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AQA A LEVEL BIOLOGY Exam | Questions with 100% Correct Answers |Verified
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AQA A LEVEL BIOLOGY Exam | Questions with100% Correct Answers |Verified
Protein Structure - ✔✔When more amino acids are added to a dipeptide, a polypeptide is
formed. A protein consists of one or more polypeptide chains folded into a highly specific 3D
shape.
4 phases:
1. Primary
2. Secondary
3. Tertiary
4. Quaternary
Primary Structure - ✔✔The sequence of amino acids in the polypeptide chain.
They are held together by peptide bonds.
Secondary Structure - ✔✔Hydrogen bonds form between amino acids in the chain, making it
coil into an alpha helix (coiling) or a beta pleated sheet (folding). The hydrogen bonds form
between the carboxyl group of 1 amino acid and the amino groups in the peptide backbone.
Tertiary Structure - ✔✔The coiled or folded chain of amino acids is often coiled or folded
further, forming a specific 3D shape for each protein. More bonds form between different parts
of the chain including hydrogen bonds and ionic bonds and disulphide bridges (wherever 2
molecules of the amino acid cysteine come close together, the sulfur atom of one cysteine
bonds to the sulfur atom in another). Disulphide bridges are very strong, ionic are slightly
weaker and rarer and hydrogen bonds are weak. For proteins made from a single polypeptide,
this is the final 3D structure.
Proteins can also be bonded by hydrophilic interactions in which the molecule folds away from
water, towards the inside of the proteins. This can be due to mutation e.g. sickle cell anaemia.
,Bonds depend on the primary structure of the protein.
Bonds - ✔✔Shape is maintained by:
Hydrogen bonds are involved in all levels of the structure and are weak.
Hydrophobic interaction are between the non polar sections of the protein and are strong.
Disulphide bonds are the strongest and most important types of bond in proteins. They occur
between two cysteine amino acids.
Quaternary Structure - ✔✔Many different polypeptide chains are held together by bonds.
This structure is the way these polypeptide chains are assembled together. They are held
together and associated with prosthetic groups (non protein groups e.g. iron ions in
haemoglobin). This is the proteins final specific 3D structure e.g. haemoglobin (4 polypeptides),
insulin and collagen.
Biuret Test - ✔✔Crush and add distilled water to make the sample a solution.
Add sodium hydroxide to make the solution alkaline.
Add copper (II) sulphate solution.
If there is protein the colour will change from blue to violet.
Function - ✔✔Shape determines function e.g. haemoglobin is compact and soluble.
Enzymes - spherical due to tight folding.
-Soluble and often have roles in metabolism e.g. digestion.
-Other enzymes help to synthesise large molecules.
Antibodies - immune response, made of 2 short polypeptide chains and 2 long and heavy chains
bonded together.
-They have a specific active site determined by the primary structure.
,Transport proteins - e.g. channel proteins which contain hydrophilic and hydrophobic amino
acids which forms the channel. These transport molecules and ions across membranes.
Structural Proteins - physically strong and consist of long polypeptide chains lying parallel to
each other with cross links between them. Collagen has 3 polypeptides wound together which
makes it strong. This is a great supportive tissue in animals.
Enzymes - ✔✔-They speed up chemical reactions by acting as biological catalysts.
-They catalyse metabolic reactions - both at a cellular level and for the organism as a whole.
-Enzymes can affect structures in an organism (e.g. the production of collagen, an important
protein in the connective tissues of animals as well as functions (like respiration).
-Enzyme action can be intracellular (within cells) or extracellular (outside cells).
-Enzymes are proteins.
-Enzymes have an active site, which has a specific shape. The active site is the part of the
enzyme where the substrate molecules (the substance that the enzyme interacts with) bind to.
-They are highly specific due to their tertiary structure.
Enzymes lower the activation energy of a reaction - ✔✔In a chemical reaction, a certain
amount of energy needs to be supplied to the chemicals before the reaction will start. This is
called the activation energy - it's often provided as heat. Enzymes lower the amount of
activation energy that is needed, often making reactions happen at a lower temperature than
they could without an enzyme. This speeds up the rate of reaction.
Enzyme-Substrate Complex - ✔✔A temporary complex formed when an enzyme binds to its
substrate molecule. It is this that lowers the activation energy. Here is why:
-If two substrate molecules need to be joined, being attached to the enzyme holds them close
together, reducing any repulsion between the molecules so they can bond more easily.
-If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on
bonds in the substrate, so the substrate molecule breaks up more easily.
, Lock and Key Model - ✔✔Enzymes only work with substrates that fit their active site. The
lock and key model is where the substrate fits into the enzyme, in the same way a key fits into a
lock.
Scientists soon realised that the lock and key model wasn't correct as new evidence showed
that the enzyme-substrate complex changed shape slightly to complete the fit. This locks the
substrate even more tightly to the enzyme.
Induced Fit Model - ✔✔The induced fit model helps to explain why enzymes are so specific
and only bond to one particular substrate. The substrate doesn't only have to be the right
shape to fit the active site, but it has to make the active site change shape in the right way as
well. This is called a conformational change which occurs when the substrate binds to the active
site of the enzyme. This is an example of how a widely accepted theory can change when new
evidence comes along.
The induced fit model is still widely accepted.
Properties of Enzymes - ✔✔-Enzymes are very specific - they usually only catalyse one
reaction e.g. maltase only breaks down maltose and sucrase only breaks down sucrose.
-This is because only one complimentary substrate will fit into the active site.
-The active site shape is determined by the enzyme's tertiary structure and so a different
shaped active site. If the substrate shape doesn't match the active site, an enzyme-substrate
complex will not be formed and the reaction won't be catalysed.
-If the tertiary structure of the enzyme is altered in any way, the shape of the active site will
change. This means the substrate will not fit into the active site and an enzyme-substrate
complex won't be formed and the enzyme will no longer be able to carry out its function.
-The tertiary structure of an enzyme may be altered by changes in pH or temperature.
-The primary structure of a protein is determined by a gene. If a mutation occurs in that gene, it
could change the tertiary structure of the enzyme` produced.
Temperature - ✔✔The rate increases when the temperature increases. More heat energy
means more kinetic energy, so the molecules move faster. This makes the enzymes more likely
to collide with the substrate molecules. The energy of these collisions also increases, which
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