Macromolecules
Note: If you have not taken Chemistry 11 (or if you’ve forgotten some of it), read the Chemistry Review Notes on your own.
A polymer is a long chain of identical units connected by covalent bonds. The individual units are called monomers, smaller pieces that are linked together to make the polymer. Most macromolecules are polymers. Monomers are linked together in a reaction called dehydration synthesis (or condensation) which releases water as the two monomers are linked. To break this bond, water must be added in a reaction called hydrolysis.
I. Carbohydrates - although they have a variety of functions, carbohydrates are the most important energy source for nearly all cells. Most carbohydrates are in one of two different categories: sugar or starch. Most sugars have an -ose suffix (e.g., sucrose and lactose)
A. Nomenclature
1. Monosaccharides - are single sugar units. Monosaccharides include the monomers of carbohydrates such as glucose, galactose, and fructose. They generally have the chemical formula CH2O. For instance, glucose has the formula C6H12O6.
2. Disaccharides - A disaccharide consists of two monosaccharides linked together by a covalent bond formed by dehydration synthesis. For example:
a glucose + fructose = sucrose (table sugar)
b glucose + galactose = lactose (found in milk)
c glucose + glucose = maltose
3. Polysaccharides are polymers consisting of many monomers linked together in long chains. For example
a starch - the carbohydrate storage for plants
b glycogen - the carbohydrate storage for animals
c cellulose - the structural material in plant cell walls
d chitin - the structural material in the exoskeleton arthropods
B. Storage Polysaccharides
1. Starch
a Formed by linking many glucose units using dehydration synthesis.
b Glucose occurs in two forms, depending on whether the hydroxyl group of C1 is above or below the ring. α-glucose has the hydroxyl group below the plane of the ring while β-glucose has the hydroxyl group above the plane of the ring. Starch is made using α 1-4 linkages between monomers of α-glucose.
c amylose - a straight chain form of starch containing up to 1000 glucose.
d amylopectin - a branched form of starch containing 24-36 glucose off the main chain. This molecule contains between 1000-6000 glucose molecules.
2. Glycogen
a Glycogen is more extensively branched than amylopectin to increase the efficiency of storage.
b Glycogen is stored in liver and muscle. Humans store enough glycogen for about 1 day.
C. Structural Polysaccharides
1. Cellulose
a About 50% all organic carbon in biosphere is tied up in cellulose. Globally plants produce 1011 t cellulose per year.
b Cellulose is formed from glucose monomers connected by dehydration synthesis in β 1-4 linkages. The β link has the oxygen on C1 above the plane of the ring so cellulose has a totally different shape from starch.
c Enzymes which digest starch by hydrolyzing α bonds cannot recognize β links and so cannot digest cellulose. Cellulose found in foods that we eat is called fiber or roughage and is an important part of a healthy diet.
2. Chitin
a It is found in the fungal cell wall rather than cellulose as in plants.
II. Lipids - are not polymers but are still fairly large molecules for the most part. They are hydrophobic and, therefore, not soluble in water. They include fats, waxes, oils, phospholipids, and steroids.
A. Fats
1. The fats found in blood are triglycerides made by dehydration synthesis of glycerol and 3 fatty acids.
2. The main purpose of fats is energy storage and contain>2x the energy per gram as carbohydrates.
3. Saturated lipids (commonly called fats):
a contain the maximum possible hydrogen atoms
b are “straight” chains with no double bonds
c are produced mostly animal sources
d are solid at room temperature
e include bacon grease, lard, butter, etc.
4. Unsaturated lipids (commonly called oils):
a are missing one or more hydrogen atoms
b the missing hydrogen causes double bonds which cause the chains to “kink” or “bend”
c mostly plant sources
d liquid at room temperature because kinks prevent close packing of molecules
e include canola oil, corn oil, peanut oil, etc.
B. Phospholipids
1. Phospholipids make up the membrane of all organisms.
2. They are made just as triglycerides, except that one fatty acid is replaced with a phosphate. In other words, a phospholipid is composed of a molecule of glycerol, two fatty acids (either saturated or unsaturated), and a phosphate group (rather than a third fatty acid).
3. The negative charge(s) of the phosphate makes the “head” of the phospholipid hydrophilic.
4. The long, hydrocarbon tail is non-polar and, therefore, hydrophobic.
5. In a cell membrane, the hydrophilic heads are on the outside of the membrane (facing water) and the hydrophobic tails are on the inside of the membrane (away from water).
