contents Principles of Biology 24 Biological Energy Transfer Cellular respiration involves the stepwise transfer of energy and electrons.
Large-scale municipal composting. The steam emitted from the compost as it is being turned is evidence of trillions of microorganisms performing cellular respiration, breaking down molecules in the compost and generating energy, some in the form of heat. Nancy J. Pierce/Science Source.
Topics Covered in this Module
How Do Organisms Obtain Energy? Redox Reactions An Outline of the Stages in Cellular Respiration
Major Objectives of this Module
Describe the relationships among photosynthesis, respiration, producers, and consumers. Explain how reduction-oxidation (redox) reactions work. Describe how aerobic cellular respiration breaks down fuel molecules and releases energy for cellular work.
page 121 of 989 4 pages left in this module
contents Principles of Biology
24 Biological Energy Transfer How Do Organisms Obtain Energy? No matter how wealthy you become in life, you will always work for a living. That is, your cells will always be working to keep you alive. From synthesizing proteins to producing gametes to chasing down prey, living cells are constantly at work, and all that work requires energy. Where does that energy come from? Ultimately it comes from our Sun as light energy. Energy flows through all living systems. Plants, algae, and photosynthetic bacteria use energy from sunlight to generate sugar molecules through the process of photosynthesis. Such organisms, known as photoautotrophic producers, convert the radiant energy of sunlight into chemical energy that they store in sugars and other organic compounds. Other organisms, termed heterotrophic consumers, must acquire the chemical energy they need by ingesting or absorbing organic molecules from other organisms. In both autotrophs and heterotrophs, cellular respiration is the process that releases energy through the breakdown of these food sources. This energy is then used to fuel cellular processes. A common misconception is that producers, such as plants, only conduct photosynthesis and need not perform cellular respiration. However, producers have to conduct respiration to break down the organic molecules they have produced (Figure 1), just as consumers do in order to release chemical energy from the food they consume. Carbohydrates, fats, and proteins can all be metabolized via cellular respiration. Each of these large molecules is broken down into its smaller components, namely, sugars, smaller fats, and amino acids. These smaller molecules are fed into the cellular respiration process, where chemical energy is generated. Fats are particularly generous energy producers. A gram of fat yields more than twice as much ATP as a gram of carbohydrate. A gram of fat has 9 kcal or 38 kJ. (Nutritional calories, which are the values reported on food packages, are actually kilocalories or kcal.) Carbohydrates and protein each have 4 kcal per gram. Figure 1: Overview of photosynthesis and aerobic respiration. Photosynthetic organisms — plants, algae, and some bacteria — use light energy to generate organic molecules from carbon dioxide and water. All organisms, including photosynthetic organisms, perform some form of cellular respiration to break down organic molecules. In aerobic respiration, molecular oxygen is consumed and water and carbon dioxide are produced. Some of the energy released from the organic molecules is temporarily stored in ATP. The exergonic hydrolysis of ATP can then be coupled to endergonic reactions; some energy is always released as heat. © 2013 Nature Education All rights reserved.
Energy flows into an ecosystem from the Sun and moves from producers to consumers through a food web. We can follow the energy flow through an ecosystem a bit more closely using an energy pyramid diagram, which shows how much energy is present at different levels within a food web (Figure 2). Photosynthesizers (the plants in Figure 2) capture only a small fraction of the energy from the solar radiation that strikes the Earth (usually less than 1%), which they convert into stored chemical energy. Once producers convert radiant energy into chemical energy, this energy becomes available to consumers. At the next level, primary consumers are herbivores, which can range in size from tiny worms and insects to massive elephants and bison. Although each species converts consumed plant energy into useful energy at a particular efficiency, 10% efficiency of transfer is a typical rule of thumb. Likewise, only a small proportion of the energy tied up in herbivores makes it up to the level of secondary consumers, which typically feed on herbivores but are themselves often prey to larger, tertiary, or top level, consumers. In all, only the tiniest fraction of energy that enters a given ecosystem through photosynthesis at the base makes it up to these top consumers (Figure 2).
Figure 2: An idealized pyramid of energy flow from the Sun into a hypothetical terrestrial food web. Typically, less than 1% of the solar radiation that is available to primary producers is converted into stored chemical energy through photosynthesis. After producers store chemical energy (given as 100% because it can vary among ecosystems), a rule of thumb is that consumers can convert ~10% of the energy from lower trophic levels to their own stored energy (with the rest being lost to metabolism, waste, and other processes). Thus, primary consumers (herbivores) contain ~10% of the energy that was present in the producers they consumed; secondary consumers (predators) have 1% of the energy that was present in the consumed producers; and tertiary consumers have 0.1% of the energy that was present in the consumed producers. Food webs generally end at tertiary consumers because of the exponential loss of energy at each higher level. © 2014 Nature Education All rights reserved.
