biology 101l laboratory 10: photosynthesis & the carbon ... · biology 101l – spring 2018...

Biology 101L Laboratory 10: Photosynthesis & the Carbon Cycle

(Exercises and parts of the manual are adapted from Keeley, S., Botany 101 Lab Manual, Kendall/Hunt Publishing Company 1999)

Student Learning Outcomes: (1) Gain knowledge about the importance of the carbon cycle (2) Gain knowledge about photosynthesis (3) Know how plants produce oxygen during Photosynthesis (4) Know how plants use carbon dioxide to produce glucose (sugar) (5) Understand how sugar can be stored in plants as carbohydrates such as starch or cellulose

I. Introduction Photosynthesis and the Carbon Cycle All organisms must take in energy, because they cannot make energy on their own. And, unlike resources such as nutrients, energy is not recycled among living organisms. Therefore, living things must constantly take in energy. The source of energy for most living organisms is the sun. Organisms can utilize the sun’s energy either directly as autotrophs, or indirectly as heterotrophs. Autotrophs (such as plants) take in carbon dioxide (CO2) and water (H20) as raw material and, with the energy provided by sunlight, assemble these into carbon-containing, sugar molecules, eventually releasing oxygen (O2) back into the atmosphere as a byproduct. Thus, the energy from the sun is converted into chemical energy in the carbon molecules. Some of the carbon molecules are used for metabolic function and growth, and others are used for storage. Autotrophic organisms include plants, phytoplankton, marine algae, and cyanobacteria. All other organisms are heterotrophs, that is, organisms that cannot produce their own carbohydrates (e.g., sugars, starches, and cellulose) and must feed on autotrophs. While energy cannot be recycled, carbon is recycled and we call this the carbon cycle. Through photosynthesis, the carbon from the atmosphere (CO2) is stored as carbohydrates in the plant, until it is released back into the atmosphere through respiration, decomposition, or combustion (Fig. 1).

Biology 101L – Spring 2018 Photosynthesis & the Carbon Cycle 2

Figure 1. The Carbon Cycle. The carbohydrates produced during photosynthesis can be stored as reserves of carbon. Such natural carbon reservoirs include not only forests and standing crops of all living plants, but also coal and fossil fuels, which are formed by natural processes, such as decomposition of buried, dead organisms. The balance of CO2 for the last 10,000 years, up until the Industrial Revolution, remained relatively constant. However, since the Industrial Revolution, more and more CO2 is being released into the atmosphere each year as a result of deforestation and the burning of wood, coal and other fossil fuels, and is contributing to the greenhouse effect.

Importance of Light and Chlorophyll in Photosynthesis Photosynthesis is a chemical process that an autotrophic organism uses to convert carbon dioxide (CO2) into carbohydrates (C6H12O6). The basic equation for photosynthesis is:

CO2 + H20

Light

Chlorophyll C6H12O6 + O2 +H20


Plant leaves contain special pigments (chlorophyll) that absorb light energy for use in photosynthesis. Photosynthesis can only happen in the presence of light energy and chlorophyll. These special pigments are stored in oval-shaped organelles called chloroplasts. Chloroplasts are numerous; between 20 and 40 can be found moving around within a single cell of a green plant. Chlorophyll a is the most important pigment that a plant cell uses to obtain light energy; it absorbs light in the red and blue wavelengths (Fig. 2). However, a typical plant cell also contains accessory pigments that assist with harvesting light at different wavelengths. Accessory pigments such as chlorophyll b, carotenes, and xanthophylls, can also be found in the chloroplast. Plants do not use the green wavelength of the visible spectrum of light, and thus this wavelength is reflected back to the atmosphere and that is why plants appear green. The byproduct of light harvesting is the release of oxygen (O2) into the atmosphere. Chloroplasts are able to move throughout the cell and this is called chloroplast streaming. Chloroplast streaming ensures that the chloroplasts are able to absorb maximum amounts of light energy to maintain cellular and plant functions.

Figure 2. Visible light spectrum. The visible light spectrum is a small portion of the electromagnetic spectrum. Energy is inversely proportional to wavelength, so that long wavelengths, such as radio waves carry less energy than short wavelengths such as gamma rays.


