Cellular Respiration: Creating Energy

Transcription

1 : Creating Energy Introduction: The Cell and the Automobile What happens when you are on a long trip in a car and the fuel light comes on? Will the car continue to work much longer? Hopefully there is a gas station nearby, right? Your car needs fuel to keep working. The needed fuel is just one component of a combustion reaction that occurs inside the engine. The combustion reaction also requires a spark and oxygen. The reaction creates pressure, which makes the piston in the engine move, and thereby makes the wheels on the car move. Much like your car, we know that cells of living organisms (this includes plants, animals, and humans) also need fuel in order to move and function correctly. The combustion reaction that takes place inside these cells is called cellular respiration (see def.) and is similar to the reaction that takes place inside an engine. Cellular respiration is a combustion (aerobic [see def.]) reaction by which energy from food source molecules (called simply food molecules or fuel) is obtained and captured by a cell. The process of cellular respiration requires fuel, oxygen, and a spark to produce the energy the cell needs, much like your car. In this installment of Phenomenal Physics we will discuss cellular respiration and even perform an experiment to see the effects of cellular respiration in action. Using the Definitions section in the back of the article as you read will make the article more interesting and easier to understand. Let s get started! 1 P age

2 Background A first step to understanding cellular respiration is gaining a basic understanding of a larger process, metabolism (see def.). Metabolism is the way an organism converts food to the energy it needs. It involves a set of chemical transformations that occurs within an organism s cells (Figure 1). Cellular respiration is just one of those transformations, but it is an essential part of metabolism because it converts biochemical energy held in carbohydrates, fats, proteins, and glucose, into usable energy for the cell (Pearson 2014). This usable energy product is called adenosine triphosphate (ATP) (see def.). ATP is a coenzyme (see def.) that carries chemical energy within the cells for metabolism. That sounds Fun Fact Instead of the phrase fast metabolism, scientists use the phrase high basal metabolic rate, when describing the number of calories the body needs at rest. It is true that muscle burns three times as many calories as fat. In fact, because the relative amounts of fat and muscle are such a highly determining factor, an obese person with a great deal of fat can have a higher basal metabolic rate than a lean person with little fat but also little muscle (Mahan 2012). like a daunting concept, so for our purposes we ll refer to ATP as a high-energy molecule that stores the energy we need. We could also say that it is what makes the pistons go up and down. Figure 1. Metabolism simple overview (graphic courtesy of James R. Jackson, Portage, Inc.). Cellular respiration is sometimes called aerobic respiration because it requires oxygen. Like a car engine that requires oxygen to burn fuel, cellular respiration requires a certain amount of oxygen to reduce food molecules into something that can be used later in the process. Just like a car, a cell has a spot where the combustion reaction occurs. The mitochondria (Figure 2) is often called a cell s power plant because it is where a cell s energy is produced. The majority of cellular respiration occurs within the mitochondria. 2 P age

3 Figure 2. Inner structure of mitochondria (graphic courtesy of Microsoft ). Getting Down to Business Cellular respiration can be broken down into three main steps. The first step is called glycolysis (see def.). In glycolysis, a glucose molecule is broken down and turned into pyruvate (see def.), which is a molecule that is required in the next step of cellular respiration. Glycolysis occurs just outside of the mitochondria. The cool thing about glycolysis is that via this process, two ATP molecules are produced without using any oxygen (Phelan 2010). It is like finding a five dollar bill in front of the ice cream store. The second step is called the Krebs cycle (see def.), or the citric acid cycle. In this step, the pyruvate resulting from glycolysis enters the mitochondria where it is turned into acetyl CoA (see def.), a helper compound that is used to make two more ATP molecules, as well as and NADH and FADH 2 (see def.), which are compounds that are essential for the next/final step (Phelan 2010). A byproduct of this step is carbon dioxide (CO 2 ). In the first and second steps, some ATP is produced, but as noted above only a total of four ATP molecules. The last step is where the rest of the much needed ATP is created. This step is called oxidative phosphorylation (see def.),which is a series of chemical reactions that takes place in the mitochondria. Oxidative phosphorylation is fueled by electron transport, and is sometimes called the electron transport chain. It combines the electrons from NADH and FADH 2 from the previous step with oxygen to form or synthesize ATP. In fact, molecules of ATP are produced for each glucose molecule used at the beginning of glycolysis. A byproduct of this step is water (H 2 O) (Pearson 2014) (see all three steps in Figure 3). Figure 3. Simplified illustration of cellular respiration (graphic courtesy of James R. Jackson, Portage, Inc.). 3 P age

