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SH1631 Transportation of Energy in Organisms Bioenergetics Autotrophs vs. Heterotrophs Two (2) types of autotrophs Organisms that get energy from the sun – the best-known autotroph that harness solar energy through the process of photosynthesis. Through photosynthesis, these autotrophs use light energy to power chemical reactions that convert carbon dioxide and water into oxygen and energy-rich carbohydrates such as sugars and starches. Organisms that use energy from chemicals – these autotrophs produce energy without light. They rely on energy within the chemical bonds of inorganic molecules such as hydrogen sulfide. Chemosynthesis is the process where organisms use chemical energy to produce carbohydrates. This process is performed by several types of bacteria. Some chemosynthetic bacteria live in very remote places on Earth, such as volcanic vents on the deep ocean floor and hot springs. Others live in more common places, such as tidal marshes along the coast. There are many organisms such as animals, fungi, and many bacteria, cannot harness energy directly from the physical environment as autotrophs do. These organisms rely on other organisms for their energy and food supply. They are called heterotrophs or consumers. Types of Heterotrophs Herbivores – eat only plants. Examples are cows, caterpillars, and deer. Carnivores - eat animals. Examples are snakes, owls, and dogs. Omnivores – eat both plants and animals. Examples are bears, crows, and humans. Detritivores – feed on plant and animal remains and other dead matter. Examples are mites, earthworms, snails, and crabs. Decomposers – breaks down organic matter. Examples are bacteria and fungi. Chloroplasts and Photosynthesis Chloroplasts are organelles found in plant cells and some other eukaryotic organisms. Aside from conducting photosynthesis, they also carry out almost all fatty acid synthesis in plants, and are involved in a plant's immune response. A chloroplast is a type of plastid that specializes in photosynthesis. The chloroplasts contain saclike photosynthetic membranes called thylakoids. Thylakoids are arranged in stacks called grana (singular: granum). Thylakoids contain clusters of chlorophyll and other pigments and protein called photosystems that are able to capture the energy from the sunlight. Photosynthesis Photosynthesis is one (1) of the most vital biochemical processes since almost all the living organisms depend on it for nutrition directly or indirectly. It is the process by which several living organisms utilize solar energy (that is, sunlight) for growth and metabolism. By definition, photosynthesis is described as the process of converting light energy into chemical energy by living organisms. 05 Handout 1 *Property of STI Page 1 of 7 SH1631 The process of photosynthesis uses raw materials like carbon dioxide, water, and solar energy to produce oxygen and carbohydrates. Higher plants, phytoplankton, algae, as well as some bacteria carry out the process of photosynthesis. Because photosynthesis usually produces 6-carbon sugars (C6H12O6) as its final products, the overall equation for photosynthesis can be shown as follows: Commons.wikimedia.org To understand photosynthesis, scientists break the reaction into two (2) stages: the light-dependent reactions and the light-independent reactions, or Calvin cycle. Before going to details of the stages of photosynthesis, discuss electron carriers: NADP+ and NADPH When sunlight excites electron in chlorophyll, the electrons gain a great deal of energy. These highenergy electrons require a special carrier. Think of a high-energy electron as being similar to a red-hot coal from a fireplace or campfire. If you wanted to move the coal from one (1) place to another, you wouldn’t want to pick it up with your own hands. You would use a pan or bucket, a carrier, to transport it. Cells treat high-energy electrons the same way. Cells use electron carriers to transfer high-energy electrons to other molecules. One (1) of these carrier molecules is a compound known as NADP+ (nicotinamide adenine dinucleotide phosphate). NADP+ accepts and holds two (2) high energy electrons along with the hydrogen ion (H+). This converts the NADP+ to NADPH. The conversion of NADP+ into NADPH is one way in which some of the energy from the sunlight can be trapped in chemical form. The NADPH can carry high-energy electrons produced by light absorption in chlorophyll to chemical reactions elsewhere in the cell. The high-energy electrons are used to build variety of molecules the cell needs, including carbohydrates like glucose. Two (2) Stages of Photosynthesis: I. Light-dependent Reactions The light-dependent reactions use energy from the sunlight to produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH. The light-dependent reactions take place within the thylakoid membranes of the chloroplasts. Steps in Light-dependent Reactions: a) Photosystem II (The first photosystem in the light-dependent reaction is called photosystem II because it was discovered after photosystem I) - Light is absorbed by chlorophyll or other pigments in photosystem II. The energy from this light is transferred to electrons, which are then passed on to the electron transport chain. Separately, enzymes break up water molecules into electrons, hydrogen ions (H+), and oxygen. b) Electron Transport Chain – High energy electrons from photosystem II move through the electron transport chain to photosystem I. The molecules in the electron transport chain use energy from electrons to transport hydrogen ions from the stroma into the inner thylakoid. c) Photosystem I – As in photosystem II, pigments add energy from light to the electrons. The high-energy electrons are then picked up by NADP+ to form NADPH. 