Detecting Photosynthesis- Analyzing Other Scientists' Data
Grade level(s):Grade 5, Grade 6, Grade 7, Grade 8, Grade 9, Grade 10, Grade 11, Grade 12
Using someone else's data to think critically about what a plant needs in order to photosynthesize.
carbon dioxide, oxygen, photosynthesis, starch, glucose, evidence
What you need:
Leaf print outs (attached as pdf or available in SEP's Resource Center in K274 "Detecting Photosynthesis" binder)
Photosynthesis and other overheads
Pairs for 1st part, then use a jigsaw for small group discussion (if have scientist or other partners to help with this) or have a large group discussion facilitated by the teacher. Could also have a series of centers, with one being for discussion of the task.
45 to 60 minutes total
15 minutes: introduction (or longer depending how much info you provide)
15 minutes: students in pairs analyzing their leaves
15 to 30 minutes: students sharing results, implications for photosynthesis, questions they have
Students will analyze the results of another scientist's experiment by examining leaves that have been exposed to different treatments, and draw conclusions about the process of photosynthesis.
Students will experience analyzing someone else's results, a key part of and regular occurrence in the scientific community.
Students will think critically about the results and what the results provide evidence for.
Students will understand that light and air are necessary for photosynthesis.
• In the late 1600’s, a physician named Jan Baptista van Helmont wanted to know how plants increased in size. What was the food that they ate? He performed an experiment to determine where plants get their mass. He grew a willow tree and measured the amount of soil, the weight of the tree and the water he added. After five years the plant had gained about 164 pounds. Since the amount of soil was basically the same as it had been when he started his experiment, he deduced that the tree's weight gain had come from water. Since it had received nothing but water and the soil weighed practically the same as at the beginning, he argued that the increased weight of wood, bark and roots had been formed from water alone.
• Then in 1772, Joseph Priestly demonstrated that green plants can replenish stale, or oxygen-poor, air so that it can again support a flame (a candle was extinguished in the “stale” air) and a living thing (a mouse, I believe, in this case). While a mouse in a closed system would die, one in a closed system with a plant would live.
• Inspired by Priestly, Jan Ingenhousz showed that only the green parts of plants can revitalize stale air and that they do so only in the presence of sunlight.
• Then that the increase in weight of a plant was due to both the intake of carbon dioxide (CO2 ) and of water
• In 1845 it was proposed that plants take in light energy and convert it to chemical energy that is stored in compounds that make up more than 90% of all plant substance.
Overheads and handouts (pictures of leaves, task cards) need to be printed out.
Lesson Implementation / Outline
Ask: "What is food? What do you think of when someone says 'food'?"- hear from students...
Food=useful substances that an organism needs to maintain life. [write on board]
Think about where your own food comes from. Think about what you had for breakfast this morning (or if you didn't eat breakfast, what you had for dinner last night). In your notebooks start with two things you ate and trace back the path it took for that food to get to you.
What kinds of resources were required to bring your food to your table? [write responses on the board]
Now imagine that, instead of all the farms and trucks and stores and cars, you wake up in the morning, you go outside and you sink your toes into the soil to suck up water. Your arms reach out, your fingers spread to catch as much sunlight as possible. Carbon dioxide or CO2 from the air flows in through your pores. And from theses things alone, sunlight, water and carbon dioxide, you can make all the food that you need. [can act this out for students, raising arms up, etc. if desired. Could also have students do it with you.]
This is the wonder of plants.
So how does this work? (overview of photosynthesis - see attachment)
Now what does it look like chemically? - Walk through photosynthesis equation...these concepts are extremely complex and counter intuitive...share history of our scientific understanding of photosynthesis as appropriate (see background for teacher).
Today we are going to be looking at photosynthesis. Scientists look at other scientists' data all of the time. Today, you will look at other people's data and make conclusions.
Much of science is doing one’s own investigations, but another part of science is being able to analyze someone else’s results. And so today you will do that, beginning by looking at evidence of specimens that have been prepared and we want to share with you. This experiment was done to hopefully help you better understand the process of photosynthesis.
First, I want to talk some about starch. You have been introduced to the idea that plants photosynthesize and, in doing so, they produce glucose. The plant then puts many, many glucoses together to form starch.
Each ring is a glucose and a chain of them is starch [can draw on board, model with plastic links hooked together, etc]. So, plants photosynthesize and make glucose, they then put lots and lots of glucoses together to make starch, and they store the starch. Thus, in plants, presence of starch provides indirect evidence that the plant was photosynthesizing. [write on board]
This investigation was done on the UCSF campus using ivy leaves [hold up example leaf, if you have one]. Describe the different set ups for the leaves (control and treatments) and the method for preparing the leaves to test for starch. Do this by handing out and going through the Detecting Photosynthesis Methods Summary attached here.
Give students the Task Card and their photo set [there are 10 sets- make as many copies as you'll need for your classroom]. Tell students to go through the questions on the Task Card with their partner and strongly encourage them to use the Photosynthesis Overview [attached here] as they do. This overhead is a tool to help you think through this experiment. And, I want you to be brave and share with each other what you don’t understand and what is confusing to you. Think critically about what you do and do not have evidence for.
If you would like to process the leaves yourself, please consider the following:
1) we have found ivy leaves to work very well. It does also work with other leaves (we have used nasturtium).
2) covering the leaves for one week when it is sunny works well; longer time may needed when it is overcast.
3) how long the leaves need to be boiled depends on plant species with the age and size of leaf also being factors to consider
4) the process requires boiling the leaves in alcohol and alcohol is highly flammable
5) the leaves are very fragile after they have been boiled in alcohol
6) after processing, the leaves can be stored in paper towels in ziplocs or tupperware in the fridge for a few days, but you will need to reapply the iodine right before use
Discuss in small groups (6-8 students) or a whole class discussion (depending on # of adults available).
Ask students to have their photosynthesis diagrams in front of them.
Have students talk about what results they see in their leaf set. Have students share their observations/results ONLY. For example the control/untreated leaf is black and that indicates presence of starch, the leaf in the bag showed little to no starch, etc. Be sure students do NOT share their interpretations (this will be hard from some students who will want to include everything they have talked about). In hearing from other students, students will likely find that not all the results are identical.
Then move to the questions on the task card:
Given the results of these investigations and what we have discussed about photosynthesis…
• What do these investigations tell you about photosynthesis? What do they not tell you?
• What, if any, evidence is present in these investigations that…
– light is important for photosynthesis?
– carbon dioxide ?
– other things?
• What is confusing to you about the process of photosynthesis? What questions do you have?
In particular, focus on the second question about evidence – people will usually understand what it tells us about light, but jump to the conclusion that it shows us that carbon dioxide is important, when really all we can conclude is that air is important.
What further experiments could you do to prove it is carbon dioxide, not just air?
Discussion of evidence for air being important (but no evidence, directly, that carbon dioxide is important)
Evidence for the rest of the parts of the process of photosynthesis (no evidence for water)
Starch as the storage product, not directly glucose (so can sometimes see starch)
Extensions and Reflections
Emphasize critical thinking and that one should not necessarily believe text books, and there is value in finding evidence for yourself.
When you are reading, prove ideas to yourself to help build understanding. Thought experiments are good experiments too.
Weblinks and References
|Leaf Treatment Sets.pdf||7.69 MB|
|Photosynthesis overview.pdf||948.75 KB|
|Detecting Photosynthesis Methods Summary.pdf||85.44 KB|
|Detecting Photosynthesis Task Card.pdf||72.43 KB|