IB MYP Sciences – Metabolism Revision Notes

For full explanations and worked examples, read the metabolism study guide first. These notes are for the final weeks before tests or the eAssessment.

They cover IB MYP Sciences metabolism, aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which lists “metabolism” among the topics explored in the on-screen examinations. There is no SL/HL split in MYP Sciences. The notes suit MYP years 4 and 5. The MYP has no prescribed content list – schools design their own units – so check the detail of your own unit with your teacher, who will share the task-specific clarifications for assessed work.

Course links: MYP Sciences hub · printable checklist · metabolism practice questions.

Key definitions

Term	Meaning
Metabolism	The sum of all chemical reactions in an organism
Anabolic	Builds large molecules from small ones; needs energy (e.g. photosynthesis, protein synthesis)
Catabolic	Breaks large molecules down; often releases energy (e.g. respiration, digestion)
Enzyme	A protein that acts as a biological catalyst
Active site	The region of an enzyme with a specific shape that the substrate binds to
Enzyme–substrate complex	Formed when a substrate is bound in the active site
Denatured	Active site permanently changed in shape, so the substrate no longer fits
Optimum	The temperature or pH at which an enzyme works fastest
Limiting factor	The factor in shortest supply, which holds back the rate
Oxygen debt	Extra oxygen needed after exercise to break down lactic acid
Emulsification	Bile breaking fat into small droplets to increase surface area

Equations to know

Process	Word equation	Symbol equation
Aerobic respiration	glucose + oxygen → carbon dioxide + water	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Anaerobic (muscle)	glucose → lactic acid	–
Anaerobic (yeast, plants)	glucose → ethanol + carbon dioxide	–
Photosynthesis	carbon dioxide + water → glucose + oxygen	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Useful data relationships:

Relationship	Use it for
rate = 1/time (or 1000/time)	“Time for starch to disappear” or “time for a colour change”
rate = amount ÷ time	Gas volume in a gas syringe, bubbles counted per minute
light intensity ∝ 1/d²	Lamp moved to distance d from a plant
% change = (new − old)/old × 100	Comparing two conditions

Enzymes in four lines

1. Substrate fits active site (complementary shape) → enzyme–substrate complex → products released → enzyme reused.
2. Temperature up to optimum: more kinetic energy, more collisions, more complexes per second.
3. Above optimum: bonds in the protein break, active site changes shape, enzyme denatured, rate falls fast.
4. pH away from optimum also changes the active site. Pepsin: acidic (about pH 2). Salivary amylase: near neutral.

Digestion summary

Enzyme	Substrate → products	Where made
Amylase	Starch → maltose (then glucose)	Salivary glands, pancreas
Protease	Protein → amino acids	Stomach (pepsin), pancreas, small intestine
Lipase	Lipids → fatty acids + glycerol	Pancreas, small intestine
Bile (not an enzyme)	Emulsifies fats; neutralises stomach acid	Made in liver, stored in gall bladder

Why digest? Large insoluble molecules → small soluble ones that can be absorbed through the villi of the small intestine into the blood.

Must-know distinctions

• Respiration vs breathing. Respiration is a chemical reaction in cells. Breathing is moving air.
• Aerobic vs anaerobic. Aerobic uses oxygen and releases much more energy per glucose; anaerobic does not use oxygen and breaks glucose down only partly.
• Muscle vs yeast anaerobic. Muscle: lactic acid only. Yeast: ethanol and carbon dioxide.
• Photosynthesis vs respiration. Photosynthesis is anabolic, in chloroplasts, only in light. Respiration is catabolic, mainly in mitochondria, all the time, in every living cell – including plant cells.
• Denatured vs “killed”. Enzymes are molecules, not organisms. Say denatured.
• Mechanical vs chemical digestion. Mechanical breaks food into pieces; chemical breaks bonds using enzymes.
• Rising vs flat part of a limiting-factor graph. Rising: the factor on the x-axis is limiting. Flat: another factor is limiting.

Respiration and exercise

• Every living cell respires, all the time. Energy released is used for muscle contraction, building molecules, active transport and, in mammals and birds, keeping body temperature steady.
• During hard exercise, breathing rate and heart rate rise to deliver more oxygen and remove carbon dioxide faster.
• If oxygen supply still cannot keep up, muscles also respire anaerobically. Lactic acid builds up and muscles tire.
• After exercise, you keep breathing hard to repay the oxygen debt.
• Yeast fermentation is used in bread (carbon dioxide makes dough rise) and brewing (ethanol).

