Comparing & Contrasting Cellular Respiration & Photosynthesis

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

In this lesson we will compare and contrast respiration and photosynthesis by considering the major steps occurring in each process, where these processes take place, and the organisms that use these forms of metabolism.

Overview of the Metabolic Interactions Between Plants and Animals

To really understand a scientific concept you need to build a solid foundation and then add to it. Start with what you know. Here is a simple picture illustrating some of the inputs and outputs of photosynthesis and respiration. When us humans carry out respiration, we breathe oxygen in and break down sugars into carbon dioxide that is exhaled. When plants carry out photosynthesis, they take in carbon dioxide to build sugars and produce oxygen. You might have already known some of that! Keep this simple overview in mind as we explore both processes in more detail.

The major inputs and outputs of respiration and photosynthesis and the link between humans and plants.
Cartoon overview of photosynthesis and respiration in plants and humans.

Respiration and Photosynthesis

Metabolism includes both the breakdown of molecules to obtain energy and the use of energy to build molecules. All living organisms carry out both parts of metabolism but here we are just focusing on two different kinds of metabolism: respiration and photosynthesis. Like we saw in our simple overview, in many ways respiration and photosynthesis are the opposite of each other. Respiration breaks down molecules like sugar, fat, and protein, and captures their energy to do work inside the cell. In contrast, photosynthesis uses the energy of light from the sun to build sugars to be used for plant growth and energy.

Mitochondria and Chloroplasts

Both respiration and photosynthesis occur in specialized compartments inside cells. Respiration occurs in mitochondria and photosynthesis occurs in chloroplasts. We usually think about respiration happening in animals and photosynthesis happening in plants but really it isn't that simple. While all animals use respiration, plants use both photosynthesis and respiration. And, there are microorganisms that are capable of both processes!

Metabolic Pathways

A metabolic pathway is a group of enzymes that work one after another to build or break down a molecule. Both respiration and photosynthesis use metabolic pathways, but these pathways work in the opposite direction. In the mitochondria, where respiration takes place, there is a metabolic pathway called the Citric Acid Cycle that takes pieces of organic molecules (like sugar, fat, or protein) and breaks their bonds to steal the energy and electrons. The waste product of this cycle is carbon dioxide.

Now something really great happens. That waste carbon dioxide gets taken up by plants and ends up in the chloroplast. In the chloroplast, where photosynthesis takes place, there is a metabolic pathway called the Calvin Cycle that takes carbon dioxide and uses energy and electrons to make chemical bonds and rebuild sugar molecules.


Okay, we have thought about what's happening with carbon dioxide, but what's happening with energy? We living organisms have a molecule called ATP or adenosine triphosphate that can store energy in chemical bonds and release it when the cell needs it. The trick is that organisms have to capture some energy and use it to make those ATP molecules.

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