Photosynthesis: Capturing Light as Chemical Energy

Estimated Time: 45-60 minutes Materials: Computer or tablet with internet access, calculator or spreadsheet, CSV export tool.

Part 1: Engage (Anchoring Phenomenon)

A tiny acorn grows into a massive oak tree weighing thousands of kilograms. The tree’s trunk, branches, leaves, and roots are all made of organic matter. But where does all that mass actually come from? Conventional wisdom suggests soil — but the mass of the soil barely changes as the tree grows. What if the tree’s mass comes from somewhere else entirely?

1. Observations and Questions:

If the tree doesn’t get most of its mass from the soil, where could it be coming from?
What role do you think gases in the air (CO₂, O₂, N₂) might play in building plant mass?
The chemical equation for photosynthesis is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Based on this equation, which molecules provide the atoms that make up the tree’s wood and leaves?
Generate at least two “need to know” questions about how plants convert light energy into chemical energy in the form of sugar.

Part 2: Explore (Simulation Investigation)

Open the Photosynthesis simulation. The simulation includes four sliders (Light Intensity, Temperature, CO₂ Concentration, Light Wavelength), a bubble counter measuring oxygen production (a proxy for photosynthetic rate), a real-time data table, a graph showing photosynthetic rate over time, and a CSV export button.

2. Data Collection:

Investigation A: Effect of Light Intensity

Set Temperature to 25°C (optimal), CO₂ to 400 ppm (ambient), Wavelength to 430 nm (blue light)
Set Light Intensity to 0% (dark)
Run the simulation for 30 seconds. Record the oxygen bubble count (bubbles/min)
Repeat at 20%, 40%, 60%, 80%, and 100% Light Intensity
Record all data in Data Table 1
Observe the graph for any patterns in the rate of change

Investigation B: Effect of Temperature

Reset the simulation
Set Light Intensity to 80%, CO₂ to 400 ppm, Wavelength to 430 nm
Set Temperature to 5°C
Run for 30 seconds. Record the oxygen bubble count
Repeat at 15°C, 25°C, 35°C, and 45°C
Record all data in Data Table 2

Investigation C: Effect of CO₂ Concentration

Reset the simulation
Set Light Intensity to 80%, Temperature to 25°C, Wavelength to 430 nm
Set CO₂ to 100 ppm
Run for 30 seconds. Record the oxygen bubble count
Repeat at 200, 400, 600, 800, and 1000 ppm CO₂
Record all data in Data Table 3

Investigation D: Effect of Light Wavelength (Color)

Reset the simulation
Set Light Intensity to 80%, Temperature to 25°C, CO₂ to 400 ppm
Set Wavelength to 430 nm (blue)
Run for 30 seconds. Record the oxygen bubble count
Repeat at 475 nm (cyan), 530 nm (green), 580 nm (yellow), 630 nm (red), and 680 nm (far red)
Record all data in Data Table 4

Data Tables:

Data Table 1: Effect of Light Intensity on Photosynthetic Rate | Light Intensity (%) | Oxygen Bubbles (bubbles/min) | Observations | |:—|—:|—| | 0 | | | | 20 | | | | 40 | | | | 60 | | | | 80 | | | | 100 | | |

Data Table 2: Effect of Temperature on Photosynthetic Rate | Temperature (°C) | Oxygen Bubbles (bubbles/min) | Observations | |:—|—:|—| | 5 | | | | 15 | | | | 25 | | | | 35 | | | | 45 | | |

Data Table 3: Effect of CO₂ Concentration on Photosynthetic Rate | CO₂ Concentration (ppm) | Oxygen Bubbles (bubbles/min) | Observations | |:—|—:|—| | 100 | | | | 200 | | | | 400 | | | | 600 | | | | 800 | | | | 1000 | | |

Data Table 4: Effect of Light Wavelength on Photosynthetic Rate | Wavelength (nm) | Color | Oxygen Bubbles (bubbles/min) | Observations | |:—|—:|—:|—| | 430 | Blue | | | | 475 | Cyan | | | | 530 | Green | | | | 580 | Yellow | | | | 630 | Red | | | | 680 | Far Red | | |

Graph Analysis:

Use the simulation’s export feature to save your data as CSV
Plot the data from at least two investigations on the graph
Describe the shape of each curve

Part 3: Explain (Sensemaking)

3. Analyzing Photosynthetic Rate Patterns:

Look at your Light Intensity data. How does the rate of oxygen production change as light intensity increases? Does it increase linearly, or does it level off? Why might it level off at high light intensities?
Examine your Temperature data. Why does the rate increase from 5°C to 25°C but then decrease at 45°C? What does this tell you about the enzymes involved in photosynthesis?
From your CO₂ data, at what concentration does the photosynthetic rate begin to plateau? What factor might be limiting the rate at high CO₂ levels?
Look at your Wavelength data. Which wavelengths give the highest photosynthetic rate? Which give the lowest? How does this relate to the absorption spectrum of chlorophyll (which absorbs strongly in blue ~430 nm and red ~660 nm)?

