Photosynthesis and Celullar Respiration

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Photosynthesis & Cellular Respiration

Photosynthesis and cellular respiration are connected by an important relationship. This relationship allows life to survive as we know it. The products of one process are the reactants of the other.

Cellular respiration and photosynthesis are important parts of the carbon cycle.

The carbon cycle is the pathway through which carbon is recycled in the biosphere. While cellular respiration releases carbon dioxide into the environment, photosynthesis removes carbon dioxide from the atmosphere. The exchange of carbon dioxide and oxygen during photosynthesis and cellular respiration throughout the world helps maintain atmospheric oxygen and carbon dioxide at stable levels.

Cellular respiration and photosynthesis are direct opposites. Energy from the sun enters a plant and is converted into glucose during photosynthesis. Some of the energy is used to produce ATP in the mitochondria during cellular respiration, and some is lost to the environment as heat.

Photosynthesis

Photosynthesis is the biochemical process by which plants, algae, and photosynthetic bacteria convert inorganic matter (carbon dioxide and water) into organic matter (sugars), harnessing energy from sunlight.

This is the primary nutritional mechanism for all autotrophic organisms that possess chlorophyll, the essential pigment for the photosynthetic process.

Photosynthesis is one of the most important biochemical mechanisms on the planet, as it involves the production of organic nutrients that store light energy from the sun into various useful molecules (carbohydrates). In fact, the name of this process comes from the Greek words photo, "light," and synthesis, "composition."

After photosynthesis, the synthesized organic molecules can be used as a source of chemical energy to sustain vital processes, such as cellular respiration and other reactions that are part of the metabolism of living beings.

To carry out photosynthesis, the presence of chlorophyll is required, a pigment sensitive to sunlight, which gives plants and algae their characteristic green color. This pigment is found in chloroplasts, cellular organelles of various sizes that are characteristic of plant cells, especially foliar cells (of the leaves). Chloroplasts contain a set of proteins and enzymes that allow the development of the complex reactions that are part of the photosynthetic process.

The process of photosynthesis is fundamental to the ecosystem and to life as we know it, since it allows the creation and circulation of organic matter and the fixation of inorganic matter. In addition, oxygenic photosynthesis produces the oxygen needed by most living beings for respiration.

Types of photosynthesis

Two types of photosynthesis can be distinguished, depending on the substances used by the organism to carry out the reaction:

Oxygenic photosynthesis: This is characterized by the use of water (H2O) to reduce the carbon dioxide (CO2) consumed. In this type of photosynthesis, not only are sugars useful for the organism produced, but oxygen (O2) is also obtained as a product of the reaction. Plants, algae, and cyanobacteria carry out oxygenic photosynthesis.

Anoxygenic photosynthesis: The organism does not use water to reduce carbon dioxide (CO2), but instead uses sunlight to break down hydrogen sulfide (H2S) or hydrogen gas (H2) molecules. This type of photosynthesis does not produce oxygen (O2) and instead releases sulfur as a product of the reaction. Anoxygenic photosynthesis is carried out by green and purple sulfur bacteria, which contain photosynthetic pigments grouped together under the name bacteriochlorophylls, which are different from the chlorophyll found in plants.

Characteristics of Photosynthesis

It is a biochemical process that harnesses sunlight to obtain organic compounds, that is, the synthesis of nutrients from inorganic elements such as water (H2O) and carbon dioxide (CO2).

It can be carried out by various autotrophic organisms, as long as they have photosynthetic pigments (the most important being chlorophyll). It is the nutritional process of plants (both terrestrial and aquatic), algae, phytoplankton, and photosynthetic bacteria. A few animals are capable of photosynthesis, including the sea slug Elysia chlorotica and the spotted salamander Ambystoma maculatum (the latter doing so through symbiosis with an alga).

In plants and algae, photosynthesis takes place in specialized organelles called chloroplasts, which contain chlorophyll. Photosynthetic bacteria also possess chlorophyll (or other similar pigments), but do not have chloroplasts.

There are two types of photosynthesis, depending on the substance used to fix carbon from carbon dioxide (CO2). Oxygenic photosynthesis uses water (H2O) and produces oxygen (O2), which is released into the surrounding environment. Anoxygenic photosynthesis uses hydrogen sulfide (H2S) or hydrogen gas (H2), and does not produce oxygen but rather releases sulfur.

