Chapter 6Biomolecules

Monosaccharides have sweet taste; disaccharides are less sweet; polysaccharides are tasteless.

Monosaccharides: glucose and fructose (also galactose, ribose). Disaccharides: sucrose and maltose (also lactose).

Sucrose (table sugar) is made of two monosaccharides, i.e. glucose and fructose.

Starch is a storage polysaccharide found in plants. It is composed of straight as well as branched chains of glucose units (e.g. in wheat, rice, maize, potato).

An amino acid is an organic molecule made of an amino group (NH₂), a carboxyl group (COOH), a hydrogen (H) and a side group (R) which are attached to a central carbon atom. Amino acids join in specific number and sequence to make proteins.

Lipids are composed of glycerol and fatty acids. Glycerol is an alcohol having 3 carbon atoms, each with a hydroxyl group. Fatty acids are long hydrocarbon chains with a carboxyl group (COOH) at the end.

DNA contains the hereditary information in the form of a sequence of nucleotides. This sequence determines the order of amino acids during protein synthesis. A segment of DNA that codes for a protein is a gene. During reproduction DNA is passed to the next generation, so it carries hereditary information.

All the chemical reactions that occur in an organism are collectively called metabolism. Anabolism: simpler substances combine to form complex substances; energy is used. Catabolism: complex molecules are broken down into simpler ones; energy is released.

Essential: the 9 amino acids which cannot be synthesized by our body and are supplied by foods. Non-essential: the 11 amino acids that can be synthesised in our bodies.

James Watson and Francis Crick, in 1962 (shared with Maurice Wilkins). They proposed the model in 1953.

Polysaccharides; cellulose is the most abundant carbohydrate.

Biochemistry is the study of the chemical processes that occur within living organisms. Molecular biology is the study of the structure and function of the biomolecules.

The molecules produced by organisms are called biomolecules: carbohydrates, lipids, proteins and nucleic acids. They are mostly large in size, so they are called macromolecules.

Proteins 50 %, nucleic acids 18 %, carbohydrates 15 %, lipids 10 % (total 93 %); the remaining 7 % is vitamins and inorganic substances.

DNA: nucleus (eukaryotes), nucleoid region (prokaryotes), mitochondria and chloroplasts. RNA: nucleus, ribosomes and cytoplasm.

Because in them the ratio of H and O is 2 : 1, the same as in water; general formula Cₙ(H₂O)ₙ.

Ribose C₅H₁₀O₅, deoxyribose C₅H₁₀O₄, glucose (also fructose, galactose) C₆H₁₂O₆.

Pentoses are monosaccharides with 5 carbon atoms (ribose, deoxyribose); hexoses have 6 carbon atoms (glucose, fructose, galactose).

Maltose is made of two glucose molecules. It is found in cereals.

Chitin is a modified form of cellulose. It is found in the exoskeletons of crabs, lobsters and insects, and makes the cell wall of fungi.

Dietary fibre contains undigestible carbohydrates, e.g. cellulose. It helps for proper bowel movements.

Plants convert their monosaccharides to disaccharides like sucrose to transport them between body parts.

Sucrose: sugar beet, sugar cane and fruits. Lactose: milk and dairy products. Maltose: cereals.

Proteins are the polymers of amino acids. They are the most abundant biomolecules in cell.

Insulin has 51 amino acids and haemoglobin has 574 amino acids.

They differ only in the side group (R): in glycine it is H, in alanine it is CH₃.

Meat (mutton, beef, chicken), fish, eggs, milk, pulses and beans.

Collagen and keratin make almost whole structures of cartilage, hair and nails.

Actin and myosin are the main components of muscle cells and are responsible for muscular movements. Fibrin forms a blood clot to prevent the loss of blood after an injury.

Enzymes catalyse all biochemical reactions in organisms; antibodies (immune system) defend the body against harmful pathogens.

Lipids are organic compounds that are insoluble in water but are soluble in organic solvents (alcohol, ether, benzene). They are composed of glycerol and fatty acids.

Fats contain saturated fatty acids and so are solid; oils contain unsaturated fatty acids and so are liquid at room temperature.

A phospholipid consists of one glycerol, two fatty acids and a phosphate group. Phospholipids make the core of all membranes.

Animals: meat and dairy products. Plants: nuts, seeds, olive oil; oils stored in seeds such as sunflower, coconut, groundnut and corn.

Adipose tissue around organs provides cushioning and insulation. Lipids help in the absorption of vitamins A, D, E and K in the digestive system.

