Free IB Biology Practice Exam (SL/HL) with Answers

This free IB Biology practice exam covers the four themes of the 2023 syllabus — unity and diversity, form and function, interaction and interdependence, continuity and change — with fifteen Paper 1A-style multiple-choice questions and two Paper 2-style structured questions: an osmosis data-analysis question with the data given in the text, and a sex-linked inheritance question that extends into meiosis and natural selection. It targets the places IB candidates lose marks: vague osmosis language, swapped hormones and ions, and "explain" answers that only describe, with IB-style mark allocation on every part.

1. Question 11 mark · Molecules of life
   Water is pulled up the xylem of a tall tree as a continuous column. Which property of water is most directly responsible for the column not breaking under tension?
   • A.
     Adhesion of water molecules to the cellulose walls of the xylem
   • B.
     Cohesion between water molecules
   • C.
     The high specific heat capacity of water
   • D.
     The ability of water to dissolve ions
2. Question 21 mark · Molecules of life
   In a sample of double-stranded DNA, 30% of the nucleotides contain adenine. What percentage of the nucleotides contain guanine?
   • A.
     20%
   • B.
     30%
   • C.
     40%
   • D.
     70%
3. Question 31 mark · Cells & membranes
   In a photomicrograph, a cell measures 50 mm across. The actual diameter of the cell is $20\ \mu\text{m}$. What is the magnification of the image?
   • A.
     $\times 25$
   • B.
     $\times 250$
   • C.
     $\times 2500$
   • D.
     $\times 25\,000$
4. Question 41 mark · Diversity & evolution
   According to the biological species concept, two populations belong to different species if they:
   • A.
     live in different habitats and never meet
   • B.
     show visible differences in body size or colour
   • C.
     cannot interbreed to produce fertile offspring
   • D.
     have different numbers of chromosomes in their body cells
5. Question 51 mark · Enzymes & metabolism
   An enzyme-catalysed reaction is carried out at temperatures above the enzyme's optimum. Which statement best explains why the rate of reaction falls?
   • A.
     The substrate molecules move too fast to bind to the enzyme
   • B.
     Bonds maintaining the tertiary structure break, changing the shape of the active site
   • C.
     The substrate molecules are denatured by the heat
   • D.
     The activation energy of the reaction increases
6. Question 61 mark · Enzymes & metabolism
   Which statement correctly describes a difference between anaerobic and aerobic cell respiration in human muscle cells?
   • A.
     Anaerobic respiration produces ethanol and carbon dioxide
   • B.
     Anaerobic respiration yields far less ATP per molecule of glucose
   • C.
     Aerobic respiration takes place only in the cytoplasm
   • D.
     Anaerobic respiration uses oxygen as the final electron acceptor
7. Question 71 mark · Enzymes & metabolism
   A student measures the rate of photosynthesis of pondweed at a high, constant light intensity and a constant carbon dioxide concentration. When the temperature is raised from $15^{\circ}\text{C}$ to $25^{\circ}\text{C}$, the rate doubles. Which factor was limiting the rate at $15^{\circ}\text{C}$?
   • A.
     Light intensity
   • B.
     Carbon dioxide concentration
   • C.
     Temperature
   • D.
     Wavelength of the light
8. Question 81 mark · Exchange & transport
   Which feature of the alveoli in human lungs increases the rate of gas exchange by reducing the diffusion distance?
   • A.
     A wall made of a single layer of flattened cells
   • B.
     A very large total surface area
   • C.
     A dense network of surrounding capillaries
   • D.
     A film of moisture lining the inner surface
9. Question 91 mark · Homeostasis & signalling
   Shortly after a meal rich in carbohydrate, the blood glucose concentration rises. Which hormone is released in response, and by which cells?
   • A.
     Insulin, released by alpha cells of the pancreas
   • B.
     Glucagon, released by alpha cells of the pancreas
   • C.
     Insulin, released by beta cells of the pancreas
   • D.
     Glucagon, released by beta cells of the pancreas
10. Question 101 mark · Homeostasis & signalling
    Which event occurs during the depolarisation phase of an action potential in a neuron?
    • A.
      Potassium ions diffuse into the axon through open channels
    • B.
      Sodium ions diffuse into the axon through voltage-gated channels
    • C.
      Sodium ions are pumped out of the axon by sodium–potassium pumps
    • D.
      Potassium ions diffuse out of the axon through voltage-gated channels
11. Question 111 mark · Defence against disease
    Why does vaccination give long-term protection against a pathogen?
    • A.
      The vaccine contains antibodies that remain in the blood for many years
    • B.
      Antigens in the vaccine stimulate the production of memory cells
    • C.
      The vaccine kills any pathogens already present in the body
    • D.
      The vaccine prevents the pathogen from entering the body
12. Question 121 mark · DNA & gene expression
    Bacteria grown for many generations in a medium containing only heavy nitrogen ($^{15}\mathrm{N}$) are transferred to a medium containing only light nitrogen ($^{14}\mathrm{N}$) and allowed to divide twice. What proportion of the DNA molecules then consist of one heavy strand and one light strand?
    • A.
      0%
    • B.
      25%
    • C.
      50%
    • D.
      100%
13. Question 131 mark · DNA & gene expression
    The template strand of a gene has the base sequence 3'-TACGGA-5'. What is the sequence of the mRNA transcribed from it?
    • A.
      5'-AUGCCU-3'
    • B.
      5'-UACGGA-3'
    • C.
      5'-ATGCCT-3'
    • D.
      5'-UCCGUA-3'
14. Question 141 mark · Diversity & evolution
    In an industrial region, a moth population changed over a century from mostly pale to mostly dark forms as soot darkened the tree bark on which the moths rest and birds preyed on them. Which statement correctly describes what happened?
    • A.
      The pollution caused mutations that turned pale moths dark
    • B.
      Individual moths became darker during their lives to match the bark
    • C.
      Dark moths, already present as a rare variant, were eaten less often and left more offspring, so the dark allele became more common
    • D.
      Pale moths migrated away from the polluted region
15. Question 151 mark · Ecosystems & climate
    Which of the following is an example of positive feedback in global warming?
    • A.
      Faster plant growth at higher carbon dioxide concentrations removes more carbon dioxide from the atmosphere
    • B.
      Thawing permafrost releases methane, which causes further warming and further thawing
    • C.
      Increased evaporation produces more cloud, which reflects more sunlight back into space
    • D.
      Warmer oceans dissolve more carbon dioxide from the air
16. Question 1610 marks · Cells & membranes
    A student investigated osmosis in potato tissue. Five cylinders were cut from the same potato, blotted dry and weighed, then each was placed for 30 minutes in a sucrose solution of a different concentration. Each cylinder was then blotted and reweighed. The percentage changes in mass were: $0.0\ \text{mol dm}^{-3}$, +8.0%; $0.2\ \text{mol dm}^{-3}$, +3.5%; $0.4\ \text{mol dm}^{-3}$, $-1.0$%; $0.6\ \text{mol dm}^{-3}$, $-5.0$%; $0.8\ \text{mol dm}^{-3}$, $-8.5$%.
    (a)

