light energy to generate chemical energy to fuel the organism’s activities. Using this process, the sun’s energy is stored in the form of organic molecules, glucose. The process of cellular respiration will employ glucose molecules to harness the energy stored in the form of sugar.

Photosynthesis takes place mostly in leaves. The chloroplasts, a specialized organelle of plant cells, contain specific proteins. Such specialized proteins are termed cytochromes which interact with light. The cytochromes also have a heme group similar to hemoglobin in blood cells.

However, cytochrome’s heme group binds to magnesium instead of iron and interacts with the light. The chloroplast uses the energy captured from these photons by interacting with cytochromes and other proteins to synthesize glucose molecules. Chloroplasts carry out synthesis of glucose by combining carbon dioxide units into chains of six carbons, twelve hydrogens, and six oxygens.

This newly synthesized glucose may be converted in other forms or mixed with other glucose molecules to be stored in fructose or starch.


Process of Photosynthesis

There are two parts of the photosynthesis process known as the Light Reactions and the Calvin Cycle. The complete photosynthesis process is depicted below.

Photosynthesis Diagram
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The initial reaction starts by combining light and water in the chloroplast, where the hydrogens are separated from oxygen from proteins that start with energy-collecting cytochromes and auxiliary pigments. The ADP and NADP+ bind to hydrogens, electrons, and related energy.

These molecules can bind hydrogen, electrons, and energy and become the major products of the light reactions, NADPH and ATP. As a by-product, oxygen is released. The ATP and NADPH are utilized during Calvin Cycle. Following a set of defined chemical reactions, glucose molecules are produced.

Calvin Cycle Diagram
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Throughout the cycle, the energy inside and the hydrogen molecules are utilized to energize the processes. Carbon fixation, reduction, and ribose regeneration are the three steps of the Calvin Cycle. The figure below depicts these responses. One carbon dioxide is added in one round of the process, yielding the 3-carbon molecule 3-phosphoglycerate. In addition to producing other products, two molecules are combined to generate a glucose molecule.

The Cellular Respiration Process

The chloroplasts produce glucose, which may then be utilized to fuel other processes inside the cell. It’s also possible to export it to other cells within the same organism. The process of cellular respiration takes control at this point. Four separate pathways drive the production of ATP during cellular respiration.

This newly produced ATP may be utilized in various cellular activities, including the proper functioning of enzymatic activities. The mitochondria is a tiny organelle comparable to the chloroplasts where cellular respiration occurs. The mitochondria are present in all living eukaryotes, but chloroplasts are exclusively found in plants.

The cells need glucose which the plants provide. However, the extra glucose is stored in the form of starches and complex sugar. In a nutshell, the plants produce glucose which animals use, and other food chains.

The Reaction of Cellular Respiration

The first step in cellular respiration is called glycolysis, which is exactly what it sounds like. The prefix “glyco-” means glucose, while the “-lysis” means “to divide or split into two.” Glycolysis takes place outside the mitochondria and in the cytoplasm of the cell.

The six-carbon glucose molecule is broken into two pyruvate molecules during this process. In the following step, the three-carbon molecules are transformed to Acetyl CoA. This molecule is further taken up by the Krebs cycle. The Acetyl CoA may also enter the mitochondria, where it will participate in the Krebs cycle and oxidative phosphorylation. The graphic below illustrates the reaction pathways.

Cellular Respiration
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The Krebs cycle is similar to the Calvin cycle, where it uses specific molecules to maintain ATP synthesis and electron generation. Following this, the electrons are sent to the inner mitochondrial membrane. This mitochondrial inner membrane is densely packed with specific proteins that may transfer energy generated by the electrons traveling across its potential gradient.

A series of chemical reactions happen where the specific enzymes attach the free phosphate groups to ADP in this electron transport chain. As a result of this chemical reaction, ATP is produced, storing energy in the bond between these molecules.

These ATP molecules are subsequently exported from the mitochondria and may be utilized to supply energy in various processes throughout the cell. For example, ATP is used to push ions out of cells, which creates the electrical potential required for neurological processes.

