Biology IA · A · Unity and diversity · topic 1 of 25

Capillary rise: can xylem lift a tree by itself? · IB Biology IA idea: guide, sample graph and step-by-step plan

physics meets plants. neat!

Water climbs a narrow tube with nothing pushing it. So how high does it go, and is that enough for a 100 m tree?

Difficulty●●○ Medium
KitBasic: glass capillary tubes, beaker, ruler, thermometer; plus published xylem data
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like physics meeting biology
  • ✓ you can measure tiny widths carefully
  • ✓ you want a model to test against real plants

The big idea

← this is your RQ hook!

Water rises in a narrow tube because it sticks to the glass and to itself. The narrower the tube, the higher it climbs. The simple model (Jurin's law) says the rise is inversely proportional to the inner diameter. Test that with real tubes first. Then ask the biology question: at the widths of real xylem vessels, how much of a tree's height can capillarity explain, and what else must be doing the work? Measure the tubes yourself, don't trust the labels. A clear prediction, a straight-line graph and a real link to plant transport make this a strong IA.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Change the liquid

Compare water with a dilute salt or detergent solution of known concentration.

→ Surface tension and contact angle change, so the gradient of your graph changes. You test the same model on a second liquid.
Make tube cleanliness the variable

Clean some tubes thoroughly and rinse others in a different way, then compare the rise at one diameter.

→ The contact angle becomes your data, not a hidden error. Your graph compares groups, with a statistical test.
Mine published xylem widths

Build a small dataset of vessel diameters from papers for a few plant types (climbers, trees, herbs).

→ This is a database investigation: your method is about source, filtering and sampling rules, and your graph is a prediction from your lab equation.
Try real stems

Stand freshly cut stems of one plant species in dyed water and measure how far the dye climbs in a set time.

→ Brings in transpiration, so capillarity alone cannot explain the result. Humidity and light need controlling, and the model is no longer clean.
Read the rise two ways

Compare height read at the bottom of the meniscus by eye with height read from a photograph.

→ Your evaluation gets real numbers on reading error instead of a guess.
Change the temperature

Run the same tubes with water at three safe temperatures (not above about 40 °C).

→ Surface tension falls slowly with temperature, so the effect is small. A good test of whether your method is sensitive enough to see it.

Your Biology IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable (inner diameter, with a range in mm) and the dependent variable (rise height, and exactly how it is read).
  • Says how the inner diameter will be measured directly, and with what uncertainty.
  • Says what is held constant (liquid, temperature, tube cleanliness) and how you will check each.
  • Says what model you compare with, and what the plant (xylem) link adds.
  • Is narrow enough for about 10 hours, with at least 5 diameters and repeats at each.

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The set-up

One way to set it up. Draw your own version, with your real tubes and measurements, for the Method section.

hsame spot on the meniscus,every timemeasure the inner diameterthermometer:same watertemperatureruler: zero atthe water level

How your Biology IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why xylem width matters for a tall plant; what you expect to see and why.
Background biology and physics~450
Cohesion, adhesion and the model for rise in a tube (cite it); the predicted shape of the graph; what other forces plants use (cite).
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or hold it, and why it matters.
Method and data sources~400
How you measured each inner diameter; how you filled, read and repeated; which published sources gave vessel widths and how you chose them.
Safety, ethics, environment~60
Specific: glass slivers and broken tubes, spilt water, disposal. No organisms harmed.
Raw datatable
Diameter and rise for each repeat, units and uncertainties in the headers; a separate table of published widths with references.
Processing~250
Mean rise with spread; uncertainty in diameter carried through; one worked example of each calculation type.
Graphs and analysis~350
Rise against diameter, and a linearised version; error bars; what the fitted gradient means and how it compares with the model.
Conclusion~450
Answer the question with values and uncertainties; how far the model held; what your equation predicts at xylem widths and what that tells you about plants.
Evaluation~650, the biggest part!
Weaknesses ranked by their effect on your result (contact angle, diameter, meniscus reading), the limits of applying a glass-tube model to living vessels, and linked improvements.
Referencesnot counted
Every source for theory, physical constants and plant data.

What your Biology IA graph could look like

two models drawn: yours?

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

0.00.20.40.60.81.01.21.41.61.82.0020406080100120140160Inner diameter d / mm (± 0.03 mm)Rise height h / mm (± 1 mm)model, perfectly wetted glassmodel, partly dirty glassyour data, with error barspoints sit below the clean-glass curve:what could do that?ILLUSTRATIVE · MADE-UP DATA
  • Both axes carry units and the uncertainty of each measured quantity.
  • Error bars on diameter matter most for the thinnest tubes, where a small width error is a big fractional error.
  • Two versions of the model drawn so the reader can see which one your data follow.
  • The curve is steep at small diameters, so choose your tubes to fill that region, not only the wide end.
  • Another graph to consider: plot h against 1/d. If the model holds this is a straight line, and the gradient can be compared with a calculated value and given an uncertainty.

Which uncertainty matters most?

Rank by the effect on your result (here, h × d, which the model says is constant), for one 0.5 mm tube. A chart like this, with your own numbers, shows where to spend effort.

Dirty tube (cos θ 1.0 to 0.9)10%Diameter (± 0.03 mm on 0.5 mm)6%Meniscus reading (± 1 mm on 59 mm)1.7%Water temperature drift (± 3 °C)0.6%Water density0.02%biggest effect: discuss this first!% effect on h × d for a 0.5 mm tubeILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

measure the diameters FIRST

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictFind the model for capillary rise and a source for the surface tension of water at your temperature. Write down what shape of graph you expect, and what rise it predicts for a vessel tens of micrometres wide.
  2. Trial run (1 lesson)Fill two or three tubes. Does water rise to a steady height? Can you see the meniscus bottom clearly? Does a tube you rinsed behave differently from one you did not? Fix problems now.
  3. Decide how to measure diameterLabels on tubes are not measurements. Choose a method (travelling microscope, calibrated eyepiece, or a weighed water thread of known length) and test it on one tube twice.
  4. Decide range, repeats and controlsChoose at least 5 diameters across your range, and how many refills per tube. Note how you will keep temperature and cleanliness the same. Write down why for each choice.
  5. Collect the dataRecord diameter and rise straight into the table with units and resolution. Note anything odd as it happens: a bubble, a tube that did not fill, a slow rise.
  6. Gather the plant dataChoose rules for which xylem measurements you will include, and record the source, species and how the width was measured. Say why that sample answers your question.
  7. Process and graphAverage the repeats, carry the uncertainty through, and plot with error bars. Try a linearised graph and extract a gradient with an uncertainty.
  8. Compare with the model and with plantsDoes the model hold, and by how much, compared with your uncertainties? What does your equation predict at xylem widths, and what does that leave unexplained?
  9. EvaluateRank the weaknesses by their effect on your result. Make every improvement specific to a weakness you found.
  10. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Biology you'll need

Traps to avoid in this Biology IA

  • Using the diameter printed on the packet instead of measuring each tube.
  • Tubes that are not clean: the contact angle changes, and every point shifts the same way.
  • Reading the meniscus at the bottom in one run and the top in another.
  • Measuring the rise from the table, not from the water level in the beaker.
  • Stopping at wide tubes where the rise is tiny and the reading error is large.
  • Claiming capillarity explains tall trees without comparing the predicted rise with the real height.

More Biology IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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