A: Biological Molecules, Cells and Levels of Organisation

Introduction

This section covers the fundamental biological molecules essential for human physiology, focusing on their structure, bonding, and roles in cellular and systemic function. Understanding these molecules is foundational for topics in anatomy, physiology, and pathology.

A1 Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. They serve as a primary energy source and play structural roles in cells.

• Monosaccharides: The simplest form of carbohydrates, consisting of single sugar units.

  • Hexose sugars: Six-carbon sugars such as glucose and fructose. These are crucial for cellular respiration and energy production.

  • Pentose sugars: Five-carbon sugars such as ribose and deoxyribose, important in nucleic acids (RNA and DNA).

• Disaccharides: Formed by the condensation of two monosaccharides, linked by glycosidic bonds.

  • Formation of glycosidic bonds: A dehydration reaction joins two monosaccharides, releasing water.

  • Important disaccharides: Sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose).

  • Structural formulae: Disaccharides have distinct chemical structures based on their monosaccharide components and bond positions.

  • Dietary sources: Sucrose is found in table sugar, maltose in malted foods, and lactose in milk.

• Polysaccharides: Large polymers of monosaccharides, primarily alpha-glucose in humans.

  • Starch: A plant polysaccharide and major dietary energy source, composed of amylose and amylopectin.

  • Glycogen: The main energy storage polysaccharide in animals, stored in liver and muscle cells for rapid energy release.

Example: After a meal, excess glucose is converted to glycogen in the liver for storage and later use during fasting or exercise.

A2 Proteins

Proteins are complex polymers made from amino acids. They perform a vast array of functions, including catalysis (enzymes), structure (collagen), transport (hemoglobin), and regulation (hormones).

• Proteins (polypeptides) as polymers of amino acids: Amino acids are linked by peptide bonds to form polypeptide chains.

• Structure of amino acids (backbone): Each amino acid has a central carbon (alpha carbon) bonded to an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain).

• Formation of peptide bonds: A condensation reaction between the amino group of one amino acid and the carboxyl group of another forms a peptide bond, releasing water.

• Structure of proteins and polypeptides:

  • Primary structure: The unique sequence of amino acids in a polypeptide chain, determined by genetic code.

  • Secondary structure: Local folding into alpha helices and beta pleated sheets, stabilized by hydrogen bonds.

  • Tertiary structure: The overall 3D shape of a polypeptide, maintained by hydrogen bonds, van der Waals forces, ionic bonds, and disulphide bridges.

  • Quaternary structure: The assembly of multiple polypeptide subunits, as seen in proteins like insulin (two chains) and hemoglobin (four subunits).

Example: Hemoglobin's quaternary structure allows it to efficiently bind and transport oxygen in the blood.

Additional info: The understanding of protein structure is crucial for interpreting how mutations can affect protein function and lead to diseases such as sickle cell anemia (a single amino acid change in hemoglobin).