The relationship between sunlight and plants was postulated as early as Ancient Greece. However, advances in the study and understanding of photosynthesis began to gain importance thanks to the contributions of a successive group of scientists in the 18th, 19th, and 20th centuries. For example, the first to demonstrate the generation of oxygen in plants was the English clergyman Joseph Priestley (1732-1804), and the first to formulate the basic equation of photosynthesis was the German botanist Ferdinand Sachs (1832-1897). Later, the American biochemist Melvin Calvin (1911-1997) made another enormous contribution, elucidating the Calvin cycle (one of the phases of photosynthesis), which earned him the Nobel Prize in Chemistry in 1961.

In plants and algae, photosynthesis takes place in organelles called chloroplasts.

Photosynthesis Equation

The general equation for oxygenic photosynthesis is: 6CO2 + 6H2O → C6 H12 O6 + 6O2

To better understand the formula, we can break it down into its 4 parts, which are CO2, H2O, C6 H12 O6, and O2.

CO2: Photosynthesis contains 6 molecules of carbon dioxide, better known as CO2, this is a gas found in the atmosphere, and is an inorganic molecule.

H2O: Photosynthesis contains 6 molecules of water, also known as H2O, whose substance contains 2 hydrogen molecules and 1 oxygen molecule, and is an inorganic molecule.

C6H12O6: Photosynthesis contains glucose, a monosaccharide containing 6 carbon molecules, 12 hydrogen molecules, and 6 oxygen molecules. Glucose is an organic molecule.

That is, for photosynthesis to take place, it needs carbon dioxide, water, glucose, and oxygen.

Phases of Photosynthesis

Photosynthesis as a chemical process occurs in two distinct stages: the light stage and the dark stage, so named because only the presence of sunlight directly intervenes in the former (which does not mean that the latter necessarily occurs in the dark).

Light Stage: During this phase, light-dependent reactions occur inside the plant. That is, the plant captures solar energy through chlorophyll and uses it to produce ATP and NADPH.

It all begins when the chlorophyll molecule comes into contact with solar radiation and the electrons in its outer layers are excited, generating an electron transport chain (similar to electricity), which is used for the synthesis of ATP (adenosine triphosphate) and NADPH (nicotine adenine dinucleotide phosphate).

The breakdown of a chlorophyll molecule Water in a process called "photolysis" allows a chlorophyll molecule to recover the electron it lost when excited (the excitation of several chlorophyll molecules is required to carry out the light phase).

As a result of the photolysis of two water molecules, an oxygen molecule is produced, which is released into the atmosphere as a byproduct of this phase of photosynthesis.

Dark Phase: During this phase, which takes place in the matrix or stroma of the chloroplasts, the plant uses carbon dioxide and harnesses the molecules generated during the previous stage (chemical energy) to synthesize organic substances through a circuit of very complex chemical reactions known as the Calvin-Benson Cycle.

During this cycle, and through the intervention of different enzymes, the previously formed ATP and NADPH, glucose is synthesized from the carbon dioxide that the plant absorbs from the atmosphere. The incorporation of carbon dioxide into organic compounds is known as carbon fixation.

Importance of Photosynthesis

Photosynthesis is a vital and central process in the biosphere for multiple reasons. The first and most obvious is that it produces oxygen (O2), an essential gas for respiration in both water and air. Without plants, most living beings could not survive.

On the other hand, by absorbing it from the surrounding environment, plants fix carbon dioxide (CO2) and convert it into organic matter. This gas, which we exhale when we breathe, is potentially toxic if not kept within certain limits.

Because plants use carbon dioxide to make their own food, the decline in plant life on the planet influences the increase in this gas in the atmosphere, where it acts as an agent of global warming.

For example, CO2 acts as a greenhouse gas, preventing excess heat that reaches the Earth from radiating out into the atmosphere. It is estimated that photosynthetic organisms fix around 100 billion tons of carbon as organic substances each year.

Cellular Respiration

Cellular respiration is the process by which cells obtain energy from sugars or other organic molecules by reacting carbon and hydrogen bonds in the presence of oxygen.

The result of cellular respiration is carbon dioxide, water, and adenosine triphosphate (ATP). Carbon dioxide is removed from the cell, and ATP is the molecule the cell uses as chemical energy to perform its functions.

Cellular respiration consists of several chemical reactions that take place in the cytoplasm and mitochondria. The chemical reactions are carried out by enzymes, proteins specialized for each reaction.

Stages of Cellular Respiration

Cellular respiration consists of a series of reactions that can be grouped into three stages.

