3 Energy and Cellular RespirationSpecific Expectations In this chapter, you will learn how to . . .
C1.1 analyze the role of metabolic processes in the functioning of and interactions between biotic and abiotic systems (3.2, 3.3)
C1.2 assess the relevance, to your personal life and to the community, of an understanding of cell biology and related technologies (3.2, 3.3)
C2.1 use appropriate terminology related to metabolism (3.1, 3.2, 3.3)
C2.2 conduct a laboratory investigation into the process of cellular respiration to identify the products of the process, interpret the qualitative observations, and display them in an appropriate format (3.2, 3.3)
C3.1 explain the chemical changes and energy conversions associated with the processes of aerobic and anaerobic cellular respiration (3.2, 3.3)
C3.3 use the laws of thermodynamics to explain energy transfer in the cell (3.1, 3.2, 3.3)
C3.4 describe, compare, and illustrate the matter and energy transformations that occur during the process of cellular respiration, including the roles of oxygen and organelles such as the mitochondria (3.2, 3.3)
The ruby-throated hummingbird (Archilochus colubris) expends a great deal of energy to stay in motion and maintain body functions. For example, its heart rate can reach over 1000 beats per minute as it flies or hovers above a flower, as shown here. As a result, the hummingbird requires a significant amount of energy to stay alive. It acquires the energy it needs by eating insects caught in flight, as well as by consuming sweet nectar from the flowers of plants.
All organisms need energy to survive, grow, reproduce, and carry out daily activities. Energy to support these functions is released from carbohydrates and other energy-rich organic molecules. In animals, plants, and most other organisms, the process that releases this energy is cellular respiration. For a small number of species that live in environments in which there is little or no oxygen, the processes that release this energy are anaerobic respiration and/or fermentation.
112 MHR Unit 2 Metabolic Processes
AFlutterofActivityThe ruby-throated hummingbird is found throughout Canada. Ranging in mass from 2.5 g to 4.8 g, it is one of Canadas smallest birds. The hummingbird flaps its wings between 55 and 75 times each second and reaches speeds of 80 km/h or more. Not surprisingly, the ruby-throated hummingbird works up an incredible appetite. It can consume up to three times its body mass in a single day. In this activity, you will compare the basal, or resting, metabolic rate of the hummingbird to that of other animals, as well as yourself. Metabolic rate refers to the rate at which the sum total of all chemical reactions necessary to maintain cellular functions occurs.
a calculator graph paper or computer graphing software
Procedure 1.Study the table below.
Hummingbird Cat Dog Horse Elephant
Basal metabolic rate (kJ/day) 42 4.6 10
2 1.5 103 5.0 104 1.7 105
Body mass (kg) 0.003 4 13 1500 5000
2.Graph the data in this table, plotting body mass on the x-axis and basal metabolic rate on the y-axis.
3.Basal metabolic rate is often reported in terms of per body mass (for example, kJ/day/kg). Calculate the energy (in kJ) needed per day per kg of body mass for each animal. Graph this data, plotting your calculated values on the y-axis and body mass on the x-axis.
4.Estimate your own body mass, or use a scale to measure it. 5.Using your estimate or measurement of your body mass from step 4,
interpolate your basal metabolic rate from your graph.
Questions 1.Based on the graph you plotted in step 1, what conclusion can you
make about the relationship between basal metabolic rate and body size?
2.Based on the graph you plotted in step 2, what conclusion can you make about the relationship between basal metabolic rate per kilogram of body weight and body size?
3.Compare your estimated metabolic rate to the metabolic rates of other animals from the table above. What do you notice?
4.Based on the data, infer what factors might influence basal metabolic rate.
Chapter 3 Energy and Cellular Respiration MHR 113
3.1 Metabolism and Energy
All living cells continuously perform thousands of chemical reactions to sustain life. The word metabolism comes from a Greek word that means change. Metabolism refers to all the chemical reactions that change or transform matter and energy in cells. These reactions occur in step-by-step sequences called metabolic pathways, in which one substrate or more is changed into a product, and the product becomes a substrate for a subsequent reaction. A unique enzyme catalyzes each of these reactions. In the absence of enzymes, the reactions would not occur fast enough to sustain the life of the cell.