The Calvin Cycle Once the light energy has been harvested by chlorophyll, it is transferred into a chemical cycle, the Calvin-Benson Cycle, to synthesize carbohydrates (sugars) from CO2 (Fig. 3). CO2 is obtained through small pores (stomata, pl., stoma, s.) in the outer tissue (epidermis) of a plant (Fig. 4) during the day. However, as the plant receives CO2 through the stomata, it also loses water (H20) through the same pores. Thus, photosynthesis can also be thought of as a balance between the gain of carbon and loss of water. Once the CO2 is inside the plant, it travels through different types of cells (spongy and palisade parenchyma). The spongy parenchyma is formed of loosely packed cells that allow CO2, O2 and water vapor to diffuse into and out of the leaf (a.k.a. gas exchange). The palisade parenchyma is formed of columnar-shaped, closely packed cells that occur beneath the epidermis and is the principal site of photosynthesis. The Calvin- Benson Cycle and the majority of light harvesting occur in the palisade parenchyma cells. The carbohydrates produced from the Calvin-Benson Cycle can be stored as sugar or starch until the plant breaks them down to use for metabolic activities.

Figure 3. The inter-relation of the light reactions and the carbon reactions inside a chloroplast of a land plant. Chemical energy produced by photosynthesis is used to convert carbon dioxide to sugar in the Calvin-Benson cycle.


Figure 4. Leaf Anatomy. Leaf Anatomical Terms Epidermis – the covering of the leaf. This layer is found on the upper and lower surfaces of a leaf.

Cuticle – a waxy layer that is usually on top of the epidermis to prevent water loss.

Stomata – are specialized epidermal cells that open and close to allow for gas exchange to occur within the leaf. Stomata are typically kidney bean shaped and are regulated by guard cells that respond to changes in temperature, CO2, sunlight, and humidity. There are typically more stomata on the lower surface on the leaf than on the upper surface.

The Mesophyll – The cells in the body of the leaf. The two types of cells found in the mesophyll are the palisade and the spongy parenchyma.

Palisade parenchyma – these are columnar shaped closely packed cells that are below the upper epidermis and are the principal site of photosynthesis.

Spongy parenchyma – these are loosely packed cells that can be found below the palisade parenchyma and allow for gas and water vapor to diffuse into and out of the leaf.

Vascular Tissue – is composed of veins or pipes that run through the mesophyll tissue of the leaf. It is composed of both xylem and phloem cells. Xylem transports water in the plant, while phloem transports sugars and minerals throughout the plant.

Name:

Section: Name


II. Lab Exercise (10 pts.) For the following exercises work in groups according to your TA’s instructions.

A. Necessity of CO2

Carbon dioxide (CO2) is used by plants during photosynthesis to make sugar and starch. To demonstrate that this is true we use an indicator dye, phenol red, to evaluate changes in CO2.

1. Obtain a falcon tube and put 3 drops of phenol red in the tube. Note the color of the solution.

2. Using a straw, blow into the solution. When you breathe, you expel CO2 as a product of respiration. Thus, you will be blowing CO2 into the solution.

3. Place a piece of Elodea in the test tube into which you have blown CO2 and place it in a rack in the spot designated by your TA.

4. Check the tube in 45 minutes or so. What color is the solution now? Do plants use CO2 in photosynthesis? (1 pt)

B. Leaf Anatomy

Microscope

Today in lab we are going to use a microscope to observe plant cells. There are two types of light microscopes: the compound and the dissecting microscope. The dissecting microscope allows us to look at large objects that we could see with our naked eye. However, the compound microscope allows us to look into cells and this is the type of microscope that you will be using today in lab. Your TA will go over the general procedures on operating the compound microscope before you get started.

