~~Getting Connected
Tell Students: Every cook needs ingredients to make a meal. Consider a simple sandwich: cheese, tomato, and all the ingredients that go into the bread: flour, water, salt and yeast. Oh, and don’t forget the pickle. But if you’re a plant, you’ll make your meal through photosynthesis—and all you’ll need is a little light, water, and carbon dioxide.
Sharing the Wealth of Knowledge
The Big Idea
Explain to Students: We have all held leaves, driven miles to see their fall colors, eaten them, raked them, sought their shade. Since they are everywhere, it’s easy to take them for granted.
But even when we do, they continue in their one occupation: turning light into life. When rays of sunlight strike green leaves, wavelengths in the green spectrum bounce back toward our eyes. The rest—the reds, blues, indigos, and violets—are trapped. A leaf is filled with chambers illuminated by gathered light. In these glowing rooms photons bump around, and the leaf captures their energy, turning it into the sugar from which plants, animals, and civilizations are built.
Chloroplasts, fed by sun, water, carbon dioxide, and nutrients, do the leaf’s work. They evolved about 1.6 billion years ago when one cell, incapable of using the sun’s energy, engulfed another cell—a cyanobacterium—that could. That cyanobacterium became the ancestor of every living chloroplast. Without their chloroplasts plants would be left like the rest of us, to eat what they find. Instead they hold out their green palms and catch light. If there is magic in the world, surely this is it: the descendants of tiny creatures in leaves, capable of ingesting the sun.
Plants use the process of photosynthesis to convert light energy into glucose, a carbohydrate that plant cells use for food. Photosynthesis occurs in the chloroplasts of plant cells. For photosynthesis to occur, the energy from the sun needs to be trapped in special molecules. One such molecule is chlorophyll, which mostly absorbs light in the red and blue ranges of the visible light spectrum. Green light is largely reflected, which is why most plants are green.
Photosynthesis can be divided into two parts, light dependent reactions and light independent reactions. During the light dependent reactions, sunlight excites electrons in chlorophyll and these high-energy electrons are used to make ATP. The high-energy electrons do not return to chlorophyll, but water can donate electrons to replace those lost. The process of losing electrons converts water to oxygen and H+. In the light independent reactions (also called the Calvin cycle), the ATP formed from sunlight provides the energy to make glucose from CO2.
The overall equation for photosynthesis can be written as:
6CO2 + 6H2O + light energy C6H12O6 + 6O2
Ask Students: Look at the equation. Can you think of a way to measure the rate of photosynthesis?
Making it Happen
Activity 1: Floating Leaf Disk Assay
Materials: (2 student team)
- Plastic cups, 8 oz.
- Bag of baking soda (1 tsp per student team)
- 16 straws
- 16 10-mL syringes
- Spinach leaves, 2 per team
- 2 containers of water (at least 500 mL each)
- Lamps with 60 watt bulb
- 16 Sharpies, Black
- 16 stop watches
Procedures:
1. Explain: The leaf is composed of layers of cells. The spongy mesophyll layer is normally infused with gases, oxygen and carbon dioxide. Leaves will normally float in water because of these gases. If you draw the gases out from the spaces, then the leaves will sink because they become denser than water.
The leaf disks will be put into a solution of baking soda (sodium bicarbonate, NaHCO3) and detergent. The baking soda is a source of carbon, and the detergent acts as a wetting agent that makes it easier for the solution to infiltrate the leaf disk.
NaHCO3 +H+ → Na + H2O + CO2
By using a syringe to create a vacuum, the gases in the intercellular air spaces will be pulled out of the leaf disk. The gases are replaced by the baking soda/detergent solution. This makes the leaf disks denser than the solution and they should sink. As the cells in the leaf disks photosynthesize, the liquid inside the leaf will be replaced by oxygen gas. The gas will increase the buoyancy of the disk and it will float to the surface. Oxygen and carbon dioxide are exchanged through openings in the leaf called stoma.
While this is going on, the leaf is also carrying out cellular respiration. This respiration will consume the oxygen that has accumulated and possibly cause the plant disks to sink. The measurement tool that can be used to observe these counteracting processes is the floating (or sinking) of the plant disks. In other words, the buoyancy of the leaf disks is actually an indirect measurement of the net rate of photosynthesis occurring in the leaf tissue; in other words, we are measuring the energetic profit made by the plant.
