Photosynthetic Pictures Are Worth More Than a Thousand Words
Type of Entry:
Type of Activity:
- hands-on activity
- inquiry lab
- group/cooperative learning
- Life Science
- Advanced /AP Biology
- Special Needs
Notes to Teacher:
Refer to articles by Julian Sacha, the first scientist thought to have developed this starch picture process. Photography manuals which describe fundamental film developing processes are also helpful for students to know. Other helpful reminders/tips are:
- Be sure your boiling alcohol set-up is in a well ventilated area.
- When hydrating your leaf following alcohol bleaching, gently apply water to the leaf and avoid potential splitting or breaking due to its brittle condition.
- Check each set-up to make sure the slide image is in focus or a poor starch picture will result.
Required of students:
Students must be able to accurately follow a lab protocol, and perform an open-ended investigation. When executing this lab, safety precautions involving boiling alcohol should be strictly followed.
Preparation time needed:
Organization and assembly of the materials and physical set-up for this lab take about 30 minutes once everything is gathered. This includes a minimum of six slide projectors. Much time is spent in positioning and focusing the image on the glass enclosed leaf. This is unavoidable and necessary.
Class time needed:
Procedures described in this activity assume implementation in a regular 45-50 minute class period. If using a block or modular schedule, you may want to set up and develop the starch pictures during the same 90 minute class period.
In ordinary photography, a print is obtained by illuminating a piece of photographic paper through a negative. A chemical reaction occurs where the paper is illuminated, and fine grains are deposited. A starch picture is made similarly. Based on early experiments by Julian Sacha a procedure using Geranium leaves as photographic paper was developed. If a leaf is illuminated through a photographic negative, starch is formed (by photosynthesis) only in illuminated areas. When starch grains are stained to make them visible (and chlorophyll is removed) a black or blue picture is seen against a white background.
Remember carbon dioxide, water, and light are need for photosynthesis to occur. Keep a Geranium plant in the dark for 48 hours (to free leaves from starch). Remove leaf, and place between two pieces of glass, along with a piece of black cloth soaked in 5% bicarbonate solution (carbon dioxide source). Position the petiole in a dish of water beneath your glass. Focus a slide image onto the glass/leaf set-up for 40 minutes. Follow this by bleaching out chlorophyll from leaf (boiling ethanol) and stain with iodine solution in potassium iodide. Wait for 10 minutes to develop, and a starch picture will appear. Just like magic.
What question does this activity help students to answer?
This activity provides an opportunity for students to observe and examine how carbon dioxide, water, and light produce glucose/starch through a process called photosynthesis. This process is validated through the production of starch picture images produced on a geranium leaf. The fundamental question answered by this activity is: "What actually occurs in plant leaves as a result of photosynthesis?"
Photosynthetic Pictures--Are More Than a Thousand Words
- Geranium Plant (mature with non-variegated leaves at least 3 inches x 3inches)
- Slide Projector, with carousel and selected slides (very contrasting and distinct)
- Glass Plates (3" x 3" in area)
- 5% Sodium Bicarbonate Solution
- Lugol's Solution
- Test Tube Clamps
- Rings and Screens
- Paper Clamps (1-2" in length)
- Watch Glasses or Small Glass Dishes (must hold approximately 8-10ml of water)
- Forceps and Tongs
- Hot Plates
- Large Glass Dishes (must easily accommodate 3"x3" leaf and 50ml of water)
- Water Bottles (optional)
People sometimes say that pictures "tell a thousand words." But can you tell me in fewer words what actually makes these pictures? Let me help you. We know that light can be captured by pigments in leaves, and that leaves use this light to manufacture glucose which is stored as starch. If plants are deprived of light for at least 48 hours, what should happen to the leaves? If we introduce light once again onto the leaves using a slide projector and slide, what do you think will happen? How do you think the slide image will compare to the starch picture it produces?
Problem: How does photosynthesis in plants produce starch pictures?
***Note: The above procedures will be carried out for the class that follows each class. However, you need to set up the first starch picture BEFORE next period! In order to execute both procedures, half of your team members must set up the apparatus with new leaves for the next class, and to also dismantle leaves from the apparatus according to following the next procedures:
- Select a leaf from a geranium plant which has been kept in the dark for at least 48 hours. Break off the leaf and its petiole carefully.
- Soak a piece of black cloth with a 5% solution of sodium bicarbonate and place this cloth and the leaf on the glass plate. Place the second glass plate over the top of the leaf and cloth, being careful to sandwich the materials between the two plates and to keep the leaf as flat as possible with the petiole (leaf stem) projecting well out of the "sandwich".
- Now place the "glass sandwich" in a clamp on a stand keeping the petiole projecting downward. Place the petiole in the watch glass so that the leaf is well supplied with water. Use small paper clamps to fasten the far side of the "glass sandwich."
- Select a photographic negative or slide with a simple, clear design. Contrasting images and background work best. Place it into the slide projector. Project this image on the leaf for approximately 40 minutes to one hour.
- Carefully drop the leaf into a solution of boiling alcohol (ensure adequate ventilation). Use tongs to hold the petiole, gently turn the leaf until it has lost all of its pigments. Boil GENTLY so as not to break the leaf. (change the alcohol)
- Place the white leaf in a large petri dish and use distilled water to GENTLY flood the leaf to remove the alcohol.
- Pipette iodine solution (10 drops at a time) onto the leaf and gently swirl the petri dish every few minutes until a starch picture is present (usually takes 10-15 minutes).
- Dry your leaves by laying them out on paper towels.
Design Question 1: Illustrate and describe below the scientific design of your starch picture- making apparatus.
Design Question 2: Design another lab procedure that could produce starch pictures, and describe it below.
Observations: Draw the slide image and the resulting starch picture below.
|Slide Image||Starch Picture
- Was your hypothesis confirmed or not confirmed according to your Observations drawing? Explain.
- Why is the geranium plant placed in the dark for 48 hours prior to lab?
- In your photosynthetic picture, what chemically occurs to make the images you see?
- Why are leaves bleached with alcohol before you develop them?
- Describe the similarities and differences between how camera photographs and starch photographs are made.
- What other possible ways can the starch picture process be applied or used?
Method of Assessment/Evaluation
Students will be assessed on their ability to:
- illustrate and describe their starch picture-making apparatus that results in starch pictures.
- design an alternative procedure that could produce starch pictures.
- generate logical answers to reflection questions from lab sheet.
- thoughtfully write a conclusion that includes improvements, possible errors and extensions of lab methods.
- Design lab to be open-ended investigation. Test variables such as water, light, or sodium bicarbonate concentration to determine their effects in starch pictures. Or have students determine differences in using various types of leaves in addition to geraniums.
- Ask students to develop alternative methods in transferring an image to leaf, other than slide projection.
- Ask teams to make a formal presentation to persuade their peers of the validity or limitations of their open-ended/alternative investigation.
- Assign students to interview the photography advisor/students, and submit a paper which examines both picture making-processes. There is a potential multi-disciplinary connection between these two courses.