1. First, you need to prepare a slide to view plant stomata.

a. Take a leaf of Tradescantia discolor and gently blot it dry.

b. Paint a thin layer of clear nail polish on the bottom of the leaf (the purple side), over an area about the size of a small fingernail (1 cm x 1 cm).

c. Wait for the polish to dry completely, approximately 5 minutes.

d. Gently press a piece of clear tape over the polish and, leaving one end of the tape free as a handle, gently peel the tape and polish from the leaf.

e. Press the tape on to a glass slide. Now you have a mold of the surface of the leaf.

f. Return the leaf to the beaker of water.

Name:

Section: Name


2. Second, obtain a prepared slide of plant mesophyll tissue.3. Sketch with pencil the overall appearance of the structures visible in both slides in the space below. Label the structures that you see using Figure 4 in the introduction and the leaf anatomical terms. (4 pts)

Name:

Section: Name


C. Necessity of Chlorophyll Variegated leaves lack chlorophyll in areas that are white, pink, yellow, or red. If chlorophyll is needed for photosynthesis, then areas that don’t have chlorophyll will not produce sugar/starch. Remember that chlorophyll is green, so areas that are not green in the leaf will not be able to produce sugar/starch.

1. Obtain a variegated leaf of Coleus.

2. Draw the leaf indicating the green and non-green areas in the space below. (1 pt) 3. Kill the leaf by placing it in a beaker of boiling water for 30-60 seconds.

4. Remove the leaf from the water and place it into a beaker of boiling alcohol. Allow the leaf to boil until the color is gone, approximately 3-5 minutes.

5. Rinse the leaf in distilled water and place it in a petri dish. Cover the leaf with iodine solution (IKI) and allow it to sit until a dark coloration appears (5-15 minutes). The IKI stains for the presence of starch. The areas that have starch should turn blue/black when stained with the IKI solution.

6. Draw the distribution of the dark color (starch) on your leaf below and compare it with that of the colors in the unboiled leaf in your drawing above. Where is the starch? Explain using your drawings. (2 pts)

Name:

Section: Name


D. Necessity of Light

1. Obtain a leaf of Impatiens. We used paper stars to block the light from part of the leaf – be sure to select a leaf with stars on both sides of the leaf. These leaves are entirely green, so all the parts should function equally well in photosynthesis.

2. Kill the leaf by placing it in a beaker of boiling water for 30-60 seconds.

3. Remove the leaf from the water and place it into a beaker of boiling alcohol. Allow the leaf to boil until the color is gone.

4. Place the rinsed leaf in a petri dish and cover with iodine solution (IKI). Allow this to sit for a minute or so until a dark coloration appears. The IKI stains for the presence of starch. The areas that have starch should turn blue/black when stained with the IKI solution.

5. Where is the starch? Sketch below. (1 pt)

6. What is the explanation for this distribution? (1 pt)

7. Discard the leaf into the trashcan. Rinse the petri dish and set on drain rack.

Name:

Section: Name


III. Homework (30 pts.)

Answer the following questions, using your lab manual and any outside sources you wish. 1. Why are plants green? (2 pts)

2. What is the difference between autotrophs and heterotrophs? Why are photoautotrophs

the key to life on earth? (3 pts) 3. Chlorophyll a is the most abundant leaf pigment, but why are there other pigments? (1 pt)

4. Why are there relatively few variegated plants in nature? (1 pt)

Name:

Section: Name


5. A) Fill in the leaf section with the appropriate anatomical terms. (5 pts)

B) Then, write a paragraph about how photosynthesis happens in the leaf. In your answer, describe how gas exchanges occurs and the function of each of the different leaf parts. Use the back of this page if you need more room. (10 pts)

Name:

Section: Name


6. What compounds pass through the stomata? (1 pt) 7. What happens to all of the carbon stored in a forest when it is burned? (2 pts)

8. A) Carbon dioxide from the atmosphere is one of the raw materials needed to make

sugar (and eventually starch) during photosynthesis. Using what you know from the experiments in this lab, propose a hypothesis to explain what you would think would happen if you covered a leaf with Vaseline so that the stomata were not able to take in carbon dioxide. (2 pts) B) Suggest an experiment that you could do to test your hypothesis. (3 pts)

biology 101l laboratory 10: photosynthesis & the carbon ... · biology 101l – spring 2018...

Documents