1.1. Explain the molecular structure of carbohydrates, proteins and lipids

This guide will help you answer 1.1. Explain the molecular structure of carbohydrates, proteins and lipids.

Understanding the molecular structure of carbohydrates, proteins, and lipids is important in health and social care. These molecules are essential for energy, cell structure, and overall functioning of the human body. Knowing their structure can help in understanding their roles in nutrition and metabolism.

Carbohydrates

Carbohydrates are organic molecules made of carbon, hydrogen, and oxygen. Their general formula is ( (CH_2O)_n ), where “n” represents the number of repeating units. They are a major source of energy.

Monosaccharides

Monosaccharides are simple sugars and the building blocks of carbohydrates. Common examples include glucose, fructose, and galactose.

• Structure: Monosaccharides consist of a single sugar unit. They have a backbone of carbon atoms, typically between 3 and 7, arranged in a chain or ring form. Each carbon atom is bonded to hydrogen and hydroxyl (OH) groups.
• Example – Glucose: Glucose has six carbon atoms (( C_6H_{12}O_6 )) arranged in a ring. It is a hexose sugar, meaning it has six carbons.

Disaccharides

Disaccharides are made by joining two monosaccharides via a glycosidic bond. This involves a condensation reaction, where water is removed.

• Examples:
  • Sucrose (( C_{12}H_{22}O_{11} )): Made from glucose and fructose.
  • Lactose: Made from glucose and galactose.
  • Maltose: Made from two glucose molecules.

Polysaccharides

Polysaccharides consist of long chains of monosaccharides linked by glycosidic bonds. They can be branched or unbranched.

• Examples:
  • Starch: Found in plants, made of many glucose units. It stores energy for later use.
  • Glycogen: Stored in the liver and muscles of animals, also made of glucose units. It serves as a quick source of energy.
  • Cellulose: Found in plant cell walls, provides structural support. Unlike starch and glycogen, humans cannot break down cellulose for energy.

Proteins

Proteins are large, complex molecules made up of smaller units called amino acids. They are crucial for cell structure, enzymes, hormones, and immune responses.

Amino Acids

Amino acids are the building blocks of proteins. Each amino acid has:

• Carbon atom (central): Acts as a backbone for all groups attached.
• Amino group (-NH_2): Contains nitrogen.
• Carboxyl group (-COOH): Acidic group.
• R group (variable side chain): Determines the properties and functions of the amino acid.

There are 20 main amino acids that combine in various sequences to form different proteins.

Peptide Bonds

Amino acids link together via peptide bonds to form proteins. This happens in a condensation reaction where a molecule of water is removed. Chains of amino acids are referred to as polypeptides.

Protein Structure Levels

Proteins are organised into different structural levels:

• Primary Structure: The sequence of amino acids joined together.
• Secondary Structure: Folding of the amino acid chain into patterns such as alpha-helix or beta-pleated sheets. This is held together by hydrogen bonds.
• Tertiary Structure: Further folding of polypeptides due to interactions between R groups (e.g., ionic bonds, hydrogen bonds, disulfide bridges).
• Quaternary Structure: Multiple polypeptides combine to form a functional protein (e.g., haemoglobin).

Examples of Proteins

• Enzymes: Speed up chemical reactions (e.g., amylase, catalase).
• Hormones: Regulate bodily functions (e.g., insulin).
• Structural Proteins: Provide support (e.g., collagen in skin and bones).

Lipids

Lipids are hydrophobic molecules. This means they do not mix well with water. They are made of carbon, hydrogen, and oxygen but have far fewer oxygen atoms compared to carbohydrates.

Lipids are essential for insulation, energy storage, and forming cell membranes.

Types of Fatty Acids

Fatty acids are key components of lipids. They have two main parts:

1. Carboxyl Group (-COOH): One end is acidic and hydrophilic (water-loving).
2. Hydrocarbon Chain: The other end contains long chains of carbon and hydrogen atoms, making it hydrophobic (water-repelling).

Fatty acids can be:

• Saturated: No double bonds in the hydrocarbon chain. Found in animal fats, these are usually solid at room temperature.
• Unsaturated: Contain one or more double bonds, causing bends in the chain. Found in plants, these are usually liquid oils.

Triglycerides

Triglycerides are the most common type of lipid. They are formed by joining one glycerol molecule to three fatty acids via ester bonds (condensation reaction).

• Structure: Glycerol has three carbon atoms, each bonded to hydroxyl (-OH) groups. Fatty acids are attached to glycerol through ester bonds.

Phospholipids

Phospholipids are crucial for forming cell membranes.

• Structure: Similar to triglycerides, but one fatty acid is replaced with a phosphate group. The phosphate group is hydrophilic, while the fatty acid tails remain hydrophobic.
• Arrangement in Cell Membranes: In the membrane, phospholipids form a bilayer with hydrophilic heads facing outward and hydrophobic tails pointing inward. This allows the membrane to control what enters or leaves the cell.

Steroids

Steroids are a type of lipid characterised by a structure of four fused carbon rings. Cholesterol is an example of a steroid, and it serves as the precursor for other molecules like hormones.

Comparison of Carbohydrates, Proteins, and Lipids

Molecule	Main Components	Energy Source	Functions	Examples
Carbohydrates	Carbon, hydrogen, oxygen	Quick energy	Energy storage, structural roles	Glucose, starch, glycogen
Proteins	Amino acids	Low energy yield	Structural support, enzymes, hormones, immunity	Collagen, insulin, amylase
Lipids	Fatty acids, glycerol	Long-term energy	Insulation, energy storage, cell membranes, steroid hormones	Triglycerides, phospholipids, cholesterol

Final Thoughts

• Carbohydrates: Provide energy for physical activities and body processes.
• Proteins: Essential for growth, repair, and enzyme production.
• Lipids: Critical for cell membrane structure, long-term energy reserves, and hormone production.

Making healthy dietary choices can influence overall physical health. Understanding these molecules can help professionals advise on nutrition or interpret results related to metabolism and deficiencies.