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Overview
Overview
The ability of plants to produce oxygen and carbon chains via photosynthesis is central to most of Earth's food chains. In this lab students will observe the production of oxygen under different light conditions and consider how various pigments such as chlorophyll are part of this process.
Connections to Other Labs
Connections to Other Labs
Students will use a indicator reagent first introduced in the Environmental Chemistry Lab. This lab can also be coupled with the Functional Response lab to consider how bottom-up and top-down forces interact and impact communities or used in conjunction with the Competition lab to focus on plants.
Background reading on theory of photosynthesis
Background reading on theory of photosynthesis
A general overview of Net Primary Productivity (NPP)
A general overview of the process of photosynthesis
Objectives
Objectives
Students should be able to
describe the process of photosynthesis (reactants and products) and their importance to food chains
use indicator reagents to identify the location of starch in leaves
Introduction
Introduction
Organisms that can produce their own organic material from inorganic sources are known as autotrophs. Organic material produced by photoautotrophs (green plants, algae, plankton, and some microbes that can harness the light of the sun to power chemical reactions via the process of photosynthesis) form the base of most food chains. Heterotrophs, such as herbivores and carnivores cannot synthesize their own food and depend on autotrophs for their energy requirements. Most animals derive their energy directly or indirectly from green plants.
The major inputs for photosynthesis are CO2 and H2O which undergo a series of reactions in the presence of light and get converted into sugars such as glucose. Along with sugars, oxygen is released as a byproduct. Refer to the balanced equation summarizing photosynthesis, provided below.
Photosynthesis is performed inside leaves, within specific organelles called chloroplasts. These organelles appear green due to the presence of the chlorophyll pigment, which forms the reaction centers for photosynthesis.
After being produced, sugars are often converted to starch molecules and stored in the leaves of plants. Starch is a long chain carbohydrate polymer made up of several glucose (sugar) molecules joined together. We can test for the presence of starch in a substance by treating it with iodine. Starch reacts with iodine and produces a deep blue-black or brown color.
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Although we usually associate green plants with chlorophyll, it is important to note that a plant leaf typically contains several other pigments that are generally masked by the green pigment. When the chlorophyll begins to break down, the other pigments become more visible. This is why leaves "change" color in the fall, as deciduous trees break down the chlorophyll in their leaves to reduce energy loss.
Carotenoids – orange color pigment
Xanthophyll – Yellow color pigment
Anthocyanins – red color pigment
These pigments also give the unique color to flower petals.
In this exercise
1. We will learn about the effects of carbon dioxide and light, on the rate of photosynthesis in a submerged plant - Elodea.
We will vary the amount of CO2 and the kind of light received by a plant, analyze their effect of the rate of photosynthesis performed by the plant.
2. We will also extract the photosynthetic pigments of a multicolored leaf taken from a common house plant – the Coleus.
We will perform a test on this leaf for the presence of starch and look for any relationship between the location of the green pigment and the presence of starch.
Methods
Methods
Part 1: Effect of light and carbon dioxide on photosynthesis:
Part 1: Effect of light and carbon dioxide on photosynthesis:
Materials required:
Elodea plant stems (4)
Glass test tubes (4)
Conical flasks (4)
Glass marker
Labeling tape
Tap water
Water enriched with sodium bicarbonate (baking soda)
Grow light
A dark cabinet
Small flexible ruler
Procedure
1. Label 4 conical flasks based on column 1 of Table 1.
2. Measure and cut out an Elodea stem to roughly 3/4th the length of the test tubes.
3. Weigh this Elodea stem to the closest gram and note it in the table in row 1 (under weight of Elodea).
4. Insert the stem in the test tube and fill it with tap water up to the rim.
5. Carefully place an inverted conical flask (labeled tap water and grow light) on the test tube so that the rim of the tube completely touches the base of the flask.
6. In one quick motion invert this arrangement, making sure the seal between the rim of the tube and the base of the flask is not broken.
7. There should be no water escaping the test tube and you should notice a very small air bubble at the top of the inverted test tube (The air bubble should not be bigger than the curved region of the test tube).
8. Using a glass marker, make a small mark on the test tube for the initial/starting level of the air bubble.
9. Place this arrangement under the grow light for 1 hour.
10. Repeat steps 2 to 7 and place this arrangement in a dark cabinet for 1 hour.
11. Repeat steps 2 to 7, two more times, however, use sodium bicarbonate enriched water instead of tap water. One of these arrangements is placed under grow light and the other one in the dark cabinet.
12. After 40 minutes (or time determined by instructor, possibly next class period), carefully mark the size of the air bubble on the test tubes again. WHILE YOU ARE WAITING COMPLETE THE 2ND PORTION OF THE LAB.
13. Using a small flexible ruler, measure the change in size of the air bubbles in mm.
14. For this exercise, every mm increase in bubble size can be used as a direct measure of oxygen production (in cubic centimeter, or cc) in the given environmental conditions.
15. Note the amount of oxygen produced in the table for all 4 conditions.
16. To standardize oxygen produced across all conditions, divide the cubic centimeters of oxygen produced by the weight of the Elodea and divide again by the number of hours elapsed during the exercise (1 hour). Note it in the table.
This procedure allows us to see how oxygen produced varies with a change in amount of CO2 and the kind of light provided to the plant.
Questions to answer at the end of the Part 1:
1. Which condition do you predict would result in the maximum amount of oxygen produced? WHY?
2. How else might you have measure oxygen produced in this experiment? (Hint: Look at your setup carefully)
Part 2: Examining the photosynthetic pigment and its effect on sugar production
Part 2: Examining the photosynthetic pigment and its effect on sugar production
Material required:
1. Leaves from a Coleus plant (with different colors) that has been grown under light
2. Glass beakers (2)
3. Pair of tongs (1)
4. Tap water
5. Ethanol
6. Petri dish (2)
7. Hot plate
8. Oven mittens (1 per group)
9. Iodine solution with a dropper
10. Eye goggles
Procedure:
This experiment must be performed with eye protection.
Switch on the hot plate and boil some tap water in a glass beaker. Once water has boiled, turn off the hot plate.
Take a leaf from the Coleus plant that has been grown under light.
Sketch out the leaf to show the location of the different pigments on it.
Using a pair of tongs, insert the leaf into the hot water. This "kills" the leaf and stops chemical reactions.
Remove the leaf from the water after 2 minutes and spread it out on a petri dish. Be gentle as the leaf might tear easily.
Make note of any changes to the appearance of the leaf and the water.
Remove the water beaker from the hot plate. Place another beaker with about 30 ml of ethanol on the hot plate. Keep an eye on the beaker for vigorous bubbling of ethanol. If you notice bubbling, carefully remove the beaker from the hot plate using mittens. Place it again on the hot surface once the bubbles subside.
Put the same leaf in the boiling ethanol beaker. Ethanol extracts chlorophyll from the plant.
After 2 minutes, carefully remove the leaf from the ethanol and spread it out gently on a petri dish.
Another group member should switch off the hot plate and place the ethanol beaker back on the table.
Make note of any changes in the appearance of the leaf and the ethanol.
Cover this leaf with a few drops of iodine solution to identify where starch is located.
Note the change to the appearance of the leaf.
Questions to answer at the end of Part 2
Questions to answer at the end of Part 2
Which pigments were extracted in boiling water?
Which pigments were extracted in boiling ethanol?
Where was starch present in the leaf? How did you arrive at this conclusion?
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