They serve as a long-term energy reserve in the form of fats in adipose tissues; when the body needs energy, they are broken down to fatty acids and glycerol, which are used as fuel. A gram of fat gives about twice the energy of a gram of carbohydrate.

The unit of nucleic acids. It is made of a pentose sugar (ribose or deoxyribose), a nitrogenous base and a phosphate group (PO₄).

DNA: adenine, thymine, cytosine, guanine. RNA: adenine, uracil, cytosine, guanine.

In 1953, US biologist James Watson and British biologist Francis Crick.

The pairing of nitrogenous bases is specific: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds).

T A C G G T, because A pairs with T and C pairs with G.

The segment of DNA in which the sequence of nucleotides determines the synthesis of a protein (polypeptide chain) is called a gene.

Transcription: the process of making an mRNA copy of DNA. Translation: the ribosome reads the nucleotide sequence of mRNA and joins specific amino acids to form a protein.

The flow of genetic information: DNA → mRNA (transcription) → protein (translation). The book calls the working of DNA the central dogma.

Water is not a biomolecule, but every biomolecule works in water: it makes up about 60-70 % of the body's mass and most of the cell's cytoplasm.

Conclusion: water is the medium, a reactant and a transport system for biomolecules. Without enough water (dehydration), enzymes slow down, transport stops and cells cannot function.

Approximate values from standard food-composition tables (they vary with recipe and size; a food label gives the exact figure):

Comparison

Carbohydrates are organic compounds in which the ratio of H and O is 2 : 1. They are also called saccharides; general formula Cₙ(H₂O)ₙ.

1. Monosaccharides

Made of single sugar molecule; easily soluble in water; sweet taste; 3 to 7 carbon atoms. Pentoses: ribose (C₅H₁₀O₅), deoxyribose (C₅H₁₀O₄). Hexoses: glucose, fructose, galactose (C₆H₁₂O₆).

Roles: glucose is the main fuel of cellular respiration; ribose and deoxyribose are parts of RNA and DNA.

2. Disaccharides

Made of two monosaccharide units; less soluble; less sweet. Sucrose = glucose + fructose; maltose = glucose + glucose; lactose in milk.

Roles: plants transport sugar between body parts as sucrose; lactose is the sugar of milk.

3. Polysaccharides

Hundreds to thousands of monosaccharide units; insoluble; tasteless.

Proteins are the polymers of amino acids and the most abundant biomolecules in cell. They participate in everything organisms do:

Structural

Catalysis and control

Transport and movement

Protection

Good sources: meat, fish, eggs, milk, pulses and beans.

1. Saturated fatty acids

Their internal carbon atoms are bonded with maximum number of hydrogen atoms. They do not have double bonds between carbon atoms. They are solid at room temperature, e.g. in animal fats (ghee, butter).

2. Unsaturated fatty acids

They have one or more double bonds between carbon atoms. They are liquid at room temperature, e.g. in plant oils such as olive and corn oil.

Structure

Fats and oils are the most familiar lipids. They contain one glycerol and three fatty acids. Each of the three OH groups of glycerol is joined to one fatty acid.

Role in energy storage

DNA is made of deoxyribonucleotides; each has deoxyribose sugar, a phosphate group and a base: adenine (A), thymine (T), cytosine (C) or guanine (G).

Double helix model (Watson and Crick, 1953)

Base pairing

Watson and Crick received the Nobel prize in 1962 for this discovery.

The DNA molecule in a chromosome consists of thousands of nucleotides. Along its length are specific segments called genes. Each gene consists of specific sequence of nucleotides that carries information for the synthesis of a specific protein.

Step 1: Transcription (in the nucleus)

During the working of a gene, the specific sequence of DNA nucleotides is copied in the form of a molecule of messenger RNA (mRNA). The process of making mRNA copy of DNA is called transcription.

Step 2: mRNA moves to the cytoplasm

The mRNA carries the sequence of its nucleotides to the ribosome.

Step 3: Translation (at the ribosome)

The ribosome reads this sequence and joins specific amino acids to form a protein. This step is known as translation. tRNA transfers the specific amino acids to the ribosome in the correct sequence; rRNA forms the ribosome.

The molecules produced by organisms are called biomolecules; they are mostly large and called macromolecules. They make 93 % of the dry mass of protoplasm.

An amino acid is an organic molecule made of an amino group (NH₂), a carboxyl group (COOH), a hydrogen and a side group (R) attached to a central carbon atom.