Test Yourself
Calculate how many kilocalories of energy a tertiary consumer receives from a producer that takes in 120 kilocalories. Assume for your calculations that 10% of the energy at each level of the food chain is passed on.
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Catabolic processes break down fuel and release energy. So just how does the oatmeal you ate for breakfast become the energy you use to do work after class? Organic molecules in food, such as carbohydrates, fats and proteins, contain potential energy in the chemical bonds between their atoms. Catabolic processes in the cell decompose large organic molecules into simpler products, releasing the energy held in their chemical bonds. Most of this energy is transferred to and stored in the chemical bonds of adenosine triphosphate (ATP), the molecule that provides the energy to drive the majority of metabolic processes in living organisms.
The 6-carbon sugar glucose (C6H12O6) is the most important fuel for cellular respiration. The complete breakdown of glucose during aerobic cellular respiration generates a large amount of ATP in a process that requires oxygen to function. In some organisms, organic molecules can be catabolized without using oxygen. One such anaerobic process is fermentation, in which sugars are partially broken down to produce ATP. Other organisms use anaerobic respiration, in which sulfate or nitrate serve the role that oxygen does in cellular respiration. The overall catabolism of glucose in aerobic cellular respiration can be represented by the following chemical equation:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O ∆G = -686 kcal/mol (-2870 kJ/mol) The negative free energy change indicates that the reaction releases energy. However, if cellular respiration took place in a single step and all of the energy was released at once, it would be a massive release of heat that would likely destroy the cell. To avoid self-destructing and to capture this energy, the cell breaks down glucose over many steps. In this manner, the release of energy is controlled — more is captured and converted to ATP, and less is lost as heat.
Redox Reactions During the cellular respiration reactions, what happens that enables the energy of bond breaking to be captured and converted into the formation of ATP? Simply put, a series of chemical reactions moves electrons between molecules. The transfer of electrons releases stored energy that can be used to attach inorganic phosphate to ADP to form ATP. These electron- transferring chemical reactions are called reduction-oxidation reactions, or redox reactions for short. In a redox reaction, two simultaneous reactions are occurring. One reactant donates electrons to another reactant. The electron donor becomes oxidized as it loses electrons. The other reactant, the electron acceptor, becomes reduced as it gains the electrons. Redox reactions are very common in chemistry. For example, one type of redox reaction occurs when elemental magnesium (Mg) and molecular oxygen (O2) combine to form magnesium oxide (MgO). In this reaction, two magnesium atoms donate four electrons to the oxygen molecule (O2). As a result, the magnesium atoms are oxidized into Mg2+ ions, each with a 2+ charge. On the other hand, the oxygen molecule is reduced upon accepting the four electrons, resulting in two O2- ions. The oxidized Mg2+ ions and reduced O2- ions combine to form magnesium oxide (Figure 3).
Figure 3: Redox reaction of magnesium and oxygen. A reactant that donates electrons is oxidized, which results in a positive charge. The reactant that accepts the electrons is reduced, which results in a negative charge. © 2013 Nature Education All rights reserved.
Test Yourself
Determine which atom is oxidized and which is reduced in the following reaction: 2 Al + 3 Br2
→ Al2Br6
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Electronegativity and redox reactions. What drives electron movement during redox reactions? Electrons tend to be found at different distances from nuclei in different atoms. Because negatively charged electrons are attracted to positively charged nuclei, separating electrons from nuclei requires an input of energy. As a result, the potential energy of the electron increases with distance from the proton. Likewise, allowing an electron to move closer to a nucleus releases some of its potential energy, which can be used to drive other chemical reactions. When bonds between atoms change during reactions, electrons can either gain or release energy depending on how their position changes relative to nuclei. Electronegativity is a measure of the tendency of an atom to attract electrons. Because redox reactions move electrons closer to some atoms and farther away from others, differences in electronegativity allow us to predict the position of electrons in bonds and therefore the electrons' energy state. The periodic table shows electronegativity values for the elements (Figure 4).
