Biology IA · B · Form and function · topic 8 of 25

How far does a molecule diffuse in a gel? · IB Biology IA idea: guide, sample graph and step-by-step plan

simple kit, clear prediction. good!

Why can't a cell be the size of a football? A blob of dye in jelly gives a first answer, if you measure it properly.

Difficulty●○○ Gentle
KitBasic: agar, food dyes, Petri dishes, ruler, phone camera
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like simple kit and clear results
  • ✓ you can be patient and keep timings steady
  • ✓ you want a link to why cells are small

The big idea

← this is your RQ hook!

Diffusion carries molecules short distances quickly and long distances very slowly. In theory the distance moved grows with the square root of time, not in a straight line, so doubling the time does not double the distance. That is a real, testable prediction, and it connects directly to why cells stay small and need transport systems. You will measure a fuzzy dye front the same way every time, plot it against time, and decide whether the square-root idea fits. Dyes of different molar mass add a second layer.

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 →

Compare dyes of different molar mass

Use two or more food dyes whose molar masses you can look up and cite.

→ You add a second variable, so your graph needs separate curves and your analysis compares rates, not just distances.
Change the gel concentration

Make agar at three or four concentrations, keeping dye and time the same.

→ Tests how a denser mesh slows diffusion. Gel preparation becomes a critical control.
Change the temperature

Run dishes at two or three controlled temperatures (for example in water baths or rooms).

→ Adds a second prediction (diffusion speeds up with temperature) but needs careful control of the gel temperature.
Link to cell size with agar blocks

Instead of a well, soak cubes of dyed agar of different sizes and measure how deep the colour reaches.

→ Moves the focus from distance travelled to surface area to volume ratio. A classic, but make sure you add something new.
Read the front with a camera

Photograph each dish at fixed times and measure the front with image software.

→ Reduces reading error and lets you measure the same dish repeatedly. Your evaluation compares eye and camera readings.

Your Biology IA research question: run the check

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

  • Names the independent variable (time, with a range in minutes; and the dye if you use more than one) and the dependent variable (distance of the dye front, and how it is read).
  • Says how the dye front will be defined, so the reading is the same each time.
  • Says what stays fixed (agar concentration, depth, dye amount and concentration, temperature) and how you will check each.
  • Names the relationship you will test (distance against time, or against the square root of time).
  • Is narrow enough for about 10 hours, with at least 5 times and repeat dishes.

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The set-up

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

xdye front is blurry: pick a rule,use it every timesame volume of dyein every wellsame agar depth andconcentration in every dishphotograph from directlyabove, ruler in the shot

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 diffusion limits cell size; what you expect to see and why.
Background biology~450
Diffusion and the factors that affect it; the prediction for distance against time and why (cite the source); what your graph should look like if it holds.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or hold it, and why it matters.
Method~400
How you make and cast the gel; how much dye and how it is added; your rule for reading the front; times; repeats; enough detail that someone could copy it.
Safety, ethics, environment~60
Specific: hot agar scalds, food-grade dyes only, no eating or drinking in the lab, safe disposal of gels.
Raw datatable
Distance at each time for every repeat, units and uncertainties in the headers.
Processing~250
Mean distance, spread, any squaring or square-rooting; one worked example of each calculation type.
Graphs and analysis~350
Distance against time and against the square root of time; error bars; line of best fit and what its gradient means.
Conclusion~450
Answer with values: does distance follow the square-root pattern? How do the dyes compare? Compare with accepted values or theory (cite).
Evaluation~650, the biggest part!
Weaknesses ranked by effect on your result (front definition, gel consistency, temperature), limits of a gel as a model for a cell, and linked improvements.
Referencesnot counted
Every source for theory and dye data.

What your Biology IA graph could look like

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

020406080100120012345678Time t / min (± 0.5 min)Distance of dye front x / mm (± 0.3 mm)smaller dye: x proportional to √tlarger dye: sloweryour data, with error barsa curve that bends like thisis not a straight line:test it, don't guessILLUSTRATIVE · MADE-UP DATA
  • Axis labels name the quantity, unit and the uncertainty of each measured value.
  • Error bars on every point. Here reading the front is the larger uncertainty, so vertical bars are longer.
  • A straight line from the origin would underestimate early points and overestimate late ones, which a plain linear fit would hide.
  • The curve must be drawn from the data, not forced to match the theory you hoped for.
  • Another graph to consider: plot x² against t. If the square-root idea holds this is a straight line through the origin, and its gradient can be compared between dyes.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result (distance of the front after 30 minutes), not on the raw readings. A chart like this, with your numbers, shows what to fix first.

Reading the dye front (± 0.3 mm)11%Gel batch (agar 1.0 vs 1.2 %)5%Room temperature (± 2 °C)2%Timing (± 0.5 min at 30 min)0.8%biggest effect: discuss this first!% uncertainty in distance at t = 30 minILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

fix the front rule in the trial run

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

0 of 10 done

  1. Read and predictFind the theory for diffusion distance against time and what affects it. Write down your prediction before you pour a single dish.
  2. Trial run (1 lesson)Make one dish, add the dye and check it every 10 minutes. How blurred is the front? Where do you read it? Does the gel dry out or crack? Fix problems now.
  3. Decide the front ruleChoose where on the blur you will read the distance (the faint edge or the strong colour) and write the rule down. Test it twice on the same photo.
  4. Decide times and repeatsChoose at least 5 times across your range and how many dishes per condition. Write down why for each choice.
  5. Prepare the gelPrepare enough agar for all dishes from one batch if you can, with the same depth and set time. Record the mass of agar and volume of water.
  6. Collect the dataRecord raw readings with units and resolution. Note anything odd as it happens, such as bubbles, uneven wells or a warm room.
  7. ProcessTurn readings into means with a measure of spread. Decide how you will test the square-root idea (for example squaring distance).
  8. Graph and fitPlot distance against time and against its transformation. Add error bars. Decide what the line should do if the theory holds.
  9. EvaluateWhich uncertainty or systematic error mattered most to your answer? Rank them. 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

  • No rule for the dye front: the edge is blurred, so your reading drifts without you noticing.
  • Concluding 'distance increases with time' and never testing the square-root prediction.
  • A gel that is deeper or thicker in some dishes, which changes the path.
  • Hot agar poured at different temperatures, so the gel sets differently.
  • Letting dishes dry out or warm up during long runs.
  • Using dye 'size' from memory instead of a cited molar mass.

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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