IB MYP Sciences – Metabolism Study Guide

This study guide teaches metabolism for IB MYP Sciences. It is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which lists “metabolism” among the topics explored in the MYP sciences on-screen examinations. MYP Sciences has no SL/HL split, and this page suits MYP years 4 and 5, including students preparing for the eAssessment.

One thing to be clear about first: the MYP has no prescribed content list. Schools design their own units, so your teacher’s metabolism unit may be ordered or named differently. This page covers the standard science behind a topic the IB’s brief names, and the investigation skills its criteria B and C describe. Your teacher will share the task-specific clarifications for any assessed work.

Use it with the revision notes and the practice questions. For the course as a whole, see the MYP Sciences hub and the printable checklist.

What this unit covers

Area	What you should be able to do	Criteria it trains
Metabolism overview	Define metabolism; tell anabolic from catabolic reactions	A
Enzymes	Explain the active site, specificity, and the effects of temperature and pH	A, C
Respiration	Write equations for aerobic and anaerobic respiration; compare them	A
Photosynthesis	Write the equation; explain limiting factors from graphs and tables	A, C
Digestion basics	Match enzymes to substrates and products; explain why food is digested	A
Investigations	Plan an enzyme investigation (B); process and evaluate data (C)	B, C

The brief’s eAssessment 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 is a good topic for all three, because nearly every idea in it can be tested by experiment.

Metabolism: the big picture

Metabolism is the sum of all the chemical reactions that happen in a living organism. There are two types:

• Anabolic reactions build large molecules from small ones and need an input of energy. Examples: photosynthesis (glucose from carbon dioxide and water), joining glucose into starch, joining amino acids into proteins.
• Catabolic reactions break large molecules into smaller ones and often release energy. Examples: respiration, digestion.

Two of the brief’s related concepts fit this unit closely. Energy: respiration releases energy stored in glucose; photosynthesis stores light energy as chemical energy. Transformation: substances change into new substances, and energy changes form. Models is a third: the lock-and-key model of enzymes is a simplified picture you should be able to use and criticise.

Enzymes

An enzyme is a biological catalyst. It speeds up a reaction without being used up. Enzymes are proteins. Each has an active site with a specific shape. Only a substrate with a complementary shape fits, forming an enzyme–substrate complex. The reaction happens, the products leave, and the enzyme can be used again. This is why enzymes are specific: amylase breaks down starch but not protein.

Temperature

• As temperature rises towards the optimum, molecules move faster and collide more often, so more enzyme–substrate complexes form per second. Rate rises.
• Above the optimum, the enzyme’s shape starts to change. Bonds holding the protein in shape break, the active site changes shape and the substrate no longer fits. The enzyme is denatured. This is permanent.
• Many human enzymes work best at around body temperature (about 37 °C).

pH

Each enzyme has an optimum pH. Far from it, the charges in the active site change, the shape changes, and the enzyme is denatured. Pepsin in the stomach works best in strongly acidic conditions (about pH 2); amylase in saliva works best near neutral.

Worked example (criterion C): processing enzyme data

A student timed how long amylase took to break down all the starch in a sample (iodine stops turning blue-black) at five temperatures.

Temperature / °C	15	25	35	45	55
Time / s	180	90	40	60	300

Step 1 – choose a rate. A shorter time means a faster reaction, so use rate = 1/time. To avoid tiny decimals, use 1000/time (units: s⁻¹ × 10⁻³).

Step 2 – calculate.

15 °C: 1000/180 = 5.56
25 °C: 1000/90  = 11.1
35 °C: 1000/40  = 25.0
45 °C: 1000/60  = 16.7
55 °C: 1000/300 = 3.33

Step 3 – describe the trend with data. Rate rises from 5.56 at 15 °C to a peak of 25.0 at 35 °C, then falls to 3.33 at 55 °C.

Step 4 – conclude carefully. The optimum is about 35 °C. Because readings were taken every 10 °C, the true optimum could lie anywhere between 25 °C and 45 °C. An improvement: repeat with 2 °C intervals between 30 °C and 44 °C.

Step 5 – explain with science. Below 35 °C, more frequent collisions raise the rate. Above it, the active site changes shape and the enzyme is denatured, so fewer complexes form.

Respiration

Respiration is a chemical reaction in every living cell that releases energy from glucose. It is not the same as breathing, which only moves air in and out of the lungs.

Aerobic respiration (uses oxygen)

• Word equation: glucose + oxygen → carbon dioxide + water (energy released)
• Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
• Happens mainly in the mitochondria.

Anaerobic respiration (no oxygen)

• In animal muscles: glucose → lactic acid
• In yeast and plants: glucose → ethanol + carbon dioxide (in yeast this is called fermentation)

Anaerobic respiration releases much less energy per glucose molecule than aerobic, because glucose is only partly broken down. During hard exercise, muscles may respire anaerobically. Lactic acid builds up, and extra oxygen is needed afterwards to break it down (the oxygen debt).

