2.2.1Additional lessons
Formation of Proteins — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 17 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Amino Acids
Monomers
- Monomers Amino acids are the monomers of polypeptides, and 20 different types are common to all living organisms.
General structure
- General structure A central alpha carbon is bonded to an amino group, a carboxyl group, a hydrogen atom and an R group.
R group
- R group The side chain differs between amino acids, from a single hydrogen in glycine to a ring structure in phenylalanine.
Properties
- Properties R groups can be polar, non-polar, acidic or basic, which affects how the polypeptide folds.
Essential and non-essential
- Essential and non-essential Humans make 11 amino acids from others but must obtain the nine essential ones from food.
| Part of amino acid | Formula | Role |
|---|---|---|
| Amino group | -NH₂ | Basic, forms the N-terminus |
| Carboxyl group | -COOH | Acidic, forms the C-terminus |
| R group | Varies | Determines the properties of the amino acid |
Part 2 of 5Making a Polypeptide
1
Two amino acids line up so that the carboxyl group of one is next to the amino group of the other.
2
A condensation reaction removes a hydroxyl from the carboxyl group and a hydrogen from the amino group, forming water.
3
A covalent peptide bond forms between the carbon of the first amino acid and the nitrogen of the second.
4
The two linked amino acids form a dipeptide, and further condensation adds one amino acid at a time to the carboxyl end.
5
Ribosomes carry out this process during translation, using the sequence of mRNA codons to order the amino acids.
Part 3 of 5Importance of Proteins
Importance of Proteins
Enzymes
- Enzymes Proteins catalyse nearly all metabolic reactions, so their formation underlies all life processes.
Transport
- Transport Haemoglobin carries oxygen, and membrane carrier proteins move substances across membranes.
Structure
- Structure Collagen and keratin give strength to tissues such as skin, tendons and hair.
Defence and signalling
- Defence and signalling Antibodies recognise antigens and many hormones, such as insulin, are proteins.
Part 4 of 5Worked Example
Question
- A tripeptide is built from three different amino acids.
- How many peptide bonds and water molecules are involved in forming it, and how many different tripeptide sequences are possible from 20 amino acids?
- Three amino acids in a chain need 3 - 1 = 2 peptide bonds.
- Each bond releases one water molecule, so 2 water molecules are released.
- Each position can be any of 20 amino acids, so the number of sequences is 20 × 20 × 20 = 8000.
Answer Two peptide bonds form with two water molecules released, and 20³ = 8000 different tripeptides are possible.
Part 5 of 5Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Amino acid A molecule with an amino group, carboxyl group, hydrogen and R group on a central carbon.
- Peptide bond The covalent bond between the carboxyl group of one amino acid and the amino group of the next.
- Polypeptide A chain of amino acids joined by peptide bonds.
- Essential amino acid An amino acid that the body cannot make and must obtain from the diet.
Common Mistakes
- Writing that peptide bonds form by hydrolysis; they form by condensation, releasing water.
- Saying the R group is the same in every amino acid, when it is the part that varies.
- Stating there are 20 amino acids in every protein;
- 20 types exist, but each protein uses a particular sequence.
- Mixing the ends of the chainthe N-terminus has the free amino group and the C-terminus the free carboxyl group.
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