Exercises: Hypotheses, Investigations and Evaluating Evidence

Define the term hypothesis and state what makes a hypothesis testable.

Distinguish between the independent variable, the dependent variable and a controlled variable in an experiment, using the example of an investigation of the effect of temperature on the rate of an enzyme reaction.

Explain the difference between accuracy and precision in measurements.

A student wants to test whether light intensity affects the rate of photosynthesis in pondweed.

1. a) Write a testable hypothesis.

1. b) State the independent and dependent variables and how each would be measured.

1. c) State three variables that should be controlled.

A student measured the volume of oxygen released by pondweed in 5 minutes, repeating the experiment five times: 4.2 mL, 4.4 mL, 4.3 mL, 4.1 mL and 4.5 mL.

1. a) Calculate the mean volume.

1. b) Explain why repeating the experiment improves reliability.

1. c) Suggest one source of error in measuring the gas volume and how it could be reduced.

A newspaper headline states: "New tablet cures colds: 90% of people who took it recovered within a week."

1. a) Identify two pieces of information missing that you would need to evaluate this claim.

1. b) Explain why a control group is necessary.

1. c) State what is meant by bias and give one example of how it might affect this study.

A researcher claims that a new fertiliser increases the yield of potatoes. She grows 10 plants with fertiliser in a sunny field and 10 without fertiliser in a shady field, then reports a 30% greater yield for the fertilised plants.

1. a) Identify two flaws in the design of this investigation.

1. b) Redesign the investigation so that the conclusion would be valid, including sample size, controls and replication.

1. c) Explain what statistical analysis might be used to decide whether the difference is significant, and what the result would mean.

Scientists use models, such as the fluid mosaic model of the membrane or the double helix model of DNA.

1. a) Explain what is meant by a scientific model and why models are used.

1. b) Using one example, explain how a model can be modified as new evidence emerges.

1. c) Discuss one limitation of models in representing biological systems.

The results of an investigation of enzyme activity at different temperatures show a bell-shaped curve with an optimum at 37 °C, but there is a large spread in the replicate readings at 60 °C.

1. a) Suggest two reasons for the larger spread at 60 °C.

1. b) Distinguish between a random error and a systematic error, with one example of each for this investigation.

1. c) Evaluate whether a study published in a magazine with no information about the method or sample size should be given the same weight as a peer-reviewed article. Justify your answer.