Limiting factors at a glance

What you see on the graph	What it means
Line rising as light increases	Light intensity is limiting
Line flat at high light	CO₂ or temperature is limiting
Higher CO₂ curve plateaus higher	CO₂ was limiting the lower curve
Rate falls at high temperature	Enzymes controlling photosynthesis denatured
Two curves overlap at low light	Light, not CO₂, is limiting there

Remember: only one factor limits the rate at any moment. Raising a factor that is not limiting has no effect.

Linking to concepts and the eAssessment

• The brief’s example related concepts include energy, transformation and models. Energy: light energy stored as chemical energy in glucose, then released by respiration. Transformation: reactants become new products. Models: the lock-and-key picture of an enzyme, which you should be able to use and also criticise as simplified.
• The on-screen examination has three tasks: Knowing and understanding (criterion A, 25 marks), Investigation skills (criteria B and C, 50 marks) and Applying science (criterion D, 25 marks).
• Metabolism supplies ready-made investigations for the 50-mark task: enzymes and temperature or pH, yeast and temperature, pondweed and light.
• For criterion D, practise applications such as biological washing powders, lactose-free milk made with the enzyme lactase, bread-making and brewing.

Method in steps: planning an enzyme investigation (criterion B)

1. Write a focused research question naming the independent and dependent variables and their range.
2. Give a hypothesis with a scientific reason (active site, collisions, denaturing).
3. State how you will change the independent variable (water bath, buffers, dilutions).
4. State how you will measure the dependent variable, with the instrument and units.
5. List control variables and how each is kept constant.
6. Plan at least five values of the independent variable and three repeats of each.
7. Add a specific safety point.

Method in steps: processing data (criterion C)

1. Spot anomalies: a repeat far from the others. Leave it out of the mean and say why.
2. Calculate means to a sensible, consistent number of decimal places.
3. Convert to a rate if the raw data is a time.
4. Plot the independent variable on the x-axis, with labelled axes and units.
5. Describe the trend quoting numbers, then explain it with science.
6. State whether the hypothesis is supported.
7. Evaluate: one specific weakness in your method, and a specific fix.

Small worked reminders

• Starch disappears in 25 s → rate = 1/25 = 0.04 s⁻¹.
• 4.8 cm³ of oxygen in 2 min → 2.4 cm³ min⁻¹.
• Lamp moved from 15 cm to 30 cm → intensity × (15/30)² = × 1/4.
• Repeats 31, 29, 33 bubbles per minute → mean 31.

Quick self-test

1. Define metabolism.
2. Is photosynthesis anabolic or catabolic? Give a reason.
3. Give the symbol equation for aerobic respiration.
4. What are the products of anaerobic respiration in yeast?
5. Explain why an enzyme only catalyses one reaction.
6. Why does rate fall steeply just above an enzyme’s optimum temperature?
7. Name the products of lipid digestion.
8. Give two roles of bile.
9. The time for a colour change is 50 s. Calculate the rate as 1/time.
10. A lamp is moved from 15 cm to 30 cm from a plant. By what factor does light intensity change?
11. A rate rises from 20 to 26 units. Calculate the percentage increase.
12. On a graph of photosynthesis rate against light intensity, the line is flat. Suggest two factors that could be limiting.

Answers

1. The sum of all chemical reactions in an organism.
2. Anabolic: it builds glucose from smaller molecules (carbon dioxide and water), using energy.
3. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
4. Ethanol and carbon dioxide.
5. The active site has a specific shape; only a substrate with a complementary shape fits.
6. The enzyme is denatured: the active site changes shape, so substrates no longer fit and fewer complexes form.
7. Fatty acids and glycerol.
8. Emulsifies fats (larger surface area for lipase); neutralises stomach acid.
9. 1/50 = 0.02 s⁻¹.
10. (15/30)² = 1/4, so intensity falls to a quarter.
11. (26 − 20)/20 × 100 = 30%.
12. Carbon dioxide concentration and temperature.

Where marks are usually lost

• Writing “enzymes are killed” instead of “denatured”.
• Explaining the rise in rate with temperature without mentioning collisions or enzyme–substrate complexes.
• Giving carbon dioxide as a product of anaerobic respiration in muscle.
• Forgetting that plants respire, day and night.
• A hypothesis with no scientific reason, which is only a prediction.
• Control variables listed with no method for keeping them constant.
• Including an obvious anomaly in a mean, or dropping it without saying why.
• Describing a graph without quoting values from it.
• Treating 1/d² as 1/d when a lamp is moved.
• Stating an exact optimum from readings taken 10 units apart.

Next: try the metabolism practice questions, or revisit how the criteria are judged in the criteria in practice revision notes.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief names metabolism as an on-screen examination topic and lists the four criteria: A Knowing and understanding, B Inquiring and designing, C Processing and evaluating, D Reflecting on the impacts of science.