University of Idaho University of Idaho
Cells & Evolution of Life



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University of Idaho
Dept. Biological Sciences




Aerobic Respiration

Aerobic respiration refers to the metabolic pathways by which organisms break down glucose to produce ATP. Respiration involves four different pathways, each pathway feeding into the next. Ultimately, respiration results in the complete oxidation of glucose, and the transfer of energy from the chemical bonds of glucose to the chemical bonds of ATP.


Aerobic respiration, as described above, happens in the presence of oxygen. In the absence of oxygen, some organisms go through a process called fermentation, which allows them to produce energy under anaerobic conditions. Fermentation produces relatively less energy than respiration, because fermentation does not result in the complete oxidation of glucose.

Learning Objectives

  • Be able to follow the flow of energy through the pathways of respiration and fermentation, from glucose to ATP.

  • Understand the important characteristics of the pathways that comprise respiration and fermentation.

  • Understand the differences between aerobic respiration and fermentation, and why each process may be more or less useful under different conditions.

  • Understand the role of redox reactions in oxidative phosphorylation.

  • Be familiar with the electron/energy shuttles used by the respiration and fermentation pathways.

Topics covered in this Lesson

Metabolic Pathways

Metabolism PathwaysMetabolism refers to the sum total of the biochemical reactions that take place within an organism. Metabolic reactions are useful primarily for two reasons: 1) they allow energy to be stored, transferred or released in useable amounts, and 2) they synthesize and break down important carbon molecules, such as the 12 key intermediates and macromolecules. Metabolic pathways occur in a series of enzyme-catalyzed steps, so that small amounts of energy are invested or released at each step. In addition, having multiple steps makes it possible to more effectively control a pathway and allows more flexibility to link to other metabolic pathways.


Respiration and photosynthesis are two of the most ancient and important metabolic pathways. In some ways, photosynthesis and respiration are almost complete opposites of each other. Photosynthetic organisms, such as plants, use the energy of sunlight to reduce carbon dioxide into carbohydrates. During respiration, glucose is oxidized back to carbon dioxide, in the process releasing energy that it is captured in the bonds of ATP.


Photosynthesis and respiration have many features in common, including the process of oxidative phosphorylation, the use of similar energy shuttles and some of the 12 key intermediates.

Learning Objectives

  • Understand that metabolic pathways occur in several to many steps, and why this is so.

  • Understand that energy flows through the pathways of respiration and photosynthesis from their beginning to their end.

  • Understand the similarities between the Calvin-Benson cycle of photosynthesis and the Krebs cycle of respiration.

  • Be able to describe how electron transport chains are used to produce ATP, and what role ATP Synthase plays in this process.

  • Understand that the intermediate compounds found in the pathways of photosynthesis and respiration can feed into other metabolic pathways.

Audio Slides


GlycolysisGlycolysis is an evolutionarily ancient process found in all eukaryotic and many prokaryotic organisms. Glycolysis consists of 10 enzyme-catalyzed steps. During this process, glucose is partly oxidized, releasing some of its energy. This energy is captured in the chemical bonds of NAD and ATP.

Learning Objectives

  • Understand how glycolysis requires an initial investment of energy, but results in a net release of energy.

  • Be able to describe the role of substrate level phosphorylation in the production of ATP during glycolysis.

  • Explain how one molecule of glucose can produce two molecules of pyruvate.

  • Understand how the names of the enzymes in glycolysis relate to the chemical reactions they catalyze.

  • Know where in cells glycolysis takes place.

  • Give one reason (or two, if you’re savvy) why glycolysis is believed to be an ancient pathway.

Audio Slides


Aerobic Respiration

Aerobic RespirationGlycolysis releases energy by partially oxidizing glucose. As a result, ATP, reduced NADH and pyruvate are produced. In the presence of oxygen, many organisms continue on and fully oxidize pyruvate to produce much more ATP, in a process called aerobic respiration.


Aerobic respiration consists of three stages – pyruvate oxidation, the Krebs cycle and oxidative phosphorylation – that result in the net production of around 36 ATP molecules per molecule of glucose.

Learning Objectives

  • Know where the different stages of aerobic respiration occur in prokaryotic and eukaryotic cells.

  • Understand the inputs and outputs of pyruvate oxidation.

  • Know the three important results of the Krebs cycle.

  • Know what molecules ‘feed’ electrons into the electron transport chain, and where those molecules are produced.

  • Understand the role of redox reactions in the electron transport chain, and how they work to build a proton gradient.

  • Know the final electron acceptor of the electron transport chain, and what happens to it when it is reduced.

  • Understand how a gradient of potential energy is used to synthesize ATP.

  • Be able to compare the net ATP gain of glycolysis alone and glycolysis followed by aerobic respiration, and understand why they are different.

Audio Slides



FermentationWhile aerobic respiration produces a lot of ATP, it is also dependent on oxygen. In many environments, however, oxygen is not available. Under anaerobic conditions, many organisms are still able to metabolize glucose to produce ATP using a process called fermentation.


Organisms that ferment produce pyruvate by glycolysis. This is where they obtain their ATP. However, the pyruvate produced is then reduced, not oxidized. This reduction is coupled to the oxidation of NADH, which is necessary for glycolysis to continue in the absence of aerobic respiration.

Learning Objectives

  • Know that fermentation occurs in a wide variety of organisms, including humans, and that some organisms ferment even in the presence of oxygen.

  • Be able to describe the link between the reduction of pyruvate and the oxidation of reduced NADH.

  • Understand why glycolysis requires the oxidation of reduced NADH.

  • Understand why aerobic respiration does not work in the absence of oxygen.

  • Know the products of two of the more common fermentative pathways, and give an example of an organism that produces each.

  • Be able to explain why fermentation produces less ATP than aerobic respiration.

Audio Slides