Photosynthesis Teacher Guide

Unit

Photosynthesis

Subject

Life Science

Grade Level

MS

Activity Names

• Leaf Photosynthesis
• Transpiration

Thinking about the Discovery Questions

This lesson module will address the following essential questions:

1. What happens in the process of leaf photosynthesis?
2. What is transpiration and why is the process important in living plants?

Misconceptions

Students of all ages hold misconceptions about plant nutrition. They tend to think plants get their food from the environment rather than manufacturing it internally, and that food for plants is taken in from the outside. These misconceptions are particularly resistant to change, often following students well into their college years. One of the most important distinctions among organisms is between plants and animals -- plants use sunlight to make their own food whereas animals consume energy-rich foods. Even after traditional instruction, students struggle to understand that plants use the energy in light to make sugars out of carbon dioxide and water. Finally, students often believe that green plants are the only organisms capable of photosynthesis. Actually, phytoplankton and cyanobacteria are also photosynthetic organisms that release significant amounts of oxygen into the atmosphere via their ocean habitats.

Learning Objectives

NGSS

• Performance Expectations
  • MS-LS1-6. Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy into and out of organisms.
  • MS-LS1-7. Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism.
  • MS-LS2-2. Construct an explanation that predicts patterns of interactions among organisms across multiple ecosystems.
• Disciplinary Core Ideas
  • Chemical Reactions
    • Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants.
  • Cycles of Matter and Energy Transfer in Ecosystems
    • Matter cycles between the air and soil and among plants, animals, and microbes as these organisms live and die. Organisms obtain gases, and water, from the environment, and release waste matter (gas, liquid, or solid) back into the environment.
    • Photosynthesis and cellular respiration (including anaerobic processes) provide most of the energy for life processes.
  • Energy in Chemical Processes and Everyday Life
    • The chemical reaction by which plants produce complex food molecules (sugars) requires an energy input (i.e., from sunlight) to occur. In this reaction, carbon dioxide and water combine to form carbon-based organic molecules and release oxygen.
    • Cellular respiration in plants and animals involve chemical reactions with oxygen that release stored energy. In these processes, complex molecules containing carbon react with oxygen to produce carbon dioxide and other materials.
  • Organization for Matter and Energy Flow in Organisms
    • Plants, algae (including phytoplankton), and many microorganisms use the energy from light to make sugars (food) from carbon dioxide from the atmosphere and water through the process of photosynthesis, which also releases oxygen. These sugars can be used immediately or stored for growth or later use.
    • The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen.
• Practices
  • Analyzing and Interpreting Data
    • Analyze and interpret data to provide evidence for phenomena.
  • Constructing Explanations and Designing Solutions
    • Construct a scientific explanation based on valid and reliable evidence obtained from sources (including the students' own experiments) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
  • Developing and Using Models
    • Develop a model to describe unobservable mechanisms.
  • Obtaining, Evaluating, and Communicating Information
    • Integrate qualitative scientific and technical information in written text with that contained in media and visual displays to clarify claims and findings.
  • Planning and Carrying Out Investigations
    • Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a range of conditions.
  • Science and Engineering Practices
    • Science knowledge is based upon logical connections between evidence and explanations.
• Crosscutting Concepts
  • Energy and Matter
    • Within a natural system, the transfer of energy drives the motion and/or cycling of matter.
  • Patterns
    • Graphs and charts can be used to identify patterns in data.
    • Patterns in rates of change and other numerical relationships can provide information about natural systems.
  • Scientific Knowledge Assumes an Order and Consistency in Natural Systems
    • Science assumes that objects and events in natural systems occur in consistent patterns that are understandable through measurement and observation.
  • Stability and Change
    • Explanations of stability and change in natural or designed systems can be constructed by examining the changes over time and processes at different scales, including the atomic scale.
  • Structure and Function
    • Complex and microscopic structures and systems can be visualized, modeled, and used to describe how their function depends on the relationships among its parts, therefore complex natural structures/systems can be analyzed to determine how they function.
  • Systems and System Models
    • Models can be used to represent systems and their interactions—such as inputs, processes and outputs— and energy, matter, and information flows within systems.

