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Unlock This CourseCarbohydrates are one of the four major groups of biological molecules, alongside proteins, lipids and nucleic acids. They're made up of carbon, hydrogen and oxygen atoms, usually in the ratio 1:2:1 (CHO). Think of them as nature's fuel and building materials - they power our cells and provide structure to plants.
Key Definitions:
These are the building blocks of all carbohydrates. The most important one is glucose (C₆H₁₂O₆), which is like the petrol for your body's engine. Other examples include fructose (found in fruits) and galactose (found in milk).
Carbohydrates come in three main sizes, each with different jobs in living things. Understanding these helps explain why we need different types of food and how plants stay upright.
Monosaccharides are single sugar molecules that can't be broken down into simpler sugars. They're small enough to pass through cell membranes and provide quick energy.
The most important sugar for energy. Your brain uses about 120g per day! Found in blood and made during photosynthesis.
The sweetest natural sugar, found in fruits and honey. Gets converted to glucose in your liver.
Found in milk sugar. Babies need special enzymes to digest it properly.
When two monosaccharides join together through a process called condensation, they form disaccharides. This reaction removes a water molecule (H₂O) and creates a glycosidic bond.
Table sugar! Made from glucose + fructose. Plants transport this around their bodies.
Milk sugar made from glucose + galactose. Some people can't digest this (lactose intolerance).
Malt sugar made from glucose + glucose. Found in germinating seeds and used in brewing.
About 65% of adults worldwide can't properly digest lactose because they stop producing the enzyme lactase after childhood. This shows how carbohydrate structure affects function - without the right enzyme to break the glycosidic bond in lactose, people get stomach problems when drinking milk.
Polysaccharides are made of hundreds or thousands of monosaccharides joined together. They're too big to dissolve in water and serve different functions depending on their structure.
These store energy for later use. They're like nature's batteries - compact and easy to break down when energy is needed.
Plants store glucose as starch in roots, stems and seeds. It comes in two forms: amylose (straight chains) and amylopectin (branched chains). We digest starch using enzymes like amylase in our saliva.
Animals store glucose as glycogen in liver and muscle cells. It's more branched than starch, allowing faster release of glucose when needed. Humans can store about 400-500g of glycogen.
These provide strength and support. Their structure makes them incredibly strong but impossible for most animals to digest.
The most abundant organic molecule on Earth! Forms plant cell walls and gives wood its strength. The glucose units are joined differently than in starch, making long straight chains that bundle together like cables.
Found in insect exoskeletons and fungal cell walls. Similar to cellulose but contains nitrogen. It's what makes crab shells hard!
Humans can digest starch but not cellulose, even though both are made of glucose. The difference is in the bonds: starch has α-1,4 glycosidic bonds that our enzymes can break, while cellulose has β-1,4 bonds that we can't. This is why we can't digest grass or wood, but cows can - they have special bacteria in their stomachs that produce cellulase enzymes.
The shape and structure of carbohydrates directly affects what they can do. This is a key principle in biology - structure determines function.
Small changes in molecular structure can completely change what a carbohydrate does in the body.
Branched structures like glycogen release energy faster because enzymes can attack multiple points simultaneously.
Straight chains like cellulose can pack tightly together, creating incredibly strong fibres.
Small sugars dissolve easily for transport, while large polysaccharides don't dissolve, making them perfect for storage.
Understanding carbohydrates helps explain many everyday experiences, from why athletes eat pasta before races to why paper is made from trees.
Different carbohydrates affect your body differently. Simple sugars give quick energy but can cause blood sugar spikes, while complex carbohydrates provide steady energy release.
Athletes often "carb-load" before events, eating lots of starch to fill their glycogen stores. During long events, they consume simple sugars for quick energy.
People with diabetes have problems controlling blood glucose levels. Understanding carbohydrate structure helps explain treatment: simple sugars raise blood glucose quickly (useful for treating low blood sugar), while complex carbohydrates provide steadier glucose release. This is why diabetics often choose wholemeal bread over white bread.
Scientists use specific chemical tests to identify different types of carbohydrates in food and biological samples.
Tests for reducing sugars (glucose, fructose, maltose). The solution changes from blue to brick red when heated with reducing sugars present.
Tests for starch. Iodine solution changes from yellow-brown to blue-black in the presence of starch.
Carbohydrates are essential biological molecules that serve two main functions: providing energy and giving structural support. Their structure - from simple monosaccharides to complex polysaccharides - directly determines their function in living organisms. Understanding this relationship helps explain everything from nutrition to why plants stand upright, making carbohydrates one of the most important molecule groups in biology.