Comprehensive AQA A-Level Biology Notes: Biological Molecules

Biological Molecules: Monomers to Polymers

Ever wonder how tiny molecules build the structures that make life possible? It all starts with monomers, the small units that join together to form polymers. These molecules connect through condensation reactions (which eliminate water) and break apart through hydrolysis reactions (which use water).

Carbohydrates are essential energy molecules that come in different sizes. Monosaccharides like glucose, galactose, and fructose are the simplest forms. When two monosaccharides join, they form disaccharides like maltose (glucose + glucose), sucrose (glucose + fructose), and lactose (glucose + galactose) through glycosidic bonds.

Plants and animals store glucose differently. Plants use starch, a mixture of unbranched amylose $25-30$ and branched amylopectin $70-75$. Animals store glucose as glycogen, which is more branched than starch, allowing for faster energy release. Cellulose, another plant carbohydrate, forms strong microfibrils that provide structural support through numerous hydrogen bonds.

Quick Tip: To test for carbohydrates in the lab, use Benedict's reagent for reducing sugars (creates a red precipitate) and iodine solution for starch (turns blue-black).

Lipids are another crucial biological molecule made of carbon, hydrogen, and oxygen. Triglycerides consist of one glycerol molecule and three fatty acid tails connected by ester bonds. These can be saturated (no double bonds) or unsaturated (contain double bonds), affecting whether they're solid or liquid at room temperature.

Membrane Lipids and Proteins

Lipids are amazing energy storage molecules! They contain many carbon-hydrogen bonds, storing more energy per gram than carbohydrates. Their water-insoluble nature also means they don't affect water potential in cells, making them perfect for compact energy storage.

Phospholipids form cell membranes with their unique structure: a glycerol molecule, two fatty acids, and a phosphate group. This creates a molecule with a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. In water, they naturally arrange into a bilayer with heads facing outward and tails inward—the foundation of all cell membranes!

Proteins are perhaps the most versatile biological molecules, formed from chains of amino acids connected by peptide bonds. Each protein has four structural levels that determine its function:

• Primary structure: The sequence of amino acids
• Secondary structure: Folding into alpha-helices or beta-pleated sheets held by hydrogen bonds
• Tertiary structure: The complex 3D shape held together by various bonds
• Quaternary structure: Multiple polypeptide chains working together

Remember This: Proteins can be globular (compact, water-soluble) like enzymes and antibodies, or fibrous (insoluble structural proteins) like collagen and keratin.

Enzymes are special proteins that work as biological catalysts. According to the induced fit model, enzymes have an active site that changes shape slightly when a substrate binds, forming an enzyme-substrate complex. This lowers the activation energy needed for reactions and makes them happen much faster—without the enzyme being used up!

Enzyme Activity and Nucleic Acids

Enzymes are super picky about their working conditions! Temperature affects enzyme activity significantly—too cold and there's not enough energy for reactions, too hot and the enzyme denatures as hydrogen bonds break. Each enzyme has an optimum temperature where it works fastest.

The same applies to pH levels. At the optimum pH, enzymes have the perfect shape for their substrate. When pH changes, charges on amino acids shift, breaking bonds and potentially denaturing the enzyme permanently. Other factors affecting enzyme activity include:

• Enzyme concentration: More enzymes mean faster reactions until all substrate molecules are being processed
• Substrate concentration: More substrate molecules increase reaction rates until all enzyme active sites are filled
• Inhibitors: Molecules that slow reactions by either competing for the active site or changing enzyme shape

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are the information molecules of life. Both are made of nucleotides consisting of a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. DNA uses the bases adenine, thymine, guanine, and cytosine, while RNA uses uracil instead of thymine.

Cool Science History: In 1953, Watson and Crick (with crucial X-ray data from Rosalind Franklin) discovered DNA's double helix structure—two polynucleotide strands held together by hydrogen bonds between complementary base pairs.

DNA's stability makes it perfect for storing genetic information. Scientists like Griffith, Avery, Hershey, and Chase performed groundbreaking experiments proving DNA (not proteins) carries hereditary information from one generation to the next.

DNA Replication and Cellular Chemistry

When cells divide, DNA must replicate itself accurately. This happens through semi-conservative replication, where each strand serves as a template for a new complementary strand. The process involves several steps:

1. DNA helicase unwinds the double helix, breaking hydrogen bonds
2. Free nucleotides pair with exposed bases on the template strands
3. DNA polymerase joins nucleotides together with phosphodiester bonds
4. The result is two identical DNA molecules, each with one original strand and one new strand

ATP (adenosine triphosphate) is the cell's energy currency. Its structure—adenine, ribose, and three phosphate groups—contains unstable bonds between phosphates that release energy when broken. The ATP cycle is continuous:

• ATP + water → ADP + phosphate + energy (hydrolysis)
• ADP + phosphate → ATP + water (requires energy)

ATP provides immediate, manageable energy for cellular processes like active transport, muscle contraction, and enzyme activation. It's more efficient than glucose because it releases energy in a single reaction rather than many steps.

Biology Application: Water is incredibly important in organisms! Its properties—high heat capacity, strong cohesion, transparency, and solvent abilities—make it perfect for supporting life processes from temperature regulation to photosynthesis.

Inorganic ions play crucial roles in biological systems despite their simple structures. Iron ions (Fe²⁺) help hemoglobin carry oxygen, phosphate ions form DNA's backbone, hydrogen ions (H⁺) determine solution pH, and sodium ions (Na⁺) help transport glucose and amino acids across cell membranes.