Download 3.2 – Carbohydrates, Lipids and Proteins

3.2 – Carbohydrates, Lipids and Proteins
3.2.1 - Distinguish between organic and inorganic compounds
Organic compounds are based on carbon and can be found in living things. Exceptions
include HCO₃, CO₂ and CO. These are classed as non-organic carbon. Three types of organic
compounds widely found in living organisms are lipids, proteins and carbohydrates.
Inorganic compounds are any compounds that do not fall into the category of organic
compounds.
3.2.2 - Identify amino acids, glucose, ribose and fatty acids from diagrams showing their
structure
Amino acids
There are 20 common amino acids found in
the protein structures of living things. These
are monomers, and combine to form larger
polypeptides, which in turn form proteins.
These are the basis of enzymes, as well as
many cellular and extracellular components.
All amino acids are soluble. Each common amino acid has the same structure, except for the
R group.
Glycine (below) is the smallest amino acid.
A common source of it is sugar cane.
Glycine has an amino group, a carboxylic
acid group and an R group (H).
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Alanine (below) is a common amino acid,
similar to glycine, but the R group is CH₃
The R group of each amino acid is different,
and the amino acids have very different
characteristics as a result (and consequently
the proteins containing them).
Carbohydrates
Monosaccharides are carbohydrates with relatively small molecules. They taste sweet and
are soluble in water. All the bonds in these molecules are covalent.
Glucose is an important monosaccharide as:

All green leaves manufacture glucose using light
energy

Our bodies transport glucose in the blood

All cells use glucose in respiration - it is called
one of the respiratory substrates

Glucose is the building block for many larger
molecules in cells and organisms
The molecular formula of glucose is C₆H₁₂O₆
Ribose is an example of a pentose, or 5-carbon
sugars. Deoxyribose is a modified version of
ribose, and is known for its role in DNA as part of
the sugar phosphate backbone. Its chemical
properties are very different to ribose.
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Fatty acids
These are the basis of triglycerides and many other types of lipid. They are also the basis of
the phospholipid molecules of the phospholipid bilayer of the cell membrane. Lipids are
insoluble in water, often described as hydrophobic.
This is a basic saturated (no double bonds) fatty acid. There is a methyl group (CH₃) at one
end of the chain. The chains are made up of covalently bonded carbons, saturated with
hydrogens. The chain is non-polar and hydrophobic. These are typically made up of 16-18
carbon atoms, but can be anywhere from 14-22. The carboxyl group is polar, making the
end of the molecules hydrophilic.
In water, fatty acid molecules arrange into spheres called micelles. The tails are at the
centre, away from water. This is important to fat digestion and membrane structure.
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3.2.3 - List three examples each of monosaccharides, disaccharides and polysaccharides
Monosaccharides
Glucose - [animal] transported to cells in the blood plasma, and used as a respiratory
substrate. In plants, it is a first product of photosynthesis
Galactose - [animal] used in the production of lactose
Fructose - [plant] this is produced in cellular respiration, and in the production of
sucrose
Disaccharides
Lactose - [animal] produced in mammary glands and secreted into the milk as an
important component in the diet of very young mammals
Sucrose - [plant] produced in green leaves from glucose and fructose. It is transported
in the plant in solution, in the vascular bundles
Maltose - [plant] this is a breakdown product in the hydrolysis of starch
Polysaccharides
Glycogen - [animal] this is a storage carbohydrate formed from glucose in the liver and
other cells (except brain cells) when glucose is not immediately required for cell
respiration
Cellulose - [plant] this is manufactured in cells and laid down externally, in bundles of
fibres, as the main component of the cell walls
Starch - [plant] this is a storage carbohydrate
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3.2.4 - State one function of glucose, lactose and glycogen in animals, and of fructose,
sucrose and cellulose in plants
Animals
Glucose

Respiratory substrate
Lactose

Dietary component for young mammals, secreted from the mammary glands in
the milk
Glycogen

Storage carbohydrate for when glucose is not immediately needed
Plants
Fructose

Used in the production sucrose, and also an intermediate of glucose breakdown
Sucrose

