2.2. 3-5 Carbohydates Flashcards
What is a Carbohydrate?
a group of molecules containing C, H, and O
What is a glycosidic bond
a bond formed between two monosaccharides by a condensation reaction
What are the functions of a Carbohydrate?
- Source of energy (e.g glucose)
- Store of energy (e.g starch & glycogen)
- Structural units (e.g cellulose in plants and chitin in insects)
- Part of other molecules such as Nucleic Acids
What are Monosaccharides?
- Simplest carbohydrates
- Important for the source of energy
- Soluble in water
- Exist in straight chains or in rings or in cyclic forms
- Glucose can exist as a number of different isomers.
- H and -OH can be revered in straight chains. - Two isomers of glucose αlpha and βeta
What are Disaccharides
- Like monosaccharides they are soluble
- Most common types are:
- Maltose, Lactose (Reducing Sugars)
- Sucrose (non- reducing sugar) - Two hydroxyl groups line up next to each other, from which water is removed.
[Disaccharides]
α-glucose + α-glucose —>
maltose
[Disaccharides]
α-glucose + fructose —>
sucrose
[Disaccharides]
β-galactose + α-glucose —>
lactose
[Disaccharides]
β-glucose + β-glucose
Cellobiose
[α-glucose] What is the molecular formula, the role and the type of sugar
Molecular Formula: C6H12O6
Role in the body: Energy source, Component of starch and glycogen which act as energy stores
Type of sugar: Hexose
[β-glucose] What is the molecular formula, the role and the type of sugar
Molecular Formula: C6H12O6
Role in the body: Energy source, Component of cellulose, which provides structural support in plant cell walls.
Type of sugar: Hexose
[Ribose] What is the molecular formula, the role and the type of sugar
Molecular Formula: C5H10O5
Role in the body: Component of RNA, ATP, NAD
Type of sugar: Pentose
[deoxyRibose] What is the molecular formula, the role and the type of sugar
Molecular Formula: C5H10O4
Role in the body: Component of DNA
Type of sugar: Pentose
What are homopolysacchardies?
Polysaccharides made solely of one kind of monosaccharide e.g starch
What happens when you join lots of glucose molecules together into polysaccharides
- You create a store of energy.
- Plants store energy as starch in chloroplasts and in membrane bound starch grains
- Humans store energy as glycogen in cells of muscles and livers
Why are polysacchardies good energy stores?
- Glycogen and starch are compact, which means they do not occupy a large amount of space. —> Both occur in dense granules within cells
- Polysaccharides hold glucose molecules in chains, so they can easily be snipped off from the end of the chains by hydrolysis when required for respiration
- Some chains are unbranched (amylose) and some are branched (amylopectin and glyocgen)
- Branched chains = More compact but also offer the chance for lots of glucose molecules to be snipped off by hydrolysis at the same time, when lots of energy is required quickly. - Polysaccharides are less soluble in water than monosaccharides. —> Due to size but also regions which could hydrogen bonds are hidden away in the molecule. Sometimes the amylose molecule may form a double helix which presents a hydrophobic external surface in contact with surrounding solution
What is enzyme Amylase responsible for?
Hydrolysing 1-4 glycosidic linkages
What is the enzyme glycosidic responsible for?
Hydrolysing 1-6 glycosidic linkages
What is Cellulose?
- Found in plants
- Forms the cell walls
- It is a Tough, insoluble and fibrous substance
Why doesn’t Cellulose spiral like chains of α-glucose?
- Hydrogen and hydroxyl groups on Carbon 1 are inverted in β-glucose. —> Every β-glucose molecule in the chain is rotated by 180°. —> Prevents from spiralling
- Hydrogen bonding between the rotated β-glucose gives the chain additional strength, and stops it spiralling
- Hydroxyl group on carbon 2 sticks out, enabling hydrogen to be formed between chains –> proves additional whole structure strength
How many cellulose chains are required to form microfibrils?
60-70 which then bundle together to make macrofibrils containing up to 400 microfibrils
Why is cellulose an excellent material for plant cell walls?
- Microfibrils and macrofibrils have very high tensile strength, both because of the strength of glycosidic bonds and hydrogen bonding between chains. —> Macrofibrils are stronger than steel wire of the same diameter
- Macrofibrils run in all directions, criss-crossing the wall for extra strength
- It is difficult to digest cellulose because glycosidic bonds between the glucose molecules are less easy to break.
Why do features of the Cellulose help the plant cell wall do its job?
- Because plants do not have a rigid skeleton, each cell needs to have strength to support the whole plant
- There is a space between macrofibrils for water and mineral ions to pass on their way into and out of the cell. This makes the cell wall fully permeable.
- High tensile strength —> Prevent cell from bursting
- Macrofibril structure can be reinforced with other substances for extra support or to make walls water proof. E.g Lignin
Where else can Cellulose be used?
Cotton is 90% cellulose
Cellophane and clelluloid are also derived from cellulose