4 Energy of a Seed Both animal and plant cells use cellular respiration to create energy for themselves. Animals (and humans) get their carbohydrates, fats, proteins, and sugars by ingesting it. Plants use photosynthesis to make sugars from the energy they collect from the sun; then they metabolize these sugars through cellular respiration just as animals do. Remember that photosynthesis happens from the green leaves and other plant parts above the ground. So, since seeds don t have leaves, how do they get their energy? The amazing thing about seeds is that they are loaded with sugars before they are dropped from the parent plant. After a seed is dropped, it uses cellular respiration to metabolize the stored sugars very, very slowly until the time of germination. Fun Fact A seed must survive by itself from the time it falls from the parent plant until it has an opportunity to germinate and grow. To ensure success, the parent plant packs each seed with enough food to go on to do that. That is why seeds contain so many calories. For example, an almond has 628 calories per 100 grams. Those calories help with the cellular respiration as they are in dormancy. Let s do the following activity to observe cellular respiration while it is happening. Our activity was adapted from an activity created by the Cornell Science Inquiry Partnerships (Vaccaro 2014). Activity Objective: To watch cellular respiration, we would need an incredibly powerful microscope, because the reaction happens at the molecular level inside of a very small cell. However, since we know that cellular respiration requires oxygen, and since carbon dioxide is a by-product (two things we can measure oxygen and carbon dioxide), it is possible for us to directly observe the effects cellular respiration. In this activity, we are going to test and measure the cellular respiration occurring inside a dormant seed. Because carbon dioxide will be a byproduct, we are going to capture it in a substance called calcium hydroxide. Calcium hydroxide captures the carbon dioxide in the air and converts the gas to calcium carbonate. We will place calcium hydroxide and seeds into a test tube and invert the tubes into a beaker of water. The calcium hydroxide will react with any carbon dioxide that is produced and remove the gas from the test tube air space. As cellular respiration in the seeds occurs, the amount of air in the test tube should actually decrease, because the carbon dioxide will be absorbed by the calcium hydroxide. Less air in the test tube should allow the water level in the test tube to rise. Therefore, it will be reasonable to assume that the changes in water height are related to the amount of carbon dioxide produced by cellular respiration of the seeds. 4 P age

5 Materials Needed: At least two ml test tubes. At least viable, medium-sized seeds. Zucchini, pumpkin, bean, or pea seeds work well. It is also important that you soak the seeds prior to the experiment, so they can begin to break dormancy. Typically, soaking them in water for a few hours or overnight prior to the activity is sufficient. Approximately mg of calcium hydroxide or soda lime. These products can be purchased from a chemical supplier or a health food store. Be sure to read and follow the chemical manufacturers handling instructions to ensure your safety. If you are unable to obtain calcium hydroxide or soda lime, baking soda will work, but the results will not be as definitive. Cotton balls. A large beaker or small tub filled with water. Ruler. Rubber band. A gram scale. Laboratory notebook and pen. Procedure: 1. Place approximately mg of carbon hydroxide into each test tube. 2. Place a cotton ball atop the carbon hydroxide. This will keep the carbon hydroxide in place. 3. Weigh seeds and record initial weight. Place 3 5 soaked seeds into one of the test tubes. The other test tube will receive no seeds and is called a blank or control. 4. Loosely place another cotton ball atop the seeds to hold them in place. Place a cotton ball in the blank. 5. Use the rubber band to secure the test tubes and the ruler together. Ensure that the test tubes are lined up with a specific mark on the ruler. 6. Invert the test tubes into the large beaker filled half way with water. Make sure that the test tubes and ruler do not float. You may need to secure the ruler to a stand (Figure 4). 7. Verify that there is some air space between the water and the last cotton ball. 8. In your laboratory notebook, record the initial water height within each of the two test tubes. Also note the date and time of the initial reading. Figure 4. Illustration of experiment (graphic courtesy of James R. Jackson, Portage, Inc.). 5 P age

