The molecule of life

The topic "The Molecule of Life" encompasses the fundamental biological molecules that constitute all living organisms, highlighting their structures, functions, and interrelationships.

Key aspects of this topic include:

• Core Biological Molecules

  • The primary groups of carbon-based compounds that make up all cells and organisms are carbohydrates, lipids, proteins, and nucleic acids.

  • Water is also a vital component, making up about 80% of a cell's contents and performing numerous essential functions.

  • These molecules are often macromolecules, meaning they are large, complex molecules.

• Monomers and Polymers

  • Macromolecules are polymers, made from a large number of smaller units called monomers joined together.

  • Examples of monomers include monosaccharides (for carbohydrates like polysaccharides), amino acids (for proteins), and nucleotides (for nucleic acids like DNA and RNA).

  • The joining of monomers to form polymers typically occurs via a condensation reaction, which forms a chemical bond and eliminates a molecule of water. The reverse reaction, hydrolysis, involves the breakage of a chemical bond with the addition of a water molecule.

• Nucleic Acids (DNA and RNA)

  • DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are essential information-carrying molecules found in all living cells.

  • DNA stores genetic information, containing all the instructions needed for an organism to grow and develop.

  • RNA transfers genetic information from DNA to the ribosomes, which are the sites of protein synthesis (translation). Ribosomes themselves are composed of RNA and proteins.

  • DNA has a double helix structure, consisting of two polynucleotide strands wound around each other, held together by hydrogen bonds between complementary base pairs (adenine with thymine, guanine with cytosine). The strands are antiparallel.

  • RNA is single-stranded and contains ribose sugar and uracil (U) instead of thymine (T).

  • The genetic code is the sequence of base triplets (codons) in mRNA that code for specific amino acids. It is universal (the same triplets code for the same amino acids in all living things) and degenerate (some amino acids are coded for by more than one triplet). This shared biochemical basis across all life provides strong evidence for evolution, suggesting a common ancestry.

• Proteins

  • Proteins are made from amino acid monomers linked by peptide bonds formed through condensation reactions.

  • Their function is directly determined by their specific three-dimensional shape, which is influenced by different levels of structure: primary, secondary, tertiary, and quaternary.

  • Proteins have a vast array of functions, including acting as enzymes (biological catalysts), transport molecules (e.g., haemoglobin for oxygen transport), structural components (e.g., keratin in hair, collagen in connective tissue), hormones, and antibodies.

• Water

  • Water is a dipolar molecule with slight negative charges on its oxygen atom and slight positive charges on its hydrogen atoms.

  • These charges lead to the formation of hydrogen bonds between water molecules, which are responsible for many of its critical properties.

  • Water is an excellent solvent for ions and polar molecules, allowing important substances (like glucose, ions, amino acids) to dissolve and be transported in biological fluids (e.g., blood plasma) and facilitating most metabolic reactions.

  • Its high specific heat capacity and latent heat of vaporisation help buffer temperature changes in organisms and environments, providing stability.

• ATP (Adenosine Triphosphate)

  • ATP is a nucleotide derivative and serves as the universal energy currency of all living cells.

  • It is formed by the condensation of ADP (adenosine diphosphate) and an inorganic phosphate group (Pi), a reaction catalyzed by ATP synthase during processes like photosynthesis and respiration.

  • The hydrolysis of ATP to ADP and Pi releases energy, which is then used to power various energy-requiring cellular processes, such as active transport, muscle contraction, DNA replication, and protein synthesis.

• Cell Membranes

  • Cell membranes, including the cell-surface membrane, are crucial structures composed primarily of lipids (mainly phospholipids), proteins, and carbohydrates.

  • Their structure is described by the fluid mosaic model, where phospholipid molecules form a fluid, continuous double layer (bilayer) with proteins scattered throughout, like tiles in a mosaic.

  • The phospholipid bilayer is a barrier to water-soluble substances due to its hydrophobic center, but small, non-polar substances and water can diffuse through.

  • Proteins embedded within or associated with the membrane have various roles, including channel proteins and carrier proteins for transport, receptor proteins for cell signaling, and structural functions.

  • Cholesterol is also present in animal cell membranes, restricting the movement of other molecules and providing stability, especially at varying temperatures.