The function of many metabolic pathways is to break down energy-rich compounds such as glucose and convert the energy into a form that the cell can use. The process of breaking down compounds into smaller molecules to release energy is called catabolism, and such a process may be referred to as catabolic. Active transport is a catabolic process, for example, as is the use of energy by muscle cells to cause motion in the form of muscle contraction. Much of the energy released by catabolic processes, however, is used to synthesize large molecules such as proteins and fats. The process of using energy to build large molecules is called anabolism, and such a process may be referred to as anabolic. Figure 3.1 compares representations of catabolic and anabolic reactions.
Figure 3.1 Anabolic reactions (A) build complex molecules, while catabolic reactions (B) reverse that process.
Explain why constructing a building with bricks, boards, and cement is analogous to an anabolic reaction, and then describe a similar analogy for a catabolic reaction.
EnergyThe same scientific laws that describe energy in chemistry and physics apply to energy in cells. Therefore, a general understanding of these laws will provide a foundation for the study of metabolism. Energy can be defined as the capacity to do workthat is, to change or move matter against an opposing force such as gravity or friction. Energy is often classified as two main types: kinetic and potential. Kinetic energy is energy of motion. Moving objects perform work by causing other matter to move. Potential energy is stored energy, or energy that is available but not yet released. For example, a boulder perched on a hilltop has potential energy. As it starts to roll downhill, some of its potential energy is transformed into kinetic energy. Much of the work of living cells involves the transformation of potential energy into kinetic energy. For example, the potential energy stored in electrochemical gradients is used to move molecules into and out of cells during active transport.
metabolism the sum of all chemical reactions that occur in the cell
metabolic pathway a sequential series of chemical reactions in living cells; each reaction is catalyzed by an enzyme
catabolism the process of breaking down compounds into smaller molecules to release energy
anabolism the process of using energy build large molecules from smaller molecules
energy the capacity to do work
kinetic energy the energy of motion
potential energy stored energy
energy out energy out
energy in energy in
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Kinetic energy and potential energy may themselves be classified as different types. For example, the kinetic energy of particles moving in random directions is thermal energy. An increase in the kinetic energy of particles of an object increases the temperature of the object. Heat is the transfer of thermal energy from one object to another due to a temperature difference between the objects. Chemical energy is potential energy stored in the arrangement of the bonds in a compound.
Bond EnergyWhenever a chemical bond forms between two atoms, energy is released. The amount of energy needed to break a bond is the same as the amount of energy released when the bond is formed. This amount of energy is called bond energy. Because energy is always released when a bond forms, free (unbonded) atoms can be considered to have more chemical energy than any compound. The relative amounts of chemical energy that compounds possess can be compared by examining the amount of energy released when each compound is formed. Figure 3.2 shows the relative amounts of chemical energy in different compounds containing one carbon atom, four hydrogen atoms, and four oxygen atoms.
When carbon, hydrogen, and oxygen combine to form methane and oxygen, the methane and oxygen molecules have less chemical energy than the individual component atoms. When these same atoms combine to form carbon dioxide and water, they release even more chemical energy. Therefore, methane and oxygen have more chemical energy than carbon dioxide and water. The red arrow in Figure 3.2 shows how much energy is released when methane and oxygen react to form carbon dioxide and water.
The energy released from chemical reactions in a laboratory is usually in the form of thermal energy (heat). The energy released from chemical reactions in living cells can include thermal energy, but it can also be in the form of the movement of compounds across cell membranes, contraction of a muscle, or even the emission of light from compounds within specialized cells in certain organisms, as shown in Figure 3.3. In many cases, energy released from one reaction is used to make another reaction occur as part of a metabolic pathway.
bond energy energy required to break (or form) a chemical bond
Figure 3.2 The amount of chemical potential energy possessed by compounds is less than the amount of chemical potential energy possessed by the atoms they contain.
