Biological Molecules: Proteins

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शिक्षकों के लिए: Biological Molecules: Proteins (Biology, AQA) के लिए इस्तेमाल के लिए तैयार लेसन स्लाइड्स, रिवीज़न नोट्स — इन्हें अपने लेसन में इस्तेमाल करें, या टॉपिक को एक इंटरैक्टिव क्लास एक्टिविटी की तरह चलाएं जिसे आपके शिक्षार्थी लाइव गेम की तरह खेलें।

लेसन नोट्स

Amino Acids: The Monomers of Proteins

Proteins are polymers (and macromolecules) made of monomers called amino acids.
The sequence, type and number of amino acids within a protein determine its shape and therefore its function.
There are 20 amino acids found in proteins common to all living organisms.
The general structure of all amino acids is a central carbon atom bonded to: an amine group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and an R group.
The R group is how each amino acid differs and why amino acid properties differ, e.g. whether they are acidic or basic or whether they are polar or non-polar.

Peptide Bond Formation

To form a peptide bond, a hydroxyl (-OH) is lost from the carboxylic group of one amino acid, and a hydrogen atom is lost from the amine group of another amino acid.
The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid.
This is a condensation reaction, so water is released.
Dipeptides are formed by the condensation of two amino acids.
Polypeptides are formed by the condensation of many (three or more) amino acids.
During hydrolysis reactions, the addition of water breaks the peptide bonds, resulting in polypeptides being broken down to amino acids.
Amino acids are bonded together by covalent peptide bonds to form a dipeptide in a condensation reaction.

Chromatography: Separating Amino Acids

Chromatography separates a mixture into its components based on their solubility.
It involves two phases: mobile phase (moves through the system) and stationary phase (does not move).
As the mobile phase passes over the stationary phase, components separate.
Components that are more soluble in the mobile phase travel further, as they spend more time moving and less time bound to the stationary phase.
Paper chromatography can be used in school laboratories to separate a mixture of amino acids.

Chromatography: Method and Identification

A spot of the unknown amino acid sample mixture is placed on a line at the bottom of the chromatography paper.
Spots of known standard solutions of different amino acids are then placed on the line beside the unknown sample spot.
The chromatography paper is then suspended in a solvent.
Each amino acid will be more or less soluble in the mobile phase than others and will therefore separate, travelling with the solvent at different times/distances from the line, depending on their charge and size.
The unknown amino acid(s) can then be identified by comparing and matching them with the chromatograms of the known standard solutions of different amino acids.
If a spot from the amino acid mixture travels the same distance as a spot from a known standard, it indicates that the mixture contains that amino acid.
To view the spots, it may be necessary to first dry the chromatography paper and then spray it with ninhydrin solution; this chemical reacts with amino acids, producing an easily visible blue-violet colour.

Protein Structure: Primary and Secondary

There are four levels of structure in proteins: three are related to a single polypeptide chain, and the fourth relates to a protein that has two or more polypeptide chains.
The primary structure is the sequence of amino acids bonded by peptide bonds.
DNA of a cell determines the primary structure of a protein by instructing the cell to add certain amino acids in specific quantities in a certain sequence, during translation. This affects the shape and, therefore, the function of the protein.
The primary structure is specific for each protein; one alteration in the sequence of amino acids can affect the function of the protein.
The secondary structure is held together by hydrogen bonds that form between the -NH region of one amino acid and the -C=O region of another.
The hydrogen of -NH has an overall positive charge, while the oxygen of -C=O has an overall negative charge.
Hydrogen bonds are relatively weak, so they can be broken easily by high temperatures and pH changes.
Two shapes can form within proteins due to the hydrogen bonds: α-helix and β-pleated sheet.
The α-helix shape occurs when the hydrogen bonds form between every fourth peptide bond.
The β-pleated sheet shape forms when the protein folds so that two parts of the polypeptide chain are parallel to each other, enabling hydrogen bonds to form between the folded layers.

