What temperature is the maximum rate of photosynthesis?
Lab Exercise, individual, 10 points
Please read the chapter about Energy in What is Life? A Guide to Biology before beginning this lab.
The purpose of this lab exercise is to show how energy is important to living organisms and to demonstrate the relationship between respiration, photosynthesis, and temperature in a typical plant.
In the chapter about Energy, we study two metabolic pathways, respiration and photosynthesis. Both of these pathways involve energy in the cell. Respiration is the pathway in which plants, animals and many other living organisms make ATP (adenosine triphosphate) to stay alive by breaking down glucose to carbon dioxide and water. ATP is our main energy molecule. Just like your car may run on gasoline, your cells run on ATP to stay alive. Plant and animal cells (and all living cells) must constantly make ATP to stay alive. Cells do not store ATP or obtain ATP directly from food: cells need to make ATP, and cells (including human cells) make ATP continuously in respiration. Living is work and from physics, we know that to do any kind of work requires energy, thus this is a very important pathway for living organisms including plants and animals.
In this lab, we will study the relationship between respiration and photosynthesis in a plant, the potato. Plants make their own food (glucose) by photosynthesis. For photosynthesis, plants need light, carbon dioxide, and water in order to make glucose (their food) and oxygen (a byproduct). Plants then take the glucose and carry out respiration and make ATP (for energy to do cellular work and to stay alive), carbon dioxide (byproduct) and water (byproduct). Plants can also use the glucose to make cellulose (polymer of beta glucose) for new cell walls and make other Carbon-based molecules in the cell such as proteins, phospholipids and DNA. In potato, the plant can take excess glucose and make starch (polymer of alpha glucose) in the potato tuber, the part of the plant that we eat. Although the tuber grows underground, it is not a root, it is a specialized stem. It can only be made when the plant has been making more food (by photosynthesis), than it needs to stay alive. If the potato plant is not carrying out enough photosynthesis, the plant will not make potato tubers underground.
For a plant to grow, increase in size with cell divisions, and produce new cellular molecules and organelles, the plant needs to carry out more photosynthesis than respiration. The rate of photosynthesis is influenced by many factors, including light intensity and temperature. If a healthy growing plant is taken from the appropriate light intensity for growth and placed in the shade or conditions with reduced light intensity, leaves and stems may eventually begin to die because the cells are carrying out more respiration than photosynthesis. There is no extra glucose to make carbon-based molecules needed for growth. If the rate of photosynthesis and respiration are equal, the plant will stay alive, but will not grow and increase in size.
This lab exercise is divided into three parts. Read each part and then answer the questions.
Part A. Plant scientists studied the relationship/correlation between photosynthesis, respiration, and temperature in a potato plant. A graph similar to the one in Figure 1 was obtained. This graph shows the relative rate of photosynthesis with the blue line and the relative rate of respiration with the red line on the Y axis versus temperature on the X axis. Temperature is in ⁰C and ⁰F (⁰C is used in scientific research worldwide. In the USA, we are more familiar with ⁰F so ⁰F is also included.) Potato plants have an above ground part of the plant which has stems and leaves which we do not eat. It also has a below ground part with tubers (which are specialized stems) and roots. We eat the tubers that are produced underground. The plant will make tubers when it has extra sugar that has been made from photosynthesis. This sugar can be made into the polysaccharide starch (remember from Chapter 2: starch is a polysaccharide, the monomer unit is glucose). If potato plants are not carrying out enough photosynthesis, they will not make tubers underground. This can occur if the plant has few leaves or the leaves are not healthy and green.. Excess photosynthesis is required for tubers to form and increase in size.
Use the information in Figure 1 to answer the following questions:
1. What temperature is the maximum rate of photosynthesis?
Would you describe this temperature as hot, warm, cool or cold for East Texas? Justify your answer.
2. What temperature is the maximum rate of respiration?
Would you describe this temperature as hot, warm, cool or cold for East Texas? Justify your answer.
3. At what temperature are the rates of photosynthesis and respiration equal? What does this mean for the plant?
4. If you are growing potatoes in your garden, hoping to harvest a good crop of nice potatoes, would your potatoes grow best in the spring or summer here in East Texas? Why?
5. What is going to happen to the potato plant if the temperature remains above 45 ⁰C? Why?
6. What is going to happen to the potato plant if the temperature remains at 25 ⁰C, but the plant has a black box placed over it and it is in complete darkness for 3 weeks? Why?
7. What is going to happen to the potato plant if the plant is deprived of carbon dioxide? Why?
8. Potato bugs devour all the leaves of the plant. Do you think the plant will make nice tubers? Why or why not?
Part B. Hypothetical Plant X has the following relative rates of photosynthesis and respiration vs. temperature as seen in Table 1 below. Make a graph using the following values for relative rates of photosynthesis and respiration vs. temperature. You can make your graph with ⁰C or ⁰F or both. Then answer the following questions. The graph can be made on graph paper and photographed or scanned or a graph can be generated electronically. Submit this graph with your lab exercise.
1. What temperature is the maximum rate of photosynthesis?
2. What temperature is the maximum rate of respiration?
3. At what temperature are the rates of photosynthesis and respiration equal?
What does this mean for the plant?
4. Do you think this plant is one that grows in a warm, moderate or cold climate? Why?
5. How does this plant compare with potato?
Part C. General Questions
1. Write the general chemical equation for respiration identifying the reactants and the products. This equation can also be photographed or scanned for submission.
2. Write the general chemical equation for photosynthesis identifying the reactants and products. This equation can also be photographed or scanned for submission.
3. Describe the relationship between the two chemical equations above.
How to submit this lab exercise: Submit this lab exercise as a Microsoft Word document, PDF or Powerpoint with the answers to the questions and the graph for Part B. Do not submit as any other file type of format. SafeAssign will check for plagiarism. Plagiarism will result in a grade of 0.
This lab exercise is worth 10 points. Check the calendar for the closing date.
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