NSES

• NSES Life Science – Structure and function in living systems
  • Cells carry on the many functions needed to sustain life. They grow and divide, thereby producing more cells. This requires that they take in nutrients, which they use to provide energy for the work that cells do and to make the materials that a cell or an organism needs.
• NSES Life Science – Regulation and Behavior
  • All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.
  • Regulation of an organism's internal environment involves sensing the internal environment and changing physiologic activities to keep conditions within the range required to survive.

Being Prepared

It works well to have students work in pairs to help each other view and record observations from the computer model and data from the relative humidity sensors. A lot is going on in the Leaf Photosynthesis model, and it would be difficult for a single student to watch the screen and also record observations. Since relative humidity sensors may be in short supply in the middle school classroom, students could work in cooperative learning groups to complete the Transpiration activity. Remember that the relative humidity sensor needs to send data for 30 minutes to produce an appropriate graph. It might be wise to do the Transpiration sensor activities on Day 1 with your classroom discussion to introduce photosynthesis. Students will need to concentrate on the Leaf Photosynthesis computer model and shouldn't interrupt the activity to tend the relative humidity sensors.

Materials

• Relative Humidity Sensor, Interface, Software -- For information on obtaining probeware, click here: probesight.concord.org/vendors/index.php
• Small potted plant with broad leaves
• 2 resealable plastic bags
• Scissors
• Construction paper

What Students Need to Know

Students need a working definition of photosynthesis that can be easily understood. We like this one from yourdictionary.com: "Photosynthesis is the process by which plants use energy from sunlight, water from the soil, and carbon dioxide from the air to create their own food, grow, and release excess oxygen into the air."

Students also need to be familiar with the words "chlorophyll and chloroplast".

Be sure your students understand the difference between "plant transpiration" and "plant respiration". Plant transpiration is the passage of water through a plant from the roots through the stem and leaves and its eventual evaporation out of the plant into the atmosphere. Plant respiration is the process in which plants convert sugars back into energy for growth.

Important Distinction! Photosynthesis RELEASES oxygen from the plant into the atmosphere. Respiration is an internal process that USES oxygen. Transpiration RELEASES water vapor to the atmosphere.

Getting Started

Prior to starting the Transpiration activity, demonstrate the use of the relative humidity sensor and USB interface. The sensor itself isn't hard to use, but you want to be sure each sensor is properly connected through the USB interface or the data won't be displayed. If you're not currently using sensor devices, some vendors offer package deals. For details see our Probesight page: probesight.concord.org/vendors/index.php

The Leaf Photosynthesis activity gives explicit directions for using the model. If students report problems getting the model to start, the most frequent problem is that they clicked "Run" without first clicking on "Setup/Reset". Walk around to be sure the model is showing all reactants and products and make sure students toggled the "day-night" switch to On. Students should be allowed at least 15 minutes to fully explore the model before recording observations, clicking snapshots, or typing responses.

Suggested Timeline

We suggest allowing three class periods for this lesson module.

Day One: Full class discussion -- introduce "Setting the Stage" questions and set-up of the first sensor activity for Transpiration. This lab activity can be set up by the teacher and will need to record for 30 minutes from relative humidity sensor to computer. At the end of class, give students 10-15 minutes to write their observations from the graph data.

Day Two: Perform the Leaf Photosynthesis modeling activity. Ideally, students will work in pairs so that one can observe while one takes notes, then reverse roles. Allow 30 minutes uninterrupted. While students are at computers, teacher can set up the second Transpiration data collecting lab (requires 30 minutes to collect data). Close the class with group discussions to compare/contrast student responses to the questions posed in the Leaf Photosynthesis model.