Protein Structure: Tertiary and Quaternary

Further conformational change of the secondary structure leads to additional bonds forming between the R groups (side chains).
The additional bonds are: hydrogen bonds between R groups, disulfide bonds between cysteine amino acids, ionic bonds between charged R groups, and weak hydrophobic interactions between non-polar R groups.
This structure is common in globular proteins such as enzymes and antibodies.
Quaternary structure occurs in proteins that have more than one polypeptide chain working together as a functional macromolecule, for example, haemoglobin.
Each polypeptide chain in the quaternary structure is referred to as a subunit of the protein.

Protein Interactions and Bond Strength

A polypeptide chain will fold differently, into its tertiary structure, due to the interactions (and hence the bonds that form) between R groups.
Each of the twenty amino acids has a unique R group, and therefore, many different interactions can occur, creating a vast range of protein configurations and therefore functions.
Disulfide bonds (also known as disulfide bridges) are strong covalent bonds that form between two cysteine R groups (this is the only amino acid with an available sulfur atom in its R group).
These bonds are the strongest within a protein, but occur less frequently, and help stabilise the proteins. They can be broken by reduction.
Disulfide bonds are common in proteins that are secreted from cells e.g. insulin.
Ionic bonds form between positively charged (amine group -NH3+) and negatively charged (carboxylic acid -COO-) R groups. They are stronger than hydrogen bonds, but they are not common. These bonds are broken by pH changes.
Hydrogen bonds form between strongly polar R groups. These are the weakest bonds that form, but the most common, as they form between a wide variety of R groups.

Functions of Proteins

Proteins perform a wide range of essential roles in all living organisms due to their diverse structures.
They are vital for structure, transport, communication, defence, movement, and catalysis in all living cells.
Enzymes – biological catalysts that speed up metabolic reactions (e.g. amylase, DNA polymerase).
Transport proteins – carry substances (e.g. haemoglobin transports oxygen; channel proteins in membranes).
Structural proteins – provide support (e.g. collagen in connective tissues; keratin in hair and nails).
Hormones – regulate processes (e.g. insulin controls blood glucose levels).
Antibodies – part of the immune response, recognising and neutralising pathogens.
Contractile proteins – enable movement (e.g. actin and myosin in muscles).

Biochemical Tests: Proteins

A liquid solution of a sample is treated with sodium or potassium hydroxide to make the solution alkaline.
A few drops of copper (II) sulfate solution (which is blue) are added to the sample.
Biuret reagent contains an alkali and copper (II) sulfate and is commonly used.
If a colour change is observed from blue to lilac/purple, then protein is present.
The colour change can be very subtle, it’s wise to hold the test tubes up against a white tile when making observations.
If no colour change is observed, no protein is present.
For this test to work, there must be at least two peptide bonds present in any protein molecule, so if the sample contains only amino acids or dipeptides, the result will be negative.
The Biuret test is qualitative - it does not give a quantitative value as to the amount of protein present in a sample.

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प्रैक्टिस सवाल

फ्री प्रीव्यू — 64 में से 8 सवाल। सभी देखने के लिए साइन अप करें।

1.What are the monomers that make up proteins?
Easy
- AAmino acids
- BMonosaccharides
- CNucleotides
- DFatty acids
2.Which group is lost from the carboxylic acid group of an amino acid during peptide bond formation?
Easy
- AHydroxyl (-OH)
- BAmino group (-NH2)
- CHydrogen atom (-H)
- DCarboxyl group (-COOH)
3.In the Biuret test, what colour change indicates the presence of protein?
Easy
- ABlue to lilac/purple
- BBlue to brick-red
- CColourless to blue-black
- DYellow to red
4.The R group of an amino acid is involved in the formation of a peptide bond.
Easy
True or false?
5.Which of the following are functions of proteins in living organisms? (Select all that apply)
Medium
- AActing as enzymes
- BTransporting substances
- CStoring genetic information
- DProviding structural support
- EActing as hormones
6.Match each level of protein structure with its description.
Medium
- Primary
- Secondary
- Tertiary
- Quaternary
- Sequence of amino acids
- Folding into α-helix or β-pleated sheet
- Overall 3D shape of a single polypeptide
- Multiple polypeptide chains working together
7.Place the steps of the Biuret test in the correct order.
Medium
- Add a few drops of copper(II) sulfate solution
- Observe any colour change
- Add sodium or potassium hydroxide to make the solution alkaline
- Record the result as positive if lilac/purple appears
8.Which type of bond is responsible for holding the secondary structure of a protein together?
Medium
- AHydrogen bonds between the -NH and -C=O groups of the polypeptide backbone
- BDisulfide bonds between cysteine R groups
- CIonic bonds between charged R groups
- DPeptide bonds between amino acids

