Name: Date: Period: Photosynthesis Lab #1: Leaf Structure and Function Purpose: The purpose of this lab is to explore the structure of a leaf, specifically the stomata and guard cells. Once we have an understanding of the structure of a leaf, it will be easier to understand the process of photosynthesis and its importance. Introduction: In plants, most photosynthesis takes place in the leaf. Plants and animals both have a layer of tissue called the epidermal layer. Most stomata are on the lower epidermis of the leaves on plants (bottom of the leaf). Therefore, we will be looking at the lower epidermis to locate stomata and guard cells. Stomata are the special pores that plants contain that allow passage of material. The word stomata means mouth in Greek because they allow communication between the internal and external environments of the plant. The stomata pores are surrounded on both sides by jellybean shaped cells called guard cells. Stomata are opened and closed by guard cells. Stomata need to be open for photosynthesis, but plants also lose a lot of water through open stomata. Guard cells also contain many chloroplasts, which carry out photosynthesis. The main function of stomata is to allow gases such as carbon dioxide, water vapor and oxygen to move rapidly into and out of the leaf. During the day, the stomata pores are open allowing gaseous carbon dioxide to enter the leaves and oxygen created during photosynthesis to exit the leaves. Water vapor is also released through these pores through a process called transpiration. The number of stomata on the epidermal surface can tell you a lot about a plant. Usually, a high concentration of stomata indicates fast growth and wet climate. Lower concentrations of stomata indicate lower rates of photosynthesis and growth or adaptations for dry weather. Stomata can also be indicators of past carbon dioxide levels in the air. In this lab, we are going to view the stomata of three different plant species and make conclusions about the data we gather.
Pre-Lab: Read Chapter 23 (p. 681-684) In this pre-lab, you will complete leaf and chloroplast structure diagrams and define important terms that you need to be familiar with for fully understanding this lab and photosynthesis. 1) Photosynthesis takes place predominantly in the leaves of plants. Complete the leaf structure diagram below using your textbook (PG 681). Use the word bank below. Vein Spongy Mesophyll cell Guard Cell Cuticle Stoma Upper epidermis Lower epidermis Palisade Mesophyll cell Xylem Phloem 2) Define the following terms: Stoma: Guard Cells: Epidermis:
3) Stomata contain guard cells which regulate the opening and closing of the stomata. Guard cells also contain chloroplasts. As we learned in Outcome D, chloroplasts are the organelle in which photosynthesis takes place. Complete the chloroplast structure diagram below using the word bank and your textbook (PG 231). Inner Membrane Thylakoid Stroma Outer Membrane Granum 4) Define the following terms: Chloroplast: Thylakoid: Granum: Stroma: Chlorophyll:
Materials: To complete this lab, the following materials will be needed: 3 leaves from 3 different plant species (3 leaves per lab group) Compound Light Microscope (1 per lab group) 3 microscope slides (3 per lab group) 1 bottle of clear nail polish (1 per lab station) Transparent tape (1 per lab station) Lab: Procedure: 1) Gather all materials or make sure all materials are present for your lab group. 2) Obtain 3 leaves from 3 different plants. 3) Paint a thick patch (at least one square centimeter) of clear nail polish on the underside of the leaf surface being studied. 4) Allow the nail polish to dry completely. 5) Tape a piece of clear tape to the dried nail polish patch. 6) Gently peel the nail polish patch from the leaf by pulling on a corner of the tape and peeling the fingernail polish off the leaf. This is the leaf impression you will examine. 7) Tape your peeled impression to a very clean microscope slide. Use scissors to trim away any excess tape. Label the slide with plant name. 8) Examine the leaf impression under a light microscope at 400X (high power). 9) Search for areas where there are numerous stomata and where there is no dirt, thumb prints, damaged areas, or large leaf veins. Draw the leaf surface with stomata. 10) Count all of the stomata in one microscope field (area that is visible through the microscope). 11) Record the number in your data table. 12) Repeat counts for at least three other distinct microscope fields. 13) Record all counts in your data table. 14) Determine an average number of stomata per microscope field for each leaf. Also record this in the data table. 15) Follow procedures 3 to 15 with the other 2 leaves.
Data: To complete your data section, fill in the data table below AND draw your observations. Data Table 1: Name of Leaf # of Stomata in field 1 # of Stomata in field 2 # of Stomata in field 3 Average Stomata in field Leaf 1 Leaf 2 Leaf 3 Microscope Observations: Leaf 1: Field of View Leaf 2: Field of View Leaf 3: Field of View 400X 400X 400X
Post-Lab: After completing this lab and gathering data, answer the following post-lab questions. Remember, answers must be written in complete sentences. 1) Which leaf had the most stomata? Why do you think this was so? 2) Explain, in detail, how guard cells open and close stomata. (p. 682) 3) At what time of the day would stomata be closed and why? 4) Why does the lower epidermis have more stomata than the upper epidermis of a leaf? 5) What two gases move in and out of the leaf stomata? 6) What does a larger number of leaf stomata indicate about the growing climate of that plant? 7) Describe one way this lab has enhanced your knowledge of leaf structure and how this aids your understanding of the process of photosynthesis.