Science at a Distance

Physical Structure

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Lecture Notes

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Physical Structure - Part Two

Polymers and Polymerization

Polymers are large molecules made by joining smaller units together in long chains. The chemical and physical properties of the polymer are very different to that of the constituent smaller units.

monomers are small molecules with a variety of chemical properties that can be joined together in long chains, like beads on a string.
two monomers joined together become a dimer, three joined together are a trimer, and so on.
many monomers joined in a long chain is termed a polymer, meaning many units.
a polymer composed of monomer units which are all the same is a homopolymer.
a polymer composed of two or more different types of monomer unit is called a heteropolymer.
the chemical process of joining monomeric units together to form a polymer is termed polymerization.

Biopolymers

There are four classes of biopolymers found in and used by living cells; lipids, polysaccharides, proteins and nucleic acids. All are polymers of smaller monomer units joined together.

all the monomers used in the construction of biopolymers are made from one or more carbon atoms joined with covalent bonds to other carbon atoms and usually hydrogen and oxygen.
the main backbone of each biopolymer is a single chain of repeated monomer units. One class of biopolymer is sometimes branched, but the rest are always single, straight chains.
hydrocarbons (one of the main constituents of lipids) and some polysaccharides are considered to be homopolymers (all the monomers in a particular chain are the same).
proteins, polynucleotides and some polysaccharides are considered to be heteropolymers as their main chains consist of more than one kind of monomeric unit.

Hydrocarbons and Fatty Acids

Hydrocarbons are biopolymers. They are long chains of carbon atoms each with two hydrogen atoms. Fatty acids are hydrocarbon chains with an organic acid group (-COOH) at one end.

a hydrocarbon chain consists of a long series of carbon atoms joined to each other and also joined to two hydrogen atoms.
cells create hydrocarbon chains by joining together two-carbon units, so all natural hydrocarbon chains have an even number of carbon atoms in their backbone (i.e. 2, 4, 6, 8, 10, etc.).
hydrocarbons are strongly hydrophobic (water fearing).
Fatty Acids are modified hydrocarbons in which an organic acid group (sometimes termed a carboxylic acid and having the chemical structure -COOH), is the terminal chemical group.
carboxylic acids are strongly hydrophilic (water loving).
fatty acid molecules, therefore, have a dual character in which one end of the molecule is hydrophobic and the other end is hydrophilic.
some fatty acid molecules are missing hydrogen atoms within the hydrocarbon chain; these are termed unsaturated.
fatty acids in which all the carbon atoms of the hydrocarbon chain are bonded with two hydrogens are termed saturated.

Lipids

There are five main groups of compounds commonly termed lipids; fats, oils, phospholipids waxes, and steroids. All but one contain either hydrocarbon chains or fatty acids in combination with other groups or molecules.

fats and oils are basically very similar types of molecules consisting of one, two or three fatty acid molecules linked to a single molecule of glycerol.
glycerol is a small, three-carbon molecule.
a single fatty acid joined to a single glycerol molecule is termed a monoglyceride; two and three fatty acids joined to the same glycerol molecule are diglycerides and triglycerides.
fats are generally made from saturated fatty acids and are solids at room temperature.
oils are generally made from unsaturated fatty acids and are often liquids at room temperature.
waxes are often simpler, consisting of two hydrocarbon chains held together by a single atom of oxygen.
phospholipids are diglycerides with a phosphate group attached to the third carbon atom of the glycerol molecule.
steroids are complex molecules made up of four closely connected rings of carbon atoms with other groups attached.

Roles for Lipids

Lipids play a variety of roles in living cells and living organisms. Generally, lipids serve as energy storage molecules, waterproof coverings and highly flexible barriers or boundaries between cells are their surroundings.

fats and oils are the main energy storage molecules for animals and for some plants.
one gram of fat provides 9 kilo calories of energy.
in humans, lipids are stored in specially adapted cells (termed adipose cells) just below the skin.
waxes form part of waterproof coverings in a variety of organisms.
phospholipids (like fatty acids) have the dual character of being part hydrophobic and part hydrophilic.
phospholipids are an essential component in all cell membranes, where their form flexible, self healing, semipermiable, double layered boundaries between the cell and it surrounding environment.
steroids are regulatory molecules that pass into and out of cells easily, where they alter the metabolism or regulatory pathways within the cell, or change, as cholesterol does, the flexibility of the cell membrane.

Carbohydrates and Sugars

The term carbohydrate means carbon joined with water and is a general term for a wide range of substances which broadly consist of molecules having one carbon atom for every two hydrogen atoms and one oxygen atom (C:H:O, 1:2:1). The simplest forms of common carbohydrate are the sugars.

the term saccharide is used for a sugar; it comes from the Latin word for sugar, saccarum.
the most common sugar is glucose, which has six carbon atoms and forms a ring structure.
glucose is an almost universal fuel for cells and organisms; it is soluble in water and provides about 4 kilo calories of energy for every one gram.
single sugar molecules are monomers that can be joined together.
two sugars joined together is a disaccaride and a polymer of sugars is a polysaccharide.

Roles for Polysaccharides

Polysaccharides contain a reasonable amount of energy and are often used by plants as their fuel reserves, but other polysaccharides function as strong building materials (in plant cell walls) and as recognition molecules on cell surfaces.

starch is a polymer of one type of glucose.
starch is a branched polymer that is made and stored by plant cells as a fuel reserve.
animals eat plants to obtain starch, which they also use for fuel.
cellulose is a polymer of a different type of glucose.
cellulose is a strong building material that forms the basis of plant cell walls, and, in a modified form, wood.
animals cannot break down the cellulose molecules for food without the help of specialized micro-organisms.

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Physical Structure