Lab 5. Energy Capture and Conversion (Photosynthesis and Respiration)

The primary source of all energy on earth is the sun. The energy that you use to walk to class or read this page came, initially, from the sun. Plants capture some of that energy in the process of photosynthesis and store it as carbohydrates. Animals obtain the energy either by eating the plants directly and releasing the energy stored there, or by eating other animals that themselves ate the plants etc. In today’s lab, you will investigate both the way that plants store light energy (photosynthesis), and the ways that organisms (including plants) release the stored energy for use (respiration and fermentation).

Photosynthesis Plants store the energy of sunlight as carbohydrates (glucose). They do this by taking carbon dioxide (CO2) from the air and combining it with water (H2O) and storing sunlight energy. The chemical formula is:

6CO2 + 6H2O + light  C6H12O6 (glucose) + 6O2

Notice that, in the process of building glucose, oxygen (O2) is given off. This is where the free oxygen on earth comes from. Prior to the evolution of photosynthesis, there was no free oxygen in our atmosphere.

Experiment 1. In this experiment, you will investigate photosynthesis by an aquatic plant, Elodea. Elodea leaves, when placed into water in a light environment, will remove CO2 from the water and produce glucose by photosynthesis. When CO2 dissolves in water, it combines with the water to form carbonic acid (H2CO3), making the water acidic (pH < 7). As carbon dioxide is removed from the water (say, by an actively-photosynthesizing Elodea plant), the amount of carbonic acid decreases, and the water eventually becomes slightly basic (pH > 7). We will use an indicator dye (phenolthalein - PT) to determine when the water that the Elodea is in is acidic, and when it is basic. PT is clear in acidic solutions, and pink in basic solutions.

Procedure – Work in pairs. Fill two test tubes ½ full with water from the beaker provided. This beaker already has PT in it, and has been adjusted to be slightly acidic by blowing into it through a straw. The CO2 has combined with water forming carbonic acid. Place a short stalk of Elodea (about 6-8cm) in one test tube (the other is the control), and cover both tubes with parafilm. Record the colors of your two tubes on the table on the next page (they should both be clear). Place the tubes under the light. Check for color changes every 10 minutes until a change is noted (it should only be in the tube with the Elodea, right?). Record the colors on the table each time you check them. Table 1. Photosynthesis Experiment Results Tube Initial Color Color at 10 Color at 20 Color at 30 Color at 40 Color at 60 min. min. min. min. min.

Control Elodea

Why did the color change? What was the Elodea doing to make the water less acidic?

Respiration When most eukaryotic organisms (plants, animals, fungi, protists) liberate the energy stored in carbohydrates (i.e. glucose), they do so through the process of aerobic cellular respiration. Essentially, this chemical process is the reverse of photosynthesis.

C6H12O6 (glucose) + 6O2 6H2O + 6CO2 + energy

It is because of your need for oxygen to carry out this chemical reaction that you perform organismal respiration (breathing). This is also why when you breathe out, your breath is high in CO2. Your lungs exchange the oxygen in the air, for the waste product (CO2) from cellular respiration.

Experiment 2. In this experiment, you will measure CO2 production by goldfish. By doing this, you will essentially be measuring the rate of respiration or metabolism of these fish. You will measure the CO2 production by measuring the acidity of the water in which the goldfish are swimming. Greater levels of acidity mean greater amounts of carbonic acid in the water, and that means that the goldfish have produced greater amounts of CO2.

Procedure: Each lab group will use two small beakers (100ml) and one 50 ml graduated cylinder. Fill each of the beakers with 75 ml of water directly from the fish tank. Place exactly 25 ml of water from the tank into the graduated cylinder. Get a goldfish from the tank and put it into the graduated cylinder (being careful not to introduce any extra water). Record the volume of the goldfish in Table 2 below (fish volume = final volume – 25 ml). Pour the water back into the tank through a net (so that you ‘catch’ the fish). Place the goldfish into one of the two beakers (the other will be your control). Cover both beakers with plastic wrap and allow the fish to respire for 25 minutes. After 25 minutes, remove the fish (just reach in with your fingers to get it) and return it to the tank.

Add one drop of phenolphthalein to each beaker. Phenolphthalein is clear in acidic solution and red in basic solution, so the water should remain clear because it is acidic (full of carbonic acid, right??) Make note of the level of NaOH in the burette. Use the burette at your table to dispense NaOH (2.5mM) drop by drop to the contents of the fish beaker. Count the number of drops added. Be sure to swirl the beaker between drops to thoroughly mix the contents. When the beaker contents turn pink, stop, and record the number of milliliters of NaOH used.

Repeat this procedure for the control beaker.

To calculate the Relative Respiration Rate for the fish, subtract the number of ml required to turn the control beaker pink from the number required to turn the fish beaker pink. Record this value in Table 2.

To calculate the Respiration Rate per ml of organism, divide the relative respiration rate by the volume of the fish. Record this value in Table 2.

Collect the data from the other lab tables, and include them in Table 2.

Table 2. Aerobic Respiratory CO2 Production by Goldfish Sample Fish Volume ml of NaOH Relative Resp. Relative Resp. (ml) (ml) Rate per fish Rate per ml Fish 1 Fish2 Fish 3 Fish 4 Fish 5 Fish 6 Total (Fish 1-6) Control 0 xxxx xxxx

Was there a difference among the respiration rates for different fish? Can you suggest a reason (a testable hypothesis) why?

Was there a correlation between volume and relative respiration rate per fish?

Was there a correlation between volume and relative respiration rate per ml?