Spinach Leaf Disk Experiment: Watch Photosynthesis Make Oxygen
By Ahmed, founder
Plants make their own food, but how do you see that happening? The spinach leaf disk experiment gives a visible, countable result: leaf pieces rising through water as they make oxygen.
What you need
- Fresh spinach leaves
- A hole punch or straw (to cut small disks)
- Baking soda, water and a drop of dish soap
- A clear cup or two
- A plastic syringe (10 mL, no needle) to remove air from the disks
- A bright lamp, and a timer
The method
- Mix about ¼ teaspoon of baking soda in 200 mL of water. This supplies carbon dioxide, which the leaf needs. Add a tiny drop of dish soap so the water wets the leaf.
- Punch 10 disks from the leaves, avoiding veins.
- Put the disks in the syringe with some of the solution, hold the tip closed and pull the plunger back gently to make a partial vacuum. Air bubbles out of the leaf tissue. Repeat until the disks sink.
- Put the sunk disks in a cup of solution under a bright lamp.
- Count how many disks have floated every minute, for 10 to 15 minutes.
Control: set up a second cup of disks in the dark, or in plain water without baking soda.
What happens
In the light, with baking soda, disks start to rise within minutes. In the dark, or without carbon dioxide, they stay at the bottom.
Why
Removing air makes the disks sink. In the light, the leaf does photosynthesis: it uses light energy, carbon dioxide and water to make sugar, and it releases oxygen. Oxygen gas gathers in the spaces inside the leaf and makes the disks buoyant, so they float. So floating disks are evidence that photosynthesis made oxygen.
Fair test ideas
- Change the light: closer or farther lamp, or different colours.
- Change the carbon dioxide: more or less baking soda.
- Change the temperature (a very small change).
Keep everything else the same, and look at how long it takes for half the disks to float.
Where does a plant get its food?
Many students think plants take food from the soil. Try growing bean seedlings without soil, in water or damp paper, and weigh them twice. They still gain mass, giving evidence that plants make food from air and water.
Related lessons
The full lesson is Where does a plant get its food?, the first in Food Webs: Who Eats Whom? For older students, see Where does a tree’s mass come from? Sign up for three lessons free.