और सवाल

9.Which of the following substances in the table below are proteins or contain protein? (Select all that apply)
Easy
- AStarch granules
- BHormones
- CStomach enzymes
- DGlycogen
- EMuscle tissue
- FHaemoglobin
10.Figure 1 shows the structure of an amino acid. Identify the group labelled X.
Easy
- AAmine group
- BCarboxylic acid group
- CR group
- DHydrogen atom
11.Figure 2 shows a dipeptide. Which of the following correctly identifies the peptide bond?
Easy
- AThe bond between the carbon of the carboxylic acid group and the nitrogen of the amine group
- BThe bond between the central carbon and the R group
- CThe bond between the nitrogen of the amine group and the hydrogen atom
- DThe bond between the central carbon and the carboxylic acid group
12.Match each property of an amino acid to its predicted Rf value.
Medium
- High solubility
- Small molecule
- Low solubility
- Large molecule
- High Rf
- Low Rf
13.Match each aspect of protein structure with its correct level of protein structure.
Medium
- α-helix
- Disulfide bridge
- Sequence of amino acids
- β-pleated sheet
- Separate polypeptide chains interacting
- Hydrogen bonds
- Ionic bonds
- Secondary
- Tertiary
- Primary
- Quaternary
- Secondary and tertiary
14.The Biuret test is a biochemical test for proteins. State the colour change observed in a positive Biuret test.
Easy
- ABlue to lilac/purple
- BColourless to blue-black
- CBlue to brick-red
- DYellow to colourless
15.The amino acid phenylalanine is non-polar. State whether phenylalanine is more likely to be found at the centre of a globular protein or at its surface.
Medium
- AAt the centre
- BAt the surface
- CEqually at both
- DOnly in the primary structure
16.Figure 1 shows the structure of collagen protein at various levels of detail. Which two features of collagen make it a strong, structural protein? (Select all that apply)
Medium
- AIt has a triple helix structure
- BIt has covalent cross-links between polypeptide chains
- CIt is a globular protein
- DIt is soluble in water
- EIt has a high proportion of non-polar amino acids
17.Which of the following correctly describes the primary structure of a protein?
Medium
- AThe sequence of amino acids joined by peptide bonds
- BThe folding of the polypeptide chain into an α-helix or β-pleated sheet
- CThe overall 3D shape formed by interactions between R groups
- DThe association of multiple polypeptide chains
18.Place the following steps of paper chromatography to separate a mixture of amino acids in the correct order.
Medium
- Spot the unknown amino acid mixture and known standards onto the baseline of the chromatography paper.
- Suspend the chromatography paper in the solvent.
- Allow the solvent to travel up the paper.
- Dry the paper and spray with ninhydrin to visualise the spots.
- Compare the distance travelled by the unknown spots with the known standards.
19.The Biuret test can be used to detect the presence of amino acids.
Easy
True or false?
20.Match each type of bond with its description.
Medium
- Disulfide bond
- Ionic bond
- Hydrogen bond
- Hydrophobic interaction
- Strong covalent bond between cysteine R groups
- Bond between positively and negatively charged R groups
- Weak bond between polar R groups
- Weak interaction between non-polar R groups
21.A polypeptide has 150 amino acids. How many water molecules are released when it is formed by condensation reactions?
Medium
- A149
- B150
- C151
- D300
22.Which of the following statements about the R group of an amino acid is correct?
Medium
- AIt determines the properties of the amino acid.
- BIt is involved in the formation of the peptide bond.
- CIt is always polar.
