Comparison of Photosynthesis and Cellular Respiration

By Le Bach Pham

Cellular respiration and photosynthesis are two important chemical processes that are similar in many ways. However, cellular respiration occurs in animals, while photosynthesis happens in plants.

Producing Energy

All living organisms must find a way to harvest energy in order to maintain life. Animals use cellular respiration in order to convert food into chemical energy. Plants release chemical energy through photosynthesis. There are various ways to compare the processes of photosynthesis and respiration. These two cellular processes are similar in that they both produce energy. However, the method in which each accomplishes this differs in various aspects.

Redox Reactions

Redox reactions are an important process for cellular respiration and photosynthesis. These reactions are also known as oxidation-reduction. In a redox reaction one reactant transfers partially or completely one or more electrons to another. The electron-losing reactant is known as the reducing agent and is considered oxidized in the reaction. The substance gaining electrons is referred to as the oxidizing agent and is considered reduced.

Oxidation-reduction in cellular respiration differs from photosynthesis in the direction of the electron transfer. In respiration, electrons are transferred from glucose molecules to oxygen. Therefore, glucose is oxidized, while oxygen is reduced in cellular respiration. However, in photosynthesis, electrons travel from water to CO2. In cellular respiration electrons travel from organic molecules to oxygen, while in photosynthesis electrons travel from oxygen in water to a carbon-based molecule.

Cellular Respiration: Glycolysis and the Krebs Cycle

Energy is created in cellular respiration through the oxidization of glucose and its derivatives. This occurs during two chemical processes known as glycolysis and the Krebs cycle. Glycolysis breaks glucose into two molecules of pyruvate. This process occurs in the cytosol. The Krebs cycle, occurring in the mitochondrial matrix, converts a derivative of pyruvate into carbon dioxide.

Throughout various steps in glycolysis and the Krebs cycle electrons are transported along a specific chain of molecules, known as the electron transport chain. This chain ends in the formation of water through combining hydrogen with oxygen. Energy is released in each step of the chain and forms adenosine triphosphate (ATP).

Photosynthesis: Light Reactions and the Calvin Cycle

Photosynthesis converts solar energy into chemical energy in light reactions. Chlorophyll absorbs energy and transfers electrons and hydrogen from water. This releases oxygen from the water molecule. The electrons and hydrogen reduced NADP+ to NADPH. ATP is also formed during the light reactions. In the Calvin cycle, carbon dioxide incorporates into organic compounds in order to form carbohydrate. NADPH and ATP from the light reactions are then used as the reducing agent while releasing chemical energy.

Chemiosmosis in Cellular Respiration and Photosynthesis

Both cellular respiration and photosynthesis use chemiosmosis to create ATP. Chemiosmosis refers to specific steps within the electron transport chain utilized to create ATP. In this particular part of the electron transport chain some molecules also accept and release protons, pumping them into the intermembrane space creating a proton gradient. Potential energy, also known as the proton-motive force, is stored in the gradient. This energy is then used to drive ATP synthesis. In cellular respiration, food is converted into ATP, while photosynthesis uses light energy to release ATP.


How are photosynthesis and cellular respiration related to each other? In this study guide you have learned that cellular respiration and photosynthesis act as parallel processes. They are both similar in that both are designed to harvest energy through synthesis of ATP. However, the direction of electron movement is opposite in the electron transfer chain. Additionally, cellular respiration uses food to harvest energy, while photosynthesis uses light energy.


Study Guide for Campbell's BIOLOGY, Fourth Edition written by Martha R. Taylor

"The Cell, Respiration, and Photosynthesis." University of Michigan Global Change Curriculum.


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