Systems: Find the Limiting Factor

Lab Brief #2

🎯 Your mission

Run the experiment. Find out what really happens.

⚡ The twist

Nature loves to surprise the careful watcher.

What you'll learn

Any process running on several inputs is capped by whichever input is currently scarcest, never by their sum. Adding light to a plant already short of carbon dioxide changes nothing at all, which is why a graph of rate against one input rises and then flattens — the plateau is the signature of some other factor taking over as the cap. The useful question to ask of any system is therefore not what it needs, but which of its inputs is limiting right now.

Key words

limiting factor
The input in shortest supply, which alone caps the rate of a process regardless of the others.
plateau
The flat region of a rate graph, showing that a different factor has become limiting.

Make a hypothesis first

Before you start: write down what you think will happen, and why.

Say your guess out loud — then find out if you were right.

Get ready

🧰 What you need

pondweed
lamp
beaker
sodium hydrogencarbonate
stopwatch
ruler

Get ready

⚠️ Safety first

Use a low-voltage lamp and keep it away from the water.
Wash your hands thoroughly after handling pond material.

Step 1 of 4 · Read

Think about what you just learned: say what "limiting factor" and "plateau" mean in your own words before you start.

Step 2 of 4 · Do

Measure the bubbling rate of pondweed at five light intensities at room temperature, then repeat the brightest setting with added sodium hydrogencarbonate. Identify from your graph where light stopped being the limiting factor and what took over.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

A rate graph that rises then flattens shows that:

How sure are you?😐 maybe

Question 2 of 3

Adding more light to a plant short of carbon dioxide will:

How sure are you?😐 maybe

Question 3 of 3

The rate of a process running on several inputs is set by:

How sure are you?😐 maybe

Mind = blown

🔬 The more you look, the more nature shows you.

Nice work!

You got 0 of 3 right.

Next: Structure and Function Are Read Off Each Other All lessons

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✨ More to explore

🌍 Where you see this in real life: This shows up in your everyday life more than you'd think.

🏆 Stretch challenge (try it this week): Try one experiment in real life this week — even a tiny one.

👨‍👩‍👧 For the dinner table: “What's the most surprising thing you learned today?”

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Systems: Find the Limiting Factor

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