Question

Get results from experiment or make them up realistically.

Exercise #2: Photosynthesis

Introduction

In the process of photosynthesis, plants capture a small fraction of the sun’s energy and store it on the chemical bonds of carbohydrates.

chloroplast

6CO2 + 6 H2O → C6H12O6 + 6O2

light

This simple equation conceals a multitude of complex processes and disguises the enormity of the importance of photosynthesis. Virtually all the energy used by all organisms on earth (the exception being a few chemosynthetic organisms) is made available by photosynthesis, a process in which light energy from the sun is absorbed and converted into chemical energy. Plant pigments are responsible for selective absorption of the sun’s energy. In this lab, we will determine which wavelengths of light are effective in causing photosynthesis.

Purpose

To become familiar with the process of photosynthesis by measuring the effects of a light dependent reaction under different visible light conditions.

Objectives

Define pigment. Explain the meaning of absorption and transmission of light by pigments.

Explain how we are measuring the occurrence of photosynthesis based on the equation of a light dependent reaction.

Discuss effect of light quality on photosynthetic activity.

Task 1: Exploring the Action Spectrum of Visible Light for Photosynthesis

The visible light spectrum, or white light, may be broken down into bands of colors which correspond to specific wavelengths (measured in nanometers) as shown below.

emspectrum

In this experiment you will vary the quality (color or wavelength) of light to determine which wavelengths are most effective for photosynthesis. You will determine the rate of photosynthetic activity by indirectly measuring the products, in this case O2. The light dependent reactions are summarized in the equation:

light

H2O + ADP + NADP + → O2 + ATP + NADPH

takes place in chloroplast

In this equation, light energy is used to split water. Some energy is captured in ATP and NADPH, and O2 is released as a by-product.

In this experiment we will cut spinach leaf discs of equal area. Leaf tissue is riddled with gas-filled intercellular spaces. By putting the discs in a NaHCO3 (sodium bicarbonate) solution and subjecting them to a vacuum, we will replace the gases in these spaces with NaHCO3. Replacement of gas by liquid will cause the discs to sink to the bottom of the flask. As the light reactions of photosynthesis proceed, O2 is evolved and diffuses into the intercellular spaces. When enough O2 accumulates, each leaf discs will regain its buoyancy and turn on edge or float to the surface.

Basis for leaf disk

Our objective in the experiment is to determine which wavelength of light produces the most photosynthesis. We will put the leaf discs in NaHCO3 solution in plastic cups covered by filters, which transmit only certain wavelengths. After 15 minutes, we will count the number of discs floating in each dish.

You will need a lamp to carry out the experiment. Do the light treatments sequentially (i.e. white light, then blue, etc.), using a different set of leaf discs each time. Keep discs in the dark until use.

Materials:

Sodium bicarbonate (Baking soda)

Liquid soap

Plastic syringe (10 cc or larger)—remove needle

Spinach leaves

Hole punch or straw parts

Plastic cups

Clear and Colored Cellophane or filters (red, blue, green, and black)

Timer

Materials image

Procedure:

Note: It is best to do these experiments in subdued light.

Because the filters absorb different amounts of light, adjust the lamp – filter distance to make all light intensities equal. BE sure the lamp to the correct distance for each treatment.

No Filter (white light)	27.5cm
Blue filter	23.5cm
Red filter	26.5cm
Green filter	27cm
Dark filter	In Drawer

Prepare 300 ml of bicarbonate solution for each trial.

Bicarb photo

The bicarbonate serves as an alternate dissolved source of carbon dioxide for photosynthesis. Prepare a 0.2% solution. (This is not very much it is only about 1/8 of a teaspoon of baking soda in 300 ml of water.)

Add 1 drop of dilute liquid soap to this solution. The soap wets the hydrophobic surface of the leaf allowing the solution to be drawn into the leaf. It’s difficult to quantify this since liquid soaps vary in concentration. Avoid suds. If your solution generates suds then dilute it with more bicarbonate solution.

soap

Cut 10 or more uniform leaf disks for each trial.

Leaf Disks

Single hole punches work well for this but large plastic straws will work as well.

Avoid major veins.

Infiltrate the leaf disks with sodium bicarbonate solution.

Remove the piston or plunger and place the leaf disks into the syringe barrel. Replace the plunger being careful not to crush the leaf disks. Push on the plunger until only a small volume of air and leaf disk remain in the barrel (< 10%).

Disks in syringe

Pull a small volume of sodium bicarbonate solution into the syringe. Tap the syringe to suspend the leaf disks in the solution.

Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold this vacuum for about 10 seconds. While holding the vacuum, swirl the leaf disks to suspend them in the solution. Let off the vacuum. The bicarbonate solution will infiltrate the air spaces in the leaf causing the disks to sink. You will probably have to repeat this procedure 2-3 times in order to get the disks to sink. If you have difficulty getting your disks to sink after about 3 evacuations, it is usually because there is not enough soap in the solution. Add a few more drops of soap.

Evacuating leaf disks

Sunk

Pour the disks and solution into a clear plastic cup. Cover the top with clear cellophane or filter. Add bicarbonate solution to a depth of about 3 centimeters.  Shallower depths work just as well.

Start

Place under the light source and start the timer. After 15 minutes, count the number of discs floating in the dish.

Repeat the procedure steps, using a different color of cellophane/ filter each time. Use the same depth for each trial.  

Rising

Predict, in order, which filters will result in the greatest number of disks floating after 15 minutes. (1 pt.)

Record your data in the table below. (6 pts.)

Spinach Disks	Clear Filter (white light)	Blue filter	Red filter	Green filter	Dark filter
Number
Percent

Did your data support your prediction?

Construct a graph that includes your results below. (3 pts)

Answer 1