2.2.3Inquiry Process
Protein Structure: Effect of pH and Temperature — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 45 of 160
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.
Section 1 of 5What Holds the Shape Together
A protein works only while it keeps its specific folded shape, which depends mainly on weak bonds.
Hydrogen bonds
- Hydrogen bonds Weak bonds between polar groups help hold folds and helices in position.
Ionic bonds
- Ionic bonds Attractions between oppositely charged R groups help maintain the three-dimensional shape.
Hydrophobic interactions
- Hydrophobic interactions Non-polar R groups cluster in the protein interior away from water.
Disulfide bridges
- Disulfide bridges Strong covalent links between cysteine side chains add stability to the shape.
| Bond | Strength | Disrupted by |
|---|---|---|
| Hydrogen | Weak | Heat, pH change |
| Ionic | Weak to moderate | pH change, heat |
| Hydrophobic interaction | Weak | Heat |
| Disulfide | Strong | Reducing agents |
Section 2 of 5Temperature Effects
Each protein has a temperature range in which it works best.
1
Heating increases the kinetic energy and vibration of atoms in the protein.
2
The weak bonds that stabilise folding begin to break.
3
The protein loses its three-dimensional shape and unfolds partly or fully.
4
Active sites change shape so substrates can no longer bind, and the enzyme stops working.
5
The change is usually permanent: cooling does not restore the original shape.
Section 3 of 5pH Effects
pH Effects
Change of charge
- Change of charge Extra H⁺ or OH⁻ ions alter the charges on R groups and on the amino and carboxyl ends.
Broken ionic bonds
- Broken ionic bonds New charges disrupt ionic and hydrogen bonds, so the protein folds differently.
Optimum pH
- Optimum pH Pepsin works best at about pH 2 in the stomach, while pancreatic enzymes work best in alkaline conditions.
Narrow range
- Narrow range Small shifts from the optimum slow the enzyme, and large shifts denature it.
Section 4 of 5Denaturation in Practice
Primary structure survives
- Primary structure survives Peptide bonds are not broken by mild heating or pH change, so the amino acid sequence remains.
Egg white
- Egg white Heating makes soluble albumin unfold and cluster, turning clear egg white solid and white.
Fever
- Fever Very high body temperature is dangerous because enzymes in the body begin to denature.
Heat-stable enzymes
- Heat-stable enzymes Taq polymerase from thermophilic bacteria remains active near 95 °C and is used in PCR.
Section 5 of 5Exam Checklist & Tips
2.2.3 – Key Points for the Exam
- Denaturation The loss of a protein's three-dimensional shape and function through disruption of bonds.
- Optimum The temperature or pH at which the protein works most effectively.
- Active site The region of an enzyme where substrate binds.
- Hydrogen bond A weak attraction between a slightly positive hydrogen and a slightly negative atom.
Common Mistakes
- Saying heat breaks peptide bonds; denaturation affects weak bonds and not the primary structure.
- Writing that the enzyme is killed; proteins are not alive.
- Assuming denaturation is always reversible.
- Thinking low temperature denatures enzymes; it only slows the reaction.
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