    The cylinder placed in the $0.6\ \text{mol dm}^{-3}$ solution had an initial mass of 2.40 g and a final mass of 2.28 g. Show that the percentage change in mass is $-5.0$%.

    [1]
    (b)

    Describe the relationship between the sucrose concentration and the percentage change in mass.

    [2]
    (c)

    Estimate the sucrose concentration at which there would be no change in mass, and state what this concentration represents.

    [2]
    (d)

    Explain the change in mass of the cylinder placed in the $0.8\ \text{mol dm}^{-3}$ solution.

    [3]
    (e)

    State two variables, other than sucrose concentration, that the student should have kept constant, and explain why one of them matters.

    [2]
17. Question 1710 marks · Genetics & meiosis
    Red–green colour blindness in humans is caused by a recessive allele of a gene carried on the X chromosome. Use $X^B$ for the allele giving normal colour vision and $X^b$ for the colour-blindness allele. Maya has normal colour vision; her father is colour blind. Her partner Tom also has normal colour vision.
    (a)

    Explain why red–green colour blindness is more common in males than in females.

    [2]
    (b)

    State the genotypes of Maya and Tom, giving a reason for Maya's genotype.

    [2]
    (c)

    Determine the probability that their first child is a colour-blind boy. Show your working.

    [2]
    (d)

    Outline two processes in meiosis that produce genetic variation among gametes.

    [2]
    (e)

    Explain how natural selection can change the frequency of an allele in a population over many generations.

    [2]

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