Gardnerella vaginalis

Gardnerella Vaginalis

Definition Gardnerella vaginalis is the name of a micro-aerophilic coccobacillus found in the vaginal flora. Gardnerella vaginalis does not cause bacterial vaginosis (vaginal infection) unless...
Acetic Acid

Acetic Acid

Definition Acetic acid is a mildly corrosive monocarboxylic acid. Otherwise known as ethanoic acid, methanecarboxylic acid, hydrogen acetate or ethylic acid, this organic compound is...
Amino Acids

Amino Acids

Definition Amino acids are the building blocks of polypeptides and proteins and play important roles in metabolic pathway, gene expression, and cell signal transduction regulation....
BCAA supplements: a muscle myth?

Branched Chain Amino Acids

Definition The branched-chain amino acids or BCAAs, leucine, isoleucine, and valine are three of the nine nutritionally essential amino acids. These three ingredients form a...
Sulfuric acid

Sulfuric Acid

Definition Sulfuric acid (sulphuric acid) is a corrosive mineral acid with an oily, glassy appearance that gave it its earlier name of oil of vitriol....
Bile salt action in the gut

Bile Salts

Definition Bile salts are found in bile, a secretion produced by liver cells to aid digestion. Although bile is 95% water, bile salts are its...
The salivary glands

Submandibular Gland

Definition Submandibular glands are the second-largest salivary gland type, producing around 65% of our saliva when unstimulated (at rest). Located under the jaw, the exocrine...
Metaphase I

Metaphase I

Definition The first metaphase of meisosis I encompasses the alignment of paired chromosomes along the center (metaphase plate) of a cell, ensuring that two complete...
Prophase II

Prophase II

Definition During prophase II of meiosis II, four important steps occur. These are the condensing of chromatin into chromosomes, disintegration of the nuclear envelope, migration...


Definition Aldosterone (C21H28O5) is a mineralocorticoid hormone compound secreted by the adrenal gland cortex. It is part of the renin angiotensin aldosterone system or RAAS...


Catabolism Definition

Catabolism is the part of the metabolism responsible for breaking complex molecules down into smaller molecules. The other part of the metabolism, anabolism, builds simple molecules into more complex ones. During the catabolism energy is released from the bonds of the large molecules being broken down. Typically, that energy is then stored in the bonds of adenosine triphosphate (ATP). The catabolism increases the concentration of ATP in the cell as it breaks down nutrients and food. The ATP, in such high concentrations, becomes much more likely to give up its energy in the release of a phosphate. The anabolism then uses this energy to combine simple precursors into complex molecules that add to the cell and store energy for cell division.

Many pathways in the catabolism have similar versions in the anabolism. For example, large fat molecules in an organism’s food must be broken down into the small fatty acids that it is comprised of. Then, for the organism to store energy for winter, large fat molecules must be created and stored. Catabolic reactions break the fats down, and anabolic pathways rebuild them. These metabolic pathways often use the same enzymes. To decrease the chance that the pathways will undo each other’s progress, the pathways often inhibit each other and are separated into different organelles in eukaryotes.

Examples of Catabolism

Carbohydrate and Lipid Catabolism

Almost all organisms use the sugar glucose as a source of energy and carbon chains. Glucose is stored by organisms in larger molecules called polysaccharides. These polysaccharides can be starches, glycogen, or other simple sugars like sucrose. When an animal’s cells need energy, it sends signals to the parts of the body that store glucose, or it consumes food. Glucose is released from the carbohydrates by special enzymes, in the first part of the catabolism. The glucose is then distributed into the body, for other cells to use as energy. The catabolic pathway glycolysis then breaks glucose down even further, releasing energy that is stored in ATP. From glucose, pyruvate molecules are made. Further catabolic pathways create acetate, which is a key metabolic intermediate molecule. Acetate can become a wide variety of molecules, from phospholipids, to pigment molecules, to hormones and vitamins.

Fats, which are large lipid molecules, are also degraded by the metabolism to produce energy and to create other molecules. Similar to carbohydrates, lipids are stored in large molecules, but can be broken down into individual fatty acids. These fatty acids are then converted through beta-oxidation into acetate. Again, acetate can be used by the anabolism, to produce larger molecules, or as part of the citric acid cycle which drives respiration and ATP production. Animals use fats to store large amount of energy for future use. Unlike starches and carbohydrates, lipids are hydrophobic, and exclude water. In this way, a lot of energy can be stored without the heavy weight of water slowing the organism down.