6 9. Continue recording the water height every hour. Leave overnight and record observations the following day. 10. After the final water height has been recorded, determine the difference between the treatment and control test tubes. The test is now concluded. a. Was there a difference in the water heights recorded for the two tubes? 11. Carefully remove the seeds from the treatment test tube and weigh. Record the weight of the seeds and determine the difference between the initial and final seed weight. a. Was there a difference between the before- and after-experiment seed weight? Questions to Consider Consider the following questions, then read our scientist s hypotheses in the table below for more information. 1. Would more seeds produce more carbon dioxide, thereby causing the water height in the tube to be greater? 2. What would happen if a dry seed was used? 3. Are there any differences between different species of seeds, regarding the outcome of this activity? 4. What if a seed had already sprouted? Would it cause a difference in the carbon dioxide release? 5. Do the water or ambient temperatures make a difference? 6. Do light levels make a difference? 7. Would the water height start to decrease the longer we left the tubes inverted? Could you make a chart showing the water height? Question Scientist s Hypotheses 1. If we used a greater number of seeds, then the water height will be higher than the tube using fewer seeds because of the greater amount of carbon dioxide. 2. If we use dry seeds, the water height will be lower than the tube with the soaked seeds because the dry seeds would not respire as much. 3. If seed size contributes to the amount of carbon dioxide that is respired and absorbed, then larger seeds will cause the water height to be greater than smaller seeds. 4. If a seed has already sprouted, then the water height would be lower, because the sprout will be using the carbon dioxide for photosynthesis. 5. If temperature affects the carbon dioxide absorption, then colder temperatures will have smaller water heights than warmer temperatures 6. If the light levels impact cellular respiration, then the water height will remain lower when it is darker and higher when it is lighter. 7. If the time the seeds are left to respire impacts the carbon dioxide levels, then the longer the test tubes are inverted, the less carbon dioxide would be available because the seeds would have used up their available food molecules. Source: James R. Jackson, Portage, Inc. 6 P age

7 Conclusion Metabolism is an important part of how an organism is able to sustain life. It is a process of chemical interactions and transformations that allow each cell to receive the energy it needs and to use it. Cellular respiration is a vital part metabolism, because it is the process of taking food molecules and converting them to chemical energy (ATP). During this lesson you had the opportunity to learn about the steps involved in cellular respiration and some of the chemical processes that occur at each step. Also, you were able to see cellular respiration in action by performing a scientific activity focused on measuring a byproduct of the process. The intent of this lesson was to provide a basic understanding of cellular respiration, one that you can use to learn more about the unique chemical interactions, their by-products, and about how cells use ATP. Fun Fact Some plants provide additional growing material around their seeds. For example, the fruit around an apple seed is designed to provide extra nutrients for the seed after it falls from the tree. It just so happens that those extra nutrients are also pretty good to eat! Some seeds, like a coconut, are designed to float so they can find new areas to grow, and they have coconut water inside to give the giant seed moisture after it reaches new ground. Sometimes a coconut sapling will already have germinated while it is still floating in the ocean! Definitions Acetyl CoA. An important molecule in metabolism that is made from pyruvate and is used in many biochemical reactions, including the Krebs cycle. Adenosine triphosphate (ATP). A coenzyme that carries chemical energy within the cells for metabolism. Aerobic. Any process that requires oxygen. Cellular respiration. A combustion (aerobic) reaction that takes place in the cells or organisms to convert the energy available from a food source into adenosine triphosphate (ATP) and then release specific waste products. Coenzyme. A small molecule that cannot cause a reaction by itself, but it can help an enzyme (see def.) to do so. In technical terms, coenzymes are organic, nonprotein molecules that bind with a protein molecule (called an apoenzyme) to form an active enzyme (called a holoenzyme). Enzyme. A biological molecule (protein) that acts as a catalyst and helps complex reactions occur everywhere in life. For example, when an average person eats a piece of meat, various proteases (a type of enzyme within the person s digestive tract), go to work to help break down/digest the protein. FADH 2. Flavin adenine dinucleotide reduced. A molecule that is an important carrier of electrons. It is essential for the creation of ATP. 7 P age

8 Glycolysis. The metabolic pathway (meaning a series of chemical reactions) that converts glucose into pyruvate. The free energy released in this process is used in the (Krebs cycle [see def.]) to form the highenergy compounds ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide). Krebs cycle. A group of metabolic processes that breakdown all of the sugars, amino acids, and fatty acids in order to prepare them for energy extraction. Metabolism. The set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. NADH. Nicotinamide adenine dinucleotide. A molecule that is essential for the creation of ATP. Oxidative phosphorylation. A metabolic pathway (meaning series of chemical reactions) in which the mitochondria in cells use their structure, enzymes, and energy released by the oxidation of nutrients to form ATP. Pyruvate. A molecule that supplies energy to living cells through the Krebs cycle when oxygen is present but ferments to produce lactate (lactic acid) when oxygen is lacking. Illustration courtesy of James R. Jackson, Portage, Inc. 8 P age

9 References Pearson, 2014, BioCoach Activity Cell Respiration, Pearson Education, Inc., website visited February 1, 2015, Phelan, Jay, 2010, What is Life? A Guide to Biology, W.H. Freeman & Co., and Sumanas, Inc., website visited February 1, 2015, Vaccaro, Lynn, 2014, What Makes a Seed Breathe Faster? Teachers Guide, Cornell Science Inquiry Partnerships, webpage visited, February 4, 2015, Contact us Website: Physics@portageinc.com Mahan, Rachel, 2012, What Does Fast Metabolism Mean?, Live Science, published December 2012, webpage visited February 4, 2015, Physics/ ?ref=br_tf 9 P age