What Are Carbohydrates?

Think of carbohydrates as the body's main fuel source - they're organic compounds made up of only carbon, hydrogen, and oxygen with the general formula Cx(H2O)y. The name literally breaks down as carbo-hydr-ate, telling you that oxygen is always present.

Monosaccharides are the building blocks - single sugar molecules that larger carbohydrates are made from. Common examples include glucose, fructose, and galactose. These simple sugars are like individual LEGO bricks that can be joined together.

Carbohydrates serve three main purposes in living organisms: providing immediate energy (like glucose in respiration), storing energy for later use (such as starch and glycogen), and providing structural support (like cellulose in plant cell walls). When monosaccharides join up through condensation reactions, they form disaccharides (two sugars) or polysaccharides (many sugars), releasing water molecules in the process.

Key Point: The glycosidic bonds that link sugars together are formed by condensation reactions and can be broken by hydrolysis reactions.

Structure and Classification

Carbohydrates follow a clear hierarchy based on size and complexity. Monosaccharides are single sugars with the formula (CH2O)n where n ranges from 3-7, making them the simplest form of carbohydrate.

Disaccharides are "double sugars" formed when two monosaccharides bond together. Polysaccharides are large molecules containing many monosaccharides linked in long chains or branched structures.

The structural diagrams show how these molecules increase in complexity. Each level up requires more condensation reactions to form and more hydrolysis reactions to break down. This size difference directly affects their properties - monosaccharides dissolve easily in water, whilst polysaccharides are typically insoluble.

Remember: The more monosaccharides joined together, the less soluble the molecule becomes - this is why starch doesn't dissolve in water but glucose does.

Glucose Isomers

Glucose exists in two forms called structural isomers - alpha glucose and beta glucose. These molecules have identical molecular formulas but different arrangements of atoms, specifically around carbon atom number 1.

In alpha glucose, the -OH group on carbon 1 points below the carbon ring. In beta glucose, this same -OH group points above the carbon ring. This might seem like a tiny difference, but it has huge consequences for the properties of larger molecules made from these sugars.

The carbon atoms in glucose are numbered 1-6, and you need to memorise the simplified ring structure for your exams. The difference between alpha and beta glucose determines whether the resulting polysaccharide will be used for energy storage (like starch from alpha glucose) or structural support (like cellulose from beta glucose).

Exam Tip: Draw the -OH group below the ring for alpha glucose and above for beta glucose - this simple rule will help you remember which is which.

Forming Disaccharides

When two monosaccharides join together, they form a disaccharide through a condensation reaction. This process removes a water molecule and creates a glycosidic bond between the sugars.

The most common disaccharides you'll encounter are maltose $glucose + glucose$, sucrose $glucose + fructose$, and lactose $glucose + galactose$. Each forms through the same basic process but results in different properties.

The glycosidic bond creates a C-O-C linkage between the two sugar molecules. These bonds can be named by their position - for example, a 1,4-glycosidic bond connects carbon 1 of one sugar to carbon 4 of another. Some bonds are 1,6-glycosidic bonds, creating branch points in larger molecules.

This reaction is reversible - hydrolysis can break the glycosidic bond by adding water back, splitting the disaccharide into its component monosaccharides.

Key Concept: Condensation builds up carbohydrates (removing water), whilst hydrolysis breaks them down (adding water back).

Glycosidic Bond Formation

The formation of glycosidic bonds involves two hydroxyl groups $-OH$ combining through condensation. This creates the characteristic C-O-C linkage that holds carbohydrates together.

Looking at maltose formation, two alpha glucose molecules combine when their hydroxyl groups react. The process removes one water molecule and creates an alpha-1,4-glycosidic bond between carbon 1 of the first glucose and carbon 4 of the second glucose.

The reaction is straightforward: monosaccharide + monosaccharide → disaccharide + water. This same principle applies when forming longer chains - each additional monosaccharide requires another condensation reaction and releases another water molecule.

Understanding this mechanism is essential because it explains how energy storage molecules like starch and glycogen are built up, and how they can be broken down when energy is needed.

Visual Aid: Think of the -OH groups as sticky hands that grab onto each other, squeezing out water in the process.

Functions of Major Polysaccharides

Starch serves as the main energy storage molecule in plants. Its helical, compact structure makes it perfect for storage within plant cells, whilst its insoluble nature means it doesn't affect the cell's water potential.

