Each tube (cuvette) contains a spinach chloroplast suspension that has been mixed with the chemical DPIP which acts as an electron receptor for the light reactions of photosynthesis. As it receives electrons (reduction) it progressively turns from blue to colorless. Every 5 minutes the tubes are placed in a spectrophotometer and the light transmittance is measured. The rate of photosynthesis is thus measured by an increase in light transmittance in the tubes in which DPIP is changing color. Tube #1 is a blank for calibrating the spectrophotometer. Tube #2 is a tube enclosed in foil to prevent light from hitting the chloroplasts. Tube #3 is live chloroplasts in the light. Tube #4 is boiled (denatured) chloroplasts in the light. The last tube is a control containing only DPIP and no chloroplasts.