Biological Molecules: Proteins (AQA A Level Biology): Flashcards
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Define amino acid.
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Define amino acid.
An amino acid is the monomer unit of proteins, consisting of a central carbon atom bonded to an amine group (-NH₂), a carboxylic acid group (-COOH), a hydrogen atom, and an R group.
What determines the shape and function of a protein?
The sequence, type and number of amino acids in a protein determine its shape and, therefore, its function.
Define peptide bond.
A peptide bond is a chemical bond formed between the carboxylic acid group of one amino acid and the amine group of another, releasing water in a condensation reaction.
Proteins are polymers made from monomers called .
Proteins are polymers made from monomers called amino acids.
Name three types of proteins found in cells and give an example of each.
Examples include: enzymes (e.g. catalase), structural proteins (e.g. collagen), and transport proteins (e.g. haemoglobin).
A dipeptide is formed by the condensation of amino acids; a polypeptide is formed by or more amino acids.
A dipeptide is formed by the condensation of two amino acids; a polypeptide is formed by three or more amino acids.
True or False?
The R group of an amino acid is involved in the formation of peptide bonds.
False.
The R group is not involved in the formation of peptide bonds; only the amine and carboxylic acid groups participate.
During hydrolysis, the is added to break a peptide bond, resulting in .
During hydrolysis, the addition of water is added to break a peptide bond, resulting in amino acids.
Define chromatography.
Chromatography is a technique used to separate a mixture into its individual components based on their differing solubilities in the mobile phase.
What are the two phases involved in chromatography?
Chromatography involves a mobile phase that moves through the system and a stationary phase that does not move.
In chromatography, the phase moves through the system, while the phase does not move.
In chromatography, the mobile phase moves through the system, while the stationary phase does not move.
True or False?
Amino acids that are more soluble in the mobile phase travel a shorter distance on the chromatography paper.
False.
Amino acids that are more soluble in the mobile phase travel further because they spend more time moving and less time bound to the stationary phase.
Define Rf value.
An Rf value is the ratio of the distance travelled by a component to the distance travelled by the solvent front in chromatography.
What is the purpose of spraying chromatography paper with ninhydrin solution after separating amino acids?
Spraying the paper with ninhydrin solution reacts with amino acids to produce a visible blue-violet colour, making the separated spots easy to see.
To identify an unknown amino acid, compare its chromatogram spot with solutions of different amino acids.
To identify an unknown amino acid, compare its chromatogram spot with standard solutions of different amino acids.
Which two properties of amino acids affect how far they travel during chromatography?
The two properties that affect how far amino acids travel are their charge and size.
After confirming the presence of protein using the Biuret test, can be used to identify which amino acids are present.
After confirming the presence of protein using the Biuret test, chromatography can be used to identify which amino acids are present.
Define primary structure of a protein.
The primary structure of a protein is the sequence of amino acids in a polypeptide chain, bonded together by peptide bonds.
Which level of protein structure involves more than one polypeptide chain working together?
The quaternary structure involves more than one polypeptide chain working together as a functional protein, such as in haemoglobin.
The two common shapes of secondary protein structure are the and the .
The two common shapes of secondary protein structure are the α-helix and the β-pleated sheet.
Define disulfide bond in proteins.
A disulfide bond is a strong covalent bond formed between the sulfur atoms of two cysteine amino acids, stabilising the tertiary structure of a protein.
What type of bond holds together the secondary structure of a protein?
The secondary structure of a protein is held together by hydrogen bonds between the -NH and -C=O groups of amino acids.
The sequence of amino acids in a protein is determined by in the cell, which gives instructions during .
The sequence of amino acids in a protein is determined by DNA in the cell, which gives instructions during translation.
True or False?
Hydrophobic interactions play a role in the secondary structure of proteins.
False.
Hydrophobic interactions are involved in the tertiary structure of proteins, not the secondary structure.
Define globular protein.
A globular protein is a protein with a roughly spherical shape, usually soluble in water, and often functions as an enzyme, antibody, or hormone.
Define tertiary protein structure.
A tertiary protein structure is the overall 3D folding of a polypeptide chain, stabilised by interactions and bonds between R groups of amino acids.
Why do polypeptide chains fold into different tertiary structures?
Polypeptide chains fold differently because the R groups of the twenty amino acids interact in various ways, forming different bonds and creating a wide variety of protein shapes and functions.
Each of the twenty amino acids has a unique , which allows for many different interactions within a polypeptide chain.
Each of the twenty amino acids has a unique R group, which allows for many different interactions within a polypeptide chain.
What are disulfide bonds in proteins?
Disulfide bonds are strong covalent bonds that form between two cysteine R groups containing sulfur, helping to stabilise protein structures.
True or False?
Disulfide bonds are broken by changes in pH.
False.
Disulfide bonds are broken by reduction, not by pH changes.
Disulfide bonds form between R groups of cysteine amino acids.
Disulfide bonds form between sulfur-containing R groups of cysteine amino acids.
Define ionic bond in the context of protein structure.
An ionic bond in proteins is an attraction between a positively charged amine group (NH₃⁺) and a negatively charged carboxyl group (COO⁻) on the R groups.
Describe the hydrogen bonds found in tertiary protein structure.
The hydrogen bond is the weakest but most common bond in tertiary protein structure, forming between strongly polar R groups.
Ionic bonds in proteins are broken by changes.
Ionic bonds in proteins are broken by pH changes.
True or False?
Hydrophobic interactions are strong covalent bonds between amino acids in proteins.
False.
Hydrophobic interactions are weak interactions, not strong covalent bonds, and help stabilise protein tertiary structure.
Define Biuret test.
The Biuret test is a qualitative biochemical test used to detect the presence of proteins in a sample.
A protein test solution is first made by adding sodium or potassium hydroxide.
A protein test solution is first made alkaline by adding sodium or potassium hydroxide.
What reagent is commonly used to test for proteins in a sample?
The Biuret reagent is commonly used to test for proteins in a sample.
A positive Biuret test is indicated by a colour change from to .
A positive Biuret test is indicated by a colour change from blue to lilac/purple.
Why should you hold test tubes against a white tile when observing the Biuret test?
Holding test tubes against a white tile helps to see the subtle colour change more clearly in the Biuret test.
If no colour change is observed in the Biuret test, are present in the sample.
If no colour change is observed in the Biuret test, no proteins are present in the sample.
True or False?
The Biuret test will give a positive result for free amino acids.
False.
The Biuret test requires at least two peptide bonds, so free amino acids or dipeptides will not give a positive result.
The Biuret test is a test and does not give a value for the amount of protein present.
The Biuret test is a qualitative test and does not give a quantitative value for the amount of protein present.
What must be present in a protein molecule for the Biuret test to work?
For the Biuret test to work, there must be at least two peptide bonds present in the protein molecule.
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