Unit 1, 3.1.2 Biological molecules and digestion

Just a few notes I typed up for revision about biological molecules and digestion, not fully complete.

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3.1.2 Biological molecules and digestion
Carbohydrates
These are used as an energy source in aall living organisms. They are also structural
molecules, used in call membranes and cell walls e.g. cellulose and chitin. Carbohydrates
contain three elements ­ carbon, hydrogen and oxygen.
Monosaccharides
These are monomer of which larger carbohydrates
are made. They are soluble and can be classified by
the number of carbons they contain.
Glucose exists in two different form ­alpha glucose
(left) and beta glucose (they are structural isomers ­
have the same formula but different structures). The
groups attached to carbon-1 are the opposite way
around.
All monosaccharides are reducing sugars. Reduction
reactions involve gain of electrons. Reducing sugars
donate electrons to other chemicals. When a
reducing sugar is heated with Benedict's reagent electrons are transferred to Cu 2+ ions:
Cu2+ + e- Cu+
Blue Red
An insoluble brick red precipitate of copper oxide forms.
Disaccharides
These sugars are also soluble in water. They are formed by condensation reactions between
2 monosaccharides ­ involving the removal of water. The bond between the
monosaccharides called a gylcosidic bond.
-glucose + -glucose maltose + water
-glucose + fuctose sucrose + water
-glucose + galactose lactose + water
A disaccharide can be split up by boiling
with dilute acid. This type of reaction is
called hydrolysis, because the glycosidic
bond is broken by adding water. In living
cells enzymes control these hydrolysis
reactions.

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Some disarccharides (maltose and lactose) are reducing sugars. Sucrose is a non-reducing
sugar. It would firstly need to be hydrolised to split it into -glucose and fructose before
testing it with Benedict's reagent.
Polysaccharides
These are polymers formed when many monosaccharides join by condensation reactions.
They are insoluble in water ­ this prevents storage cells absorbing water by osmosis.
Polysaccharides are either storage compounds (e.g. starch and glycogen) or structural
materials (e.g. cellulose).…read more

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­ removing one molecule of water. When 2 amino acids join they
form a dipeptide.
A dipeptide had a free amino acid group at one end and a free carboxyl group at the other ­
these can join with more amino acids to form a longer chain called a peptide. When many
amino acids join together a polypeptide forms.
In the body digestive enzymes hydrolise peptides by breaking the peptide bonds. The body
can then absorb free amino acids into the blood stream.…read more

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Disulphide bridge formation
Tertiary protein structure describes the overall 3D shape of a polypeptide chain. The shape is
stabilised by hydrogen bonds (-C=O --- H-N-), ionic bonds and disulphide bridges which form
between R groups (e.g. a disulphide bridge can form between two cystine amino acids.)
Quaternary protein structure describes how different polypeptide chains bind together in a
protein (e.g. haemoglobin (4 polypeptides) and insulin (2 polypeptides)).
A quaternary protein structure occurs when a protein consists of more than one amino acid
chain.…read more

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Enzymes are globular proteins that act as biological catalysts ­ they are not used up in the
reactions, and usually increase the rate. Enzymes control almost every chemical reaction
inside organisms (metabolism). There are 2 types of metabolic reaction:
1. Anabolic reactions ­ larger molecules are built from smaller ones (e.g. proteins built
from amino acids).
2. Catabolic reactions ­ larger molecules are broken down into smaller ones (e.g.
polysaccharides digested into monosaccharides).…read more

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Q10
The change in the rate of reaction for each 10°C rise in temperature is called the
temperature coefficient or Q10.
pH
Each enzyme has an optimum pH at which it catalyses its
reaction fastest ­ the active site of the enzyme and the
complimentary substrate collide and bind most frequently
at this pH so enzyme-substrate complexes form most
frequently.
Enzymes that work inside cells usually work best at a pH
of 7.3-7.4.…read more

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To reduce the effect of competitive inhibitors
the substrate concentration can be increased.
Non-competitive inhibitors
These bind with the enzyme away from the active site (their shape is not
complementary to the active site). Once it has bound the enzyme undergoes a shape
change (conformational change), which includes a change in the active site.
Enzyme-substrate complexes can no longer form and adding more substrate has no
effect.
Digestion
Digestion starts in the buccal cavity.…read more

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The pancreas is a large gland found below the stomach. It produces pancreatic juice, which
contains the following enzymes:
Proteases that hydrolyses proteins into amino acids
Amylase that hydrolyses starch into maltose
Lipase which hydrolyses lipids into fatty acids and glycerol.
The duodenum is the first part of the small intestine where the bulk of chemical digestion
occurs. In the duodenum chime is mixed with pancreatic juice and bile (from the liver).…read more

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Carbohydrate digestion
Chemical digestion is carried out by enzymes. All digestive enzymes function by hydrolysis ­
the splitting up of molecules by adding water to the chemical bonds that hold them
together.
The enzymes amylase is produced by the salivary glands and the pancreas. Amylase
hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide
maltose.
In the mouth food is chewed and mixed with saliva. Saliva has a pH of 7, which is the
optimum pH for amylase to work.…read more

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