7.24: Summary- Energy and Metabolism - Biology

7.24: Summary- Energy and Metabolism - Biology

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Learning Outcomes

  • Identify different types of metabolic pathways
  • Distinguish between an open and a closed system
  • State the first law of thermodynamics
  • State the second law of thermodynamics
  • