Cellular Respiration NAME ONE THING we have in common with plants. If you said cellular respiration, you are right. That is one thing we have in common with plants, slugs, slime mold, and spiders. Living organisms conduct cellular respiration. Objective: Explain the processes of cellular respiration. Key Terms: aerobic respiration anabolism anaerobic respiration catabolism cellular respiration chemiosmosis citric acid cycle cytosol electron transport system (ETS) enzymes fermentation glycolysis metabolism oxidation redox reactions reduction The Processes of Cellular Respiration The sugars produced during photosynthesis provide the energy used to run life processes in plant cells. The transfer of energy from molecules to produce adenosine triphosphate (ATP) is known as cellular respiration, and it occurs in living cells. METABOLISM Living plants are active. Plant cells grow, divide, develop, make substances, and conduct many processes for the maintenance of life. Plants transform energy and matter for their own use/purpose. All of the physical and chemical processes involved in plant life fall under the term metabolism. Page 1 www.mycaert.com
Anabolism and Catabolism Metabolic processes entail the combining and the splitting of molecules. The process called anabolism is the assembly of complex molecules from simple molecules. Anabolism is the process by which plants store energy during photosynthesis, produce new materials for the cellular activities, and grow. Anabolic reactions usually require chemical energy, mainly in the form of ATP. The breakdown of larger molecules into smaller molecules is an element of metabolism known as catabolism. An example of a catabolic reaction is the breakdown of carbohydrates manufactured during photosynthesis to release ATP. Photosynthesis CO 2 and H 2 O are used. Food and O 2 are produced. Energy from light is trapped by chlorophyll and food. Only cells containing chlorophyll carry out photosynthesis. Photosynthesis occurs only in light. Respiration O 2 and food are used. CO 2 and H 2 O are produced. Every living cell carries out respiration. Respiration occurs in light and dark. AEROBIC RESPIRATION The most common catabolic reaction in plants involves the breakdown of glucose. This process requires oxygen and is known as aerobic respiration. The process occurs in the mitochondria. Aerobic respiration is essentially the reverse of photosynthesis. Oxygen is one critical ingredient to the reaction, and water is another component. In the process, chemical energy is released when the molecular bonds of the sugar molecules are broken. The extracted energy, ATP, drives a variety of chemical reactions that are important to plant growth and development. Byproducts of the reaction are carbon dioxide and water. A simple equation for aerobic respiration is as follows: C 6 H 12 O 6 +6O 2 +6H 2 O 6CO 2 +12H 2 O + Energy (as ATP) Plant growth takes place primarily at night when photosynthesis is shut down. It is fueled by aerobic respiration. With signals from hormones, enzymes (chemical activators) are produced. Each enzyme has one specific job. With split-second timing, the enzymes break down sugars and recombine them with nitrogen and other minerals. Many complex molecules are produced, including starches; pectin to bind cells; lignin, which is a tough, durable substance; cellulose; lipids; proteins; pigments; hormones; vitamins; and alkaloids and tannins, which protect plants from pests and diseases. Stages of Aerobic Respiration Aerobic respiration of glucose involves four primary stages. Page 2 www.mycaert.com
The first stage of aerobic respiration is glycolysis, which is the splitting of sugar. A specific enzyme is the catalyst for this reaction that involves approximately 10 steps and does not require oxygen. Glycolysis occurs in the cytosol, which is the fluid of a cell in which organelles are suspended. Six-carbon glucose molecules are split into two three-carbon pyruvate molecules with a net profit of two ATP and two NADH molecules. Stage two involves the formation of acetyl coenzyme A (acetyl CoA). Pyruvate molecules enter the mitochondria. Then each pyruvate molecule becomes a two-carbon molecule (acetate) through oxidation. The acetate combines with coenzyme A to form acetyl CoA. In the process, carbon dioxide is removed and hydrogen atoms are contributed to the production of NADH. The third stage is the citric acid cycle (also known as the Krebs cycle) and consists of eight steps that occur in the mitochondria. Acetyl CoA combines with a four-carbon molecule to form a six-carbon citrate molecule. In a series of events, the citrate reforms a four-carbon molecule. With each turn of the cycle, one ATP and two carbon dioxide molecules are released. NADH and FADH2 are produced, too. The cycle must turn twice to process one glucose molecule. Throughout the first three steps, hydrogen atoms were transferred to hydrogen acceptors NAD and FAD, forming NADH and FADH2. In the final stage of aerobic respiration, these reduced compounds enter the electron transport system, which is a series of chemical reactions by which electrons are passed from one acceptor molecule to another. Oxygen is the final hydrogen acceptor. During this pathway, ATP is synthesized during the chemiosmosis process in which hydrogen atoms cross the thylakoid membrane and travel down a protein gradient, producing ATP. The fourth stage sees the production of 32 ATP molecules. In addition, water is made and released. Transfer of Energy NADH NADH CO 2 FADH 2 Glycolysis Pyruvate molecules Acetyl CoA Citric acid cycle (Kreb s cycle) Electron transport chain NADH Energy in cells is transferred by the flow of protons and electrons. A chemical reaction in which a substance loses electrons is called oxidation. In contrast, substances gain electrons through the chemical process of reduction. Electrons released during oxidation cannot exist ATP CO 2 ATP 32 ATP STAGE 1 STAGE 2 STAGE 3 STAGE 4 FIGURE 1. Aerobic respiration of glucose involves four primary stages. Page 3 www.mycaert.com
Oxidation independently. Therefore, every oxidation reaction is accompanied by a reduction reaction. The reactions are called oxidation-reduction or redox reactions. Oxidation-reduction reactions are crucial to photosynthesis, cellular respiration, and other chemical reactions that occur in cells. During aerobic respiration, glucose or other forms of carbohydrates are broken down in the presence of water and oxygen. The products are energy in the form of ATP, carbon dioxide, and water. In aerobic respiration, glucose is oxidized and oxygen is reduced. In the multi-step reaction, hydrogen is transferred from the glucose molecule to oxygen. Energy from the hydrogen electrons is used for ATP synthesis. + + Electron Reduction + + FIGURE 2. Substances lose electrons during oxidation and gain electrons through reduction. Factors Affecting Respiration The speed at which respiration and other metabolic reactions occur is influenced by numerous factors. For example, environmental factors (e.g., oxygen levels, light, water, and temperature) influence respiration. Even the age of the plant influences the rate of aerobic respiration. In some instances, people in agriculture want to increase the rate of cellular respiration; in other situations, people strive to lower the rate of cellular respiration. Oxygen Oxygen is required for aerobic respiration. If levels of oxygen are reduced in the atmosphere or soil, respiration slows. Compacted soils and soils saturated with water impact the aerobic respiration rate. In the case of flooding, the soil pores are filled with water, and oxygen is lacking. Without available oxygen, respiration does not occur in the root cells. The roots of most plants can survive only a short time without oxygen. In time, however, root cells and ultimately root systems die in waterlogged soil. The first symptom of stress is wilting, which is the same symptom that occurs with drought. The symptom occurs because the roots are unable to absorb water. Another symptom is discoloration of the roots as the tissues die. In another example, harvested fruits and vegetables are sometimes placed in storage facilities where oxygen levels are artificially reduced.a1to3percent drop in oxygen levels is enough to slow respiration and ripening rates. Page 4 www.mycaert.com
Light Low light conditions influence respiration as photosynthesis operates at a slower pace. As a result, plants growing in low light conditions produce fewer carbohydrates. Low levels of carbohydrates, in turn, mean low rates of respiration and less energy for metabolic reactions. Plants grown in lower than ideal light conditions look less vigorous and lack the health of counterparts grown under higher light conditions. Water Water is essential for cellular respiration and photosynthesis. Enzyme activity grinds to a halt without water, and the plant essentially stops growing. Temperature FIGURE 3. The growth of new plant leaves is fueled by aerobic respiration and takes place primarily at night when photosynthesis is shut down. Temperature has a role in most metabolic processes. The rate of cellular respiration increases as the temperature rises. Conversely, the rate slows in cool or cold temperatures, which is why plant growth is more rapid in warm weather than in cold weather. The effect of temperature on aerobic respiration is applied in the agriculture industry. Harvested fruits and vegetables are refrigerated to slow cellular respiration and delay ripening. Cut flowers in the horticulture industry also are kept in coolers to slow natural aging. The seeds are planted when soil temperatures increase to improve germination success rates. HELP WANTED ON THE JOB CAREER CONNECTION: Plant Physiologist Plant physiologists are interested in the functions of plants. They study the chemical and physical processes that take place in plants. Studies range from the molecular level to the entire plant. Plant physiologists might specialize in plant reproduction, plant growth, protein synthesis, photosynthesis, cellular respiration, tropisms, or plant nutrition. They might also choose to study cells, cell membranes, enzyme activity, plant growth regulators, and material transport. Plant physiologists are required to have advanced training in botany, chemistry, biology, and mathematics. Plant physiologists find employment with educational institutions, state and federal agencies, and corporations. Page 5 www.mycaert.com
Stage of Growth The stage of growth and the age of the plant influence the rate of respiration needed to maintain the life processes. Young actively growing plants tend to have higher rates of respiration than older plants. ANEROBIC RESPIRATION Some cells perform a less efficient form of respiration called anaerobic respiration, which does not require oxygen. Cells that rely on anaerobic respiration are found in the soil or in water where oxygen is in short supply. Some bacteria and fungi adapted to anaerobic respiration convert energy by fermentation (a chemical change with effervescence). Humans recognize the value of fermentation in the making of silage, beer, and wine. Summary: The transfer of energy stored in molecules to produce ATP is known as cellular respiration. All of the physical and chemical processes involved in the life of a plant fall under the term metabolism. The assembly of complex molecules from simple molecules is a process called anabolism. The breakdown of larger molecules into smaller molecules is an element of metabolism known as catabolism. Aerobic respiration is the most common catabolic reaction in plants and involves the breakdown of glucose. Aerobic respiration of glucose involves four primary stages: glycolysis, the formation of acetyl coenzyme A, the citric acid cycle, and the entry of reduced compounds to the electron transport system. Oxidation-reduction reactions are crucial to photosynthesis, cellular respiration, and other chemical reactions that occur in cells. The speed at which respiration and other metabolic reactions occur is influenced by oxygen levels, light, water, temperature, and the age of the plant. Checking Your Knowledge: 1. What is metabolism? 2. How does cellular respiration compare to photosynthesis? 3. What are the stages of aerobic respiration? 4. What are oxidation and reduction? 5. What are factors that influence cellular respiration? Page 6 www.mycaert.com
Expanding Your Knowledge: Conduct an activity whereby you sow seeds in two different pots. Place one pot at room temperature and another in a refrigerator. Watch what happens to the two pots over 7 to 10 days. Another activity would be to obtain fresh cut flowers. Cut the stems of two flowers and place them in separate vases with water. Keep one vase at room temperature and the other in a refrigerator for 7 to 10 days. Analyze the results. Web Links: Cellular Respiration http://encarta.msn.com/encyclopedia_1741500937/cellular_respiration.html Cellular Respiration http://www.eng.auburn.edu/~wfgale/usda_course/section0_2_page_3.htm #aerobicrespiration Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways http://www.uic.edu/classes/bios/bios100/lectf03am/lect11.htm Respiration and Photosynthesis http://staff.gc.maricopa.edu/~bcooper/prrespho.htm Agricultural Career Profiles http://www.mycaert.com/career-profiles Page 7 www.mycaert.com