1. Biological Molecules: Definitions (AQA A-Level Biology): Revision Notes
1. Biological Molecules: Definitions
Adenosine triphosphate (ATP): The universal energy currency molecule found in all living cells, comprising ribose sugar, adenine base, and three phosphate groups that store energy in their bonds.
Amino acid: The fundamental building blocks of proteins, each containing an amino group (), a carboxyl group (), and a distinctive variable side chain (R group).
Amino acids are essential components of all proteins. The unique R group determines each amino acid's properties and how it will interact with other amino acids in protein formation.
Benedict's test: A qualitative biochemical test that detects reducing sugars by producing a colour change from blue to brick-red when heated with the reagent.
Biuret test: A biochemical assay that identifies the presence of proteins by forming a purple-coloured complex with peptide bonds.
Cellulose: A structural polysaccharide composed of beta-glucose units linked together, providing rigidity and strength to plant cell walls.
Condensation reaction: A chemical process where two molecules combine to form a larger molecule with the simultaneous removal of a water molecule, creating covalent bonds.
Condensation reactions are fundamental to biology - they create all the major biological macromolecules including proteins, carbohydrates, lipids, and nucleic acids. Understanding this process is essential for grasping how complex biological molecules are formed.
Deoxyribonucleic acid (DNA): The hereditary material consisting of deoxyribonucleotide monomers arranged in a double-stranded helical structure, storing genetic information.
Dipeptide: A compound formed when two amino acids join together through a peptide bond via condensation reaction.
Disaccharide: A carbohydrate molecule created by linking two monosaccharide units together through glycosidic bonds.
DNA helicase: A specialised enzyme responsible for unwinding and separating the two complementary strands of DNA during replication by breaking hydrogen bonds.
DNA polymerase: The key enzyme that facilitates DNA synthesis by catalysing the formation of phosphodiester bonds between nucleotides in the growing DNA chain.
DNA helicase and DNA polymerase work together during DNA replication. Helicase opens up the double helix, while polymerase builds the new complementary strands.
Enzyme: A biological catalyst protein that accelerates biochemical reactions by lowering activation energy without being consumed in the process.
Glycogen: A highly branched storage polysaccharide made from alpha-glucose monomers, serving as the primary energy reserve in animal cells.
Glycosidic bond: The covalent linkage formed between two carbohydrate molecules through a condensation reaction, connecting monosaccharides.
Heat capacity: The quantity of thermal energy required to increase the temperature of a given mass of substance by one degree Celsius.
Hydrolysis: The breakdown of chemical bonds between molecules through the addition of water, effectively reversing condensation reactions.
Hydrolysis is the opposite of condensation reactions. While condensation builds complex molecules by removing water, hydrolysis breaks them down by adding water. This process is crucial for digestion and cellular metabolism.
Induced-fit model: The current theory explaining enzyme function, where the enzyme's active site changes shape to accommodate the substrate more precisely upon binding.
Iodine test: A specific biochemical test used to detect starch presence, producing a distinctive blue-black colour when positive.
Lactose: A disaccharide sugar formed by the condensation of glucose and galactose molecules, commonly found in mammalian milk.
Latent heat: The amount of thermal energy required to change the physical state of a substance without altering its temperature.
Lipid emulsion test: A biochemical assay that identifies lipids by creating a cloudy white emulsion when the sample is mixed with ethanol and water.
The four main biochemical tests (Benedict's, Biuret, Iodine, and Lipid emulsion tests) each produce distinctive colour changes or visual results, making them reliable tools for identifying different types of biological molecules.
Maltose: A disaccharide composed of two glucose molecules joined by a glycosidic bond, often produced during starch digestion.
Metabolite: Any small molecule that participates in or results from metabolic processes within living organisms.
Monomers: The individual small molecules that serve as building blocks for larger, more complex macromolecules.
Monosaccharide: The simplest form of carbohydrate, consisting of single sugar units that cannot be hydrolysed into smaller carbohydrates.
Phospholipid: A complex lipid molecule consisting of glycerol, two fatty acid chains, and a phosphate group, forming the basic structure of cell membranes.
Polymers: Large macromolecules constructed from numerous monomers linked together through covalent bonds.
Polypeptide: An extended chain of amino acids connected by peptide bonds, forming the basic structure of proteins.
Polysaccharide: Complex carbohydrates composed of many monosaccharide units joined together through glycosidic bonds.
Primary structure: The specific linear sequence of amino acids in a polypeptide chain, determining all higher levels of protein structure.
Quaternary structure: The three-dimensional arrangement found only in proteins containing multiple polypeptide chains, describing how these separate chains interact.
Ribonucleic acid (RNA): A nucleic acid molecule composed of ribonucleotide monomers, typically single-stranded and involved in protein synthesis and gene regulation.
Secondary structure: The local folding patterns within a polypeptide chain, including alpha-helices and beta-pleated sheets, stabilised by hydrogen bonds.
Protein structure has four levels of organisation: primary (amino acid sequence), secondary (local folding), tertiary (overall 3D shape), and quaternary (multiple chain interactions). Each level builds upon the previous one.
Semi-conservative replication: The mechanism of DNA duplication where each new double helix contains one original strand and one newly synthesised strand.
Solvent: A substance capable of dissolving other materials (solutes) to form a solution.
Starch: A storage polysaccharide found in plants, composed of alpha-glucose monomers and serving as the primary energy reserve.
Sucrose: A disaccharide formed by the condensation of glucose and fructose molecules, commonly known as table sugar.
Tertiary structure: The overall three-dimensional folding pattern of a complete polypeptide chain, stabilised by various intramolecular forces.
Triglyceride: A lipid molecule consisting of one glycerol molecule chemically bonded to three fatty acid chains, serving as an energy storage compound.
Key Points to Remember:
- Building blocks matter: Monomers like amino acids, monosaccharides, and nucleotides form the foundation of all biological macromolecules through condensation reactions
- Structure determines function: The four levels of protein structure (primary, secondary, tertiary, quaternary) directly influence how proteins work in biological systems
- Biochemical tests are specific: Each test (Benedict's, Biuret, Iodine, Lipid emulsion) detects particular molecule types through characteristic colour changes
- Energy storage varies: Plants use starch, animals use glycogen, and both use lipids for long-term energy storage, while ATP provides immediate energy
- DNA replication is semi-conservative: Each new DNA molecule contains one original and one newly synthesised strand, ensured by specific enzymes like helicase and polymerase