- DIt is the same in all amino acids.
23.Place the levels of protein structure in order from simplest to most complex.
Medium
- Primary
- Secondary
- Tertiary
- Quaternary
24.Hydrogen bonds in the secondary structure of a protein form between the R groups of amino acids.
Medium
True or false?
25.Which of the following are functions of proteins? (Select all that apply)
Medium
- AActing as enzymes to catalyse metabolic reactions
- BTransporting substances such as oxygen
- CProviding structural support
- DStoring genetic information
- ERegulating processes as hormones
26.A protein has a quaternary structure made of four identical polypeptide chains. Each chain contains 120 amino acids. Calculate the total number of peptide bonds in the protein.
Medium
- A476
- B480
- C484
- D120
27.Some washing powders contain molecules that break down and hydrolyse the peptide bonds between amino acids. The degree of hydrolysis (DH) is calculated using: DH = 100 × (number of peptide bonds hydrolysed / total number of peptide bonds present). A washing powder was applied to a protein and the DH value after exposure was 28, with a total of 140 peptide bonds hydrolysed. Calculate the number of amino acids in the protein.
Hard
- A500
- B501
- C499
- D140
28.Collagen protein molecules have a quaternary structure. One of the polypeptide chains that forms a molecule of collagen contains 1049 peptide bonds. Assuming that all the chains in collagen are identical in length, calculate the number of amino acids that would be found in a single molecule of collagen.
Hard
- A3147
- B3150
- C3153
- D1049
29.Each haemoglobin molecule can bind 4 oxygen molecules. There are 300 million haemoglobin molecules in each red blood cell. An adult human is estimated to have 3 × 10¹³ red blood cells in circulation at any one time. Calculate the theoretical maximum number of oxygen molecules that could be carried in the red blood cells of an adult human.
Hard
- A3.6 × 10²²
- B9 × 10²¹
- C1.2 × 10²²
- D3.6 × 10²¹
30.In the secondary structure of a protein, hydrogen bonds form between the -NH group of one amino acid and the -C=O group of another.
Easy
True or false?
31.Which of the following statements about the Biuret test are correct? (Select all that apply)
Medium
- AIt requires at least two peptide bonds to give a positive result.
- BA positive result is a colour change from blue to lilac/purple.
- CIt gives a quantitative measure of the amount of protein present.
- DIt can be used to detect the presence of amino acids.
- EIt involves adding copper(II) sulfate solution to an alkaline sample.
32.Match each property of an amino acid to its predicted Rf value in chromatography.
Medium
- High solubility in the mobile phase
- Low solubility in the mobile phase
- Small molecule
- Large molecule
- High Rf
- Low Rf
33.A scientist uses chromatography to separate a mixture of amino acids. The Rf value of an unknown spot is 0.65. A standard amino acid, leucine, has an Rf value of 0.65 under the same conditions. What can the scientist conclude?
Medium
- AThe unknown spot is likely to be leucine.
- BThe unknown spot is definitely not leucine.
- CThe unknown spot is a protein.
- DThe unknown spot is a dipeptide.
34.Sickle cell anaemia results from a mutation that replaces glutamic acid (polar) with valine (non-polar) in the haemoglobin molecule. This makes the haemoglobin less soluble. Which statement best explains this observation?
Hard
- AThe non-polar valine causes the protein to fold incorrectly, exposing hydrophobic regions that reduce solubility.
- BThe polar glutamic acid is replaced by a non-polar amino acid, making the protein more soluble.