Day Three: You may want to consider having the 3rd relative humidity investigation set up when class arrives. In cooperative groups, let students analyze graph patterns from the second lab activity in Transpiration. Ask them to predict what will happen to the transpiration rate when the plant is moved from shade to sunlight. There should be about 10 minutes at the end of class to compare graphs from the last 2 experiments.

Discussion: Setting the Stage

Why are plants known as "Producers" in the food chain while animals are known as "Consumers"?

Through photosynthesis, plants convert the energy of the sun to make their own food internally. Animals must rely on the environment for their food, which provides them with the fuel and building materials to grow and thrive.

Why are plants so important to an ecosystem?

Many organisms eat plants and break down the plant structures to provide energy to survive and grow. In turn, plant-eaters are consumed by carnivores. In addition, plants release oxygen during the photosynthesis process. (So do other organisms that can undergo photosynthesis, like cyanobacteria and phytoplankton). Together, all this released oxygen makes up a large percentage of the oxygen in our air. Without plants, human life would be very difficult. We would have to generate all that oxygen some other way!

What happens inside the plant during photosynthesis?

Ask students to brainstorm their ideas, but avoid leading the discussion much. Explain that they will be exploring a computer model that shows them exactly what happens inside the plant during photosynthesis.

When it rains or you water a plant, what happens to extra water?

Again, let the students brainstorm ideas without much direction. They will explore this very question in their Transpiration activity.

Discussion: Formative Questions

Leaf Photosynthesis Model

1. What inputs and outputs can you observe from the Leaf Photosynthesis model?

The inputs in the photosynthesis process are sunlight, carbon dioxide, and water. The outputs (products) are oxygen, sugar, and water vapor.

2. What is happening when a chloroplast moves into an excited state?

When the chloroplast switches from green to red (its excited state in the model) it absorbs light, carbon dioxide and water. It releases oxygen. Teachers: Inside the chloroplast, the captured solar radiation provides energy to strip hydrogen atoms from oxygen atoms in a water molecule. The oxygen atoms form twosomes and are released into the atmosphere. Carbon dioxide from the air enters the plant through its stoma and then reacts with the remaining hydrogen atoms in the chloroplast. Carbon dioxide loses an O, which stick to two H's -- together they form H2O. Next in the atomic shuffle, the plant builds a structure of carbon, hydrogen, and oxygen atoms to form glucose (the plant's food).

Transpiration

Single Leaf Sealed in Plastic Bag

Was there any change in the amount of moisture in the bag over time?

Sealed Plastic Bag over part of Live Plant in Shade

Was there any change in the amount of moisture in the bag over time? Why?

Sealed Plastic Bag over part of Live Plant in Sunlight

Was there any change in the amount of moisture in the bag over time? Why

Discussion: Wrapping Up

1. Based on the Leaf Photosynthesis Model, explain why plants are what scientists call "autotrophs". (Definition of autotroph: An organism capable of synthesizing its own food from inorganic substances, using light or chemical energy.)

Plants are able to capture solar radiation from the sun and convert its energy into sugar molecules. The plants use these molecules as food to grow and thrive. Plants don't have to eat food like animals do. Animals are not autotrophs because they must consume food from another organism to survive. Teachers: The process goes like this: Light enters the plant and encounters water molecules. In a chemical reaction, the hydrogen atoms are stripped from each oxygen atom in the water molecule. The H atoms stay in the plant and the oxygen atoms form pairs to be released into the environment. Next, carbon dioxide enters the plant leaf through a stoma (small opening on the surface of the plant). the CO2 undergoes breaking of molecular bonds. It loses an O, which sticks to two H's to form water (H2O). Next, the plant builds glucose out of the structure of Cs, Hs, and Os.

