Photosynthesis Investigation Notes

Photosynthesis Investigation

Background

• Photosynthesis fuels ecosystems and replenishes atmospheric oxygen.

• The rate of photosynthesis can be measured by:

  • Production of $O<em>2$ (moles of $O</em>2$ per mole of sugar).

  • Consumption of $CO<em>2$ (moles of $CO</em>2$ per mole of sugar).

• The general equation for photosynthesis is: $2H<em>2O + CO</em>2 + light \rightarrow carbohydrate (CH<em>2O) + O</em>2 + H_2O$

Leaf Disk Technique

• Leaves normally float due to gases (O2 and CO2) in the spongy mesophyll layer.

• Vacuum infiltration replaces these gases with water, causing leaf disks to sink.

• In bicarbonate solution, photosynthesis produces oxygen, causing the disks to float.

• The rate of photosynthesis is indirectly measured by the rate of rise of leaf disks.

• Cellular respiration consumes oxygen, counteracting oxygen accumulation from photosynthesis.

• The buoyancy of leaf disks measures the net rate of photosynthesis.

Learning Objectives

• Design experiments to explore factors affecting photosynthesis.

• Connect concepts: cell structure, energy capture/storage, gas diffusion, and gas laws.

Materials

• Baking soda (sodium bicarbonate).

• Liquid soap.

• Syringes.

• Living leaves.

• Hole punch.

• Clear plastic cups.

• Timer.

• Light source.

Procedure Summary

1. Prepare 0.2% bicarbonate solution (CO2 source).

2. Add a drop of dilute liquid soap (surfactant).

3. Cut leaf disks, avoiding major veins.

4. Infiltrate leaf disks with bicarbonate solution (or water for control) using a vacuum.

5. Place disks in cups under a light source and record the number of floating disks per minute.

6. Determine the ET50 (Estimated Time for 50% of disks to float).

Key Steps

• Creating a vacuum is critical for sinking the disks.

• The choice of plant material is critical.

Data Analysis

• ET50 is a reliable reference point.

• There is an inverse relationship between ET50 and the rate of photosynthesis.

• To show a direct relationship, use 1/ET50.

• Include variability in data (error bars, box plots).

Experimental Design Considerations

• Environmental variables (light, temperature).

• Leaf features/variables.

• Procedural variations.

• Combining variables can reveal differences between plants (e.g., photosynthesis light response curves).