Review Questions Photosynthesis 1. Describe a metabolic ...

Review Questions Photosynthesis

1. Describe a metabolic pathway. In a factory, labor is divided into small individual jobs. A carmaker, for example, will have one worker install the front windshield, another install the doors, and another the wheels. Each person has a specific task on the assembly line and they do that same task all day long. A metabolic pathway is similar. Cells often have to build, degrade or transform complicated molecules. Like a factory assembly line, these molecules are altered one part at a time by a series of enzymes, with each enzyme performing only one specific task. In a metabolic pathway, you may have an enzyme that only adds a hydrogen, or one enzyme that breaks a double bond. Enzymes, by their nature, are specific like this because of their active sites. Each enzyme has an active site that, into which, only one particular substrate will fit. At the start of a pathway the first substrate is altered by the first enzyme and becomes the first product. That first product, in turn, then becomes the substrate for the next enzyme. This alternating substrate and product substrate sequence continues until the end of the pathway and the end product. See figure below.

2. How does negative feedback work in metabolic pathways? Cells, like factories, need to match their production with demand. A factory wouldn't stay in business long if it continued to manufacture more goods than it could sell. Cells are like efficient factories. They turn off their pathways when they have an excess of product.

We often associate enzyme inhibitors with poisons, toxins, and drugs. But cells control their biochemical pathways using feedback inhibition. The end product acts as an inhibitor of the first enzyme in the pathway. By shutting down the first enzyme, the pathway is turned off and the product ceases to be made. Most cell pathways are regulated this way. See previous figure.

3. What is the general chemical formula for photosynthesis? CO2 + H2O + sunlight C6H12O6 + O2

4. Why is photosynthesis so important to the biosphere? Photosynthesis is one of the most important chemical reactions in the biosphere. Photosynthesis forms the food base of the majority of earth's food webs. In addition, we can thank photosynthesis for our oxygenated atmosphere.

5. Where does photosynthesis occur in plants? Describe the structures of the chloroplast. In plants and algae (photosynthesizing protists), the chloroplast is the site of photosynthesis. A chloroplast is a double membrane organelle. The inner membrane is specialized into stacks of thin, flattened, membranous bags called thylakoids. The thylakoid membrane contains the photosynthesizing pigments and converts light energy into chemical energy. Surrounding the thylakoids is a fluid-filled space called the stroma. Glucose is made here.

6. What is the electromagnetic spectrum? What is color? What is the relationship between energy and wavelength? Light is electromagnetic radiation and comes in a range of wavelengths. The visible spectrum is the wavelengths we can see as well as the wavelengths of light plants use in photosynthesis. The visible spectrum ranges from ~400 nm (violet) to ~700 nm (red). Light waves in the visible spectrum have enough energy to be efficiently harvested but are not powerful enough to damage biological molecules. Remember, the shorter the wavelength, the more energy the photons contain. So violet at 400 nm has more energy than red at 700 nm.

7. Define pigment. What is the dominant plant pigment? What is an accessory pigment? Give an example. A pigment is any substance that absorbs visible light. The color white is said to be unpigmented; all the wavelengths of visible light are reflected and so we see white. The color black absorbs all the visible wavelengths and reflects none. So we see black. The color green absorbs all the visible wavelengths of light except green which is reflected. The colors of the world we see around us are the wavelengths of light not absorbed but reflected. The dominant plant pigment is chlorophyll A and it reflects mostly in the green part of the spectrum. So we live in a green world. There are other plant pigments as well. These are called accessory pigments: such as chlorophylls B & C, carotenoids (reds and oranges), phycoerythrins (reds), phycocyanins (blues), etc.

The accessory pigments allow the plant to absorb energy from the entire visible spectrum. They are also phytoprotective. The carotenoids for instance are antioxidants. They help eliminate superoxide free radicals that can damage plant cell DNA and cell membranes.

8. Why do tree leaves change color in the fall? In autumn, green leaves of certain plants change colors; brilliant reds, yellows, oranges, and browns. The pigments that reflect those colors were in the leaf all along. They were masked by the dominant chlorophyll A. In the fall the plants stop making chlorophyll A and that allows the accessory pigments to appear.

9. What wavelengths of light are absorbed the most by green plants? Below is an absorption spectrum for chlorophyll A plus some accessory pigments. These are the wavelengths each pigment has it greatest absorption and reflection. You'll notice that chlorophyll A has its greatest absorption in the violet-blue part of the spectrum and then a second peak in the red part. You can see that there is very little absorption in the green area. Those wavelengths are reflected. You'll notice that the accessory pigments absorb wavelengths outside the peaks of chlorophyll A. In this way, plants have a combination of chlorophyll A and accessory pigments can exploit most of the visible spectrum.

Below is an absorption spectrum of a typical green plant. We can measure rate of photosynthesis at different wavelengths by measuring O2 production. Again as predicted, you'll see the two peaks indicate chlorophyll A. You may notice that there is still photosynthesis in the green range indicating the presence of active accessory pigments.

10. Describe how electrons can be "energized". Electrons can store energy. They do it by increasing their potential energy by moving out from near the nucleus to a higher energy level. Electrons with more energy are able to overcome the attraction of the nucleus and inhabit a higher energy level. An electron can store the energy of a photon. Photons are bundles of light energy. When a photon with the right wavelength strikes a pigment, it excites an electron. The electron jumps to a higher energy level. Most of the time, the photoexcited electron only stays at the higher level for a fraction of a second and then drops back down to its ground state. When the electron falls back down it gives off energy. Often, the energy released is heat. On a hot day, a black car

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