🌱 Photosynthesis Investigation: Green Onion Experiment
A 10-Day Investigation into Light and Plant Growth (with daily measurements)
📋 About This Lab: This experiment requires daily data collection for 10 days (Days 0-6, 8, 9, 10). You'll measure four variables: water + cup mass, plant mass, height, and color. Be prepared to commit to daily measurements for the best results!
🔬 Investigation Question
How does light availability affect the growth, color, and mass of green onion plants over a 10-day period?
In this investigation, you'll compare two green onion plants placed in cups (or beakers) with water—one kept in complete darkness (inside a cabinet) and one exposed to normal classroom lighting. You'll collect daily measurements to track changes in height, color, plant mass, and water mass over 10 days, discovering why plants need light for photosynthesis.
🌿 Background: Why This Experiment Works
Plants use photosynthesis to convert light energy into chemical energy (glucose) using the equation:
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
What happens without light?
🚫 No chlorophyll production: Plants can't make the green pigment needed to capture light
📏 Etiolation: Plants grow tall and spindly searching for light
💛 Pale yellow color: Without chlorophyll, plants appear yellowish-white
⚖️ No mass gain: Without glucose production, plants can't build new biomass
🔋 Using stored energy: Plants survive by burning stored starches (like us using savings)
What happens with light?
✅ Active photosynthesis: Plants produce glucose and store it as starch
🌳 Compact growth: No need to "stretch" for light—energy goes into healthy growth
💚 Dark green color: High chlorophyll content for maximum light capture
📈 Mass increase: New cells built from photosynthetically-produced sugars
🌱 Thriving plant: Energy production exceeds consumption
📦 Materials Needed (Per Group)
🧅 2 green onion bulbs (similar size)
🥤 2 clear plastic cups OR beakers (identical)
💧 Water
📦 Saran Wrap (3-4 inch squares)
📌 Tape (4 small pieces per container)
🚪 Access to a dark cabinet/cupboard
📏 Ruler (cm)
⚖️ Balance/scale (grams)
🎨 Green color palette (for comparison)
✏️ Permanent marker (for labeling)
✂️ Scissors
🧪 Procedure
Prepare your onions (Day 0 - Setup):
Select 2 green onion bulbs that are as similar as possible in size and appearance
Trim the plants to roughly the same height
Cut off any excess plant material that is not looking healthy
You want the plants starting as equally as possible
Set up the cups/beakers:
Get 2 cups or beakers (identical containers)
Label one container "DARK" and one "LIGHT" on the bottom
Fill both containers ¾ of the way full with water
Cut 2 squares of Saran Wrap (3-4 inches each)
Place each onion bulb in its container with roots submerged
Cover the top with Saran Wrap and secure with 4 small pieces of tape (finger-sized pieces) around the edges
The Saran Wrap helps reduce water evaporation
Use the teacher's example setup to see how your setup should look
Initial measurements (Day 0):
Measure the mass of water + cup/beaker together (record in table)
Carefully remove plant and measure its mass separately
Measure the height of the plant in cm (from base to tallest leaf tip)
Compare the green color to the color palette and record
Return plant to container and re-cover with Saran Wrap
Acclimation period (Weekend):
Place BOTH plants near the Hour 1 cabinets over the weekend
This is an acclimation period to adjust to room conditions
Allows plants to adjust after being in a store refrigerator
Both plants start in the same environment initially
Put away any stereoscopes or microscopes neatly before placing plants
Move to experimental conditions (After acclimation):
DARK container: Place inside a closed cabinet or cupboard that receives NO light
LIGHT container: Place on a windowsill, lab bench, or desk with normal classroom lighting
Ensure both locations have similar temperature (normal room temp)
Daily measurements (Days 1-6, then Day 8, 9, 10):
Every day: Record measurements for both plants
Measure mass of water + container
Measure mass of plant (remove from water, dry gently, weigh, return)
Measure height in cm
Compare and record green color vs. palette
Add water as needed to maintain level
Work quickly when measuring the DARK plant to minimize light exposure
⚠️ Important: When measuring the DARK onion, work quickly to minimize light exposure. Return it to the cabinet immediately after measurements. Handle plants gently when removing for weighing.
📝 Lab Report
Write your complete lab report below. Your work auto-saves and will be loaded when you return. Paste is disabled - all work must be original.
🎯 Introduction
Explain what you're investigating and why photosynthesis is important. State your hypothesis about what will happen to each onion.
🧪 Materials & Methods
Describe what materials you used and how you set up your experiment. Write in your own words - what did YOU actually do?
📊 Results - Data Collection
Record your measurements daily for Days 0-6, then Days 8, 9, and 10. All sections auto-save when you click the button below.
🌑 DARK Onion Data
Day
Mass of Water + Cup (g)
Mass of Plant (g)
Height (cm)
Green Color vs Palette
Day 0
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Day 8
Day 9
Day 10
☀️ LIGHT Onion Data
Day
Mass of Water + Cup (g)
Mass of Plant (g)
Height (cm)
Green Color vs Palette
Day 0
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Day 8
Day 9
Day 10
💡 Conclusion
Analyze your results and explain what you learned. Connect your observations to the concept of photosynthesis.
💭 Analysis Questions
Which onion grew taller by Day 10? Why do you think this happened?
Which onion gained more mass? Explain the reason using the photosynthesis equation.
Why did the dark onion turn yellow while the light onion stayed green?
Calculate the percent change in plant mass for each onion from Day 0 to Day 10: % Change = [(Day 10 Mass - Day 0 Mass) / Day 0 Mass] × 100
The dark onion grew tall without gaining mass. Where did it get the energy to grow?
Look at your water + cup mass measurements. Did the water level change differently in the light vs dark containers? Why?
On which day(s) did you notice the most dramatic changes in color? What does this tell you about when chlorophyll breakdown occurs?
What would happen to the dark onion if you moved it into light after 10 days? Would it recover? Design an experiment to test this.
Based on your data, estimate how long a plant could survive without light using only stored energy.
In nature, why might a seedling growing under leaf litter exhibit etiolation?
How does this experiment demonstrate that light is essential for photosynthesis?
🔬 Going Further
Extension Ideas for Advanced Investigation:
🌈 Test different light wavelengths (red, blue, green filters)
⏱️ Compare light duration (6 hours vs 12 hours vs 24 hours)
💡 Test light intensity (window vs overhead light vs grow light)
🧪 Extract and compare chlorophyll content using alcohol
📊 Graph your class data to identify trends and patterns
🌱 Try different plants (radish, bean, grass) to compare responses
✅ Lab Safety Reminder: Plants are living organisms. While these onions are being used for education, treat them with care. After the experiment, you can plant them in soil to continue growing!