AP Biology Investigation 5: Photosynthesis

Lab Notes General summary equation for photosynthesis: 2H2O + CO2 + light  carbohydrate (CH2O) + O2 + H2O Ways to measure the rate of photosynthesis: Production of O2 Consumption of CO2 In this lab, we will use a system to measure the accumulation of oxygen

Spongy mesophyll layer of leaves normally infused with gases (O2 and CO2) Leaves will normally float in water

Floating Leaf Disk Technique Draw gases from leaves using a vacuum and replacing gases with water Sunken leaves placed in solution with source of carbon dioxide (sodium bicarbonate) Photosynthesis occurs in leaves  oxygen accumulates  leaves become buoyant and rise in the water Rate of photosynthesis measured indirectly by rate of rise of the leaf disks

Materials

Procedure1: Structured Inquiry Label 2 cups: “With CO2” and “Without CO2” With CO2: pour 0.2% sodium bicarbonate (NaHCO3) solution into plastic cup to a depth of about 3 cm Without CO2: distilled water (control)

Procedure 1 Add 1 drop of dilute liquid soap into each cup Soap acts as a surfactant or “wetting agent” Wets hydrophobic surface of leaf, allowing solution to be drawn into leaf enabling leaf disks to sink in fluid

Procedure 1 Using hole punch (or plastic straw), punch out 10 uniform leaf disks for each cup. Avoid major leaf veins

Procedure 1 Draw gases out of leaves and infiltrate with solution (either with or without CO2) Remove plunger from syringe. Place 10 leaf disks into each syringe barrel. Tap to put disks at bottom of syringe. Carefully replace plunger. Push down close to leaf disks. Draw small volume (~ 5 cc) of solution into syringe. Tap to suspend disks in solution. Invert and expel excess air from syringe.

Create a vacuum in plunger using finger to draw air out of leaf tissue. Hold finger over syringe  draw back plunger 10 sec  swirl tube to resuspend disks  release Repeat 2-3 times to get all disks to sink (No more than 3+ times! Damage to disks can occur.)

Pour disks and solution from syringe into appropriate cup. Disks infiltrated with sodium bicarbonate go into “With CO2” cup Disks infiltrated with distilled water go into “Without CO2” cup

Place both cups under light source and start timer. Record # floating disks at the end of each minute. Swirl disks to dislodge any that stick to sides of cups. Continue until all disks are floating in “With CO2” cup

Data Analysis Need standard point of reference to make comparisons ET50 = Estimated Time it takes 50% of leaf disks to float Graph data  determine ET50 from graph ET50 = time it takes for half the disks (5) to float # floating leaf disks Time (minutes)

Class Data Create a data table for class ET50 data

Comparison of ET50 across treatments Inverse relationship: ET50 decreases as rate of photosynthesis increases (negative slope) To correct this representation, graph the inverse (1/ET50) vs. rate of photosynthesis

What are some factors that could affect photosynthesis? Brainstorm What are some factors that could affect photosynthesis? FROM THIS LIST YOU WILL DECIDE HOW TO PROCEED WITH YOUR INQUIRY INVESTIGATION (Procedure 2)

Procedure 2: Inquiry Investigation Problem (choose a specific factor to test) Experimental Design (IV, DV, control, contants, etc.) Hypothesis (If…, then…. because….) Materials Procedure (flowchart) Data Table Record # floating disks each minute until all disks are floating

Analysis and Conclusion Determine ET50 (estimated time it takes 50% of disks to float) Graph lab group data Class Data Conclusion (typed and taped into notebook) How do different treatments affect the rate of photosynthesis?