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Photosynthesis Lab

Measure net O₂ production rates under different light intensities, CO₂ concentrations, and temperatures. Identify limiting factors, find the light compensation point, and explore enzyme-driven saturation curves in real time.

Guided Experiment: Light Response Curve

What do you predict will happen to the net O₂ production rate as light intensity increases from 0 to 2000 μmol/m²/s? Will the relationship be linear?

Write your hypothesis in the Lab Report panel, then click Next.

Controls

Light Intensity(independent variable)500 μmol/m²/s
CO₂ Concentration400 ppm
Temperature25.0 °C
Measurement Noise0.5 σ

Results

Net O2 Rate=?  μmol/m2/s\text{Net } O_2 \text{ Rate} = ? \; \mu\text{mol/m}^2\text{/s}
Net O₂ Rate
— μmol/m²/s
Limiting Factor
Co-limited
Light
500 μmol/m²/s
CO₂
400 ppm
Temperature
25.0 °C

Light Response Curve

Theoretical curveRecorded dataCurrentCompensation point

Data Table

(0 rows)
#TrialLight Intensity(μmol/m²/s)Net O₂ Rate(μmol/m²/s)Limiting Factor
0 / 500
0 / 500
0 / 500

Reference Guide

Light Reactions

Light reactions occur in the thylakoid membranes and convert light energy into chemical energy (ATP and NADPH). The overall equation is

2H2O+2NADP++3ADP+3Pi→lightO2+2NADPH+3ATP2\mathrm{H_2O} + 2\mathrm{NADP^+} + 3\mathrm{ADP} + 3\mathrm{P_i} \xrightarrow{\text{light}} \mathrm{O_2} + 2\mathrm{NADPH} + 3\mathrm{ATP}

The rate of O₂ evolution is directly proportional to light intensity at low intensities, following a Michaelis-Menten saturation curve.

v=Vmax⁡⋅IKm+Iv = \frac{V_{\max} \cdot I}{K_m + I}

Calvin Cycle

The Calvin cycle (light-independent reactions) fixes CO₂ into organic molecules using ATP and NADPH from the light reactions. RuBisCO catalyzes the key carbon fixation step.

3CO2+9ATP+6NADPH→G3P+9ADP+6NADP+3\mathrm{CO_2} + 9\mathrm{ATP} + 6\mathrm{NADPH} \rightarrow \mathrm{G3P} + 9\mathrm{ADP} + 6\mathrm{NADP^+}

When CO₂ is limiting, the Calvin cycle cannot consume ATP and NADPH fast enough, so increasing light beyond the CO₂-saturation point gives diminishing returns.

Limiting Factors

Blackman's law of limiting factors states that the rate of photosynthesis is determined by whichever factor is in shortest supply. The three main limiting factors are

  • Light intensity limits the rate of light reactions
  • CO₂ concentration limits the Calvin cycle
  • Temperature affects enzyme activity (especially RuBisCO)

The overall rate equals the minimum of individual factor-limited rates, weighted by temperature

vgross=min⁡(vlight,  vCO2)×f(T)v_{\text{gross}} = \min(v_{\text{light}},\; v_{\mathrm{CO_2}}) \times f(T)

Compensation Point

The light compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of cellular respiration, so net gas exchange is zero.

Net O2=Gross photosynthesis−Respiration=0\text{Net } O_2 = \text{Gross photosynthesis} - \text{Respiration} = 0

Below the compensation point, the plant consumes more O₂ than it produces. Above it, net O₂ evolution is positive. The compensation point shifts with temperature and CO₂ because these factors change both photosynthesis and respiration rates.