A-Level Biology: Unit 1 Biological Molecules Revision Booklet

Biological Molecules Basics

All living organisms are remarkably similar when you look closely at their molecular structure. Whether you're examining a human, a houseplant, or bacteria, they're all built from the same types of biological molecules.

These molecules fall into two main categories: small molecules called monomers and massive molecules called polymers. Think of monomers as individual LEGO bricks, whilst polymers are the complete structures you build from joining loads of bricks together.

The most important groups you need to know are carbohydrates, lipids, and proteins. Each group has specific jobs - carbs provide energy, lipids store energy and form cell membranes, and proteins do almost everything else from building muscle to speeding up reactions.

💡 Key Point: Carbon atoms are special because they can form four bonds, allowing them to create the complex molecular frameworks that make life possible.

Building Big from Small

Macromolecules are built through a process called condensation, where two smaller molecules join together and kick out a water molecule in the process. It's like molecular matchmaking - one molecule loses a hydrogen (H), the other loses a hydroxyl group (OH), and together they form water (H₂O).

The opposite process is hydrolysis - breaking big molecules back down by adding water. Your digestive system uses hydrolysis constantly to break down food into smaller, usable pieces.

Carbohydrates contain carbon, hydrogen, and oxygen in a 2:1 ratio (twice as many hydrogens as oxygens). They come in three sizes: monosaccharides (single sugars like glucose), disaccharides (two sugars joined together like sucrose), and polysaccharides (massive chains like starch).

💡 Remember: Condensation reactions build molecules UP by removing water, whilst hydrolysis breaks molecules DOWN by adding water.

Glucose and Sugar Chemistry

Glucose has the formula C₆H₁₂O₆, but there are actually two different versions: α-glucose and β-glucose. These are called isomers - same formula, different arrangement of atoms. The difference is tiny but crucial for how they behave.

When two glucose molecules join, they form a glycosidic bond through condensation. Two α-glucose molecules create maltose, whilst glucose can also join with other sugars like fructose to make sucrose (table sugar).

Reducing sugars like glucose, fructose, and maltose will turn Benedict's solution orange when heated - this happens because they can donate electrons (get reduced). Sucrose won't do this directly, but if you break it down with acid first, then it will test positive.

Different sugars have identical formulas but completely different properties because of how their atoms are arranged. It's like having the same ingredients but following different recipes.

💡 Lab Tip: Benedict's test turning orange = reducing sugar present. Blue-black with iodine = starch present. These are essential tests to remember!

Starch Structure and Function

Starch is made from two components: amylose and amylopectin. Amylose forms long, straight chains of α-glucose that coil into spirals, held together by hydrogen bonds. Amylopectin has branches because some glucose molecules link differently (1,6 bonds instead of 1,4).

This branched structure makes starch incredibly compact - perfect for storage. Plants pack starch into tiny granules that don't take up much space but store loads of energy.

Starch is ideal for storage because it's insoluble (won't dissolve and leak out of cells) and doesn't affect osmosis. Plants can quickly build it up when they have excess glucose, and break it down when they need energy. Your digestive enzyme amylase breaks down dietary starch into maltose, which then gets converted to glucose for energy.

The coiled, compact structure means plants can store massive amounts of energy in small spaces - think of a potato or grain of rice packed full of starch granules.

💡 Storage Smart: Starch's insoluble nature means it won't mess with a cell's water balance whilst storing energy efficiently.

Glycogen and Cellulose

Glycogen is animals' version of starch - it's like amylopectin but with way more branches. This makes it even more compact and means it can be broken down super quickly when you need energy fast. Your muscles store glycogen locally for instant energy, whilst your liver maintains bigger reserves.

Cellulose is completely different even though it's also made from glucose. It uses β-glucose instead of α-glucose, and every other glucose molecule is flipped upside-down. This creates long, straight chains that line up parallel to each other.

Multiple hydrogen bonds between cellulose chains create microfibrils that are incredibly strong - as strong as steel fibres of the same thickness. These microfibrils criss-cross in plant cell walls, making them resistant to stretching in any direction.

Humans can't digest cellulose because we lack the right enzymes. Only certain bacteria and fungi can break it down, which is why cows need special gut bacteria to digest grass.

💡 Structure = Function: Glycogen's extra branches = faster energy release. Cellulose's straight, bonded chains = maximum strength.

Cellulose Structure Details

The alternating "upside-down" arrangement of β-glucose molecules in cellulose creates a unique structural advantage. Unlike the coiled chains of starch, cellulose forms perfectly straight chains with -CH₂OH groups sticking out on alternating sides.