Most catabolic pathway are convergent in that they end in the same molecule. This enables organisms to consume and store energy in a variety of different forms, while still being able produce all the molecules it needs in the anabolic pathways. Other catabolic pathways, such as protein catabolism discussed below, create different intermediate molecules are precursors, known as amino acids, to build new proteins.

Protein Catabolism

All proteins in the known world are formed of the same 20 amino acids. That means that the proteins in plants, animals, and bacteria are all just different combinations of the 20 amino acids. When an organism consumes a smaller organism, all of the protein in that organism must be digested in the catabolism. Enzymes known as proteinases break the bonds between the amino acids in each protein, until the acids are completely separated. Once separated, the amino acids can be distributed to the cells of the body. According to the organism’s DNA, the amino acids will be recombined into new proteins.

If no source of glucose is present, or there are too many amino acids, the molecules will enter further catabolic pathways to be broken down into carbon skeletons. These small molecules can be combined in gluconeogenesis to create new glucose, which the cells can use as energy or store in large molecules. During starvation, cellular proteins can go through the catabolism to allow an organism to survive on its own tissues until more food is found. In this way, organisms can live with only small amounts of water for extremely long times. This makes them much more resilient to changing environmental conditions.

  • Anabolism – The part of the metabolism that build large molecules from smaller ones.
  • Metabolism – The anabolism and catabolism combined, or all of the enzyme-driven reactions in a cell.
  • Metabolic Pathway – Consecutive chemical reactions organized within cells.
  • Catabolic Pathway – A single series of reactions that breaks down a specific molecule.


1. Yeast are a single celled organism used to create alcohol. In an environment with little to no oxygen, yeast create alcohol as a byproduct of release of energy from glucose. Is the production of alcohol part of an anabolic pathway, catabolic pathway, or neither?
A. Anabolic Pathway
B. Catabolic Pathway
C. Neither

Answer to Question #1
B is correct. Although alcohol is a byproduct, it occurs during the catabolism of glucose. Like all cells, yeast must use glucose for energy. Without oxygen, yeast have developed a catabolic pathway known as fermentation in which energy can still be harvested, but without oxygen. Instead, alcohols are created and released into the environment. Breweries, vineyards, and distilleries use this neat trick of glucose to create alcohol from sugars. Different sources of sugar produce beverages with different tastes. Wine uses the sugar from grapes, beer uses the starches of barley, and other spirits use a variety of different sugars, such as potatoes in some vodka and rice in sake.

2. Carnivores can produce all the glucose they need from animal protein. Herbivores obtain all the glucose they need from plants. Why can’t obligate carnivores eat plants, or obligate herbivores eat meat to get their energy?
A. They don’t know how.
B. They don’t produce the required enzymes.
C. They can! An omnivore is just a predator that learned to eat plants.

Answer to Question #2
B is correct. Obligate carnivores can only eat meat because they lack the required catabolic pathways that break down plants. Evolution, by selecting against unused and inefficient pathways, selects for organisms that fill certain niches. If that niche offers very little plant material, the catabolism changes, and certain pathways are lost. Thus, even if you taught a carnivore how to eat and gather plants, their body could not process the nutrients. In the same way, an herbivore can only get nutrients from plant materials. Omnivores have evolved in a niche that requires energy from both sources to be utilized. In these animals, the catabolism is capable of digesting both kinds of food.

3. Bacteria, having no specialized compartments within their cells, must regulate the anabolism and catabolism to work together. A scientist adds a chemical to the bacteria that shuts off the anabolism, permanently enabling only the catabolism. What will happen to the cell?
A. It will die.
B. It will grow.
C. It will produce a lot of energy.

Answer to Question #3
A is correct. While the catabolism will produce a lot of energy, it will eventually run out of molecules to break down, and the energy will stop. The cell would not be able to grow without the anabolism creating new molecules. Thus, even though the cell could provide energy, without the process that repair and add to the cell, it will eventually fall apart. Both anabolism and catabolism are needed to create a functioning metabolism in an organism.