- CThe mutation changes the primary structure, which always increases solubility.
- DThe non-polar valine forms more hydrogen bonds with water, reducing solubility.
35.Which of the following bonds is the strongest in a protein and can be broken by reduction?
Medium
- ADisulfide bond
- BHydrogen bond
- CIonic bond
- DPeptide bond
36.A student carries out the Biuret test on four different solutions. Which solution would give a negative result?
Medium
- AA solution containing a polypeptide made of 10 amino acids
- BA solution containing a single amino acid
- CA solution containing a protein with quaternary structure
- DA solution containing a dipeptide
37.In paper chromatography of amino acids, the Rf value is calculated as the distance moved by the amino acid divided by the distance moved by the solvent front. A student obtains an Rf value of 0.75 for an unknown amino acid. Which conclusion is correct?
Medium
- AThe amino acid is highly soluble in the stationary phase.
- BThe amino acid is highly soluble in the mobile phase.
- CThe amino acid is large and therefore moves slowly.
- DThe amino acid has a high charge and binds strongly to the paper.
38.In paper chromatography, amino acids with higher solubility in the mobile phase have higher Rf values.
Easy
True or false?
39.Match each property of an amino acid with its predicted Rf value in chromatography.
Medium
- High solubility in mobile phase
- Low solubility in mobile phase
- Small molecule size
- Large molecule size
- High Rf value
- Low Rf value
40.A protein has a quaternary structure made of four identical polypeptide chains. Each chain contains 150 amino acids. Calculate the total number of peptide bonds in the protein.
Hard
- A600
- B596
- C599
- D150
41.Name the bond that joins two amino acids together in a dipeptide.
Easy
42.Complete the sentence about the general structure of an amino acid.
Easy
Every amino acid has a central carbon atom bonded to an amine group, a carboxylic acid group, a hydrogen atom, and a variable ____ group.
43.A polypeptide contains 288 peptide bonds. Calculate the total number of amino acids in this polypeptide.
Medium
44.In a chromatography experiment, alanine moved 3.0 cm from the baseline, and the solvent front moved 6.0 cm from the baseline. Calculate the Rf value of alanine.
Medium
45.Each haemoglobin molecule can bind 4 oxygen molecules. There are 300 million haemoglobin molecules in each red blood cell. An adult human is estimated to have 3 x 10¹³ red blood cells in circulation at any one time. Calculate the theoretical maximum number of oxygen molecules that could be carried in the red blood cells of an adult human. Give your answer in standard form.
Hard
46.Polypeptides are formed from hundreds of amino acids joined together. A polypeptide contains 288 peptide bonds. Calculate the total number of amino acids in this polypeptide.
Medium
47.Some washing powders contain molecules that break down and hydrolyse the peptide bonds between amino acids. The degree to which a protein is broken down can be called the degree of hydrolysis (DH). The DH value is calculated using the equation: DH = 100 × (Number of peptide bonds hydrolysed) / (Total number of peptide bonds present). Washing powder was applied to a molecule of protein. The DH value after exposure to the washing powder was 28 and a total of 140 peptide bonds were hydrolysed. Calculate the number of amino acids in the protein.
Hard
48.Collagen protein molecules have a quaternary structure. One of the polypeptide chains that forms a molecule of collagen contains 1049 peptide bonds. Assuming that all the chains in collagen are identical in length, calculate the number of amino acids that would be found in a single molecule of collagen. (Collagen is composed of three polypeptide chains.)