Glycogen functions as the animal equivalent of starch, storing glucose in liver and muscle cells. Its highly branched structure provides many end points for rapid hydrolysis when energy is needed quickly during exercise or between meals.

Cellulose provides structural support in plant cell walls. Made from beta glucose, its straight, unbranched chains align parallel to each other and are held together by hydrogen bonds, creating incredibly strong microfibrils.

The key to understanding these molecules is that structure directly relates to function - storage molecules are compact and branched for easy access, whilst structural molecules are straight and strongly bonded for maximum strength.

Function Focus: Energy storage requires quick access (branching), whilst structural support requires strength $parallel chains with cross-links$.

Structure-Function Relationships

Starch (amylose) forms a helical structure that's compact for efficient storage. Being a large, insoluble molecule, it can't leave the cell or cross membranes, keeping the stored glucose safely locked away until needed.

Glycogen and starch (amylopectin) both have branched structures that serve dual purposes. The branching creates a compact shape that fits more molecules into small spaces, whilst also providing multiple end points for enzymes to work on during hydrolysis.

Cellulose has a unique structure where every other beta glucose molecule is inverted, creating long, straight, unbranched chains. Multiple chains align parallel to each other and form hydrogen bonds between them, creating strong microfibrils that give plant cell walls their strength.

The insoluble nature of these polysaccharides is crucial - they don't affect the cell's water potential, preventing osmotic problems that would occur if large amounts of soluble sugars were stored.

Structure Secret: Branching = fast access for energy, straight chains = maximum strength for support.

Testing for Reducing Sugars

The Benedict's test identifies reducing sugars like monosaccharides, maltose, and lactose. This test works because these sugars can donate electrons to copper ions, causing a colour change.

Start with 2cm³ of your liquid sample and add 2cm³ of Benedict's reagent (which appears blue). It's crucial to use excess Benedict's solution to ensure there's enough copper(II) sulfate to react with any sugar present.

Heat the mixture gently in a boiling water bath for five minutes. A positive result shows as a colour change from blue through green, yellow, orange, to brick red precipitate. The more sugar present, the more intense the red colour becomes.

Benedict's reagent contains copper(II) sulfate in alkaline conditions. When reducing sugars are heated with this reagent, they reduce the copper(II) ions to copper(I) ions, forming the characteristic red precipitate of copper(I) oxide.

Colour Guide: Blue = no sugar, green/yellow = small amount, orange/red = large amount of reducing sugar present.

Testing for Non-Reducing Sugars

Non-reducing sugars like sucrose don't give a positive Benedict's test initially because their glycosidic bonds prevent them from acting as reducing agents. However, you can still detect them with an extended procedure.

First, perform the standard Benedict's test - if it stays blue, no reducing sugars are present. Then take a fresh sample and add dilute hydrochloric acid before heating for 5 minutes. This acid hydrolysis breaks down the disaccharides into their constituent monosaccharides.

After hydrolysis, you must neutralise the acid with sodium hydrogen carbonate because Benedict's reagent doesn't work in acidic conditions. Once neutralised, repeat the Benedict's test with the same procedure as before.

If non-reducing sugars were present in the original sample, the acid will have broken them down into reducing sugars, so you'll now see the characteristic blue to brick red colour change. This confirms that sucrose or other non-reducing sugars were present initially.

Key Step: Always neutralise the acid before adding Benedict's reagent, or the test won't work properly.

Quantitative Sugar Analysis

To measure sugar concentration accurately, you can use a colorimeter with Benedict's test. Create a dilution series of known sugar concentrations and perform Benedict's tests on each, measuring the absorbance of light through each sample.

Plot a calibration curve with concentration on the x-axis and absorbance on the y-axis, drawing a line of best fit through your data points. Test your unknown sample using the same Benedict's procedure, measure its absorbance, and read off the concentration from your calibration curve.

Alternatively, you can filter and dry the precipitate from Benedict's test, then weigh it. More precipitate indicates higher sugar concentration, though this method is less precise than colorimetry.

For starch detection, use iodine dissolved in potassium iodide solution. Add this orange-brown solution to your sample and stir. A positive result gives a distinctive blue-black colour that's easy to spot.

Precision Tip: Always use the same volumes and timing for all samples when creating your calibration curve - consistency is key for accurate results.