Worked example: rate of yeast respiration

Yeast in sugar solution at 30 °C produces 18 cm³ of carbon dioxide in 5 minutes, collected in a gas syringe.

Rate = volume ÷ time = 18 ÷ 5 = 3.6 cm³ min⁻¹.

Always give units. A rate with no unit can cost marks even if the number is right.

Photosynthesis

Plants and algae make glucose by photosynthesis in the chloroplasts, which contain green chlorophyll that absorbs light.

• Word equation: carbon dioxide + water → glucose + oxygen (light energy, chlorophyll)
• Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Notice it is the reverse of the aerobic respiration equation. Plants respire all the time; they photosynthesise only in the light.

The glucose is used for respiration, stored as starch, turned into cellulose for cell walls, or combined with nitrate from the soil to make amino acids and proteins.

Limiting factors

The rate of photosynthesis depends on light intensity, carbon dioxide concentration and temperature. A limiting factor is the one in shortest supply: increasing it increases the rate.

• On a graph of rate against light intensity, the rising part means light is limiting.
• Where the line levels off, something else is limiting (carbon dioxide or temperature). If raising carbon dioxide lifts the plateau, carbon dioxide was limiting.
• Temperature affects the enzymes that control photosynthesis, so too high a temperature lowers the rate.

Worked example: light intensity and distance

Many experiments change light intensity by moving a lamp. Light intensity is proportional to 1/d², where d is the distance from the lamp.

Lamp moved from 10 cm to 20 cm: intensity changes by (10/20)² = 1/4 of its value. Lamp moved from 10 cm to 5 cm: intensity changes by (10/5)² = 4 times its value.

A common error is to assume doubling the distance halves the intensity.

Digestion basics

Large, insoluble food molecules cannot pass through the gut wall. Digestion breaks them into small, soluble molecules that can be absorbed into the blood, mainly through the villi of the small intestine.

• Mechanical digestion: teeth and stomach muscles break food into smaller pieces, increasing surface area for enzymes.
• Chemical digestion: enzymes break bonds in large molecules.

Enzyme group	Substrate	Products	Made in
Amylase (a carbohydrase)	Starch	Maltose (then glucose)	Salivary glands, pancreas
Protease (e.g. pepsin)	Proteins	Amino acids	Stomach, pancreas, small intestine
Lipase	Lipids (fats)	Fatty acids and glycerol	Pancreas, small intestine

Bile is made in the liver and stored in the gall bladder. It is not an enzyme. It emulsifies fats into small droplets (more surface area for lipase) and neutralises stomach acid.

Digestion is catabolic. The products feed anabolic reactions in cells: amino acids are rebuilt into your own proteins, and glucose is respired or stored.

Designing an investigation (criterion B)

Worked plan: How does pH affect the activity of catalase in potato? Catalase breaks down hydrogen peroxide into water and oxygen.

• Research question: How does pH (4, 5, 6, 7, 8, 9) affect the volume of oxygen released in 60 s when potato catalase breaks down hydrogen peroxide?
• Hypothesis with reasoning: Oxygen volume will be highest near pH 7 and lower at either extreme, because pH away from the optimum changes the shape of the active site, so fewer enzyme–substrate complexes form.
• Independent variable: pH, set with buffer solutions.
• Dependent variable: volume of oxygen in 60 s, read from a gas syringe.
• Control variables: mass and surface area of potato (same-sized cylinders cut with a borer), volume and concentration of hydrogen peroxide, temperature (water bath), time.
• Method outline: equilibrate tubes in the water bath; add buffer then peroxide; add potato and start timing; read the syringe at 60 s; repeat three times per pH.
• Safety: hydrogen peroxide irritates skin and eyes – wear eye protection and gloves.

For more on how each criterion is judged, read the criteria in practice and the investigation skills exam preparation.

Common errors

• Saying enzymes are “killed” by heat. They are proteins, not living things: they are denatured.
• Saying high temperature denatures the substrate. It is the enzyme’s active site that changes.
• Confusing respiration with breathing.
• Writing “anaerobic respiration produces lactic acid and carbon dioxide” for muscles. Muscle anaerobic respiration makes lactic acid only.
• Claiming plants photosynthesise instead of respiring. They do both.
• Reading an optimum as an exact value from widely spaced data.
• Calling bile an enzyme.
• Describing a graph without quoting any numbers from it.

Where next

Test your recall with the metabolism revision notes, then try the metabolism practice questions. For the eAssessment structure, see the assessment revision notes and the subject guide. Mitochondria and chloroplasts as cell structures are in cells and organisms; photosynthesis and respiration as parts of the carbon cycle are in cycles.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists metabolism as a topic explored in the on-screen examinations and names energy, movement, transformation and models as example related concepts.