C. Steroids
1. Most lipid steroids are made of 4 interconnected carbon rings.
2. Cholesterol, although it gets a bad rap in health and diet circles, is the precursor for most lipid steroids and is an important component in the cell membrane.
D. Waxes
1. Long chain lipids joined to an alcohol or carbon ring.
2. Waxes function in waterproofing; e.g., plant cuticle, feathers.
III. Proteins - make up about 50% of the dry weight of cells and as they are the primary structural and functional components of cells, they have many functions.
A. The most important of proteins are enzymes, which help in regulating metabolism (all the reactions occurring in your cells) by acting as catalysts. Catalysts speed up the rate of a reaction by decreasing the amount of energy needed for the reaction to proceed
B. Proteins are polymers made of amino acid monomers.
C. Each of the 20 different amino acids contains a carboxyl group, an amino group, and an R group. The R group is the variable part of the amino acid and each amino acid has a different R group.
D. Amino acids are categorized into three types as determined by the R group. Each type has characteristics which cause the amino acid to behave differently in different environments. This is important for the formation of the three dimensional shape of proteins. The shape of the protein is important for its specific function. See Structure below.
1. Polar (hydrophilic) - the R group contains oxygen, nitrogen, or sulphur
2. Non-polar (hydro-phobic) - the R group contains only carbon and hydrogen
3. Charged - the R group carries a charge
E. Uses
1. Support - collagen, elastin, keratin
2. Storage of amino acids - ovalbumin, casein
3. Transport - hemoglobin
4. Communication
a hormones - insulin
b neurotransmitters - dopamine
5. Receptors - cell membrane proteins
6. Movement - actin, myosin
7. Defense - antibodies
8. Reactions - enzymes
F. Formation
1. Proteins are made from linking together long chains (sound familiar?) of amino acids.
2. Dehydration synthesis forms a peptide bond between two adjacent amino acids.
3. Many amino acids linked together is called a polypeptide.
G. Structure
1. A polypeptide folds spontaneously into a specific shape; the shape is determined by the amino acid sequence and is reinforced by interactions between R groups.
2. There are four levels of protein structure
a Primary structure - the specific sequence of amino acids. A single change in the primary structure of the chain can cause a different protein shape, thus different function. It can even deform the protein (e.g. a single substitution of amino acid in red blood cells will result in sickle-cell anemia).
b Secondary structure - H-bonds cause segments of the protein to be coiled or folded into one of two specific shapes:
(1) α-helix
(2) pleated sheet
c Tertiary structure - results from interactions between amino acid side chains.
(1) hydrophobic/hydrophilic - a polar amino acid will “prefer” to be in a polar environment. For example, imagine a protein in an aqueous environment (i.e., polar) which has a series of non-polar amino acids as part of its primary structure. This section of the polypeptide will be found inside the protein away from the polar environment. This contributes to its overall shape.
(2) electrostatic - segments of the polypeptide can be held together by ionic bonds using amino acids of opposite charge.
(3) disulfide bridges - very strong chemical bonds formed between the -SH groups of two cysteine monomers.
(4) H-bonds - weak interactions which can be used to reinforce sections of three dimensional shape.
d Quaternary structure - 2 or more polypeptide chains associate to form the complete protein
3. Chaperone proteins - the shape of most proteins is too complex to spontaneously form correctly without some help. Chaperone proteins hold the polypeptide in the correct configuration so that interactions can occur to for the correct shape.
H. Denaturation
1. Disrupting the native (or natural) conformation of a protein is called denaturation.
2. If the denaturation is not too great, the protein may return to its native conformation. This is called renaturation.
3. Proteins can be denatured in several ways.
a pH - changing the pH can disrupt electrostatic interactions between amino acids
b salt - adding salt can disrupt electrostatic interactions between amino acids
c heat - increasing the temperature can disrupt the weak H-bonds
d different solvent - changing from a polar to non-polar solvent (or vice versa) interferes with the hydrophobic and hydrophilic amino acids.
e chemical treatment - disulfide bridges can be disrupted using harsh chemicals
IV. Nucleic Acids - include the polymers deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
A. They are polymers formed by linking together long chains (here we go again) of nucleotide monomers.
B. A nucleotide consists of:
1. A nitrogenous base, which can be either adenine, guanine, cytosine, or thymine (in the case of RNA, thymine is replaced by uracil).
2. A five-carbon sugar, which can be either ribose or deoxyribose.
3. One or more phosphate groups.