Photosynthesis
6 Minutes
Photosynthesis for students: learn how plants gain weight from air, sunlight, and water using Van Helmont's experiment. An English science lesson.
- Van Helmont's soil experiment on where trees get their weight
- How water is lost through tiny leaf openings called stomata
- How plants combine carbon dioxide, water, and light into glucose
What this lesson teaches
Photosynthesis is the process by which plants build their mass and create food by combining carbon dioxide from the air with water in the presence of sunlight.
Van Helmont's experiment
Jan Baptist van Helmont showed that soil is not what makes plants grow heavier. He placed 90.9 kilograms of dried soil into a container and grew a 2.3 kilogram plant in it for five years, giving it only water. When he weighed them again, the plant weighed 77 kilograms, while the soil lost only 0.1 kilograms, dropping to 90.8 kilograms. This demonstrated that trees do not build their bodies by consuming soil.
The role of water and stomata
- Plants take in water through their roots, but around 95 percent of that water leaves the plant as waste.
- This water loss through transpiration can be observed when moisture gathers on a cover wrapped around a leafy branch.
- Water exits and carbon dioxide enters through tiny pores on the underside of leaves called stomata.
- Stomata also allow the oxygen produced during food production to exit back into the surrounding air.
How glucose is made
Inside green leaves, a pigment called chlorophyll absorbs sunlight to power chemical reactions. The plant takes six molecules of carbon dioxide from the surrounding air and six molecules of water to produce one molecule of glucose. Glucose is a simple sugar that plants store and use for energy and growth. This carbon captured from invisible air is the main reason a seed weighing only 0.5 grams can become a tree weighing over 150 kilograms.
Nighttime respiration and the food chain
- Plants cannot perform photosynthesis at night because the process requires light energy.
- During the night, plants undergo respiration by using their stored glucose, taking in oxygen, and releasing carbon dioxide.
- Because plants produce food from sunlight rather than eating other organisms, they are called primary producers.
- These primary producers form the start of the food chain, passing stored energy to herbivores, carnivores, and decomposers.