Figure 3.3 Chemical reactions within the cells of these jellyfish release energy in the form of light, or phosphorescence.
Predict what other form of energy is released by the chemical reactions responsible for phosphorescence.
energy releasedwhen bonds form
energy releasedwhen bonds form
net energy released whenmethane is burned with oxygen toform carbon dioxide and water
4 hydrogen 4 oxygen
methane 2 oxygen
carbon dioxide 2 water
Chapter 3 Energy and Cellular Respiration MHR 115
The Laws of ThermodynamicsAll activities those that are necessary to enable and sustain life processes as well as those that occur in the non-living world and anywhere else in the universeinvolve changes in energy. Thermodynamics is the study of these energy changes. Two laws of thermodynamics, called the first and second laws of thermodynamics, describe how energy changes occur. Both laws apply to a system and its surroundings. A system can be a whole organism, a group of cells, or a set of substrates and productswhatever object or objects are being studied. Surroundings are defined as everything in the universe outside of the system.
In terms of thermodynamics, biological systems are considered to be open systems, meaning that the system and its surroundings can exchange matter and energy with each other. The laws of thermodynamics describe how a system can interact with its surroundings and what can, and cannot, occur within a system.
The First Law of Thermodynamics The first law of thermodynamics concerns the amount of energy in the universe. The first law is also called the law of the conservation of energy. In this context, conservation refers to maintaining the same amount of energy throughout a process. (This is different from the popular concept of energy conservation as preserving energy or reducing its usage so that it is not wasted or used excessively.)
The First Law of ThermodynamicsEnergy cannot be created or destroyed, but it can be transformed from one type into another and transferred from one object to another.
Thus, when a chemical reaction occurs and energy is released, some of the energy can be transformed into mechanical energy, such as the motion of a contracting muscle, and the rest can be transformed into heat or other forms of energy. All of the energy is accounted for. If thermal energy leaves a system such as a living organism, the same amount of thermal energy must enter the surroundings. The energy cannot just disappear or be lost to the surroundings.
The first law of thermodynamics also states that energy cannot simply appear. For example, you cannot create the energy you need to go jogging. Chemical energy stored in food molecules must be transformed into kinetic energy in your muscles to enable you to move.
The Second Law of ThermodynamicsAccording to the first thermodynamics law, the total amount of energy in the universe remains constant. Despite this, however, the energy available to do work decreases as more of it is progressively transformed into unusable heat. The second law of thermodynamics concerns the transformation of potential energy into heat, or random molecular motion. It states that the disorder in the universemore formally called entropyis continuously increasing. Put more simply, disorder is more likely than order. For example, it is much more likely that a stack of books will tumble over than that a pile of books will arrange themselves spontaneously to form a tidy stack. Similarly, it is much more likely that a tidy, orderly room will become more untidy and more disorderly over time than that an untidy room will spontaneously tidy itself.
The Second Law of ThermodynamicsDuring any process, the universe tends toward disorder.
thermodynamics the science that studies the transfer and transformation of thermal energy (heat)
entropy a measure of disorder
116 MHR Unit 2 Metabolic Processes
Energy transformations proceed spontaneously to convert matter from a more ordered, less stable condition to a less ordered, more stable condition. For example, in Figure 3.4, you could put the pictures into their correct sequence using the information that time had elapsed with only natural processes occurring. Common experience supports thisdisorganized rooms do not spontaneously become organized. Thus, the second law of thermodynamics also can be stated simply as entropy increases. When the universe formed, it held all the potential energy it will ever have. It has become increasingly more disordered ever since, with every energy exchange increasing the amount of entropy.
Because organisms are highly ordered, it might seem that life is an exception to the laws of thermodynamics. However, the second law applies only to closed systems. While they are alive, organisms remain organized because they are not closed systems. They use inputs of matter and energy to reduce random...