Figure 4: Periodic table with electronegativities. This periodic table includes the electronegativity of most of the elements. Notice that oxygen and fluorine have especially high electronegativities. The electronegativities of the noble gases are zero and are not shown. © 2011 Nature Education All rights reserved. Figure Detail
Many redox reactions in the cell involve organic molecules such as carbohydrates. In these molecules, carbon and hydrogen atoms tend to become oxidized, while oxygen atoms tend to become reduced. Why? In a bond between atoms with a large difference in electronegativity, such as a hydroxyl group formed between oxygen and hydrogen, electrons tend to spend more time closer to the nucleus of the more electronegative oxygen atom (Figure 5).
Figure 5: Electron position in different bonds. In C–O and H–O bonds, the oxygen atom attracts electrons closer to itself because of its high electronegativity. © 2011 Nature Education All rights reserved. Figure Detail Now compare the electronegativity values of oxygen, carbon and hydrogen. Because oxygen has a higher electronegativity (3.5) than either carbon (2.5) or hydrogen (2.1), we predict that electrons will tend to be closer to the oxygen atom in either an O–C bond or an O–H bond. Thus, reactions that break bonds between hydrogen and carbon and create bonds with oxygen release energy because electrons are being allowed to move to a position of lower potential energy — closer to the more electronegative oxygen atom. The released energy can then be used to drive other chemical reactions in the cell. Test Yourself
Compare the electronegativity values of oxygen, carbon and sulfur. A chemical reaction breaks a C–S bond in an organic molecule and forms a C–O bond in its place. What is the energetic outcome of this reaction? Explain.
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Electrons and energy can be transferred through mobile carriers. Now let's apply the principles of redox reactions to cellular respiration. Recall the overall chemical equation for the catabolism of glucose during aerobic cellular respiration:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O ∆G = -686 kcal/mol (-2870 kJ/mol) During cellular respiration, energy is released through a series of redox reactions, instead of all at once as in direct combustion. As electrons move from higher energy states (i.e., farther from protons in a nucleus) to lower energy states (i.e., closer to protons in a nucleus), energy is released. Through a series of steps in aerobic respiration, glucose is oxidized into carbon dioxide, and oxygen is reduced into water. Because electrons are not transferred directly from glucose to molecular oxygen in aerobic respiration, electron carriers are required during the intermediate steps. Nicotinamide adenine dinucleotide (NAD+) is the most versatile electron carrier in aerobic cellular respiration (Figure 6). In an early step of cellular respiration, enzymes remove two hydrogen atoms — equivalent to two protons and two electrons — from an intermediate of glucose breakdown. One of the hydrogen atoms is released as a H+, while the remaining hydrogen atom and the two electrons are transferred to NAD+, reducing it to NADH. Additional molecules of NADH are formed in later steps. The NADH molecules carry both electrons and energy to the final steps of cellular respiration (Figure 7). In aerobic respiration, molecular oxygen ultimately accepts the electrons, and some of the energy can be used to synthesize ATP. Figure 6: NAD+ moves electrons. Nicotinamide adenine dinucleotide (NAD+) has two nucleotides. One of them has an adenine base, and the other has a nicotinamide group. The two nucleotides are connected by a diphosphate bridge. In simple terms, nicotinamide adenine dinucleotide can exist in its oxidized form (NAD+) or in its reduced form (NADH). By alternating between these forms, NAD+/NADH functions as an electron and energy carrier within the cell. © 2014 Nature Education All rights reserved.
Figure 7: NAD+/NADH functions as an energy and electron carrier in cellular respiration. NAD+ switches back and forth between its oxidized form and its reduced form, NADH, as it carries electrons and energy between phases of cellular respiration. © 2013 Nature Education All rights reserved. Test Yourself
Flavin adenine dinucleotide (FAD) is another electron acceptor used in cellular respiration. The
reduced form of FAD is FADH2. What does FAD accept in the redox reaction that transforms it + to FADH2? How does this differ from NAD /NADH?
Submit
IN THIS MODULE
How Do Organisms Obtain Energy? Redox Reactions An Outline of the Stages in Cellular Respiration Summary Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
Synthetic Biology: Making Life from Bits and Pieces Scientists are combining biology and engineering to change the world.
PRIMARY LITERATURE
How carbon dioxide in the atmosphere affects other greenhouse gases Increased soil emissions of potent greenhouse gases under increased atmospheric CO2. View | Download
Growing new heart cells to treat damaged hearts Conversion of mouse fibroblasts into cardiomyocytes using a direct reprogramming strategy. View | Download
Engineering polymers from E. coli, not petrochemicals Metabolic engineering of Escherichia coli for direct production of 1,4-butanediol. View | Download
Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download
How can nematodes help reduce obesity in humans? A whole-organism screen identifies new regulators of fat storage. View | Download
SCIENCE ON THE WEB
A Brief History of Mitochondria Learn about the origin and evolution of mitochondria
Create a High Energy Diet Explore the USDA's 2011 recommendations about a proper human diet
page 122 of 989 3 pages left in this module
contents Principles of Biology
24 Biological Energy Transfer An Outline of the Stages in Cellular Respiration The major stages of aerobic cellular respiration are glycolysis, the citric acid cycle and oxidative phosphorylation (Figure 8).
