Test scheduled for .
As we move through this unit you will be learning a large amount of detail about the process of photosynthesis. However, there are some general points that you should always keep in mind to put these details in perspective. First and foremost is the observation that photosynthesis is one of the most remarkable of biological processes. The ability to convert energy from the environment (sunlight) into large and complex carbon based molecules (glucose and starch) is fundamental to the origin of plant life and all other life forms that ultimately depend upon plants. Second, the similarities between photosynthesis and cell respiration demonstrate the interdependence of the complex cellular processes both at the structural and at the chemical level. If you keep these points in mind as you learn the following material, the details will make much more sense to you.
Learning Objectives: the successful student will be able to ...
- generally describe the range of waves found in the electromagnetic spectrum and explain the relationship between wave length and energy of these waves.
- describe the range of waves in the visible light spectrum and relate them to the action spectra of plant pigments.
- write the complete balanced equation for photosynthesis.
- name and identify the common cells and tissues of a typical plant leaf and describe their function.
- name and identify the major components of the chloroplast and relate them to the events of photosynthesis.
- describe the interdependence of the light dependent and light independent phases of photosynthesis. In particular, name the starting reactants and products of each phase and how they are recycled during the process of photosynthesis.
- explain how the structure and location of the photosystems, electron transport systems, and ATP synthase are correlated to their function in the light dependent reactions.
- describe the process of carbon fixation in the light independent reactions (Calvin cycle).
- (AP students only) compare the variations in the Calvin cycle and relate them to changes in leaf structure and function (the C3, C4, and CAM pathways).
Lesson One: The Electromagnetic Spectrum and the Action Spectrum.
Go to the Online Biology text and read the section on the nature of light. Be sure you understand the inverse relationship between wave length and energy. In addition, appreciate the location of the visible light spectrum within the electromagnetic spectrum and the relationship between perceived color and wave length. Finally, understand the dual nature of light as wave and particle (photon). It is the photon particle that interacts with plant tissues in photosynthesis.
Now go to the section on chlorophyll and the accessory pigments. We will take time to study the structure of the chlorophyll molecule latter. For now focus on the two graphs in this section; the absorption of light vs wave length and the rate of photosynthesis vs wave length. These graphs are called "action spectra" and represent the amount of light of various colors actually absorbed by plant pigments and the rate of photosynthesis that takes place under different colors (wave lengths) of light. Notice that there is more than one form of chlorophyll and several other pigments in addition to chlorophyll (accessory pigments). Be sure you understand the relationship between the amount of light absorbed by these pigments and the rate of photosynthesis.
Homework, Due 13 December. Answer the following questions and email them using your FBA account.
- What is the peak absorption wave length for the two types of chlorophyll?
- What is the relationship between the amount of absorption of light by the photosynthetic pigments and the rate of photosynthesis?
- What is the relationship between the wave length of light and its energy?
- Is there any correlation between the rate of photosynthesis and the amount of energy (or wave length) of light?
Lesson Two: Leaf Structure and the Chloroplast.
Go to the Kimball page the outlines the structure of a typical leaf. You should be able to identify the various structures discussed here and describe their functions. Be aware that there are several important variations of this structural plan that are associated with alternative pathways of photosynthesis. We study these latter.
Now review the structure of the chloroplast. You can use the pages in Kimball or the Online Biology text. Be sure you understand the relationship between the various membranes of the chloroplast and the spaces they create (the lumen and stroma). Just as in cell respiration these membranes and associated spaces become very important.
Homework, Due 15 December. Email the answers to the following questions using your FBA account.
- For each of the leaf structures labeled in the diagram you studied above give its specific function.
- Briefly discuss the similarities between the organization of the membranes in the chloroplast to that of the mitochondria.
Lesson Three: Photosystems & the Light Dependent Reactions.
Photosynthesis is usually divided into two distinct phases, the light dependent and the light independent reactions. The light dependent reactions accomplish three important functions.
- Water is split into H+, O2, and electrons. The electrons produced replace those ejected by chlorophyll when this pigment absorbs light energy.
- The energy from these electrons is used to create a high concentration of H+ in the lumen of the thylakoids and to reduce NADP to NADPH.
- This concentration gradient, in turn, is used to make ATP.
Go to Kimball's pages for a review of the function and molecular components of the photosystems. Note that photosystems I and II are very similar but there are significant differences especially in their function and central chlorophyll pigments. Note the similarities in the light dependent reactions and the events of the ETS of the mitochondria during cell respiration. The ATP and NADPH produced will be the source of energy needed to run the next stage of photosynthesis, the light independent or Calvin Cycle.
Homework, Due 19 December. Email the answers to the following questions using your FBA account.
- During cell respiration oxygen gas is used up during the very last step of the process. When is oxygen gas produced during the process of photosynthesis? Explain why it is produced at this point?
- ATP synthase is an enzyme that plays an important role in both cell respiration and photosynthesis. What is the function of this enzyme and where does it get the energy to perform this function?
- Photosystems I and II each have different functions. Explain the function of each.
- Although Photosystems I and II have different functions, they are linked together. Explain.
- Only a few pigment molecules actually produce a high energy electron in response to light. What is the function of the other surrounding pigments?
Lesson 4: Carbon Fixation, the Calvin Cycle.
Up to this point we have accounted for the water consumed and the oxygen gas produced during photosynthesis. The next stage, the light indpendent reactions, will account for the CO2 consumed and the glucose (and other carbohydrates) produced during photosysthesis. In its simplest form this process involves combining CO2 with a five carbon molecule (RuBP) which then undergoes a series of reactions to produce glucose and regenerates RuBP. Go to the Kimball section on the Calvin Cycle to review this process. Notice how CO2 enters this process (six times/each glucose molecule produced), the role of ribulose bisphosphate, and the enzyme Rubisco. Don't worry about the names of the other molecules involved.
Homework, Due 4 January.
- Kimball suggests that the enzyme Rubisco is the most abundant protein in the world. Explain the rationale for this claim.
- If you follow the fate of a single CO2 in the Calvin Cycle diagram, it eventually becomes part of the glucose molecule produced in this process. If that's the case, why are 5 more CO2 molecules needed in the overall equation for photosynthesis?
- The Calvin Cycle is a highly endergonic reaction. What is the source of energy for this reaction? Be specific.
