Metabolism/Energy Overview Glycolysis Citric Acid Cycle  Elmhurst College
Cell Structure Energy Storage - ATP FAD, CoQ, CoA  Chemistry Department
Energy Overview NAD+ Electron Transport  Virtual ChemBook

Cell Structures


Although this description of cellular components is about a general eukaryotic cell, it applies to most cells. Of course, there are some differences between liver cells and fat cells and other cells. The chemical composition by weight in the cell is: water (70%), proteins (15%), nucleic acid (7%), carbohydrates (3%), lipid (2%), inorganic minerals (1%), and miscellaneous organic molecules (2%).

Eukaryotic cells are composed of distinct subcellular particulate bodies called organelles surrounded by a membrane. These organelles include the cell nucleus, mitochondria, lysosomes, peroxisome, endoplasmic reticulum, and Golgi apparatus. A simplified sketch of the cell is shown in Figure 2.

The nucleus of the cell is the largest and most conspicuous organelle. The nuclear membrane is a double membrane with pores which allow the exit of large RNA molecules. Inside of the nucleus in the nucleoplasm, there are areas rich in RNA and other areas containing the DNA chromosomes which store genetic information.

The endoplasmic reticulum is a netlike system localized in the cytoplasm of the cell. Two types are known: rough endoplasmic reticulum (RER) with small granules attached and smooth endoplasmic reticulum (SER) with no granules. The RER contains ribosomes of RNA and enzymes necessary for the synthesis of proteins that are destined to be used outside of the cell. The SER participates in packaging of the proteins in small vesicles and transporting them out of the cell.

The Golgi apparatus also packages proteins to be transported out of the cell. In addition, this may be the site for the biosynthesis of complex carbohydrate materials.

Lysosomes contain enzymes which can degrade or hydrolyze other proteins so that the cell can use the products. Peroxisomes contain enzymes which catalyze the conversion of toxic hydrogen peroxide to water and oxygen.

The cell membrane regulates the passage of substances into and out of the cell. Most membrane functions involve the participation of highly specific receptor proteins which operate by binding a specific substance on one side of the membrane. This event triggers other events in the membrane and the cell which in turn signals the cell to do (or not do) something.


Mitochondria are present in virtually every cell of the body. They contain the enzymes required for the citric-acid cycle , ATP synthesis, and the oxidation of fatty acids.

Mitochondria (from the Greek "threadlike grain") have a double membrane. The mitochondria are specialized, oval-shaped cellular compartments. The outer membrane is smooth, while the inner membrane called cristae contains numerous foldings. Attached to the cristae are small granules of unknown composition and function. The mitochondria are responsible for the aerobic (oxygen dependent) metabolism of the cell. The enzymes necessary for the citric acid cycle, respiratory chain, and beta fatty acid oxidation, are all compartmentalized in the cristae. There is also some DNA present in the mitochondria which is probably responsible for the synthesis of messenger RNA necessary to produce protein enzymes.