Back to: Organic Chemistry 400 Level
Welcome to class!
Hello amazing learner, I’m truly glad to have you here today. I hope you’re feeling refreshed and curious because today we are beginning a very important and highly relevant topic in Organic Chemistry – Carbohydrate Chemistry I. Whether you are interested in medicine, nutrition, biotechnology or biochemistry, understanding carbohydrates will help you appreciate how living organisms obtain energy and build essential biological structures.
Carbohydrate Chemistry I
Have you ever wondered why athletes take glucose drinks during sports or why garri and rice keep you full and energised for hours? The answer lies in carbohydrates – the primary energy source for most living organisms.
Meaning and Classification of Carbohydrates
Carbohydrates are organic compounds made up of carbon, hydrogen and oxygen, usually in the general formula Cₙ(H₂O)ₙ. They are also known as saccharides (from the Greek word for sugar). Their main function is to provide energy and serve as structural components in living organisms.
Carbohydrates can be classified based on the number of sugar units:
Monosaccharides – Single sugar units that cannot be hydrolysed into smaller units. Examples: glucose, fructose and galactose.
Disaccharides – Composed of two monosaccharide units linked together. Example: sucrose (glucose + fructose), lactose (glucose + galactose).
Oligosaccharides – Contain 3–10 monosaccharide units.
Polysaccharides – Long chains of many sugar units. Examples: starch, glycogen and cellulose.
Monosaccharides: Structure and Types
Monosaccharides are classified according to:
Number of carbon atoms
Trioses (3C), Tetroses (4C), Pentoses (5C), Hexoses (6C)
Type of functional group
Aldoses (contain an aldehyde group, e.g., glucose)
Ketoses (contain a ketone group, e.g., fructose)
For example, glucose is an aldohexose (six carbons + aldehyde), while fructose is a ketohexose (six carbons + ketone).
Open Chain and Ring Forms
In aqueous solution, most monosaccharides exist in dynamic equilibrium between an open-chain (straight) form and a cyclic (ring) structure.
Aldoses form hemiacetals
Ketoses form hemiketals
In the case of glucose, the ring structure contains six atoms and is called a pyranose ring, while fructose forms a five-membered furanose ring. Think of this like a long rope (open chain) that naturally curls to form a loop (ring) when dropped into water.
Reducing and Non-Reducing Sugars
Some sugars can act as reducing agents because they have a free aldehyde or ketone group. These are called reducing sugars (e.g., glucose and lactose).
Non-reducing sugars (e.g. sucrose) do not have a free carbonyl group because it is involved in the glycosidic bond.
Biological Importance of Carbohydrates
Energy Source: Glucose is the primary source of energy for cells.
Energy Storage: Starch (in plants) and glycogen (in animals) store energy for later use.
Structural Role: Cellulose in plant cell walls provides structural strength, and chitin in insects’ exoskeletons gives protection.
Summary
- Carbohydrates are organic compounds with the formula Cₙ(H₂O)ₙ and are also called saccharides.
- They are classified into monosaccharides, disaccharides, oligosaccharides and polysaccharides.
- Monosaccharides are further classified by the number of carbon atoms and by whether they are aldoses or ketoses.
- Monosaccharides exist in equilibrium between open-chain and cyclic forms (pyranose and furanose rings).
- Carbohydrates serve as energy sources, energy storage materials and structural components in living organisms.
Evaluation
- Define carbohydrates and state their general formula.
- Differentiate between monosaccharides and disaccharides with one example each.
- What is the difference between an aldose and a ketose?
- Why is glucose considered a reducing sugar?
Great job today! You’re progressing beautifully, and Afrilearn is very proud of you. Keep up the amazing work — the next lesson will build on this foundation and make your understanding even stronger!