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? CO 2 + H 2 O + sunlight C 6 H 12 O 6 + O 2 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 O 2 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
will get hotter than a white car. The black pigment absorbs more wavelengths than white pigment. The photons are exciting the electrons in the pigment to jump to a higher energy level. They stay there for a moment and upon their return to their ground state they give off their energy as heat. Why does the white car heat up? The pigment doesn t absorb visible wavelengths but does absorb UV and infrared rays. Sometimes the energy is also released as light but at a longer wavelength. We call that fluorescence. If you expose chlorophyll A to black light (UV) it will glow red. When the photoexcited electrons drop they give off heat plus light at a longer wavelength. 11. What occurs in an oxidation-reduction reaction? When an atom or molecule loses an electron, it is known as oxidation. We say the substance has been oxidized. If an atom or molecule gains an electron, it is called reduction. We say the substance has been reduced. Hydrogen atoms often tag along with electrons as they are passed. So often a reduced substance will not only gain an electron but a hydrogen atom as well. 12. What is a photosystem? Where are they located? What is a reaction center? What happens at a photosystem? Embedded within the thylakoid membrane are thousands of clusters of chlorophyll A molecules called photosystem. Each photosystem is a bunch of around 300 chlorophyll A molecules surrounding a reaction center. The reaction is simply two specialized chlorophyll molecules. A photosystem is where light energy is converted into chemical energy for making sugar. First a photon strikes one of the chlorophyll molecules in the photosystem. Its electrons are excited. It passes the energy to a neighboring chlorophyll molecule. That molecule gets excited and passes the energy to its neighbor. This wave of energy bounces from one chlorophyll molecule to another in the photosystem until if final reaches the reaction center. The two specialized
chlorophyll molecules in the reaction center have their electrons excited. An electron from each is boosted to a higher energy level. Rather than dropping back down the ground state, these two energized electrons are stripped from the reaction center and held by a primary electron acceptor. All photosystems work this same way. 13. Describe the light reaction. The light reaction is the conversion of light energy into chemical energy in the thylakoid membrane using photosystems. There are two kinds of photosystems within the thylakoid membrane. Let me introduce you to photosystem I. This was the first one to be discovered. Just like in the previous question, a photon strikes a chlorophyll molecule in the photosystem and it passes the energy to a neighboring chlorophyll molecule. The energy keeps getting passed around the photosystem until it reaches the reaction center. In photosystem I, the reaction center is called P700 because it is most absorbent to wavelengths of light at 700 nm (red area). An electron from each of the two special chlorophyll molecules of the reaction center get excited and jump to a higher energy level. The two energized electrons are grabbed by a molecule called a primary electron acceptor. The two excited electrons don t stay long with the primary electron acceptor. They are passed to another molecule called ferrodoxin. Ferrodoxin is oxidized and the electrons attach to an electron carrier molecule called nicotinamide adenine dinucleotide phosphate (NADP + ). NADP + is reduced plus it gains a H + and becomes NADPH. The energy of a photon is now stored in the chemical bonds of NADPH. We ll see that NADPH will move out into the stroma where it will donate the electrons and hydrogen to the Calvin cycle in glucose synthesis.
Since two electrons were removed from P700, they have to be replaced. This is done by photosystem II. This is the second photosystem to be discovered. Just like photosystem I, photosystem II absorbs the energy of photon and that energy is transferred around the chlorophyll molecules until it reaches the reaction center. This reaction center has a slightly different maximum absorption wavelength of 680 nm. So it is called P680. Two electrons from P680 are boosted to a higher energy level and promptly abducted by a primary electron acceptor. The electrons don t stick around for long but enter into an electron transport chain. The chain is a series of electron carrying proteins embedded within the thylakoid membrane. They pass the two energized electrons down a sequence of proteins. The electrons are pulled down the chain because each succeeding protein is more attractive to electrons. These proteins extract a little energy from the electrons as they are passed until the electrons reach the last protein in the chain at their ground state. These spent electrons replace the two electrons lost in photosystem I. So what is done with the energy extracted from the electrons? It is used to make ATP. Now we are stuck with the same problem in photosystem II. Its reaction center is now missing two electrons. How are they replaced? One of the reactants of photosynthesis is H 2 O. Photosystem II strips two electrons from water and splits the molecule into two. Oxygen is released as a waste product. So where does the oxygen in our atmosphere come from? The oxygen we breathe arises from the splitting of water in photosystem II of the light reaction. Summary of the light reaction: Location thylakoid membrane Photosystem I energized electrons stored in NADPH
Photosystem II energized electrons travel down electron transport chain and have their energy extracted to make ATP. Water is split and oxygen is released as a waste product. NADPH and ATP leave the thylakoid membrane and move to the stroma. 14. Describe the dark reaction. The dark reaction or Calvin cycle as it is also known occurs in the stroma. Carbon dioxide enters the biochemical pathway as well as the products of the light reaction; NADPH and ATP. Carbon dioxide is reduced to form glucose. NADP+ and ADP+P are recycled and return to light reaction in the thylakoid membrane. The dark reaction is broken down into three steps. The first step is carbon fixation. Three molecules of carbon dioxide enter the pathway. The enzyme Rubisco attaches each carbon dioxide molecule to a five carbon molecule called ribulose biphosphate (RuBP). Three short-lived 6 carbon intermediates are made but quickly break into six 3 carbon molecules called 3-phosphoglycerate. ATP from the light reaction is hydrolyzed and a phosphate group is added forming 1,3- biphosphoglycerate. NADPH is oxidized and two energized electrons plus the H+ are attached forming glyceraldehyde-3-phosphate (G3P). Thus begins phase 2: reduction. Only one out of five, G3P molecules made at this time exit the cycle and join with another to make a glucose molecule. The remaining five enter into phase 3: regeneration of the CO 2 receptor (RuBP). In a complex series of reactions, the five molecules of G3P are rearranged into three molecules of RuBP. Summary of the dark reaction: Location stroma NADPH donates energized electrons and H + ATP provides energy NADP + and ADP + P are recycled back to thylakoid membrane. Carbon dioxide is reduced to glucose