These straight chains pack together tightly and are held by numerous hydrogen bonds. Although each individual hydrogen bond is weak, having thousands of them creates incredibly strong microfibrils. These bundle together to form even stronger fibres.

Plant cell walls use these fibres in a criss-cross pattern, creating walls that resist stretching in all directions. This gives plants their structural strength and allows them to grow tall and withstand environmental stresses like wind.

Cellulose is probably the most abundant carbohydrate on Earth. Without bacteria and fungi that can break it down, dead plant material would never decompose and the planet would be buried under cellulose waste.

💡 Amazing Fact: Cellulose microfibrils are as strong as steel wire of the same diameter - nature's incredible engineering!

Introduction to Lipids

Lipids include fats, oils, steroids, and waxes, but the most important ones you need to understand are triglycerides and phospholipids. Unlike carbohydrates and proteins, lipids aren't polymers - they're not made from repeating monomer units.

Triglycerides consist of one glycerol molecule joined to three fatty acid molecules. Glycerol has three -OH groups that can each bond with a fatty acid through condensation reactions, forming ester bonds.

Fatty acids come in two types: saturated (only single bonds between carbons) and unsaturated (one or more double bonds). Saturated fats are usually solid at room temperature (like butter), whilst unsaturated fats tend to be liquid (like olive oil).

The emulsion test identifies lipids - mix your sample with ethanol, then add water. If lipids are present, you'll see a white cloudy emulsion form.

💡 Memory Trick: Saturated fats are "saturated" with hydrogen atoms - they can't fit any more because all carbon bonds are single bonds.

Triglyceride Formation

When triglycerides form, each of the three fatty acid molecules joins to glycerol through condensation. This removes three water molecules - one for each fatty acid that attaches. The bonds formed are called ester bonds.

The length of fatty acid chains in animal cells is typically 14-16 carbon atoms, but this can vary. Saturated fatty acids have higher melting points than unsaturated fatty acids because they can pack together more tightly.

Think of saturated fatty acids as straight chains that stack neatly, whilst unsaturated fatty acids have kinks from double bonds that prevent tight packing. This is why butter (more saturated) is solid whilst olive oil (more unsaturated) stays liquid at room temperature.

The structure directly affects function - solid fats provide structural support and insulation, whilst liquid oils are better for energy storage and cell membrane flexibility.

💡 Visual Learning: Draw glycerol with three -OH groups, then "flip" three fatty acids so their -COOH groups point toward the -OH groups. Remove water and connect!

Phospholipids and Membranes

Phospholipids are similar to triglycerides but have a phosphate group instead of one fatty acid. This creates a molecule with two distinct ends: a hydrophilic (water-loving) "head" containing glycerol and phosphate, and hydrophobic (water-hating) "tails" made of fatty acid chains.

This dual nature is crucial for cell membranes. When phospholipids mix with water, they automatically arrange into a phospholipid bilayer - two layers with hydrophobic tails pointing inward and hydrophilic heads facing the watery environment on both sides.

This bilayer forms the foundation of all cell membranes, creating a barrier that separates the inside of cells from the outside environment. The hydrophobic interior prevents water-soluble substances from passing through easily, whilst the flexible structure allows the membrane to bend and reshape.

The phospholipid bilayer is like a molecular sandwich - water-loving bread on the outside, water-hating filling on the inside.

💡 Key Concept: Phospholipids self-assemble into membranes because of their amphipathic nature (having both water-loving and water-hating parts).

Lipids in Real Life

Understanding triglycerides helps explain nutrition and health. The table showing fatty acid concentrations in breast milk demonstrates how diet affects the lipids we produce. Polyunsaturated fatty acids (containing multiple double bonds) are essential nutrients that our bodies can't make.

Vegan mothers' milk contained more polyunsaturated fatty acids because plant-based diets are richer in these essential fats. This shows how molecular structure directly impacts nutrition and health outcomes.

Saturated fatty acids have no double bonds in their hydrocarbon chains - they're "saturated" with hydrogen atoms. Unsaturated fatty acids have one or more double bonds, and polyunsaturated means multiple double bonds.

The different properties of these fatty acids affect everything from food texture to cardiovascular health. Understanding their molecular structures helps you make sense of nutritional advice and food science.

💡 Real-World Connection: The fatty acid composition of foods directly relates to their physical properties and nutritional value - chemistry you can taste!