Biology IA · B · Form and function · topic 10 of 25

Transpiration and the air's drying power · IB Biology IA idea: guide, sample graph and step-by-step plan

a classic, but with a better variable. good choice :)

Humidity alone is a poor measure of how thirsty the air is: 60% at 15 °C and 60% at 30 °C dry a leaf very differently. You're going to test a better number.

Difficulty●●○ Medium
KitBalance (or potometer), humidity and temperature sensor, lamp, camera
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like plants and a bit of calculation
  • ✓ you can build a controlled-air chamber
  • ✓ you want a variable better than 'humidity'

The big idea

← this is your RQ hook!

Water leaves a leaf because the air inside is saturated and the air outside is not. Vapour pressure deficit (VPD) is the gap between the two, and it depends on temperature and humidity together. Your job: turn two readings into one number you can justify, vary it on purpose, and measure the water lost per unit leaf area. Theory says the relationship need not be a straight line, because stomata can respond. Clear variables, real calculation, honest controls. Examiners like that.

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 →

Humidity vs VPD

Collect two sets of data: one varying humidity at a fixed temperature, one varying temperature at fixed humidity.

→ You can plot both against humidity and against VPD, and test whether VPD collapses the two sets onto one pattern.
Potometer instead of mass loss

Measure water uptake by watching a bubble move along a capillary tube.

→ Your DV is distance moved per time converted to volume; the evaluation compares uptake with loss, which are not always equal.
Compare two species

A thin-leaved and a thick or waxy-leaved species at the same set of VPDs.

→ Your graph has two curves; you can ask whether leaf structure changes the slope or the levelling-off.
Seal one surface

Smear petroleum jelly on the lower, then the upper, leaf surfaces of matched shoots.

→ Adds a link to stomatal distribution; the DV becomes loss per cm² of open surface. Needs extra controls.
Fan on or off

Repeat one VPD with different air movement.

→ Tests the boundary layer idea. Your RQ must make clear air movement is the variable here, not a control.
Change the context

Link to a greenhouse, a drought or a heatwave, and choose VPDs that match real conditions.

→ Gives your range a reason; the analysis stays the same.

Your Biology IA research question: run the check

write it FIRST, then check it

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

  • Names VPD as the independent variable and says how it is changed (which temperatures and humidities) and over what range in kPa.
  • Names the dependent variable as water lost per unit leaf area per hour, with the units.
  • Says how you will calculate VPD, and from which equation, so a reader can check the numbers.
  • Says what stays fixed (light, species, shoot size, air movement) and how you will know each did not drift.
  • Is narrow enough for about 10 hours, with at least five VPD values and several shoots at each.

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The set-up

One way to set it up for the mass-loss method. Draw your own version, with your real measurements, for the Method section. If you use a potometer, redraw the tube and capillary.

dlamp at a fixed distance(it heats the leaves!)leaf area: photograph theleaves flat, with a rulerstem cut under water,no air bubble in the stemhumidity + temperaturesensor, close to the shootbalance: weigh at the same time each houroil layer: noevaporation fromthe water

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 plants lose water, why a greenhouse or a heatwave makes this matter, and what you expect to happen to the water lost as the air gets 'thirstier'.
Background biology~450
Transpiration, stomata and the water potential gradient; how VPD is calculated and why it combines temperature and humidity; what your graph should look like if your prediction holds (cite your source).
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant, and why it matters.
Method~450
How you created each air condition and checked it; how you cut and set up the shoots; how long you waited before weighing; how you measured leaf area; a blank tube to correct for evaporation.
Safety, ethics, environment~60
Short and specific: sharp blades, electrical kit near water, a lamp that gets hot, silica gel dust (avoid the cobalt chloride indicating type), and the plants you cut.
Raw datatable
One clear table: temperature, humidity, masses at the start and end, leaf area, with units and uncertainties in the headers. Qualitative notes (wilting, curled leaves) beside it.
Processing~250
VPD from the two readings (with uncertainty), water lost per cm² per hour, one worked example of each calculation type, and how the blank tube was used.
Graphs and analysis~350
The graph you choose and why, SD error bars, a fit justified by the shape of the data, and what the fit says about the plant's response; Spearman's rank or a regression with each shoot as one replicate.
Conclusion~400
Answer the question with numbers and uncertainties, link it to the biology, and say whether your prediction held, only over the VPD range you used.
Evaluation~600, the biggest part!
Strengths, weaknesses ranked by their effect on your result (lamp heat, leaf temperature, shoot variation), and realistic improvements linked to each.
Referencesnot counted
Every source for the VPD equation, the biology and the method.