Produced from glucose and fructose, and transported in the plant in solution
Cellulose

The main component of cell walls, laid down in bundles of fibres
3.2.5 - Outline the role of condensation and hydrolysis in the relationships between
monosaccharides, disaccharides and polysaccharides; between fatty acids, glycerol and
triglycerides; and between amino acids and polypeptides
A polymer consists of large molecules made up of a linked series of repeated simple
monomers
A monomer is a simple molecular unit.
Condensation is the process of two monomers into a dimer or polymer. A molecule of water
will also be formed as a product. This is a condensation reaction. The link between them
after the removal of H₂O is called a glycosidic linkage, which comprises of strong, covalent
bonds. This reaction is brought about by an enzyme.
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The reverse, a hydrolysis reaction, is when a molecule of water is added and the glycosidic
linkage is split. It is also catalysed by an enzyme, but a different one from in the
condensation reaction.
Sugars
Below is a condensation reaction
between two molecules of glucose
to form maltose. This can be
reversed in a hydrolysis reaction.
To form polysaccharides, many
monosaccharides are joined.
Polysaccharides form in the same
way as disaccharides.
Fats and oils
Fats and oils are triglycerides (simple lipids). At 20oC,
fats are solid and oils are liquid. Oils have a lower
density and melting point due to bends in their tails
and unsaturated bonds. Fats tend to have longer fatty
acid tails and saturated bonds. This makes them
denser and raises the melting point.
Triglycerides are not formed as in above. Instead, the chains are bonded to the molecule
glycerol. The triglyceride formed is insoluble.
Phospholipids are the principle molecules in the cell membrane that form the bilayer. Their
structure is similar to the triglyceride, except one of the fatty acids chains are replaced by a
phosphate group.
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𝑮𝒍𝒚𝒄𝒆𝒓𝒊𝒅𝒆 + 𝟑 𝑭𝒂𝒕𝒕𝒚 𝑨𝒄𝒊𝒅𝒔 → 𝑻𝒓𝒊𝒈𝒍𝒚𝒄𝒆𝒓𝒊𝒅𝒆 + 𝟑𝑯𝟐 𝑶
Amino Acids
In the formation of a dipeptide or polypeptide,
two amino acid monomers will align to form
peptide bonds by condensation reactions. This
bond can form between the carboxyl group of the
first amino acid and the amino group of the
second amino acid. Again, water is removed in the reaction. A dipeptide is formed when
there is a bond between C-N. This pattern is true for all polypeptides.
𝟐 𝑮𝒍𝒚𝒄𝒊𝒏𝒆 → 𝑫𝒊𝒑𝒆𝒑𝒕𝒊𝒅𝒆 + 𝑯𝟐 𝑶
Polypeptide chains fold into the complex, specific shapes of the protein seen below. The
shape is determined by the hydrogen bonding and some covalent bonding between R
groups. Polypeptides can be hydrolysed in the same way as polysaccharides by incubating
with acids. They are digested into amino acids by peptidases.
3.2.6 - State three functions of lipids

Energy store - Fats and oils transfer twice as much energy as carbohydrates. They are
also insoluble, so their presence does not cause osmotic water uptake
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
Metabolic water source - Energy and water are released when fats are used as a
substrate in respiration.

Buoyancy aid - fat is not as dense as muscle or bone, so [for example, the blubber in
whales] it will give buoyancy to the body.

Thermal insulation - Heat can be retained in the body through fat insulation

Water-proofing for hairs and feathers - this oil acts as a water repellent cuticle, and
prevents hair and feathers from becoming waterlogged when wet.

Electrical insulation - Myelin lipid forms sheaths around the long fibres of nerve cells,
electrically isolates the cell plasma membrane and facilitates the conduction of nerve
impulses.

Hormones - Steroids can act as hormones in the body, examples include progesterone
and testosterone

Cell receptors - Glycolipids on the surface of cells can act as receptors for hormones
and other substances

Structure - Lipids like cholesterol are essential for maintaining the structure of cell
membranes.
3.2.7 - Compare the use of carbohydrates and lipids in energy storage
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