Biomolecules: One Mark Questions, Book Back Answers and Notes

About Biomolecules

Hard ~180 min study

Biomolecules are the organic compounds found in living things. This unit applies the functional group chemistry you have already studied to real biological molecules. Glucose is a polyhydroxy aldehyde, amino acids carry both –NH₂ and –COOH groups, and a protein is a long chain of amino acids joined by amide (peptide) bonds.

The unit starts with carbohydrates, the body's main fuel, then moves to proteins, which act as enzymes, carriers, antibodies and structural material. Nucleic acids come next: DNA stores hereditary information and RNA helps turn it into proteins. The unit ends with vitamins and hormones, which are needed only in small amounts but whose absence shows up quickly as disease or disorder.

Most questions here are about structure and linkage. Know which disaccharides are reducing, which bond joins the units in starch, cellulose and proteins, which bases occur in DNA and in RNA, and which vitamin deficiency causes scurvy, rickets or night blindness. Short definitions such as zwitterion, denaturation and anomers are also asked often.

What you'll learn

• Classify carbohydrates by the products they give on hydrolysis.
• Give the evidence for the open-chain and cyclic structures of glucose.
• Identify peptide bonds and describe the four levels of protein structure.
• Describe how enzymes catalyse reactions and why they are specific.
• Distinguish DNA from RNA by sugar, bases, structure and function.
• Match common vitamin deficiencies to the diseases they cause.

Before you start

• Functional groups: alcohols, aldehydes, ketones, amines and carboxylic acids.
• Polymerisation and how monomers join to form macromolecules.
• Hydrogen bonding and polarity.

Biomolecules explained

Carbohydrates, Proteins and Nucleic Acids: What the Unit Covers

Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis. They are classified by what hydrolysis gives: monosaccharides cannot be hydrolysed further, oligosaccharides give two to ten monosaccharide units, and polysaccharides give many. Glucose is an aldohexose and fructose a ketohexose. The open-chain structure of glucose cannot explain all its reactions, for example glucose does not give Schiff's test or form a bisulphite addition product, and this is evidence for its six-membered ring (pyranose) form, drawn using Haworth projections. Monosaccharides join through glycosidic linkages: sucrose is glucose plus fructose, lactose is glucose plus galactose. Starch stores energy in plants and glycogen stores it in animals.

Proteins

Proteins are polymers of alpha-amino acids joined by peptide bonds (–CO–NH–). About 20 different alpha-amino acids occur in proteins. In water, an amino acid exists as a zwitterion, with –NH₃⁺ and –COO^– in the same molecule, which is why amino acids are amphoteric. Protein structure is described at four levels: primary (the sequence of amino acids), secondary (alpha-helix and beta-pleated sheet held by hydrogen bonds), tertiary (the overall folding of the chain) and quaternary (the arrangement of two or more chains). Heat, change in pH or some chemicals can unfold a protein and destroy its activity. This is denaturation, as seen when egg white sets on boiling.

Enzymes and Hormones

Enzymes are biological catalysts, mostly proteins. They speed up reactions by lowering the activation energy and are highly specific: urease acts on urea and nothing else. The lock and key model explains this specificity, because only a substrate of the right shape fits the active site. Hormones are chemical messengers released by endocrine glands and carried in the blood to target organs. Insulin, for example, controls blood glucose level, and thyroxine controls the rate of metabolism.

Nucleic Acids

DNA and RNA are polymers of nucleotides. Each nucleotide has a pentose sugar, a nitrogenous base and a phosphate group, and nucleotides are joined by phosphodiester bonds. DNA contains deoxyribose and the bases A, G, C and T; RNA contains ribose and has U in place of T. DNA is a double helix of two antiparallel strands held by hydrogen bonds between complementary bases, A with T and G with C. This base pairing lets DNA copy itself exactly, and it is the basis of transcription to RNA and translation into protein.

Vitamins

Vitamins are organic compounds needed in small amounts for normal growth and health, and most must come from food. Vitamins A, D, E and K are fat soluble and are stored in the liver and fatty tissue. The B vitamins and vitamin C are water soluble, are not stored for long and are lost in urine. Lack of vitamin C causes scurvy, lack of vitamin D causes rickets, and lack of vitamin A causes night blindness.

Common mistakes to avoid

• Mixing up alpha and beta glucose in Haworth projections. In alpha-D-glucose the –OH on C1 points down, on the opposite side of the ring from the CH2OH group on C5; in beta-D-glucose it points up, on the same side.
• Assuming every carbohydrate tastes sweet. Monosaccharides and most disaccharides are sweet, but starch and cellulose are tasteless.
• Pairing bases wrongly when writing the complementary DNA strand. A always pairs with T and G with C; in RNA, U takes the place of T.
• Thinking denaturation breaks the peptide bonds. It destroys secondary, tertiary and quaternary structure, but the primary sequence of amino acids is unchanged.
• Mixing up fat-soluble and water-soluble vitamins. Learn the short list: A, D, E and K are fat soluble; B group and C are water soluble.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is the difference between reducing and non-reducing sugars?

A reducing sugar has a free aldehyde or ketone group, or one that can open up from the ring, so it reduces Tollens' reagent and Fehling's solution. All monosaccharides, and maltose and lactose, are reducing. Sucrose is non-reducing because the glycosidic bond joins C1 of glucose to C2 of fructose, tying up both reducing groups.

Why are some amino acids called essential?

Essential amino acids cannot be made by the human body and must come from food. There are ten of them: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine and lysine (remembered as PVT TIM HALL). Non-essential amino acids such as glycine and alanine can be made in the body.

How does DNA store genetic information?

The information is in the order of the four bases A, G, C and T along the strand. A sequence of three bases codes for one amino acid, so the base sequence of a gene decides the amino acid sequence of a protein. Because each strand can act as a template for a complementary strand, the information is copied faithfully when a cell divides.

What happens when a protein is denatured?

Heat, a change in pH or chemicals such as strong acids disturb the hydrogen bonds and other forces that hold the protein in its folded shape. The chain unfolds and the protein loses its biological activity. The peptide bonds are not broken, so the primary structure is still there. Egg white turning solid on boiling and milk curdling are everyday examples.

What are the parts of a nucleotide?

A nucleotide has a nitrogenous base, a pentose sugar and a phosphate group. In DNA the sugar is 2-deoxyribose and the bases are A, G, C and T. In RNA the sugar is ribose and uracil replaces thymine. Nucleotides join by phosphodiester bonds between the sugar of one and the phosphate of the next, forming the sugar-phosphate backbone.

Why do vitamins have to be included in our diet?

Most vitamins cannot be made by the body in the amounts it needs, and many act as coenzymes that help enzymes work. Water-soluble vitamins (B group and C) are not stored for long and are lost in urine, so they must be taken regularly. Fat-soluble vitamins are stored in the liver and fatty tissue, but a long-term shortage still leads to deficiency diseases.