Carbohydrates and lipids

Sugars

Carbohydrates are sugars and their chains.

• A monosaccharide is a single sugar unit. Glucose, fructose and galactose are examples.
• A disaccharide is two monosaccharides joined. Sucrose is glucose and fructose. Lactose, the sugar of milk, is glucose and galactose. Maltose is two glucose units.
• A polysaccharide is a long chain of sugar units, like a thread.

Polysaccharides

Several polysaccharides are chains of one kind of sugar. Each has its own job.

• Cellulose is a chain of glucose. It makes the walls of plant cells. Paper and cotton fibre are cellulose.
• Starch is also a chain of glucose. Plants store energy in it.
• Glycogen is the energy store of animals. Its chains have branches.
• Inulin is a chain of fructose.
• Chitin is a chain of a modified sugar, the amino sugar $N$-acetyl glucosamine. It forms the hard exoskeleton of arthropods.

In a chain such as glycogen, one end is the reducing end and the other is the non-reducing end.

Starch coils into a helix, and the helix can hold iodine. The starch and iodine together are blue. Cellulose has no complex helix, so it cannot hold iodine and stays uncoloured.

Lipids

Lipids do not dissolve in water. The simplest are fatty acids. A fatty acid has a carboxyl group with an R group attached. The R group is a chain of $\text{CH}_2$ units, from 1 to 19 carbons long.

• Palmitic acid has 16 carbons, counting the carboxyl carbon.
• Arachidonic acid has 20 carbons.

A saturated fatty acid has no double bond. An unsaturated fatty acid has one or more carbon to carbon double bonds.

Glycerol is a small molecule with three hydroxyl groups. Many lipids have both glycerol and fatty acids. The fatty acids are joined to the glycerol by ester links. A glycerol with one, two or three fatty acids is a mono-, di- or triglyceride.

These are the fats and oils. Oils have a lower melting point than fats. Gingelly oil, for example, stays liquid in winter.

You should find that a fatty acid with $16$ carbons and no double bond has $32$ hydrogens, and a mass of $256\ \text{Da}$. Add a double bond, and the mass drops by $2\ \text{Da}$.

Some lipids hold phosphorus and a phosphorylated organic compound. These are phospholipids. They are found in the cell membrane. Lecithin is an example. Nervous tissue holds lipids that are more complex still.

Try it in Python

The program turns fatty acids into formulas and masses. Then it builds a fat from glycerol and three fatty acids.

Practise with new numbers

Common mistakes

• Saying lipids are polymers. They are not chains of one repeating unit.
• Mixing up starch and cellulose. Both are glucose chains. Starch stores energy and turns blue with iodine; cellulose builds cell walls.
• Thinking an oil has less fat in it. An oil differs from a fat in its lower melting point.
• Forgetting the carboxyl carbon when counting carbons. Palmitic acid has 16, and the carboxyl carbon is one of them.

Remember these

• Monosaccharide, disaccharide (sucrose, lactose, maltose), polysaccharide (starch, glycogen, cellulose, inulin, chitin).
• Starch and iodine make a blue colour; cellulose does not hold iodine.
• Fatty acid: carboxyl group and a carbon chain; saturated or unsaturated. Glycerol plus fatty acids make glycerides.
• Phospholipids, such as lecithin, are found in membranes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the two monosaccharides in sucrose.

2. Which polysaccharide holds iodine in its helix and turns blue?

3. What is the main difference between a fat and an oil?

4. What makes a fatty acid unsaturated?

5. Name a phospholipid found in cell membranes.