Chemicals Of Life - Carbohydrates, Proteins And Lipids

- These are chemical compounds that constitute the living organisms.
- Biochemistry is the branch of biology that deals with the study of the chemicals of life and their reactions.
- Chemicals of life include carbohydrates, proteins and lipids.

- Carbohydrates compounds of carbon, hydrogen and oxygen in the ratio of 1:2:1.
- They have a general formula (CH₂O)_n where n represents the number of carbon atoms.
- Carbohydrates are grouped into three categories: Monosaccharides, Disaccharides, and Polysaccharides

Monosaccharides

- These are the simplest carbohydrates.
- They include glucose, fructose, galactose.
- Their general formula is C₆H₁₂O₆.

Properties of Monosaccharides

• They are sweet tasting
• They readily dissolve in water
• They are crystallisable
• They are reducing sugars; monosaccharides reduce blue copper (II) sulphate in Benedict's solution to red brown copper (I) oxide when heated.

NOTE: Most fruits are sweet tasting because they contain a lot of monosaccharides. Monosaccharide units can be combined to form complex carbohydrate molecules through a process known as condensation. Water molecules are produced in the process.

Functions of Monosaccharides

• They are the chief respiratory substrate. They are broken down to release energy in the body.
• They are condensed to form complex important carbohydrates.

Disaccharides

- These are complex sugars formed by linking two monosaccharide units through condensation.
- They have a general formula C₁₂H₂₂O₁₁. The bond that holds two monosaccharide units is called glycosidic bond.
- Examples of disaccharides include:

• Maltose - common in germinating seeds
• Sucrose - fruits and sugar cane. Sucrose is the form in which carbohydrates are transported in plants
• Lactose - found in milk

Properties of Disaccharides

• They are sweet tasting
• They are crystallizable
• They are water soluble
• While they are non reducing sugars, some such as maltose is sugar reducing and is known as a complex reducing sugar.
• They can be broken down into their constituent monosaccharide units through hydrolysis.
  - Hydrolysis is the process through which complex molecules are broken down in the presence of water molecules.
• In living systems, hydrolysis is carried out by enzymes. However, in the laboratory, hydrolysis can be carried out by boiling the disaccharide in dilute acid such as hydrochloric acid.

Functions of Disaccharides

• They are hydrolyzed into monosaccharides and respired on to yield energy.
• They are the form in which carbohydrates are transported in plants due to their soluble and inert nature.

Polysaccharides

- These are formed through linking of numerous monosacchride units through condensation.
- Their general formula is (C₆H₁₀O₅)_n where n is a very large number.

Properties of polysaccharides

• They are non sweet
• They do not dissolve in water
• They are non crystalline
• They are non-reducing sugars

Examples of polysaccharides

• Starch- Made by linking numerous glucose molecules. It is a form in which carbohydrates are stored in plants.
• Glycogen- Is a storage carbohydrate in liver and muscles of animals. It is broken down to glucose in animals when blood glucose falls.
• Cellulose- This is a structural polysaccharide in plants. It is a component of the cell wall.
• Chitin- A structural carbohydrate found in the cell wall of fungi and arthropod exoskeletons.

Functions of polysaccharides

• They are storage carbohydrates; their insolubility and inertness makes them ideal for storing carbohydrates.
• They are structural carbohydrates e.g. cellulose forms the plant cell walls.
• They can be hydrolyzed into monosacharides and be broken down to release energy.

- These are compounds of carbon, hydrogen and oxygen.
- However, they contain lesser oxygen but higher hydrogen compared to carbohydrates.
- Building units for lipids are fatty acids and glycerol.
- To synthesize a molecule of lipid, three fatty acids and a glycerol molecule are linked through a condensation reaction.
- There is one type of glycerol but numerous fatty acids: There are different types of fatty acids.
- The property of a lipid therefore depends on the type of fatty acids that link up with the glycerol.
- There are complex lipids such as phospholipids, steroids, waxes and cholesterol. These also form through condensation.

Properties of Lipids

• Fats easily change to oil when heated while oils easily solidify when cooled.
• They are insoluble in water but readily dissolve in organic solvents such as chloroform to form emulsions.
• They are inert hence can be stored in tissues of organisms.

Functions of Lipids

1. They are a source of energy when oxidized.
   - They yield more energy compared to carbohydrates when oxidized per unit weight. However, they are less preferred as source of energy because they require a lot of oxygen to oxidize.
   - In addition, they are insoluble hence not easy to transport to respiratory sites.
2. They are a source of metabolic water.
   - When oxidized, they yield a lot of metabolic water.
   - This explains why some desert animals such as camels store large quantities of fat in their bodies.
3. Lipids offer protection to internal organs as they are deposited around them to act as shock absorbers.
4. Lipids provide heat insulation when stored underneath the skin as they are poor conductors of heat hence do not conduct heat away from the body.
   - Organisms in cold areas tend to be short and plump as they have fatter fat adipose.
5. Lipids form structural compounds for instance phospholipids in cell membrane.
6. Complex lipids such as waxes in leaves help minimize water loss through transpiration.
7. Some lipids mediate communication between cells.

- These are compounds of carbon, hydrogen and oxygen.
- In addition, they also contain nitrogen and sometimes phosphorous or sulphur or both.
- Some proteins molecules contain other elements. In particular, haemoglobin contains iron.
- Proteins are made up of amino acids. There are about twenty known amino acids. Amino acids are of two kinds;

• Essential - These are those amino acids that cannot be synthesized by the body systems hence have to be supplied in the diet.
• Non essential - These are amino acids that can be synthesized by the body mechanisms hence do not need to be supplied in the diet.

- An amino acid has an amino group, carboxyl group, hydrogen atom and an alkyl, R-group. Amino acids differ from each other by the alkyl group.
Proteins are of two kinds:

• First class proteins - Contain all essential amino acids
• Second class proteins - Proteins lack one or more essential amino acids

Protein Synthesis
- Two amino acids combine through a condensation process to form a dipeptide molecule.
- Several amino acids link up to form a polypeptide chain.
- Proteins are made up of long chain polypeptides.
- Properties of a protein depend on the type of amino acids present in its chain and the sequence in which the amino acids link up in the polypeptide chain.

Properties of Proteins

• They dissolve in water to form colloidal suspensions in which the particles remain suspended in water.
• They are denatured at temperatures beyond 40°C. Strong acids, bases, detergents and organic solvents also denature proteins.
• They are amphoteric- i.e possess both basic and acidic properties.
• This property enables them to combine with other non protein substances to form conjugated proteins such as:
• Mucus- Protein plus carbohydrate
• Haemoglobin- Protein plus iron

Functions of proteins

• They are structural compounds of the body. Cell membrane is protein in nature. Hair, nails and hooves are made up of protein keratin.
• Proteins are broken down to release energy during starvation when all carbohydrate and lipid reserves are depleted.
• Functional proteins play vital roles in metabolic regulation. Hormones are chemical messengers while enzymes regulate the speed of metabolic reactions.
• Proteins such as antibodies provide protection to the body against infections
• Some protein molecules are transport molecules. Haemoglobin molecule plays a crucial role in transportation of respiratory gases.
• Proteins play a vital role in blood clotting e.g. fibrinogen.
• Contractile proteins such as actin and myosin bring about movement.