05 Handout 1 *Property of STI Page 2 of 7 SH1631 d) Hydrogen Ion Movement – As a result of the H+ ions released during water-splitting and electron transport, the inside of the thylakoid membranes becomes positively charged and the outside of becomes negatively charged. The difference in the charges across the membrane provide energy to make ATP. e) ATP Formation – Hydrogen ions will pass through ATP synthase. ATP synthase binds ADP and converts it to ATP. Photosystem II Electron Transport Chain Photosystem I Hydrogen Ion Movement ATP Formation II. Light-independent Reactions or Calvin Cycle The ATP and NADPH formed by light-dependent reactions are not stable enough to store energy for more than a few minutes. The Calvin cycle uses ATP and NADPH from the light reactions to form high-energy sugars that can be stored for a long time. The Calvin cycle occurs in the stroma. This cycle is named after the American scientist Melvin Calvin, who worked out the details of this remarkable cycle. Because the Calvin cycle does not require light, this is also called the lightindependent reactions. Steps in Calvin cycle: a) CO2 Enters the Cycle – Six (6) carbon dioxide molecules enter the cycle from the atmosphere. The carbon dioxide molecules combine with six 5-carbon molecules. The result is twelve 3-carbon molecules. b) Energy Input – The twelve 3-carbon molecules are then converted into higher energy forms. The energy for this conversion comes from ATP and high-energy electrons from NADPH. c) 6-Carbon Sugar Produced – Two (2) of the twelve 3-carbon molecules are converted into two (2) similar 3-carbon molecules. These 3-carbon molecules are used to form various 6-carbon sugars and other compounds d) 5-Carbon Molecules Regenerated – The remaining ten 3-carbon molecules are converted back into the 5-carbon molecules to begin the next cycle. CO2 enters the Cycle Energy Input 6-Carbon Sugar Produced 5-Carbon Molecules Regenerated The Calvin cycle uses six (6) molecules of carbon dioxide to produce a single 6-carbon sugar molecule. As photosynthesis proceeds, the Calvin cycle works steadily, turning out energy-rich sugars and removing carbon dioxide from the atmosphere. The plant uses sugars for energy and to build more 05 Handout 1 *Property of STI Page 3 of 7 SH1631 complex carbohydrates such as starches and cellulose, which is important form plant growth and development. When other organisms eat plants, they can also use the energy stored in carbohydrates. Cellular Respiration All organisms need energy to function and we get this energy from the food we eat. The most efficient way for cells to harvest energy stored in food is through cellular respiration. Cellular respiration is the process that releases energy by breaking down food molecules in the presence of oxygen. The Equation for cellular respiration is: http://cronodon.com Three (3) main stages of cellular respiration I. Glycolysis Literally means “splitting sugars”. Glucose, a six-carbon sugar, is split into two (2) molecules of a three-carbon sugar. In the process, two (2) molecules of ATP, two (2) molecules of pyruvic acid, and two (2) “high energy” electron carrying molecules of NADH are produced. Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. Without oxygen, glycolysis allows cells to make small amounts of ATP. This process is called fermentation. ATP production – to get glycolysis going, the cell uses energy in the form of two (2) ATP molecules. When glycolysis is complete, four (4) ATP molecules are produced. This gives the cell a net gain of two (2) ATP molecules. NADH production – one (1) of the reactions in glycolysis removes four (4) high-energy electrons and passes them to an electron carrier called NAD+ (nicotinamide adenine dinucleotide). Like NADP+ in photosynthesis, each NAD+ accepts a pair of high energy electrons. This molecule, known as NADH, holds the electrons until they can be transferred to other molecules. Pyruvic acid production – during glycolysis, glucose is broken down into two (2) molecules of pyruvic acid. Fermentation – when oxygen is not present, glycolysis is followed by a different pathway. The combined process of this pathway and glycolysis is called fermentation. Because fermentation does not require oxygen, it is also referred to as anaerobic respiration. The term anaerobic means “not in air”. The two (2) main types of fermentation are the following: Alcoholic Fermentation – used by yeast and a few other microorganisms that forms ethyl alcohol and carbon dioxides as wastes. When yeast in dough runs out of oxygen, it begins to ferment, giving off bubbles of carbon dioxide, which form the air spaces in a slice of bread. The small amount of alcohol produced in dough evaporates when the bread is baked. Lactic Acid Fermentation – pyruvic acid that accumulates as a result of glycolysis are converted to lactic acid. Lactic acid is produced in your muscles during rapid exercise when your body cannot supply enough oxygen to the tissues. Your muscles rapidly begin to 05 Handout 1 *Property of STI Page 4 of 7 SH1631 produce ATP by lactic acid fermentation. The buildup of lactic acid causes a painful, burning sensation. This is why your muscles may feel sore after only a few seconds of intense activity. II. Citric Acid Cycle The Citric Cycle, also referred as Krebs cycle, is named after Hans Krebs, the British biochemist who demonstrated its existence in 1937. In the presence of oxygen, pyruvic acid produced