Cellular Respiration and Photosynthesis: An Evolutionary Perspective

The Theory of Evolution is strongly disputed. However, a substantial amount of data suggests that all life had a common ancestor. This progenitor then diversified into the millions of species we see today over the period of hundreds of millions of years. The endosymbiotic process would explain this intricacy.

Bacteria, the simplest living organism, are most likely a near-identical copy of the initial form of life. Bacteria do not have organelles and carry out all of their metabolic activities in a single compartment. Many bacteria are capable of completing glycolysis to obtain energy while others, such as primitive single-celled plants, use photosynthesis.

As per the Endosymbiotic hypothesis, these ancient bacteria started interacting and evolved into diverse niches within the environment. Some would use sunlight to generate energy, while others feed on it. Some of the predatory bacteria grew to be pretty huge.

As a result, they may be able to take in vast amounts of tiny germs. Rather than digesting them, they provide a safe environment for them and assist in producing energy. Therefore the endosymbiotic bacteria are considered the earliest organelles. According to this view, the chloroplasts were photosynthetic bacteria, and mitochondria were once competent in oxidative phosphorylation.

The more giant bacteria evolved into eukaryotes, which included more organelles. The fact that both chloroplasts and mitochondria are enveloped by double membranes, a presumed vestige of the primordial engulfing process, supports this idea.

Furthermore, pieces of circular DNA are detected in both mitochondria and chloroplast, which are identical to that found in bacteria. This DNA is independent of the DNA in the bacterial nucleus.

Cellular Respiration and Photosynthesis: An Ecology Perspective

Evolution has provided us what we see now hundreds of millions of years after this separation of organelles. Algae, which are connected to photosynthetic microorganisms, are related to plants. Animals are connected to ancient species that did not have photosynthetic endosymbionts and eat other organisms instead.

At the bottom of the food chain, we find photosynthetic organisms. They are by far the most biomass on the planet, restricted only by their access to sunshine, nutrients, as well as water. Herbivores, which are one step above plants and algae, use the richness that plants generate.

For example, an elephant, a relatively large animal globally, is entirely herbivorous. However, the herbivores are of different sizes, from grasshoppers to microscopic insects. Because herbivores must devour a large number of photosynthetic species to thrive, there are fewer organisms at this level of the food chain.

On the other hand, the carnivores are much less in number than the herbivores since they consume smaller species throughout their lives in order to grow and breed. In doing so, photosynthesis and cellular respiration are at the heart of the whole food chain. Ecology also involves studying how different organisms interact while performing these reactions.

Related Questions

1. What is difference between Photosynthesis and cellular respiration?

  • A. Only one employs the utilization of sunshine.
  • B. Glucose is broken down by just one person.
  • C. Only one is based on a carbon molecule cycle.

2. Glucose is required for human cells to operate. Name the source of this glucose?

  • A. Your body
  • B. Plants
  • C. Meat

3. Which of the following would have the greatest impact on an ecosystem?

  • A. A herbicide is used to destroy all of the grass in a meadow.
  • B. A pesticide is used to destroy all of the butterflies in a meadow.
  • C. Hunters slaughter all of the birds in a meadow.


Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th ed.). New York: W.H. Freeman and Company.

McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.

Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.

Answers and Explanations

Q1. C is the right answer. While the Krebs cycle and Calvin cycle produce different results, they both depend on a continuous chain of carbon molecules. Although the molecules vary, the mechanisms are fairly similar.

Q2. B is the right answer. Every single thing you eat started out as a plant. If you eat meat, the nutrients you get are the same ones that the animal consumed before it died. Even animal protein as well as fat are made out of the same protein as well as glucose found in plants.

Q3. A is the right answer. The whole food chain will collapse if grass is not there. Higher stages of the food chain are shown by the other two cases. All insects as well as birds would perish if grass were not there. But keep in mind that none of the solutions are ideal. Without the birds, insects may consume all of the grass, resulting in the same outcome.


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