Figure 8: The major features of aerobic cellular respiration. Glucose, which originates from photosynthesis or consumption of other organisms, enters glycolysis. Through a series of coupled reactions, glucose is converted into smaller organic molecules. A small amount of ATP is formed during glycolysis. The final product of glycolysis enters the citric acid cycle, where a number of redox reactions move electrons to the electron carriers NADH and FADH2. Some CO2 is generated as well. The electron carriers feed their electrons to molecules in the oxidative phosphorylation process, which eventually transfers them to the terminal electron acceptor, O2. In the process of moving electrons, a large amount of ATP is synthesized from ADP. © 2013 Nature Education All rights reserved.
Cellular respiration is composed of a series of reactions, many of them redox reactions, that release energy in steps as organic molecules (such as glucose) are broken down. Glucose enters cellular respiration in glycolysis, where it is broken into smaller molecules that feed into the next stage, the citric acid cycle. Glycolysis itself does not require O2 to function. Indeed, in the absence of O2, the end product of glycolysis can be converted to other organic molecules in a process known as fermentation, which is used by many microorganisms and by athletes during strenuous prolonged exercise. Small amounts of ATP are produced during glycolysis, but it and the citric acid cycle mainly function to transfer electrons to NAD+ and flavin adenine dinucleotide (FAD) as part of several separate redox reactions. The reduced forms of these electron carriers, NADH and FADH2, give up their electrons to oxidative phosphorylation and ultimately to O2, producing large amounts of ATP in the process. IN THIS MODULE
How Do Organisms Obtain Energy? Redox Reactions An Outline of the Stages in Cellular Respiration Summary Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
Synthetic Biology: Making Life from Bits and Pieces Scientists are combining biology and engineering to change the world.
PRIMARY LITERATURE
How carbon dioxide in the atmosphere affects other greenhouse gases Increased soil emissions of potent greenhouse gases under increased atmospheric CO2. View | Download
Growing new heart cells to treat damaged hearts Conversion of mouse fibroblasts into cardiomyocytes using a direct reprogramming strategy. View | Download
Engineering polymers from E. coli, not petrochemicals Metabolic engineering of Escherichia coli for direct production of 1,4-butanediol. View | Download
Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download
How can nematodes help reduce obesity in humans? A whole-organism screen identifies new regulators of fat storage. View | Download
SCIENCE ON THE WEB
A Brief History of Mitochondria Learn about the origin and evolution of mitochondria
Create a High Energy Diet Explore the USDA's 2011 recommendations about a proper human diet
page 123 of 989 2 pages left in this module
contents Principles of Biology
24 Biological Energy Transfer Summary OBJECTIVE Describe the relationships among photosynthesis, respiration, producers, and consumers. Photosynthetic producers convert light energy from the Sun into chemical energy to produce their own organic molecules. Producers then use cellular respiration to release energy from the chemical bonds in the organic molecules they have produced. Heterotrophic consumers acquire or absorb organic molecules from other organisms and use cellular respiration to release energy from the chemical bonds in those food molecules.
OBJECTIVE Explain how reduction-oxidation (redox) reactions work. One reactant donates electrons to the other reactant. The electron donor becomes oxidized as it loses electrons. The electron acceptor becomes reduced as it gains electrons. Electrons move from donor to acceptor based on the electronegativity of each molecule in the redox pair. The electronegativity of O is stronger than almost every other atom, meaning that O attracts electrons strongly, holding them in orbit relatively close to the atomic nuclei. When electrons are transferred from the breaking of a carbon- carbon or a carbon-hydrogen bond to a bond containing oxygen, the electrons release energy because they are in a state of lower potential energy.
OBJECTIVE Describe how aerobic cellular respiration breaks down fuel molecules and releases energy for cellular work. Cellular respiration releases energy through a series of redox reactions that release energy in steps and capture it in ATP. These reactions move electrons from organic molecules (such as glucose) to O2. The major parts of cellular respiration are glycolysis, the citric acid cycle and oxidative phosphorylation. Small amounts of ATP are produced during glycolysis, but it and the citric acid cycle mainly function to transfer electrons to NAD+ and FAD. The reduced forms of these electron carriers, NADH and FADH2, give up their electrons to oxidative phosphorylation, where large amounts of ATP are produced.