Today, we will be using spinach leaf disks to assay the rate of photosynthesis under various light conditions.
2. Collaborative Instructor: Pass out FLDA Steps Laminated Card (see below)
3. Tell Students: In order to do the assay, you will need to make an alternative carbon source for our spinach leaf disks:
• Make a solution of sodium bicarbonate by mixing 300 ml of water to a pinch of baking soda (about 100 ml to 1g)
• Make a diluted solution of liquid detergent in a small beaker by adding 3 drops of dish soap to 70 ml of water. Do not make suds!
• Add one drop of this dilute soap solution to your 300 ml bicarbonate solution. Swirl gently to avoid making suds.
4. Tell Students: We will now prepare our syringes:
A. Collect spinach leaf disks by punching holes in the leaf (try to get them between the veins) using your straws. You will need 20 leaf circles.
B. Place 10 leaf disks into the syringe and pull in a small volume of the bicarbonate and soap solution.
C. Replace the plunger and push out most of the air, but do not crush your leaves.
D. Create a vacuum by covering the tip of the syringe with your finger. Draw back on the plunger.
E. Release the vacuum so that the solution will enter the disks. It may take a few times to get the disks to sink. You may need to gently tap the syringe to dislodge discs from the sides.
F. Once they have sunk, you can put them back into the sodium bicarbonate solution and expose the disks to light. Start a timer and record how many of the disks are floating at 1 minute intervals. (See attached Data Table)
• Troubleshooting:
Gently swirl solution to dislodge disks which may become stuck at the bottom.
If no discs float within 5 minutes, add a couple more drops from your soap solution
Place your beaker as close to the light as possible!
5. Repeat your set-up from above, but this time do not place baking soda in the beaker. This is your control. Place another set of sunken disks into this solution and record data on the table. (Attached to lesson plan)
6. Both the experimental group and the control should run until all the discs are floating.
7. Analyzing Data
Explain to Students: To make comparisons between experiments, a standard point of reference is needed. Repeated testing of this procedure has shown that the point at which 50% of the disks are floating (the median or ET50) is a reliable and repeatable point of reference. In this case, the disks floating are counted at the end of each time interval. The median is chosen over the mean as the summary statistic. The median will generally provide a better estimate of the central tendency of the data because, on occasion, a disk fails to rise or takes a very long time to do so. A term coined by G. L Steucek and R. J Hill (1985) for this relationship is ET50, the estimated time for 50% of the disks to rise. That is, rate is a change in a variable over time. The time required for 50% of the leaf disks to float is represented as Effective Time = ET50.
Graph your data for the experimental group. Determine the ET50 for your leaf disks and determine the ET50 for your data.
Wrapping It Up
Ask Students:
• Which syringe serves as a control? (The syringe with water and no light source will serve as the control)
• What variables are tested in this experiment? (Light intensity and carbon dioxide levels)
• Compare the test groups. Which syringe had the most leaf disks floating after 20 minutes?
(Results may vary. The syringe with a baking soda solution and bright light should have the most leaf disks floating after 20 minutes)
• Were there any syringes without floating disks? (Results may vary. The syringes without a light source should not have floating leaf disks)
• How do floating disks correspond to the rate of photosynthesis? (As photosynthesis occurs, oxygen is released. An accumulation of oxygen in the leaf disks will cause them to float. As photosynthetic rates increase, more oxygen will be released. The rate of photosynthesis should correlate with the intensity of the light provided)
• According to your data, does light intensity affect the rate of photosynthesis? (Results may vary. As photosynthetic rates increase, more oxygen will be released. The rate of photosynthesis should correlate with the intensity of light provided)
• How did the baking soda solution affect photosynthetic rates? (Results may vary. Baking soda will release carbon dioxide when dissolved in water. Carbon dioxide is needed for photosynthesis to occur; thus an increase in CO2 may increase the rate of photosynthesis)
• Why is photosynthesis a light-dependent reaction? (Chlorophyll molecules absorb light energy and begin the process whereby carbon dioxide is fixed into more complex molecules. This reaction will occur only if light energy is available)
Cleanup
• Have children dump trash in trash cans.
• Have children replace non-trash items into kits. Wash as needed between classes.