Hard
49.Complete the following sentence about the Biuret test.
Easy
In the Biuret test, a positive result is indicated by a colour change from ____ to ____.
50.A protein has a quaternary structure made of four identical polypeptide chains. Each chain contains 120 amino acids. Calculate the total number of peptide bonds in the whole protein.
Hard
51.Name the reagent used to visualise amino acids on a chromatogram.
Easy
52.Complete the sentence about the formation of a peptide bond.
Medium
A peptide bond forms between the ____ group of one amino acid and the ____ group of another amino acid.
53.Explain why the Biuret test is described as a qualitative test for proteins.
Hard
54.Explain the meaning of the term ‘R’ in Figure 1.
Medium
55.State how a scientist might use the results of a chromatography experiment to identify an unknown amino acid.
Medium
56.Suggest one branch of science where chromatography techniques, like the one used to separate a mixture of amino acids, have a practical application.
Medium
57.The Biuret test is a qualitative test for proteins. Explain why the Biuret test is referred to as ‘qualitative’.
Medium
58.A polypeptide contains 288 peptide bonds. Calculate the total number of amino acids in this polypeptide.
Medium
- A289
- B347
- C260
- D318
59.In a chromatography experiment, alanine moved 3.0 cm from the baseline, and the solvent front moved 6.0 cm from the baseline. Calculate the Rf value of alanine.
Medium
- A0.5
- B2.5
- C1.5
- D-0.5
60.Each haemoglobin molecule can bind 4 oxygen molecules. There are 300 million haemoglobin molecules in each red blood cell. An adult human is estimated to have 3 x 10¹³ red blood cells in circulation at any one time. Calculate the theoretical maximum number of oxygen molecules that could be carried in the red blood cells of an adult human. Give your answer in standard form.
Hard
- A3.24e+22 molecules
- B3.96e+22 molecules
- C4.32e+22 molecules
- D3.6e+22 molecules
61.Polypeptides are formed from hundreds of amino acids joined together. A polypeptide contains 288 peptide bonds. Calculate the total number of amino acids in this polypeptide.
Medium
- A260 amino acids
- B318 amino acids
- C289 amino acids
- D347 amino acids
62.Some washing powders contain molecules that break down and hydrolyse the peptide bonds between amino acids. The degree to which a protein is broken down can be called the degree of hydrolysis (DH). The DH value is calculated using the equation: DH = 100 × (Number of peptide bonds hydrolysed) / (Total number of peptide bonds present). Washing powder was applied to a molecule of protein. The DH value after exposure to the washing powder was 28 and a total of 140 peptide bonds were hydrolysed. Calculate the number of amino acids in the protein.
Hard
- A551 amino acids
- B501 amino acids
- C451 amino acids
- D601 amino acids
63.Collagen protein molecules have a quaternary structure. One of the polypeptide chains that forms a molecule of collagen contains 1049 peptide bonds. Assuming that all the chains in collagen are identical in length, calculate the number of amino acids that would be found in a single molecule of collagen. (Collagen is composed of three polypeptide chains.)
Hard
- A3150 amino acids
- B3465 amino acids
- C3780 amino acids
- D2835 amino acids
64.A protein has a quaternary structure made of four identical polypeptide chains. Each chain contains 120 amino acids. Calculate the total number of peptide bonds in the whole protein.
Hard
- A524 peptide bonds
- B476 peptide bonds
- C428 peptide bonds
- D572 peptide bonds

