Chapter 3 - Carbohydrates Flashcards
(44 cards)
What are carbohydrates?
Carbohydrates are molecules which consist only of carbon, hydrogen and oxygen and they are long chains of sugar units called saccharides. There are three types of saccharides - monosaccharides, disaccharides and polysaccharides.
Name 3 roles of carbohydrates
- Energy storage - sugars can be stored as complex carbohydrates (eg: starch or glycogen)
- Structural components - cellulose and chitin are used in cell walls
- Building blocks for biological molecules - deoxyribose and ribose can be used to make nucleic acids
What are monosaccharides?
Monosaccharides are the simplest form of carbohydrates, also known as ‘simple sugars’. Monosaccharides are soluble, sweet-tasting and are found in many foods such as fruits, vegetables, and grains.
What are monosaccharides’ general formula
They have the general formula (CH2O)n where ‘n’ can be any number from 3 to 7.
How are monosaccharides classified?
According to the number of carbon atoms in each molecule:
Eg: hexose sugars = 6 carbon atoms
pentose = 5 carbon atoms
Give 3 examples of a hexose sugar
Glucose
Fructose
Galactose
Give 2 examples of a pentose sugar
Ribose
Deoxyribose
What is glucose?
Glucose is a hexose (6-carbon) sugar with the formula C6H12O6.
The atoms in glucose can be arranged in two different ways.
This means that there are two isomers of glucose:
Alpha-glucose (α-glucose)
Beta-glucose (β-glucose)
What is an isomer?
When the atoms in the molecule can be arranged in 2 different ways
What is the difference between alpha glucose and beta glucose?
The hydroxyl group (OH group) on carbon 1 of alpha glucose is below, whereas on beta glucose its above.
What are 2 features of glucose that help it function?
- It is soluble - The hydroxyl groups can form hydrogen bonds with water, so it can be transported around organisms.
- Its bonds store lots of energy - This energy is released when the bonds are broken.
How are disaccharides formed?
When two monosaccharides join together.
Give 3 examples of disaccharides
- Maltose (found in grains and cereals)
- Sucrose (used as a transport sugar in plants)
- Lactose (the main carbohydrate found in milk)
What is maltose is made up of?
Glucose joined to glucose.
What is sucrose is made up of?
Glucose joined to fructose.
What is lactose is made up of?
Glucose joined to galactose
How are disaccharides created and broken down?
Disaccharides are created via condensation reactions, and broken down via hydrolysis reactions. These reactions involve the formation or the breakdown of a covalent bond known as a glycosidic bond.
Describe a condensation reaction
When two monosaccharides join, a hydroxyl group (OH) of one monosaccharide reacts with a hydroxyl group (OH) of another monosaccharide. This forms a glycosidic bond, and a water molecule (H2O) is released.
Most of the time the hydroxyl groups on carbons 1 and 4 are reacting together, so a 1-4 glycosidic bond forms, but they can also form between other carbons.
Describe a hydrolysis reaction
When a water molecule (H2O) is added to a disaccharide, the glycosidic bond is broken to release the 2 monosaccharides.
What are polysaccharides?
Polysaccharides are complex carbohydrates made up of many monosaccharides joined via glycosidic bonds.
Give three examples of polysaccharides
- starch - amylose + amylopectin
- glycogen
- cellulose
What is starch used by?
Plants to store excess glucose. This means that starch can be hydrolysed back into glucose when they require energy.
Describe the structure of starch
Starch is made up of many alpha-glucose monomers joined via glycosidic bonds to form chains. Starch chains come in two forms: amylose and amylopectin.
Describe the structure of amylose
Amylose is a long, unbranched chain of alpha-glucose joined by 1-4 glycosidic bonds. The angles of these bonds cause the chain to coil into a helix to make a compact structure.
