Here you will find a toolbox full of inquiry investigations on photosynthesis and respiration. Rather than the detailed lesson plans provided elsewhere at My Science Box, each experiment only contains a short background, materials, procedure, and going further section. It is up to you to decide which of the many experiments you wish to try with your students and how to sequence them. You will find everything from descriptions for how to extract chlorophyll, discover that plants “breathe”, recreate the experiments of Priestly and Ingenhousz, detect carbon dioxide production, and measure the rate of yeast respiration. None of these experiments require expensive equipment such as metabolism chambers or oxygen meters although those are great tools if you can afford them.
Briefly, photosynthesis occurs in the chloroplasts of plants as a means of turning solar energy into chemical energy in the form of glucose, the primary food/energy source of cells. Through a series of biochemical reactions, sunlight energy transforms carbon dioxide and water into glucose and oxygen.
6CO2 + 6H2O + light → C6H12O6 + 6O2
Respiration releases the chemical energy stored in glucose and turns it into energy that can be used by the cell in the form of ATP (adenosine 5’triphosphate). ATP may be considered the standard currency of the cell, much as the dollar is the standard currency in American society. Nearly all cellular processes depend on ATP as their energy source. The chemical equation for respiration using glucose is the mirror image of the chemical equation for photosynthesis.
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Can recognize that all living things metabolize and thus require nutrients, take advantage of chemical reactions to release energy, and produce wastes.
Can describe the general process of photosynthesis and respiration.
Can recognize the reciprocal relationship between photosynthesis and respiration.
Can explain the jobs of chloroplasts and mitochondria and their importance to cells.
Can design, conduct, and interpret experiments on plants and animals related to photosynthesis and respiration.
Chlorophyll is the pigment in plants that captures sunlight energy and uses it to drive photosynthesis. While chlorophyll does give plants their characteristic green color, chlorophyll actually comes in many colors and subtypes ranging from green to yellow to orange to red. In this experiment, students use paper chromatography to separate the many pigments from one another. First the pigments are extracted from the plants by simply crushing the plant cells open on the filter paper with the edge of a penny. When the filter paper is then immersed in rubbing alcohol, the pigments are carried upwards through capillary action. The smallest pigments travel more quickly and thus separate from the larger pigments that remain closer to the origin line.
Chlorophyll is an amazing chemical that is the essential ingredient in photosynthesis, the process through which plants capture light from the sun to create glucose. When photons of light hit chlorophyll, the electrons in the central magnesium atom donate an electron to a series of chemical reactions – the electron transport chain – that produce ATP, the cells primary unit of energy currency. The chlorophyll gets this electron back by taking one from water, resulting in the release of oxygen gas as a byproduct.
Chlorophyll comes in two main forms: chlorophyll a and chlorophyll b. Each has a slightly different chemical structure and therefore absorbs light of different wavelengths. In this paper chromatography experiment, chlorophyll a will appear as a bright yellow-green band while chlorophyll b will appear as a dull green band. In addition, there are many other accessory pigments in plants that absorb light and help transfer photons to the chlorophyll. These include carotene (an orange band) and xanthophyll (a yellow band). A final pigment that may be detected is anthocyanin (a red-brown band) that acts as a type of sunscreen for plants, protecting the plant from UV damage.
For more information on paper chromatography and its use in chlorophyll extraction, a fabulous scientific description may be found on the Science Buddies website. Also see this photosynthesis lesson plan by Sara Swisher, Barb Wilson, and Jean Wilson. Finally, Cheryl Massengale has a great write up for chlorophyll extraction including a table illustrating how to calculate the Rf value for each band and use that as a quantitative means of identifying the various pigments.
Students often believe that only animals “breathe”, but all things exchange gases with their environment. It’s just that the process is not so obvious in plants. Elodea is a very common water plant that can be found in aquarium stores. As photosynthesis occurs, oxygen is produced as a by-product. Elodea releases bubbles of oxygen as it photosynthesizes. In fact, the number or volume of bubbles in a certain amount of time can be used as a rough measure of photosynthetic rate.
The measure is rough because oxygen dissolves in water so may not always appear as a gas. Moreover, most light sources produce heat, which causes the water temperature to increase, which in turn causes lower gas solubility, and thus may produce bubbles just by turning the light on. Finally, the size of a bubble is not constant thus counting the number of bubbles per unit time is only a very rough measure. The total volume of gas produced is a better measure but again falls victim to the other caveats. The best measure is to use a dissolved oxygen meter which unfortunately costs a fair bit of money (between $400 to over $1000). You can also try disposable dissolved oxygen tests (see the Sources section of the Water Analysis activity).
An important note is that freshly cut Elodea stems produce more bubbles than the leaves. That is because Elodea stems contain large intracellular air passageways. As oxygen is produced, the plant transports the oxygen away from the leaves towards the roots. Thus, a freshly cut stem will produce oxygen bubbles at an observable rate. The cells in the leaves are much more tightly packed together and provide greater resistance to the emerging oxygen gas than the stems. Thus, to maximize oxygen gas production in this experiment, cut the stem of the Elodea then place the plant upside-down in the test tube. For more information about why the bubbles emerge from the stems, not the leaves, see this article by David Hershey of the Mad Scientist Network.
An excellent resource with more information about the use of Elodea in this experiment can be found at the Clifton College Science School website. There you will find detailed information about Elodea, how oxygen is produced, experiments by Frost Blackman, practical advice and more.
These experiments use a bell jar (or any other very large, clear, glass jar) to determine the identity of the gas produced by plants. It mirrors the famous experiments of Joseph Priestley and Jan Ingenhousz from the 1700’s that first demonstrated the existence of oxygen and its importance to plants and animals.
