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Basic Energy Concepts
Types of Catabolism
Feremented Foods
Catabolism of Fats
Catabolism of Proteins
Amazing Respirations
Membranes and
Energy Generation

Anaerobic Respiration
Summary of Catabolism
Collecting Elements
Synthesizing Monomers
Carbon Assimilation
Nitrogen Assimulation
Other Assimilation
Formation of
Amino Acids

Lipid Synthesis
Nucleotide Synthesis
Making Polymers
Structural Assembly
Amphibolic Pathways

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Types of Catabolic Pathways

©2000 Timothy Paustian, University of Wisconsin-Madison

Catabolic pathways create energy for the cell. In catabolism the goal is to take energy out of what is gathered and store it in energy carriers (ATP) and electron carriers like NAD and Flavin-Adenine Dinucleotide (FAD). This energy and reducing power fuels growth, repair and movement. In this section we will discuss generally the two major ways of living on this planet phototrophy and chemotrophy.


Phototrophs take light from the sun and convert it directly into biochemical energy and reducing power. This is a complex, multi-protein process that always requires a membrane bound system. Energy is generated by photophosphorylation. Cell material is often built from CO2. Note that photosynthesis is not the private domain of plants; photosynthetic bacteria are common.

Dependent Chemotrophs

These chemotrophs depend on phototrophs, either directly or indirectly, to make complex organic molecules that they can oxidize and generate energy from. Energy is generated by substrate level phosphorylation (SLP) or by electron transport level phosphorylation (ETLP). They also often use these same organic molecules (or breakdown products from catabolizing them) as building blocks for more cells.

Independent Chemotrophs

Recently scientists have discovered microbes living deep in the sea on the ocean floor near areas of sea floor spreading. In these areas lava erupts into the ocean and eventually forms hydrothermal vents. The vents pump out hot sea water loaded with minerals. Microbes living here do not depend on plants or the sun at all to live, but generate their energy by oxidizing reduced chemicals in the vent plumes and build their cell material from CO2. In fact the reactions that they use to make cell material from CO2 are identical to those used by phototrophs. Follow the link to learn more about hydrothermal vents. There are also independent chemotrophs living in terrestrial environments.

Energy Generation

Catabolism is all about running reactions to make energy for the cell. Despite the great diversity of life, all organisms on this planet generate their energy using one of three processes.

Substrate level Phosphorylation (SLP)- Synthesis of ATP from ADP directly coupled to the breakdown of high energy organic substrates. A high energy phosphate molecule is transferred from the substrate being catabolized to ADP forming ATP.

Oxidative or Electron Transport Level Phosphorylation (ETLP) - High energy electrons are removed from the catabolic substrate and given to electron carriers (often NAD or FAD). These carriers eventually transfer their electrons to an electron transport chain which synthesizes ATP using the enzyme ATPase. Eventually the electrons combine with O2 (or some other terminal electron accepter) and H+ to form H20.

Photophosphorylation - The conversion of light energy in the form of photons to high energy electrons. These electrons then pass through an analogous electron transport chain as describe under ETLP, eventually resulting in the formation of ATP.

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