4. Connecting to the Tree Mass Phenomenon:

Based on the chemical equation (6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂), identify which atoms from the reactants end up in the sugar molecules.
Explain how the CO₂ gas that a tree takes in from the air becomes the carbon atoms that make up the tree’s trunk, branches, and leaves.
If a tree gains 100 kg of dry mass, approximately how much of that mass came from CO₂ in the air? (Hint: Carbon makes up roughly 40-50% of the dry mass of wood.)

Part 4: Elaborate / Evaluate (Argumentation & Modeling)

5. Model Development and CER Explanation

Your task is to develop a scientific model that explains how plants capture light energy and convert it into chemical bond energy stored in sugar molecules. Your model must integrate your simulation data with the molecular-level process of photosynthesis.

Construct a visual model (concept map, flow chart, annotated diagram, or system model) that includes:

Components: Light energy (photons), matter inputs (CO₂ and H₂O), matter outputs (C₆H₁₂O₆ and O₂), chloroplasts in plant cells, chlorophyll pigments, chemical bond breaking and bond formation, the energy transfer pathway
Relationships: Show how CO₂ and H₂O are converted into sugar and oxygen through photosynthesis. Include how light intensity, temperature, CO₂ concentration, and light wavelength affect the rate of this process based on your experimental data
Connections: Trace the flow of energy from sunlight through the photosynthetic system and into the chemical bonds of sugar. Show the transfer of matter from the abiotic environment (atmospheric CO₂, soil H₂O) into organic matter (sugar, cellulose, starch). Connect this model to the anchoring phenomenon: explain how the tree’s mass comes mostly from CO₂ in the air

Write a CER (Claim, Evidence, Reasoning) to accompany your model, answering the driving question: Where does a tree’s mass come from, and how does it use light energy to build that mass?

Claim: A clear, concise answer to the driving question that includes both matter and energy.
Evidence: Specific data from at least two of your simulation investigations. Include quantitative values (oxygen bubble rates at specific settings) and comparisons between different conditions. Reference your data tables.
Reasoning: Explain how the evidence supports your claim by describing:
- How your data demonstrates the relationship between environmental factors and photosynthetic rate
- How the chemical equation for photosynthesis accounts for the atoms needed to build plant mass
- How light energy is captured and stored as chemical bond energy in sugar molecules
- How the organization of matter and flow of energy in this system connects the organism (plant) to its environment

Teacher Notes & NGSS Alignment

Performance Expectation: HS-LS1-5. Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.

Alignment to Dimensions:

SEP: Developing and Using Models — Students develop a visual model that traces the flow of matter and energy through the photosynthetic system, showing the relationships between environmental factors (light intensity, temperature, CO₂, wavelength) and the rate of photosynthesis. They use simulation data to inform and validate their model.
DCI: LS1.C (Organization for Matter and Energy Flow in Organisms) — The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen. The sugar molecules thus formed contain carbon, hydrogen, and oxygen; their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA), used for cellular respiration, or stored as starch.
CCC: Energy and Matter — The transfer of energy can be tracked as energy flows through a natural system. Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. In photosynthesis, light energy is transferred to chemical bond energy in sugar molecules.

Evidence Statement Mapping:

1 (Components): Students develop a model that includes the components of a photosynthetic system: light energy, chemical bond breaking (absorbs energy), bond forming (releases energy), matter inputs (CO₂ and H₂O), and matter outputs (sugar and oxygen). Demonstrated in Part 4 when students construct a visual model showing all system components and the molecular equations.
2 (Relationships): Students use the model to describe the relationships between matter and energy in photosynthesis — specifically that sugar and oxygen are produced from CO₂ and H₂O, and that light energy drives the conversion. Demonstrated in Parts 2 and 3 as students collect data on factors affecting photosynthetic rate and analyze how environmental variables affect sugar/oxygen production.
3 (Connections): Students connect the model to the broader context of matter transfer and energy flow between the organism and its environment, recognizing that the energy stored in sugar represents the difference between bond energies of inputs vs. outputs. Demonstrated in Part 3 and Part 4 as students connect simulation data to the tree mass phenomenon and construct a CER explaining the origin of plant biomass.