This WebQuest was designed for high school (or late middle school) students in an introductory course on biology. The purpose of this WebQuest is to help students construct some basic knowledge of photosynthesis and its importance to the world. As a part of this WebQuest students will be required to create a PowerPoint presentation persuading a specific audience about the importance of photosynthesis to life on earth.
In this project you will learn about the plant process of photosynthesis and about its importance to the earth and all living creatures. As a part of this project, your group will be assigned a very important task to complete.
Immediatelyafter exposure to 14CO2, the plant's photosynthetic tissue iskilled by immersing it in boiling alcohol, and all of the biochemical reactions cease.
The process whereby plants make sugars is photosynthesis. The plant takes in carbon dioxide from the air though pores in its leaves and absorbs water through its roots. These are combined to make sugar using energy from the sun and with the help of a substance called chlorophyll. Chlorophyll is green which allows it to absorb the sun's energy more readily and which, of course, gives the plants' leaves their green colour. The reaction of photosynthesis can be written as the following chemical equation when sucrose is being made:
(3) Exciton Transfer (Resonance Energy Transfer): Transfer of energy to a nearby unexcited molecule with similar electronic properties. This can happen because the molecular orbital energy levels of the molecules overlap. This mechanism will play an important role in photosynthesis.
The first step in photosynthesis is the interaction of light with chlorophyll molecules. The chemical structures of the various chlorophyll molecules are based upon the cyclic tetrapyrrole that is also seen in the heme group of globins and cytochromes. Various modifications of this group, namely ring saturation characteristics and substitutions on the rings provide a series of pigment molecules that, as a group, absorb effectively over the wavelength range of 400 nm - 700 nm, the spectrum of . It is the high degree of conjugation of these molecules that makes them so efficient as absorbers of visible light.
Chapter 22 ("Electron Transport and Oxidative Phosphorylation") in Voet & Voet (3rd Edition) is one of the most important chapters in the entire text (at least in my opinion) and it would help to reread it as you look at the light reaction of photosynthesis in more detail over the next two lectures.
Photosynthesis and cellular respiration are two chemical reactions that happen inside of cells and are very important to living systems. They are both chemical reactions because in each process, matter is changed into new forms of matter. Photosynthesis occurs in the chloroplasts of plant cells (as well as algae and some bacteria). Cellular respiration occurs in the mitochondria of eukaryotic cells, both plant and animal.
You have already studied the "dark reaction" and I will refer you to Dr. Diwan's notes on the subject. As the overall process of photosynthesis involves a series of electron transfer reactions, we are in the realm of oxidation-reduction chemistry, and it would help to review the basics of these processes because we will be going into this topic in greater depth. There is a direct analogy to electron transfer in the mitochondrion, in which clumps of energy are transferred from one electron carrier to another along a "chain" and H+ ions are translocated out, across the mitochondrial membrane, thus generating an electrochemical gradient. The energy inherent in this gradient is used to synthesize ATP in the process of "oxidative phosphorylation." The same processes occur in photosynthesis and the chloroplast, the site of photosynthesis in plants and blue-green algae (but not in photosynthetic bacteria), is the analog of the mitochondrion in eukaryotes.
The reactants of photosynthesis are water and carbon dioxide, which the plant takes in from the environment and reacts to make the products glucose and oxygen. The glucose stays in the plant (which may be eaten by heterotrophs) and much of the oxygen is released into the environment (where it is taken in by animals).
(4) Photooxidation: Transfer of an electron to an acceptor molecule. In photosynthesis, an excited chlorophyll molecule, Chl*, donates an electron, thereby becoming oxidized to the cationic free radical, Chl+.
On behalf of the Plant Lovers of America (a very large non-profit lobbying group) your group has been chosen to create a presentation that will inform the President about photosynthesis and the importance of sunlight to life on earth and will persuade him to consider other options and not release the chemical into the atmosphere.
We'll look at a simpler example of photosynthesis first, and use it as an introduction to photosynthesis in plants and cyanobacteria (blue-green algae). Although the primary reactions of photosynthesis take place at "photosynthetic reaction centers," the first level of interaction of light with an organism that carries out photosynthesis is at an assembly of chlorophyll molecules that "harvest" light (the "light-harvesting complex"). Such an assembly results in a greater chance that photons will be captured and, because of the strategic arrangement of the individual chlorophyll and other accessory light-absorbing molecules, the transfer of energy to the photosynthetic reaction center is very fast (-10 s) and very efficient (>90%).