biological molecules

AS level Biology A OCR spec

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Biological Molecules
How hydrogen bonding occurs between water molecules and relate this, and other properties of
water, to the roles of water for living organisms:
Hydrogen bonding occurs between water molecules as it is a polar molecule (the charge is unevenly
distributed). The uneven sharing of electrons leads to the Oxygen having a delta negative charge
and the Hydrogen a delta positive. Due to this when water molecules come close together the delta
negative oxygen atoms form hydrogen bonds with the delta positive hydrogen atoms. These bonds
break and reform allowing the molecule can move around.
· Solvent: Because water is polar it is able to act as a solvent in which many solutes in an
organism can be dissolved. Ex: Cytosol of prokaryotes and eukaryotes is mainly water.
Organisms can take up these dissolved minerals/nutrients. Ex: CO2 is dissolved in water and
used by plants
Transport medium: water acts as a medium for chemical reactions by transporting dissolved
Ex: water transports dissolved compounds in and out of cells
Ex: blood uses water as a transport medium to get around the body.
· Coolant: water acts as a coolant which helps keep the water temperature stable. This can
happen because water has a high SHC as a lot of energy is required to break the hydrogen
bonds in water.
Ex: helps keep temperature stable so reactions can occur in prokaryotic and eukaryotic cells
Ex: maintaining constant temperatures allows reactions to occur e.g metabolic reactions as
the enzymes are able to function (require optimum temp.)
· Habitat: provides a constant environment as it does not change temperature easily. When
water freezes it turns into ice, which floats as it is less dense than in its liquid state, this is
because the molecules are more spread out which forms a lattice structure. So the ice forms
an insulating layer over the water so that the temperature of the water below is maintained
(so the water below does NOT freeze). So organisms living in that water do not freeze and
Water molecules are strongly cohesive to each other which creates surface tension on the
surface of water, which is strong enough to support small insects.
Ex: transport of water up the xylem in plants can only happen due to water having cohesive

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The concept of monomers and polymers and the importance of condensation and hydrolysis
reactions in a range of biological molecules:
Polymers: long-chain molecules made up of covalently bonded individual molecules (monomers) in
a repeating pattern. So larger biological molecules are made from smaller molecules.
Ex: in carbohydrates the monomers are sugars (saccharides), in proteins the monomers are amino
Condensation reaction: when glycosidic bonds (covalent bonds) are formed between monomers
forming polymers.…read more

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The ring structure and properties of glucose as an example of a hexose monosaccharide and the
structure of ribose as an example of a pentose monosaccharide. How the structures and
properties of glucose relate to their functions in living organisms:
Glucose (C6H12O6) is a hexose monosaccharide, meaning it contains 6 carbon atoms. The carbons
are numbered clockwise, beginning with the carbon to the right of the oxygen within the ring.…read more

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Synthesis: Maltose is an example of a disaccharide it is formed by a condensation reaction where
covalent bonds called glycosidic bonds are formed joining carbon 1 of a glucose molecule and
carbon 4 of another glucose molecule. Water is also a product of this reaction.
Breakdown: When maltose is broken down 1 molecule of water is added and the glycosidic bonds
are broken by hydrolysis.
Sucrose is also a disaccharide and is formed by the joining of the 2 monosaccharides: fructose and
glucose.…read more

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Cellulose ­ A straight chain molecule formed of beta glucose molecules the OH groups are too far away to
react so 1 of the beta molecules has to be turned upside down ­ because of this it is unable to coil or form
Structure linked to function: essential part of our diet and because it is hard to break down due to its
structure it forms `fibre' which is needed for a healthy digestive system.…read more

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Phospholipids: ideal for the formation of cell surface membranes as their inner core is hydrophobic
and the outside is hydrophilic. So the membrane is selectively permeable e.g ions have difficulty
passing through as hydrophobic core repels them
Ex: this is how membrane-bound organelles are formed in eukaryotic cells
Cholesterol: small thin molecule can fit in to lipid bilayer giving strength and stability, also regulates the
fluidity by keeping membranes fluid at low temperatures and stopping them becoming too fluid at high
temperatures.…read more

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Secondary structure: The secondary structure is when the polypeptide chain lies parallel to one
another joined by hydrogen bonds to make a beta pleated sheet OR hydrogen bonds form within
the amino acid chain to make an alpha helix structure. Is the result of hydrogen bonds.
· Tertiary structure: The folding of the protein into its final shape, often including sections of
secondary structure.…read more

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Ex: catalase is an enzyme, it has a quaternary structure containing 4 haem prosthetic groups
too. The Fe2+ from the haem group allows catalase to interact with hydrogen peroxide (a
byproduct of metabolism that can cause damage to cells) speeding up its breakdown =
preventing it from damaging cells.
Proteins without prosthetic group are called simple proteins. There are different types of prosthetic
groups: lipids/carbohydrates/vitamins/ions form prosthetic groups = cofactors when they are
necessary for the protein to carry out their functions.…read more

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This complex makes the
solution go from blue to purple.
· Benedict's test for reducing and non-reducing sugars ­
Reducing sugars: benedict's reagent is added to the solution, then heated, if reducing sugars
are present they will react with the copper ions in this reagent and a reduction reaction will
take place. Cu2 + Cu+ this will be visible in the colour change it will go from blue to brick
red indicating a positive result.…read more

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Thin layer chromatography is done exactly as it says - using a thin, uniform layer of silica gel or
alumina coated onto a piece of glass, metal or rigid plastic.
The silica gel (or the alumina) is the stationary phase. The stationary phase for thin layer
chromatography also often contains a substance which fluoresces in UV. The mobile phase is a
suitable liquid solvent or mixture of solvents.
Measurements are taken from the plate in order to help identify the compounds present.…read more


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