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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Carbohydrate Structure
Carbohydrates are one of the most important biological molecules in living organisms. They're your body's main source of energy and make up a significant portion of your diet. But what exactly are they made of? Let's explore the building blocks and structures that make carbohydrates so important for life.
Key Definitions:
All carbohydrates are built from simple sugars called monosaccharides. These are the smallest units of carbohydrates and cannot be broken down further by hydrolysis. The most common monosaccharides include:
These simple sugars have the formula (CH2O)n, where n is usually 5 or 6. Glucose, for example, has the formula C6H12O6.
Monosaccharides can exist in different forms. Glucose, for instance, can be found as:
In solution, glucose molecules form a ring structure where the carbon atoms are numbered from 1 to 6. This ring structure is crucial for how glucose molecules connect to form larger carbohydrates.
When monosaccharides join together, they form more complex carbohydrates. This joining process involves a chemical reaction called condensation, where a water molecule is removed as the bond forms.
When two monosaccharides join together, they form a disaccharide. This happens through a condensation reaction where a water molecule is removed and a glycosidic bond forms between the two sugar units.
Formed from two glucose molecules. It's found in germinating seeds and is produced when starch is broken down during digestion.
Made from glucose + fructose. This is your everyday table sugar, extracted from sugar cane or sugar beet.
Consists of glucose + galactose. It's the main sugar found in milk and dairy products.
The glycosidic bond is what holds monosaccharides together in larger carbohydrates. It forms between the -OH group on one monosaccharide and the -OH group on another, releasing a water molecule in the process. This is a type of condensation reaction.
To break this bond, a process called hydrolysis occurs, where water is added back to split the molecules apart. This is exactly what happens during digestion when your body breaks down complex carbohydrates into simple sugars.
When many monosaccharides join together (often hundreds or thousands), they form polysaccharides. These large molecules serve important functions in living organisms.
Structure: Made up of many glucose units joined by glycosidic bonds. It consists of two components:
Function: Energy storage in plants. Found in potatoes, rice, wheat and other plant foods.
Test: Starch turns blue-black when iodine solution is added.
Structure: Similar to amylopectin but more highly branched, made up of glucose units.
Function: Energy storage in animals, particularly in the liver and muscles.
When your body needs energy, glycogen is broken down into glucose which enters the bloodstream. This is why athletes often "carb-load" before competitions - to build up glycogen stores!
Cellulose is another polysaccharide made of glucose units, but with a crucial difference in how these units are connected.
Grass contains large amounts of cellulose, which makes up plant cell walls. Humans can't digest cellulose because we don't have the enzyme cellulase needed to break the β-glycosidic bonds.
Cows and other ruminants, however, have special bacteria in their digestive systems that produce cellulase. These bacteria break down the cellulose into glucose, which the cow can then use for energy. This is why cows can survive by eating grass, while humans would starve on such a diet!
This difference in digestive abilities is a perfect example of how the specific structure of carbohydrates affects their function and usefulness to different organisms.
Scientists can identify different carbohydrates using specific chemical tests:
Procedure:
Results:
Detects: Glucose, maltose, lactose (reducing sugars)
Note: Sucrose is a non-reducing sugar and gives a negative result
Procedure:
Results:
Detects: Starch
This test is often used in food science and biology labs to detect the presence of starch in various samples.
The structure of carbohydrates determines their function in living organisms:
Understanding the structure of carbohydrates helps us understand how our bodies process the foods we eat, how plants store energy and even why some animals can digest plants that others cannot. The specific arrangement of atoms and the types of bonds formed between monosaccharides are crucial to the role these molecules play in living systems.
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