रिवीज़न नोट्स

## Proteins: Amino Acids & the Peptide Bond - Proteins are **polymers** made of **amino acids** (monomers). The sequence, type and number of amino acids determine the protein's shape and function. - Proteins form: enzymes, membrane proteins, hormones, immunoproteins, transport proteins (e.g. haemoglobin), structural proteins (e.g. collagen, keratin), contractile proteins (e.g. myosin). - **General amino acid structure:** central carbon bonded to: - amine group (-NH₂) - carboxylic acid group (-COOH) - hydrogen atom - R group (varies; determines properties e.g. acidic/basic, polar/non-polar) - **Peptide bond formation:** hydroxyl (-OH) lost from -COOH of one amino acid; hydrogen lost from -NH₂ of another. Remaining carbon (with double-bonded O) bonds to nitrogen. This is a **condensation reaction** (water released). - **Dipeptide** = two amino acids; **polypeptide** = three or more. **Hydrolysis** adds water to break peptide bonds. ## Chromatography: Amino Acids - Separates mixture based on **solubility**. Two phases: mobile (moves) and stationary (does not move). - More soluble components travel further in mobile phase. - **Method:** spot unknown mixture on baseline; spot known standards alongside; suspend in solvent. - Amino acids separate based on **charge** and **size**. - Identify by comparing distance travelled with known standards. **Ninhydrin** spray produces blue-violet colour to visualise spots. ## Protein Structure & Function - **Primary:** sequence of amino acids bonded by peptide bonds. Determined by DNA. - **Secondary:** hydrogen bonds between -NH and -C=O of peptide bonds. Forms **α-helix** (H-bonds every fourth peptide bond) or **β-pleated sheet** (parallel chains). - **Tertiary:** further folding; bonds between R groups: - hydrogen bonds between R groups - disulfide bonds between cysteine amino acids - ionic bonds between charged R groups - weak hydrophobic interactions between non-polar R groups - **Quaternary:** more than one polypeptide chain (subunits) working together, e.g. haemoglobin. - **Functions:** enzymes (catalysts), transport (haemoglobin, channel proteins), structural (collagen, keratin), hormones (insulin), antibodies, contractile (actin, myosin). ## Protein Interactions - **Disulfide bonds:** strong covalent bonds between cysteine R groups; strongest, least common; broken by reduction. Common in secreted proteins (e.g. insulin). - **Ionic bonds:** between positively charged (-NH₃⁺) and negatively charged (-COO⁻) R groups; stronger than hydrogen bonds but not common; broken by pH changes. - **Hydrogen bonds:** between strongly polar R groups; weakest but most common. ## Biochemical Tests: Proteins - **Biuret test:** add sodium/potassium hydroxide to make alkaline; add copper(II) sulfate (blue). - Positive result: **blue to lilac/purple**. No change = no protein. - Requires at least **two peptide bonds**; negative for amino acids or dipeptides. - **Qualitative** – does not give quantitative value of protein amount.

Amino acid: Monomer of proteins; general structure: central carbon bonded to amine group (-NH₂), carboxylic acid group (-COOH), hydrogen atom, and R group.
Peptide bond: Covalent bond formed between amino acids by condensation reaction; hydroxyl lost from -COOH of one amino acid and hydrogen lost from -NH₂ of another; remaining carbon (with double-bonded O) bonds to nitrogen.
Condensation reaction: Reaction in which two molecules join, releasing water; forms peptide bonds between amino acids.
Hydrolysis: Reaction in which water is added to break peptide bonds, breaking polypeptides down to amino acids.
Dipeptide: Molecule formed by condensation of two amino acids.
Polypeptide: Molecule formed by condensation of many (three or more) amino acids.
Primary structure: Sequence of amino acids bonded by peptide bonds; determined by DNA.
Secondary structure: Folding of polypeptide chain held by hydrogen bonds between -NH and -C=O of peptide bonds; forms α-helix or β-pleated sheet.
Tertiary structure: Further folding of secondary structure due to bonds between R groups: hydrogen bonds, disulfide bonds, ionic bonds, hydrophobic interactions.
Quaternary structure: Structure of proteins with more than one polypeptide chain (subunits) working together, e.g. haemoglobin.
Disulfide bond: Strong covalent bond between two cysteine R groups; strongest bond in proteins, broken by reduction; common in secreted proteins.
Ionic bond: Bond between positively charged (-NH₃⁺) and negatively charged (-COO⁻) R groups; stronger than hydrogen bonds but not common; broken by pH changes.
Hydrogen bond: Weak bond between strongly polar R groups; weakest but most common bond in proteins.
Hydrophobic interaction: Weak interaction between non-polar R groups; contributes to tertiary structure.
Chromatography: Technique to separate mixture based on solubility; uses mobile phase (moves) and stationary phase (does not move).
Rf value: Ratio of distance moved by amino acid to distance moved by solvent front; used to identify amino acids.
Ninhydrin: Chemical that reacts with amino acids to produce blue-violet colour, used to visualise spots in chromatography.
Biuret test: Qualitative test for proteins: add sodium/potassium hydroxide then copper(II) sulfate; positive result is colour change from blue to lilac/purple.
Globular protein: Protein with tertiary structure that is roughly spherical, e.g. enzymes, antibodies, haemoglobin.
Fibrous protein: Protein with structural role, e.g. collagen, keratin; often has quaternary structure.

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