in glycolysis passes through the Krebs cycle. During the cycle, pyruvic acid is broken down into carbon dioxide in a series of energy extracting reactions. The first compound formed in this series of reactions is citric acid, thus the term citric acid cycle. Citric acid production – As pyruvic acid enters the mitochondrion, a carbon is removed, forming CO2, and electrons are removed, changing NAD+ to NADH. Coenzyme A joins the 2carbon molecule, forming acetyl-CoA. Acetyl-CoA then adds the two (2) carbon acetyl group to a 4-carbon compound, forming citric acid. Energy extraction – Citric acid is broken down into a 5-carbon compound, then into a 4-carbon compound. Along the way, two (2) more molecules of CO2 are released, and electrons join NAD+ and FAD (flavine adenine dinucleotide), forming NADH and FADH2. In addition, one (1) molecule of ATP is generated. The energy tally from one (1) molecule of pyruvic acid is four (4) NADH, one (1) FADH2, and one (1) molecule of ATP. The Krebs cycle spins round and round, generating high energy electrons that are passed to NADH and FADH2. III. Electron Transport The electron transport chain uses high energy electrons from the Krebs cycle to convert ADP into ATP. High energy electrons from NADH and FADH2 are passed into and along the electron transport chain. In eukaryotes, the electron transport chain is composed of a series of carrier proteins located in the inner membrane of the mitochondrion. In prokaryotes, the same chain is in the cell membrane. High energy electrons are passed from one carrier protein to the next. At the end of the electron transport chain is an enzyme that combines electrons from the electron chain with hydrogen ions and oxygen to form water. Oxygen serves as the final electron acceptor of the electron transport chain. It is essential for getting rid of low-energy electrons and hydrogen ions, the wastes of cellular respiration. Every time two (2) high energy electrons transport down the electron transport chain, their energy is used to transport hydrogen ions (H+) across the membrane. During electron transport, H+ ions build up in the intermembrane space, making it positively charged. The other side of the membrane, from which those H+ ions have been taken, is now negatively charged. The inner membranes of the mitochondria contain protein spheres called ATP synthases. As H+ ions escape through channels into these proteins, the ATP synthases spin. Each time it rotates, the enzyme grabs a low-energy ADP and attaches a phosphate, forming high energy ATP. On an average, each pair of high-energy electrons that moves down the electron transport chain provides enough energy to convert 3 ADP molecules into 3 ATP molecules. In the presence of oxygen, the Krebs cycle and electron transport enable the cell to produce 34 more ATP molecules per glucose molecule in addition to the two (2) ATP molecules obtained from glycolysis. 05 Handout 1 *Property of STI Page 5 of 7 SH1631 The 36 ATP molecules the cell makes per glucose represent 38% of the total energy of glucose. The 62% is released as heat, which is one (1) of the reasons your body feels warmer during vigorous exercise. http://cresearch.co.uk Function Location Reactants Products Equation Photosynthesis Energy Storage Chloroplasts CO2 and H2O C6H12O6 and O2 6CO2 + 6H2O C6H12O6 + 6O2 Cellular Respiration Energy Release Mitochondria C6H12O6 and O2 CO2 and H2O 6O2 + C6H12O6 6CO2 + 6H2O Photosynthesis and cellular respiration are almost opposite processes: Photosynthesis stores energy and cellular respiration withdraws energy. The equations of photosynthesis and cellular respiration are the reverse of each other. Photosynthesis removes carbon dioxide from the environment and cellular respiration puts it back. Photosynthesis releases oxygen into the atmosphere and cellular respiration uses that oxygen to release energy from food. Products of photosynthesis are similar to the reactants of cellular respiration and vice versa. Cellular respiration takes place in all eukaryotes and some prokaryotes while photosynthesis only occurs in plants, algae and some bacteria. The Energy Flow Feeding Relationships: Food Chain – illustrates that energy stored by producers can be passed through an ecosystem. This is a series of steps in which organisms transfer energy by eating and being eaten. 05 Handout 1 *Property of STI Page 6 of 7 SH1631 http://zarkanderson.com Food Webs – show feeding relationships among various organisms in an ecosystem form a network of interactions. A food web links together all the food chains in an ecosystem. Trophic Level – refers to each step in a food chain or food web. Producers make up the first level. Consumers make up the second. Each consumer depends on the trophic level below it for energy. Only about 10% of the energy available within a trophic level is transferred to organisms at the next trophic level. http://goldridge08.com For instance, 1/10 of the solar energy captured by grasses ends up stored in tissues of cows and other grazers. Only 1/10 of that energy or 1% in total is transferred to humans when they eat cows. Thus, the more levels that exist between a producer and a top-level consumer in an ecosystem, the less energy that remains from the original amount. References: Bioenergetics. (n.d.). Retrieved July 28, 2014 from: http://www.merriamwebster.com/dictionary/bioenergetics Cellular Respiration. (n.d.). Retrieved July 30, 2014 from: http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm Miller, K & Levine, J. (2002). Biology. Upper Saddle River, NJ: Pearson Education, Inc. 05 Handout 1 *Property of STI Page 7 of 7