1. Glycolysis: This is the first stage of respiration. It occurs in the cytoplasm of most cells. It involves converting one glucose molecule, with six carbons, into two pyruvate molecules, each with three carbons.

Glycolysis consists of ten reactions, each catalyzed by an enzyme, in which two ATP molecules are consumed and four ATP molecules are produced. In addition, two hydrogen carrier molecules, nicotinamide adenine dinucleotide (NADH), are generated, which will be used in the final stage.

2. Citric Acid Cycle: This is the second stage of cellular respiration, which takes place in the mitochondria of eukaryotic cells. It is also known as the tricarboxylic acid cycle or Krebs cycle.

In this cycle, carbon dioxide CO2 is produced, which is eliminated, and electron carrier molecules NADH and flavin adenine dinucleotide FADH2 move on to the next stage.

The cycle consists of 8 steps, where oxaloacetate (a 4-carbon molecule) combines with acetyl (2 carbons) from acetyl-Coenzyme A (acetyl-CoA) to form citrate (6 carbons).

From the citrate, carbon dioxide (1 carbon) is released until oxaloacetate is formed again and the cycle begins, as shown below:

3. Oxidative phosphorylation: This is the final stage of cellular respiration in which oxygen directly participates. Electron carriers, such as NADH and flavin adenine dinucleotide FADH2, deposit electrons in a sequence of proteins embedded in the inner mitochondrial membrane.

The electrons pass to oxygen molecules O2 and combine with hydrogen H+ to produce water H2O. Concomitantly, a phosphate group is added to adenosine diphosphate ADP to form adenosine triphosphate ATP. This is called phosphorylation.

Types of Cellular Respiration

Aerobic Respiration: In aerobic respiration, pyruvate, which was produced in glycolysis from sugars in the cytosol, is transported to the mitochondria in eukaryotic cells. Here, pyruvate is transformed into carbon dioxide, which is eliminated, and into acetyl-CoA, which enters the Krebs cycle.

When oxygen accepts electrons, it reduces the water molecule, which is obtained as a metabolic waste product.

Anaerobic respiration: This is cellular respiration in the absence of oxygen. Anaerobic respiration begins with the transformation of glucose through glycolysis, just like aerobic respiration. However, pyruvate is transformed into other compounds through fermentation.

Pyruvate can be transformed into lactate in muscle cells or into ethanol and carbon dioxide in alcoholic fermentation.

Anaerobic respiration produces much less energy than respiration in the presence of oxygen.

The most primitive living beings usually perform anaerobic cellular respiration.

Salts are reduced to a gas that is released into the atmosphere as a byproduct of metabolism.

Cells that perform this type of respiration are exclusively prokaryotes; they usually inhabit extreme environments with little or no oxygen.

Activity. Answer the following questions.

Remember to review the answers to the open-ended questions at the bottom of this page.

1. How many ATP molecules does the complete oxidation of a glucose molecule produce?

2. What is photosynthesis?

3. Which of the following substances is not necessary to carry out photosynthesis?

4. What is cellular respiration?

5. What is photolysis?

Once you click this button, the quizzes will close and you won't be able to change your answer.

6. What is the difference between aerobic and anaerobic respiration?

7. Where does photosynthesis take place in plants and algae?

8. Where does glycolysis take place?

9. Describe the Krebs cycle.

10. How is light used Solar radiation in anoxygenic photosynthesis?

Still have questions?

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Answers to open questions:

6. Aerobic respiration requires oxygen to occur, while anaerobic respiration occurs in the absence of oxygen.

9. Oxaloacetate combines with acetyl acetyl-Coenzyme A to form citrate. From citrate, carbon dioxide is released until oxaloacetate is formed again and the cycle begins.

10. It is used to break down hydrogen sulfide (H2S) or hydrogen gas (H2) molecules.

2. Equipo editorial, Etecé. (2024a, agosto 3). Fotosíntesis - Concepto, fases, características y ecuación. Concepto. https://concepto.de/fotosintesis/

6. Lambers, Hans, Bassham, & Alan, J. (2025, 8 abril). Photosynthesis | Definition, Formula, Process, Diagram, reactants, Products, & Facts. Encyclopedia Britannica. https://www.britannica.com/science/photosynthesis

7. The Editors of Encyclopaedia Britannica. (2025a, marzo 27). Cellular respiration | Definition, Equation, Cycle, Process, Reactants, & Products. Encyclopedia Britannica. https://www.britannica.com/science/cellular-respiration