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Percentage change in mass is calculated by dividing the change in mass (final mass - initial mass) by the initial mass. Free notes and 11 questions answered.
Measuring and Calculating Water Movement is part of Transport in cells in GCSE AQA Higher Biology.
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GCSE AQA Higher
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Percentage change in mass is calculated by dividing the change in mass (final mass - initial mass) by the initial mass. Free notes and 11 questions answered.
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Use this lesson to stay inside Transport in cells while connecting the detail back to the wider Cell biology topic.
Read the core explanations from Measuring and Calculating Water Movement before testing yourself from memory.
Plants can't survive without water - but how much do they actually use? When you water a houseplant, where does that water go? Some evaporates from leaves, some stays in cells, and some passes straight through. Understanding these movements helps farmers optimise irrigation and botanists study plant health.
In your exams, you'll need to calculate water uptake rates and percentage changes in plant mass. These aren't just abstract numbers - they reveal how plants respond to different conditions like light intensity, humidity, and temperature.
Trap: Many students think rate means 'how much water' - actually it means 'how much water per unit time'. Without the time component, you just have volume, not rate.
Trap: Students often forget to divide by the initial mass. Just subtracting final - initial gives you the absolute change, not the percentage change.
Water uptake rate tells us how quickly a plant absorbs water. This depends on factors like leaf surface area, temperature, and humidity. The basic formula is straightforward, but exam questions often include unit conversions to test your attention to detail.
Worked example: Calculating Water Uptake Rate
A potometer shows that a plant shoot absorbed 15 cm³ of water in 30 minutes. Calculate the rate of water uptake in cm³/min. [2 marks]
Answer: 0.5 cm³/min
Notice how we kept the units throughout the calculation. Examiners often award method marks for showing the formula and substitution, even if your final answer is wrong.
When plant tissue gains or loses water, its mass changes. Calculating percentage change allows us to compare results from different experiments, even when the initial masses were different. This is crucial for osmosis investigations.
Worked example: Calculating Percentage Change in Mass
Potato chips were placed in different salt solutions. One chip had an initial mass of 3.2g and a final mass of 2.9g. Calculate the percentage change in mass. [3 marks]
Answer: -9.4% (mass decreased by 9.4%)
The negative result tells us the potato chip lost mass - water moved out of the cells into the salt solution. This happens when the external solution is more concentrated than the cell contents.
Water movement calculations appear frequently in GCSE papers, often worth 3-4 marks. The key to full marks is showing your working clearly and avoiding these common traps that catch students out.
Exam depth extension: In measuring and calculating water movement, connect why measuring water movement matters to the language of water movement using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Mark-winning phrases: Use 'rate of water uptake' not just 'water uptake', and always state whether percentage changes represent 'mass gain' or 'mass loss' in your conclusion.
Common exam trap: Questions often give you volume in ml but ask for the answer in cm³, or give time in hours but ask for rate per minute. Always check the units carefully before starting your calculation.
Master these calculations and you'll confidently tackle any water movement question in your exam. Remember - show your working, include units, and double-check that your answer makes biological sense.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Percentage change in mass is calculated by dividing the change in mass (final mass - initial mass) by the initial mass. Just subtracting final - initial gives the absolute change, not the percentage change, and a negative result shows mass loss.
The rate of water uptake is how much water a plant absorbs per unit time, so the volume of water is divided by the time taken. Without the time component there is only volume, not rate, and units such as ml, cm³, hours and minutes must be checked carefully.
Absolute change in mass is just final mass minus initial mass, while percentage change in mass is (final mass - initial mass) ÷ initial mass × 100. Just subtracting final - initial without dividing by the initial mass gives the absolute change, not the percentage change.
Rate of water uptake is typically measured in cm³/min or ml/hour, because it is the volume of water absorbed per unit time. Without the time component, the value is just a volume, not a rate.
A positive percentage change in mass shows the plant tissue gained mass, while a negative value shows it lost mass. A potato chip going from 3.2g to 2.9g has a change of -9.4%, meaning water moved out of the cells into a more concentrated salt solution.
The percentage change in mass of a potato chip is (final mass - initial mass) ÷ initial mass × 100. For a chip with an initial mass of 3.2g and final mass of 2.9g, this is -0.3 ÷ 3.2 × 100 = -9.4%, a mass decrease of 9.4%.
Water uptake questions often give volume in ml but ask for the answer in cm³, or give time in hours but ask for a rate per minute. Units should always be checked carefully before starting the calculation.
Rate of water uptake does not mean just how much water a plant takes in; it means how much water per unit time. It is calculated by dividing the volume of water taken up by the time period, for example 15 cm³ in 30 minutes is 0.5 cm³/min.
A potato chip loses mass in a salt solution because water moves out of the cells into the salt solution. This happens when the external solution is more concentrated than the cell contents, giving a negative percentage change in mass.
Percentage change in mass is used because it allows results from different experiments to be compared, even when the initial masses were different. Subtracting initial mass from final mass only gives the absolute change, not the percentage change.
The rate of water uptake in plants depends on factors like leaf surface area, temperature and humidity. Measuring it shows how plants respond to different conditions, which helps farmers optimise irrigation and botanists study plant health.