Explore these detailed set of notes. Learn about the different types of lipids, including triglycerides, phospholipids, and steroids. Understand their critical roles in the body, from energy storage and thermal insulation to cellular membrane composition and hormonal functions. This guide also cove...
Lipids
Lipids are not polymers like proteins and complex carbohydrates. They are a large group of
compounds that includes triglycerides, phospholipids and steroids. Lipids are insoluble in water,
and so don’t affect cell water potential and therefore osmosis. They are stored as droplets inside
the cell.
Lipids are used for many purposes in the body, including:
● As thermal insulation
● As an energy store
● To protect organs from mechanical damage
● In the membranes around cells to control the exit and entry of molecules into the cells
● As a component of steroid hormones such as testosterone
● For buoyancy
● To waterproof parts of the body (i.e skin)
● As a source of water via respiration
● As electrical insulation around neurones
● To aid the absorption, storage, and production of fat-soluble vitamins A, D, E, and K
Triglycerides
A triglyceride is a macromolecule containing one glycerol molecule and three fatty acid chains
(see diagram). The fatty acids are attached to the glycerol by a condensation reaction. The
bonds are called ester bonds and they can be broken by hydrolysis.
Triglyceride molecules are rich in energy and used to store excess energy. When required, the
molecules can be broken down in aerobic respiration to release this energy. Water is also
released, which can be useful for animals that live in dry environments — hence camels store
fat in their humps. The stores can be held under the skin and around major organs. It has the
benefit of protecting the major organs from physical shock.
Triglycerides are also good insulators and are used to insulate animals that live in cold
environments such as polar bears and aquatic mammals such as whales. They also provide
buoyancy for these mammals.
Saturated and unsaturated fatty acids
Fatty acids are long chains of carbon atoms with hydrogen atoms bonded to them. If each
carbon has two hydrogen atoms attached, then there are no double or triple bonds in the fatty
acid. This is called a saturated fatty acid. Saturated fatty acids are found in animal fats. They
have a higher melting point and are more solid at room temperature, like butter. When eaten
they can cause an increase in low-density lipoproteins (LDLs,‘bad’ cholesterol). Mycoprotein
has less lipid than meat from animals, and more of it is unsaturated, so it results in less of an
increase in LDLs.
If there are fewer hydrogen atoms, there will be double or even triple bonds between adjacent
carbon atoms. This is called an unsaturated fatty acid, and they have a lower ratio of hydrogen
, atoms attached due to any double or triple bonds. An unsaturated fatty acid may be called
polyunsaturated if it has more than one double or triple bond.Unsaturated fatty acids are found
in plant fats and oils. They have lower melting points and are more likely to be liquid at room
temperature, like spreads and vegetable oil.
Phospholipids
Phospholipids are similar to triglycerides, but one of the fatty acid chains is replaced by a
phosphate group. The two remaining fatty acid ‘tails’ are insoluble in water and are called
hydrophobic.
The phosphate group is complex and includes choline, which is water
soluble. This alters the characteristics of the molecule. This group makes
the ‘head’ end of the phospholipid able to mix with water — it is
hydrophilic. Phospholipids form bilayers with the hydrophobic ‘tails’ in the
centre and the hydrophilic ‘heads’ pointing outwards to interact with the
surrounding aqueous solution. This is the basis of all cell membranes.
The middle of the bilayer is hydrophobic, making it difficult for polar
molecules to pass through.
Amino acids
Proteins are made up of long chains of amino acids. There are 20
different amino acids used in proteins, but all have the same basic
structure. The residual R group is the only part that differs between
different amino acids.
Proteins
Proteins are polymers consisting of long, unbranched chains of amino acids, which are held
together by peptide bonds. These bonds are formed by condensation and occur between the
amine group of one amino acid and the carboxylic acid group of another. There are four main
types of protein; enzymes, antibodies, transport proteins, and structural proteins.
Two amino acids together make a dipeptide. Many amino acids in a chain form a polypeptide.
There are four levels of protein structure.
The primary structure of a protein is the
sequence of amino acids, held together in a
chain by covalent bonds called peptide bonds.
Peptide bonds occur between the amine group
of one amino acid and the carboxyl group of
another. The H from the amine group combines
with the OH from the carboxyl group in a condensation reaction, with a molecule of water being
produced.
The secondary structure is formed when the chain of
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