2. Explain why photosynthesis is essential for almost all life on Earth.

If plants couldn't engage in photosynthesis, they couldn't make the sugar molecules they need to grow and stay alive. Plants provide food for insects and many species of animals, which would die without plants. If plant-eating animals died out, carnivores would be the next to die out. To form a complete response, students also need to discuss how photosynthetic organisms add oxygen to the atmosphere by releasing O2 as a by-product of photosynthesis. Without our oxygen-rich oceans and atmosphere, most living organisms on Earth would die.

How does plant transpiration contribute to a stable atmosphere?

Students can brainstorm, but shouldn't be expected to know the full answer. They should be able to discuss how a plant releases water into the atmosphere through transpiration. Let them come up with ideas. The answer is that plant transpiration accounts for about 10% of the moisture in our atmosphere. One acre of corn can give off 3,000-4,000 gallons of water each day and a big oak tree can transpire 40,000 gallons per year.

Additional Background

Photosynthesis is actually a highly complex process. Green plants synthesize carbon-based energy molecules from sunlight to make sugar molecules and oxygen. The sugar molecules then form the basis for more complex molecules (also made within the photosynthetic cell) such as glucose. The plants use the glucose to form energy-carrying molecules to stay alive. They give off oxygen as a by-product of the entire process. Photosynthesis doesn't just occur in plants! It also happens in phytoplankton and cyanobacteria. Photosynthesis drives the carbon cycle and creates oxygen necessary for other living organisms (like humans). Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the oceans are believed to produce 30%-50% of the oxygen in Earth's atmosphere!

Transpiration serves three primary roles in the plant:

1. Moves minerals from the roots and sugars from photosynthesis throughout the entire plant.
2. Serves as an environmental cooling system (i.e., 80% of the cooling effect of a large tree is from evaporative cooling effects of transpiration)
3. Provides turgidity so the plant can remain upright and push its roots through the soil.

For lots more information on transpiration and its role in the water cycle, try the USGS "Science for a Changing World" website. You can find it at water.usgs.gov/edu/

Analysis

Leaf Photosynthesis Model

The two steps in photosynthesis shown in the model are called light-dependent reactions (chloroplast becomes excited) and the light-independent reactions (chloroplast returns to an unexcited state).

1. Do your observations confirm that the first step requires light and the second does not?

Response to this question should be "yes". If students are not seeing the chloroplast returning to an unexcited state during the night phase, ask them to set the model to run slower and be sure they clicked the "day-night" switch to "On". This phenomenon is most easily seen in the first of the three simulations.

2. Which step of the reaction produces sugar?

This reaction is seen most clearly in the second and third of the three models. If students slow the model down enough, they should be able to see that when carbon dioxide enters the leaf, a sugar molecule can be produced. Don't expect too much sophistication in the response, because this chemical process is rather complex. Interesting Question: Is this sugar-producing reaction light-dependent? If you study the model closely, you can see that sugar molecules are still being produced at night, but production stops halfway through nighttime. Background for Teachers: Inside the chloroplast, the captured solar radiation provides energy to strip hydrogen atoms from oxygen atoms in a water molecule. The oxygen atoms form twosomes and are released into the atmosphere. Carbon dioxide from the air enters the plant through its stoma and then reacts with the remaining hydrogen atoms in the chloroplast. Carbon dioxide loses an O, which stick to two H's -- together they form H2O. Next in the atomic shuffle, the plant builds a structure of carbon, hydrogen, and oxygen atoms to form glucose (the plant's food).

3. Which step of the reaction requires carbon dioxide?

Production of the sugar molecule requires carbon dioxide. This is probably the reaction students will notice most, but a close look at the third model will help them see that when carbon dioxide enters the plant, it reacts in the chloroplast to produce water (H2O). In the screenshot above, CO2 is labeled in the left image. The water molecule is represented as the larger blue circle. The image on the right shows a water vapor molecule and release of sugar molecules that are shown traveling down the stem of the leaf. The model simplifies the process, but shows the H2O molecule breaking down to become water vapor (gray circle), which then is transformed by the plant into sugar to use as its own food.