Key Terms aerobic respiration Respiration pathway requiring oxygen. anaerobic respiration Cell respiration in which oxygen is not the final electron acceptor.
catabolic process Chemical reaction that breaks down complex molecules and releases energy.
cellular respiration Process by which energy is released from the breakdown of organic molecules. citric acid cycle Second stage of aerobic respiration; complete oxidation of glucose to CO2 generates NADH and FADH2 for use in oxidative phosphorylation. electronegativity A measure of the tendency of an atom to attract electrons. flavin adenine dinucleotide (FAD) Molecule that functions as an energy and electron carrier in the cell. A dinucleotide with an attached flavin group. Occurs in both oxidized (FAD) and reduced (FADH2) forms. food web A set of interrelated food chains in an ecosystem. glycolysis The first stage of cellular respiration, in which carbohydrates (such as glucose) are broken down into smaller molecules that are either converted to end-stage organic molecules by fermentation or fed into the citric acid cycle. heterotrophic consumer An organism that feeds on producers or other consumers; cannot make its own organic molecules. nicotinamide adenine dinucleotide (NAD+) One of the electron carriers in cellular respiration. oxidative phosphorylation Controlled electron flow is used to synthesize ATP. O2 is the terminal electron acceptor and is reduced to H2O. photoautotrophic producer An organism that carries out photosynthesis and uses the resulting organic molecules as fuel for its cellular respiration. photosynthesis Use of light energy to convert carbon dioxide and water into more complex organic molecules. redox reactions In the reaction, one participant is oxidized, the other reduced.
IN THIS MODULE
How Do Organisms Obtain Energy? Redox Reactions An Outline of the Stages in Cellular Respiration Summary Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
Synthetic Biology: Making Life from Bits and Pieces Scientists are combining biology and engineering to change the world.
PRIMARY LITERATURE
How carbon dioxide in the atmosphere affects other greenhouse gases Increased soil emissions of potent greenhouse gases under increased atmospheric CO2. View | Download
Growing new heart cells to treat damaged hearts Conversion of mouse fibroblasts into cardiomyocytes using a direct reprogramming strategy. View | Download
Engineering polymers from E. coli, not petrochemicals Metabolic engineering of Escherichia coli for direct production of 1,4-butanediol. View | Download
Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download
How can nematodes help reduce obesity in humans? A whole-organism screen identifies new regulators of fat storage. View | Download SCIENCE ON THE WEB
A Brief History of Mitochondria Learn about the origin and evolution of mitochondria
Create a High Energy Diet Explore the USDA's 2011 recommendations about a proper human diet
page 124 of 989 1 pages left in this module
contents Principles of Biology
24 Biological Energy Transfer
Test Your Knowledge
1. What is the ultimate electron acceptor in aerobic cellular respiration?
FAD ADP NAD+ hydrogen oxygen
2. Complete the following sentence: Cellular respiration is carried out by...
phototrophs. heterotrophs. producers. phototrophs, heterotrophs and producers. both producers and phototrophs.
3. Which kind of reaction converts NAD+ to NADH?
respiration phosphorylation reduction oxidation fermentation
4. Which class of food molecules contains the most energy per gram?
carbohydrates fats proteins nucleic acids All of these molecules contain an equal amount of energy per gram.
5. Which atom below is the most electronegative?
hydrogen carbon nitrogen oxygen sulfur
Submit
IN THIS MODULE
How Do Organisms Obtain Energy? Redox Reactions An Outline of the Stages in Cellular Respiration Summary Test Your Knowledge
WHY DOES THIS TOPIC MATTER? Synthetic Biology: Making Life from Bits and Pieces Scientists are combining biology and engineering to change the world.
PRIMARY LITERATURE
How carbon dioxide in the atmosphere affects other greenhouse gases Increased soil emissions of potent greenhouse gases under increased atmospheric CO2. View | Download
Growing new heart cells to treat damaged hearts Conversion of mouse fibroblasts into cardiomyocytes using a direct reprogramming strategy. View | Download
Engineering polymers from E. coli, not petrochemicals Metabolic engineering of Escherichia coli for direct production of 1,4-butanediol. View | Download
Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download
How can nematodes help reduce obesity in humans? A whole-organism screen identifies new regulators of fat storage. View | Download
SCIENCE ON THE WEB
A Brief History of Mitochondria Learn about the origin and evolution of mitochondria
Create a High Energy Diet Explore the USDA's 2011 recommendations about a proper human diet
page 125 of 989