1.4Cell Structure and Molecular Biology
Enzymes & Metabolism — EST 2 Biology Revision Notes & Topical Questions
10 min read 8 parts Lesson 4 of 11 1 quiz
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.
Lesson summary
Enzymes & Metabolism EST Biology notes: Distinguish anabolic and catabolic, exergonic and endergonic reactions.Enzymes lower activation energy but do NOT change ΔG, the overall energy change, or the equilibrium position.Enzyme names often end in -ase (lipase, amylase, DNA polymer…
- First law of thermodynamics
- Metabolic pathways
- Energy and thermodynamics
- How enzymes work
- Cofactors and coenzymes
- Environmental factors
Section 1 of 8Enzymes & Metabolism
1
Metabolic pathways.
Reactions happen in sequences, each step catalyzed by a specific enzyme. Catabolic pathways (e.g. respiration) break complex molecules down and release energy; anabolic pathways (e.g. protein synthesis, photosynthesis) build complex molecules and consume energy.
2
Energy and thermodynamics.
Energy exists as kinetic energy and potential energy (including chemical energy stored in bonds). Because energy conversions are never 100% efficient, some is always lost as heat, and living things need a constant input of energy — ultimately from the Sun.
3
ATP.
Adenosine triphosphate has adenine, ribose and three phosphates. Removing the terminal phosphate releases energy that powers muscle contraction, active transport and biosynthesis. The ATP–ADP cycle regenerates ATP through respiration (and photosynthesis in plants).
4
How enzymes work.
A reaction's activation energy is a barrier even when it is exergonic overall.
- Enzymes provide an alternate path with a lower barrier, so reactions occur quickly at body temperature.
- Enzymes are highly specificthe lock-and-key idea explains specificity,
- while induced fit refines it by showing the active site is flexible.
5
Cofactors and coenzymes.
Many enzymes need inorganic cofactors (Zn2+, Fe2+, Mg2+) or organic coenzymes, often derived from vitamins (NAD+ from niacin, FAD from riboflavin).
6
Environmental factors.
Rate increases with temperature until the optimum, then falls steeply as the protein denatures. Extreme pH also changes the enzyme's shape. With fixed enzyme, adding substrate increases rate until all enzymes are saturated, producing a plateau.
7
Inhibition.
Competitive inhibitors compete for the active site; their effect is reduced by raising substrate concentration. Noncompetitive inhibitors bind elsewhere and change the active site shape. Irreversible inhibitors (nerve gases, some antibiotics like penicillin) bind covalently.
8
Regulation.
Allosteric activators stabilize the active form; inhibitors stabilize the inactive form. In feedback inhibition, a pathway's final product shuts off an early enzyme, preventing waste. Cells also regulate by controlling how much enzyme is produced and by compartmentalizing enzymes in organelles.
Section 2 of 8Process & sequence
1
The substrate collides with the enzyme and enters the complementary active site.
2
An enzyme-substrate complex forms; the active site molds around the substrate (induced fit).
3
The enzyme lowers activation energy by straining bonds, orienting reactants or providing a favourable microenvironment.
4
The reaction occurs and the substrate is converted to product(s).
5
Products are released because they no longer fit the active site.
6
The unchanged enzyme is free to catalyze another reaction.
Enzyme catalytic cyclewrite these steps in this order; each step is a marking point.
1
Substrate enters active site
2
Induced fit tightens binding
3
Activation energy lowered
4
Substrate converted to products
5
Products released
6
Enzyme reused unchanged
Section 3 of 8Compare
| Factor | Effect | Reason |
|---|---|---|
| Temperature | Rate rises to an optimum (about 37 °C in humans), then falls sharply | More collisions at first; above optimum, bonds break and the enzyme denatures |
| pH | Each enzyme has an optimum (pepsin ~2, trypsin ~8) | Changes in H+ alter charges on R groups and the active site |
| Substrate concentration | Rate rises then levels off (plateau) | All active sites become occupied (saturation) |
| Enzyme concentration | Rate rises in proportion if substrate is plentiful | More active sites available |
| Inhibitors | Reduce rate | Block or distort active site |
| Feature | Competitive | Noncompetitive |
|---|---|---|
| Binding site | Active site | Allosteric (other) site |
| Resembles substrate? | Yes | No |
| Overcome by more substrate? | Yes | No |
| Effect on maximum rate | Unchanged at very high substrate | Lowered |
| Example | Malonate inhibiting succinate dehydrogenase | Cyanide binding cytochrome oxidase; heavy metals |
Section 4 of 8Worked examples
Fully denatured
An experiment measures catalase activity from 0 to 80 °C. Activity peaks at 40 °C and is zero at 70 °C. Explain the shape of the curve.
- From 0 to 40 °C, rising temperature increases kinetic energy, so enzyme and substrate collide more often and more complexes form.
- Above 40 °C, vibrations break the hydrogen and ionic bonds that hold the tertiary structure;
- the active site changes shape and substrate no longer fits.
- By 70 °C the enzyme is fully denatured, so activity is zero and does not recover on cooling.
- Above 40 °C, vibrations break the hydrogen and ionic bonds that hold the tertiary structure;
Competitive
How could you tell experimentally whether a new drug is a competitive or noncompetitive inhibitor?
- Measure reaction rate at several substrate concentrations with and without the drug.
- If very high substrate concentration restores the normal maximum rate, the inhibitor is competitive .
- (substrate outcompetes it)
- If the maximum rate stays lower however much substrate is added, it is noncompetitive.
- If very high substrate concentration restores the normal maximum rate, the inhibitor is competitive .
Section 5 of 8Keep going
Enzymes & Metabolism EST Biology: what to do next
These Enzymes & Metabolism EST Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Enzymes & Metabolism EST Biology understanding with the practice questions below, then reread any Enzymes & Metabolism EST Biology point you were unsure about before moving on.
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Previous lesson: 1.3 Membranes and transport
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Next lesson: 1.5 Cellular energetics
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Course: EST 2 Biology — all units
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Official syllabus: OpenStax free science textbooks
Section 6 of 8Key terms
Section 7 of 8Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Metabolism · Enzyme · Activation energy · Substrate · Active site · Induced fit · Cofactor · Competitive inhibitor · Allosteric regulation · Feedback inhibition
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Marked practiceQuizzes for this lesson
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