4. Is oxygen used up in the reaction, or is it produced?

The process of photosynthesis produces oxygen as a byproduct, which is released into the atmosphere. When sunlight meets water molecules inside the plant, the hydrogen atoms are stripped from each oxygen atom. The oxygen atoms form twosomes and are released out of the leaf as oxygen. The oxygen released by plants is a critical part of why our planet is habitable to animal life. This process is seen most clearly in the third model.

5. When animals breathe, they take in oxygen and give off carbon dioxide. How is photosynthesis like or unlike this process?

Acceptable responses will include the idea that both processes are involve input/output and both involve chemical reactions inside the organism. Expect students to recognize that the input/output seems to be opposite. But animal breathing (respiration) is quite unlike photosynthesis. Teachers: You may or may not want to bring up the fact that plants also undergo respiration -- and in this process, plants also release carbon dioxide. The definition of respiration is the conversion of oxygen by living things into the energy the need to continue life. Carbon dioxide is a product of respiration in whatever organism it occurs. Respiration and photosynthesis are completely different processes.

6. Describe the ways, in terms of chemicals and energy, that we depend on photosynthesis.

Responses will vary. Expect students to discuss release of oxygen by plants and the resulting benefit to humans. They will probably also bring up the idea that the sugars produced inside the plant will be consumed, in turn, by animals up the food chain. Without the sun to provide solar radiation to the plant, there could be no photosynthesis reaction and our producers in the food chain (plants) would die. Consumers in the food web would then die in turn.

Transpiration

1. Based on your humidity data, do you think that live plants exchange gases with the environment? Explain.

This question is addressed in either activity where a plastic bag is wrapped around part of a plant. Student responses will vary, depending on their understanding of gas exchange. Most students should be able to respond that the baggie got very foggy, meaning gas was trapped inside it. If the bag were off, this gas would be exchanged in the atmosphere. Ask students to record differences between the single leaf in a bag and an entire stem of a live plant covered in a baggie. The single leaf was snipped off the live plant and will flatline after 10-20 minutes. Why? Because it has only the moisture left inside it to transpire; nothing else is coming up from the roots. But the live whole plant with one stem covered by the baggie will continue its transpiration process, but the rate depends on whether it is in sunlight or shade. Acceptable response will include: The snipped leaf AND the live plant both exchanged gases with the environment, but the snipped leaf didn't have much to contribute because its transpiration halted when it was cut off the plant. The live plant exchanges gases with the environment continuously, but at a higher RATE when it's in sunlight.

2. What effect did it have to place your plant in a sunny window?

There will be more condensation in the plastic bag (caused by a greater rate of transpiration). Students may observe that the bag looks "foggier" or "has more water droplets".

3. What do you think the graph for the living leaf would look like over a 24-hour period?

Answers will vary. Ask students to consider whether the transpiration process is still occurring during the night.

4. How does temperature affect the rate of transpiration of a plant?

Transpiration rates go up as the temperature goes up. The graph above shows the upward trend in the graph as it recorded Relative Humidity vs. Time. This should be observable by comparing the results of the two plastic bag investigations. The bag on the plant in the sunlight should produce more condensation because the sunny window is warmer. This means its rate of transpiration is higher. Students should be able to express that plants placed in a sunny window are exposed to a higher temperature than plants in the shade. Transpiration rates go up as the temperature goes up. Regarding humidity levels -- warmer air holds more water, creating a bigger driving force for water movement out of the plant (which will increase the rate of transpiration).

Further Investigation

Try planting your plant in a jar, similar to the image shown in the Introduction. Monitor the temperature within the bottle by punching a small hole in the lid. Pass a temperature sensor through the lid and seal around the sensor with clay.

Record the temperature over a week-long period at the same time each day; click the new button next to the graph for each new data set. Label each reading for the day of the week.

Observe and record the amount moisture apparent on the inside of the bottle each day.