What your Biology IA graph could look like

error bars on EVERY point ✓

Made-up data, to show you the form of a strong graph, not the answer. Yours may curve, flatten or even turn over. Let the data say. example only

0.00.51.01.52.02.53.00123456Vapour pressure deficit, VPD / kPa (± 0.1 kPa from sensor)Water lost / mg cm⁻² h⁻¹ (mean ± SD of shoots)levelling-off curvestraight lineyour data (mean of repeats), with error barsdoes your data keep climbing,or level off? Say why.ILLUSTRATIVE · MADE-UP DATA
  • Axis labels give the quantity, its unit and the uncertainty of the variable you set. VPD is calculated, so say where its uncertainty comes from.
  • Error bars on every point. Each point is the mean of several shoots, so show the spread (SD or range) and say how many.
  • Two models drawn so the reader can see which one the data follows. A fit through the origin is only justified if zero VPD should mean zero loss.
  • The scale starts at zero for both axes, so the shape is honest.
  • Another graph to consider: plot the same data against relative humidity and against VPD in two panels. Which one gives the tidier pattern? That is your evidence for the 'better number' claim.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result (water lost per cm² per hour, here for a shoot that loses 0.40 g in an hour), not on the raw readings. Notice what is not on this chart: the effect of lamp heat on leaf temperature. You cannot put a number on it without measuring it, so measure it.

Mass loss: two weighings, ±0.01 g each, of 0.40 g lost5%Leaf area from photo (±4%)4%Timing the hour (±1 min)1.7%biggest effect: discuss this first!% uncertainty in water lost per cm²ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

don't skip the trial run! it saves you weeks

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

0 of 10 done

  1. Read and predictFind how VPD is calculated and how stomata respond to dry air. Before touching apparatus, write down what you predict will happen to water loss as VPD rises, and what shape the graph will have.
  2. Decide how to make different airThink about how you will get a range of temperatures and humidities in a small chamber (sealed box, damp towels, silica gel, a gentle heat source). Where will the sensor sit, and how long will it take to settle?
  3. Trial run (1 lesson)Set up one shoot and one blank tube. Does the mass change enough in an hour to measure? Do bubbles form in the stem? Does the lamp change the air reading? Fix the problems now.
  4. Decide range, repeats and timingChoose at least five VPD values that give a clear spread, how many shoots at each, and a fixed time. Write down why for each choice.
  5. Collect the dataRecord raw values straight into your table with units. Log temperature and humidity through the hour, not just at the start, and note anything odd as it happens.
  6. Measure leaf areaPhotograph every shoot's leaves flat with a ruler in the shot, and measure in image software. Decide now whether you count one surface or both, and say so.
  7. ProcessCalculate VPD with its uncertainty, then water lost per cm² per hour. Decide how you will handle the uncertainty of VPD itself, not just the mass.
  8. Graph and fitPlot with error bars. Decide whether a straight line, a curve or no fit is justified, and what each would mean biologically.
  9. EvaluateWhich uncertainty or uncontrolled variable mattered most for your answer? Rank them using your own data, then make every improvement specific to a weakness you found.
  10. Write, then checkWrite in the order of the layout. Tables, graphs, equations and references are not counted, so let them carry detail. Run the checklist below before your teacher's draft.

Biology you'll need

Traps to avoid in this Biology IA

  • Air bubbles in the stem or a stem not cut under water: the water column breaks and loss drops for the wrong reason.
  • Lamp heat changes leaf temperature and the air's humidity, so you are changing two things at once. Measure both.
  • Using the air temperature in the VPD equation when the leaf is warmer. State your assumption and test it if you can.
  • Leaf area measured once on one side, or by guess. Decide which area you report and apply it the same way to every shoot.
  • No blank tube: water evaporating from the tube and oil is counted as transpiration.
  • Different shoots at each VPD with no thought for size and leaf number: variation between shoots can hide the trend.
  • Forcing a straight line through zero, then calling a curved pattern 'a few anomalies'.

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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