Molecules of Life| Vocabulary Search | Self-quiz |
Molecules of LifeMost of the molecules that form cells, organic molecules, are based on a carbon skeleton structure. Carbon atoms are bonded to one another bycovalent bonds. Further, each carbon skeleton has atom groups bonded to it to give it a specific structure. Since molecule structure determines it function, each atom group is known as a functional group. These functional groups can be added or removed from each carbon skeleton to provide a cell with a very large number of different organic molecules, each with a different specific function.
You DO NOT need to know the atomic structures that are in the figures. But do know the names of common groups. Please use the objectives as a guide here.
Carbohydrates, lipids or fats, proteins and nucleic acids make up the four major categories of organic molecule or macromolecules. Each of these categories has a fundamentally different structure, hence function.
One of the major functions of carbohydrates is to act as fuel molecules providing the energy a cell needs to maintain itself and reproduce.Simple carbohydrates, monosaccarhides like glucose, act as immediate fuel sources while more complex carbohydrates, like starch in plants and glycogen in animals, act as fuel reserves. For example, when blood sugar (glucose) levels are low then glycogen will be converted to glucose by the liver.
Another major function of carbohydrates is to form structures. For instance,cellulose is used by plants to form a protective cell wall around each of its cells. Insects and crustaceans use chitin to form their exoskeleton and fungal cells use chitin in their cell walls.
Monosaccharides (e.g. glucose and fructose) have one sugar unit, disaccharides (e.g. sucrose) have two, and polysaccharides (e.g. starch, cellulose, chitin) have many.
Lipids provide important functions at both the cellular and organismal levels. Some fats are used as long term energy reserves, as cushioning against injury, and as insulation against heat loss (especially among marine mammals). Phospholipids are a major component of cell membranes as is cholesterol. Steroids form human sex hormones testosterone, estrogen, and progesterone. Waxes cover the leaves of most plants preventing excessive water loss (recall activity two). Waxes are also used by terrestrial arthropods (e.g. insects, ticks, mites) for the same purpose.
It could be said that the primary function of a cell is to produce proteins. Proteins are certainly the most structurally complex and varied of the categories with an associated huge range of vital functions.
One of the most important of protein functions is that of an enzyme-the chemical that essentially governs the structure of most other molecules in the cell because it governs all cellular reactions (controls metabolism). Most reactions occur faster when they are promoted (or mediated) by enzymes. The structure of an enzyme is very important and changes in its structure, by changes in pH or temperature, may drastically alter its effectiveness in mediating reactions.
Proteins also form fibers. The protein keratin forms hair and nails. The protein collagen is found in cartilage and skin. the protein fibrin helps clot the blood.
Proteins form microtubes that slide along one another, resulting in motion. All muscles and cellular motions result from sliding protein micrtubules.
The hormone insulin is a protein, as is hemablobin, the molecule responsible for gas transport in humans and most other vertebrates.
Nucleotides are composed of a phosphate group, a five carbon sugar, and a nitrogenous base (see Figure 3.20A).
The two most important nucleic acids, DNA and RNA are quite similar, differing in minor structural detail.
The molecule DNA , a nucleic acid, consists of many individual nucleotides bonded together in a "twisted ladder."
Sections of DNA, known as genes, act as templates for the production of enzymes. A gene will code for an enzyme that, through reactions, determine a particular cellular activity (a genetic trait). Even more interesting, each of these long DNA molecules is packaged into a structure called a chromosome which can be passed onto the cell's ancestor. So, with DNA there is the ability of a cell to receive or inherit the traits of a previously successful cell. Certainly, this ability to inherit genetic traits that promote success in the struggle for survival is an advantage.
revised 8/99 WJC