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BIOLOGICAL MOLECULES
CARBOHYDRATES
2.2.3 Carbohydrates – sugars
- Carbohydrates = a group of molecules that contain only carbon, hydrogen and oxygen
- Glyosidic bond = a bond formed between two monosaccharides by a condensation reaction
- They are “hydrated carbon” = for each 1 carbon there is a water molecule of 2 hydrogen atoms
and 1 oxygen atom
- 3 functions:
a) Source of energy e.g. glucose
b) Store of energy e.g. starch and glycogen
c) Structural units e.g. cellulose in plants, chitin in insects
- 3 main groups of carbohydrates:
a) Monosaccharides (common ones end in -ose)
b) Disaccharides (common ones end in -ose)
c) Polysaccharides
Monosaccharides
- Simplest carbohydrates often used as a source of energy
- Large number of carbon-hydrogen bonds make them well suited for this
- Molecules that are:
a) Sugars
b) Sweet taste
c) Soluble in water
d) Insoluble in non-polar solvents
- Can exist as either straight chains or ring/cyclic forms
- Different sugars have different numbers of carbon atoms e.g. pentose – 5 and triose – 3
- Monosaccharide hexose sugars e.g. glucose are monomers of more complex carbohydrates
when they bond together to form disaccharides or polysaccharides
- In solution triose and tetrose sugars exist as straight chains whilst pentoses and hexoses are
more likely to be found in ring form
- Glucose can exist as a number of different isomers – monomers with the same formula but
different arrangement of atoms – in both straight chain and cyclic forms
- Straight chain: -H and -OH can be reversed
- Ring form: when oxygen attached to carbon 5 bonds to carbon 1, this can form either a- and b-
glucose -> form is important when starch or cellulose is formed
Disaccharides
- Also sweet and soluble
- E.g. Maltose and lactose (reducing sugars) and sucrose (non-reducing sugar)
- Formed when two monosaccharides join together
a-glucose + a-glucose -> maltose
a-glucose + fructose -> sucrose
b-glucose + a-glucose -> lactose
b-glucose + b-glucose -> cellobiose
- When two monosaccharides join a condensation reaction occurs to form a glycosidic bond
,- Two hydroxyl groups (-OH) line up next to each other and a water molecule is expelled
- This leaves an oxygen atom acting as the link between 2 monosaccharide units
- Disaccharides are broken into monosaccharides by a hydrolysis reaction requiring the addition of
water (provides a hydroxyl group -OH and a hydrogen -H to break the glycosidic bond)
- E.g. monomer cellobiose is obtained from the hydrolysis of polysaccharide cellulose
2 glucose Condensa5on reac5on to join Glycosidic (covalent)
molecules them, expels water bond formed
Glycosidic (covalent) Hydrolysis reac5on to separate
Maltose
bond broken them, uses water
molecule
2.2.4 Polysaccharides as energy stores
- Polysaccharides are polymers of monosaccharides – hundreds/thousands bonded together
- Polysaccharides made of one type of monosaccharide = homopolysaccharides e.g. starch
- Polysaccharides made of multiple types of monosaccharide = heteropolysaccharides e.g.
hyaluronic acid
Energy sources vs energy stores
- Glucose is a source of energy when a reactant in photosynthesis which releases ATP – energy
currency of the cell
- Glucose + Oxygen -> releases energy used as ATP -> Carbon dioxide + Water
- Plants then form polysaccharides to have a store of any unused energy e.g. starch in chloroplasts
and membrane bound starch grains
- In humans energy is stored as glycogen in muscle or liver cells
, Why are polysaccharides good energy stores?
- Their structure make them good for this purpose
- Glycogen in animals and starch in plants (amylose and amylopectin) form large granules within
cells and are good stores of monosaccharides because:
a) Glycogen and starch are compact, form dense granules so don’t take up too much space
b) Glucose molecules are in chains so they can be easily snipped off from the end by hydrolysis
when needed for respiration
c) Some chains (e.g. amylose) are unbranched
d) Some are branched (e.g. amylopectin and glycogen) so are more compact and offer the
chance for many different glucose molecules to be snipped off at each end at once
Amylase hydrolyses type 1-4 glycosidic linkages (when carbon-1 links to carbon-4 on the
other glucose) whilst glucosidase hydrolyses type 1-6 glycosidic linkages
e) Less soluble in water than monosaccharides – useful because if lots of glucose dissolved in
the cytoplasm, the water potential would reduce and excess water would diffuse in –
disrupting the cell’s processes
Less soluble because:
i) Larger size than monosaccharides
ii) Regions that could hydrogen-bond with water are hidden inside the molecule
Amylose
- In plants
- Long chain of a-glucose molecules
- Glycosidic bonds between carbons 1 and 4
- Coils into spiral structure
- Hydrogen bonds hold spiral shape
- Hydroxyl groups on carbon 2 on inside of coil = less soluble, lets H bonds maintain structure
Amylopec:n
- In plants
- Glycosidic bonds between carbons 1 and 4, but also between carbons 1 and 6 to form branches
- Coils into spiral shape with branches emerging off
- Held together by hydrogen bonds
Glycogen
- In animals
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