In 1771 and 1772, Priestly conducted a series of experiments using a bell jar. It was known that a candle placed in a sealed bell jar would eventually burn out and could not be relighted while still in the jar. Priestly discovered that a plant can survive indefinitely within a jar. Thus, he tried placing a plant into the jar with the burning candle. The candle went out as before and could not be relit right away. Priestly waited several days and tried again. The candle could be relit! The plant had restored the air inside the jar! (Do not try the next series of experiments since it harms animals!) Next priestly investigated what would happen to animals. He found that a mouse placed inside a sealed jar will eventually collapse. However, a mouse can survive in a sealed jar with a plant since the plant restores the air. Priestly was the first to demonstrate that oxygen is necessary for fire and animals but that given time, plants can create oxygen, allowing fires to burn and animals to breathe.
A few years later, Jan Ingenhousz investigated the effect of light on a plant’s ability to restore air. He found that a plant left in darkness cannot restore the air for a candle. To demonstrate this, he burned up all the oxygen in a jar with a plant then left the plant in sunlight for a few days to restore the air. Then without relighting the candle, he put the plant into darkness for several more days. At the end of the dark period, he was unable to relight the candle. He concluded that a plant in darkness acts like animals, using up the oxygen that it had created. Ingenhousz had discovered that plants can photosynthesize and create oxygen but only in the light. If left in the dark, plants do not photosynthesize and thus no oxygen is produced. Moreover plants are always performing respiration, just like animals. In sunlight, the rate of photosynthesis outstrips respiration so there is an excess of oxygen being produced. But in darkness, no photosynthesis takes place but respiration continues to occur. Thus, by keeping a plant in darkness you can demonstrate that plants need oxygen and use it up, just like animals.
For more information, see the wonderful website of Julian Rubin which describes many of these experiments and offers links and resources on how to recreate them in the classroom. Also, NSTA has produced a great set of photosynthesis related inquiry activities including the bell jar experiments.
A candle uses up the oxygen in the jar:
A plant restores the oxygen in the jar:
A plant in darkness does not restore oxygen:
A plant in darkness uses oxygen:
Blow through a straw into bluish liquid and watch it turn green then yellow before your eyes. Put some plants into the yellow liquid, leave it in a sunny window, come back the next day and the liquid is green. What if you leave the plants in the dark? What if you put some pond snails in? What if you put both pond snails and plants? What’s going on?
The liquid is bromthymol blue (BTB) a non-toxic acid-base indicator that can be used to indirectly measure levels of dissolved carbon dioxide (CO2). The amount of CO2 in a solution changes the pH. An increase in CO2 makes a solution more acidic (the pH gets lower). A decrease in CO2 makes a solution more basic (the pH gets higher). The reason for this is that carbon dioxide that is dissolved in water is in equilibrium with carbonic acid (H2CO3).
CO2 + H2O ↔ H2CO3
In any solution, while the majority of CO2 stays as CO2, some of it is converted to H2CO3, turning the solution slightly acidic. If CO2 is added to the water, the level of H2CO3 will rise and the solution will become more acidic. If CO2 is removed from the water, the amount of H2CO3 falls and the solution becomes more basic. Thus, acid-base indicators such as BTB can indirectly measure the amount of CO2 in a solution.
For more than you ever wanted to know about carbonic acid, see the Wikipedia article on carbonic acid. For the example lesson plans developed by Bob Culler through Access Excellence at the National Health Museum. For a great time lapse video showing BTB color changes using elodea and snails, see Activity C13 from Addison-Wesley’s Science 10 curriculum.
|BTB solution||Living things
|bubbled BTB (yellow)||spring of Elodea||Sunny window/bright light|
|unbubbled BTB (blue)||5 pond snails||Dark closet/drak heavy cloth|
|both Elodea and 5 pond snails|
Yeast are a single celled fungi that are a great model organism for studying respiration in the classroom. The species Saccharomyces cerevisiae is commonly used for leavening bread and fermenting beer but other species such as Candida albicans are known to cause infections in humans (vaginal yeast infections and diaper rash being the most common). In this investigation, students fill the bulb of a disposable pipet (eyedropper) with yeast, then submerge the pipet in a test tube of water. They can then measure the rate of respiration by counting the number of bubbles of carbon dioxide gas that emerge from the tip of the pipet in a certain length of time. By varying the temperature and the nutrient source, students can discover what variables affect the rate of respiration in yeast. By submerging the pipet in bromthymol blue (see Colorful Respiration activity), students can identify the gas being produced as carbon dioxide.
For more information about yeast in classroom experiments, see this experiment from the Exploratorium or this one from PBS Kids that has snippets of what different kids saw with different manipulations. The idea of using inverted disposable pipets to contain the yeast and measure their respiration rate came from a workshop led by Steve Ribisi of the University of Massachusetts – thanks Steve!
1. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:
a. Students know cells function similarly in all living organisms.
b. Students know the characteristics that distinguish plant cells from animal cells, including chloroplasts and cell walls.
d. Students know that mitochondria liberate energy for the work that cells do and that chloroplasts capture sunlight energy for photosynthesis.
Investigation and Experimentation
7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:
a. Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.
c. Communicate the logical connection among hypotheses, science concepts, tests conducted, data collected, and conclusions drawn from the scientific evidence.
e. Communicate the steps and results from an investigation in written reports and oral presentations.
Grade 8 Life Science
5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
a. Students know reactant atoms and molecules interact to form products with different chemical properties.
b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same.
c. Students know chemical reactions usually liberate heat or absorb heat.
Grades 9-12 Biology
1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism's cells. As a basis for understanding this concept:
f. Students know usable energy is captured from sunlight by chloroplasts and is stored through the synthesis of sugar from carbon dioxide.
g. Students know the role of the mitochondria in making stored chemical-bond energy available to cells by completing the breakdown of glucose to carbon dioxide.