Lesson guide · Lesson 5 · Grades 3–8 · 6 min

Light and why it matters

How a leaf uses light, water, and air to cook its own sugar, and why plants indoors need a grow light.

Open the lessonPrintable worksheet

Students will

  • Explain that a plant makes its own sugar from light, water, and air.
  • Name what goes into photosynthesis and what comes out.
  • Describe what happens to a plant that gets too little light.

New words

photosynthesis
How a leaf uses light, water, and air to make sugar.
chlorophyll
The green stuff in leaves that catches light.

Talk about it

Where in your home or school could a plant get enough light? Where couldn't it, and what could you do about that?

Check questions and answers

  1. What does a leaf make with the help of light?

    • Sugar (answer) Right. The leaf uses light to cook sugar for the plant.
    • Soil Not quite. Plants don't make soil. Think about the leaf kitchen.
    • Water Not quite. Water goes into the kitchen. Something new comes out.

  2. A plant on a dark shelf grows tall and thin. What does it need?

    • More light (answer) Right. It is stretching to reach more light.
    • More water Not quite. Tall and thin means it is reaching for something above it.
    • A bigger pot Not quite. The plant is reaching up. What is up there?

  3. Which three things go into a leaf's kitchen?

    • Light, water, and air (answer) Right. The leaf mixes them to make sugar.
    • Soil, sugar, and light Not quite. Sugar comes out of the kitchen. It doesn't go in.
    • Oxygen, rocks, and soil Not quite. Oxygen comes out of the leaf. Think light and water.

Sources

  • Clark, M. A., M. Douglas, and J. Choi. "8.1 Overview of Photosynthesis." Biology 2e. OpenStax, 2018.
  • Clark, M. A., M. Douglas, and J. Choi. "8.2 The Light-Dependent Reactions of Photosynthesis." Biology 2e. OpenStax, 2018.
  • Moore, K. A., and L. K. Bradley, eds. "3. Botany." North Carolina Extension Gardener Handbook, 2nd ed. NC State Extension, AG-831, 2022.
  • Weisenhorn, J., and N. Hoidal. Lighting for indoor plants and starting seeds. University of Minnesota Extension (reviewed 2026).
  • Trinklein, D. H. Lighting Indoor Houseplants. G6515. University of Missouri Extension, revised 2017.
  • Pennisi, B. V. Growing Indoor Plants with Success. Bulletin 1318. University of Georgia Cooperative Extension, 2022.
  • Shaughnessy, D., and A. Pertuit. Indoor Plants: Cleaning, Fertilizing, Containers & Light Requirements. HGIC 1450. Clemson Cooperative Extension Home & Garden Information Center, 1999.
  • Steil, A. Sources of Supplemental Light for Indoor Plants. Yard and Garden, Iowa State University Extension and Outreach (reviewed 2023).
  • Meng, Q., and E. S. Runkle. Blue Photons from Broad-Spectrum LEDs Control Growth, Morphology, and Coloration of Indoor Hydroponic Red-Leaf Lettuce. Plants 12(5): 1127, 2023.
  • Torres, A. P., and R. G. Lopez. Measuring Daily Light Integral in a Greenhouse. HO-238-W. Purdue Extension.
  • Supplemental Lighting Run Time Worksheet. University of New Hampshire Extension.
  • Lanoue, J., J. Zheng, C. Little, A. Thibodeau, B. Grodzinski, and X. Hao. Alternating Red and Blue Light-Emitting Diodes Allows for Injury-Free Tomato Production With Continuous Lighting. Frontiers in Plant Science 10: 1114, 2019.
  • Matsuda, R., N. Ozawa, and K. Fujiwara. Effects of Continuous Lighting with or without a Diurnal Temperature Difference on Photosynthetic Characteristics of Tomato Leaves. Acta Horticulturae 956: 165-170, 2012.
  • Schuh, M., and J. MacKenzie. Growing lettuce, endive and radicchio in home gardens. University of Minnesota Extension (reviewed 2024).