BIOLOGICAL MOLECULES
Molecular biology is the study of the structure and function of
biological molecules. Metabolism is all of the biochemical
reactions in the body. Many biological molecules are made up of
many similar or identical monomer subunits and these are called
polymers. Polysaccharides and proteins are polymers. The
bonds within polymers can be broken by splitting with water.
These are hydrolysis reactions. Bonds are formed by removing
water during condensation reactions.
Carbohydrates have the general formula C (H O) . They can
be divided into the monosaccharides, disaccharides and
polysaccharides.
Monosaccharides are sugars, sweet and
dissolve in water. They have the general formula (CH O) .
Trioses have n = 3, pentoses n = 5 and in hexoses n = 6. Glucose
has the molecular formula C H O . The arrangement of atoms
can be drawn in a structural formula. Glucose can have ring
structures. There are 2 isomers of glucose which are alpha and
beta. Beta glucose has the H pointing down on carbon one –
remember HB pencils. A monosaccharide such as glucose or
fructose is important as an energy source as it has lots of carbon –
hydrogen bonds. When they are broken they release a lot of
energy. They are also important as building blocks to make
larger molecules.
KNOW THE STRUCTURE OF THE GLUCOSE ISOMERS.
alpha glucose
If 2 alpha glucose molecules join in a condensation reaction, a
disaccharide forms with a glycosidic bond between them.
LEARN THIS DIAGRAM.
3 examples of disaccharides are sucrose, maltose and lactose.
If many monosaccharides join a polysaccharide forms. Starch is
an example, where many alpha glucose molecules are joined. It
is a mixture of amylose and amylopectin. Amylose is a long
unbranching chain with many 1,4 links. Amylopectin is a
smaller branching chain with both 1,4 and 1,6 links. Both are
found together coiled into compact starch grains. This allows a
lot of them to be stored. Starch can be easily broken down to
release glucose for energy. It is insoluble allowing for storage
and not changing the water potential of plant cell contents and
upsetting osmosis. is the energy storage carbohydrate of animal
liver and muscle cells. It is like amylopectin but even more
branched. Cellulose is a polymer of beta glucose. In order for
the glucose molecules to join each successive molecule must be
rotated 180 degrees. The hydrogen atoms of the OH groups are
weakly attracted to oxygen atoms in the same cellulose molecule
and in neighbouring molecules. Many of these hydrogen bonds
together give great tensile strength. 60 – 70 molecules form
microfibrils. These are held in bundles by hydrogen bonds into
fibres. To increase strength a cell wall has layers of fibres
running in different directions and a glue like matrix. Cellulose
is insoluble – the cell wall would break up otherwise in water.
Cellulose fibres are permeable between the molecules so allow
passage of substances to and from the plasma membrane.
RECOGNISE DIAGRAMS OF AMYLOSE, AMYLOPECTIN
AND CELLULOSE.
Lipids are a diverse group.
The most common are the
triglycerides (fats and oils). They are formed from a glycerol
and 3 fatty acid molecules that join after condensation reactions
by ester bonds. LEARN THE DIAGRAM OF THIS
REACTION.
Lipids are insoluble in water, non-polar and hydrophobic.
Unsaturated fatty acids contain double bonds and this makes
them melt more easily. They are usually oils. Polysaturated
lipids have many double bonds, monosaturated have one and
saturated (animal fats) have none. They are good stores of
energy, providing about 2.5 times more energy per gram
compared to carbohydrates. They have many more C-H bonds.
Fat under our skin gives insulation and provides protection to
some organs e.g. the kidneys. Phospholipids are similar to
triglycerides but one fatty acid is replaced by a phosphate group.
LEARN THE DIAGRAM OF A PHOSPHOLIPID.
The phosphate group is hydrophilic (water loving) and the
fatty acids hydrophobic (water hating). If shaken with water
phospholipids can form bilayers that are the basis of
membranes.( See membrane notes)
Proteins are very important. They are found in membranes, skin
bone and tendons, and haemoglobin, antibodies and enzymes are
proteins. They are polymers of amino acids. All amino acids
have a general structure with a central carbon bonded to an amine
group –NH , a hydrogen, carboxylic acid group –COOH and a
side group R.
LEARN THIS DIAGRAM.
The R group varies in different amino acids and there are 20
naturally occurring amino acids. A -OH from the carboxylic
group and a –H from an amine group join in a condensation
reaction to form a peptide bond. Two amino acids form a
dipeptide and many joined make a polypeptide. LEARN THIS
DIAGRAM.
The primary structure of a protein refers to the order of amino
acids in a sequence. There are many different possibilities in the
order to give different proteins. The secondary structure can
form when the primary structure twists into an alpha helix held by
H bonds or a beta pleated sheet. H bonds are easily broken by
high temperatures and pH changes. The protein can coil into a
3D shape which is its tertiary structure. The structure can be
held in shape by H bonds, disulphide bridges, ionic bonds and /
or hydrophobic interactions. Disulphide bonds are the last to
break when heated.
Ionic bonds can be broken by pH changes.
A quaternary structure is made up of 2 or more polypeptide
chains held together by the same bonds as in the tertiary structure.
Proteins are globular or fibrous. Globular proteins are roughly
spherical, mostly soluble, of tertiary or quaternary structure and
metabolically active. Haemoglobin is an example. It is a
quaternary structure made up of 2 alpha chains and 2 beta chains.
The hydrophobic R groups point in and the hydrophilic ones
point out - thus haemoglobin is soluble. The hydrophobic R
groups help maintain the 3D shape. Each polypeptide contains a
haem group with an iron ion Fe . One oxygen molecule can
bind with an iron ion. A haemoglobin molecule can therefore
carry 8 O atoms. The haem group is not made of amino acids and
this is called a prosthetic group. Collagen is a fibrous protein
found in skin, blood vessels and tendons. It is a structural
protein and a collagen molecule is made up of 3 polypeptide chains,
each in the shape of a helix twisted around to form 3D ‘rope’.
Nearly every third amino acid is glycine that is small. This
allows the strands to lie close together, held by H bonds that give
it strength. Many of these triple helixes lie side by side and are
linked by covalent cross-links between the carboxyl end of one
molecule and the amine group of another. They are staggered,
which gives them more tensile strength. Many molecules make a
fibril and many fibrils make a fibre.
RECOGNISE
COLLAGEN.
DIAGRAMS
OF HAEMOGLOBIN
AND
Inorganic ions are charged particles needed by living things. For
example calcium phosphate is needed for teeth and bone
structure. Ca are needed for the passage of electrical impulses
across synapses and muscle contraction. Na and K are needed
for nerve impulse transmission along neurons. Mg are needed
for chlorophyll structure and the enzyme ATPase has them at their
active sites. Fe is found in haemoglobin for oxygen transport.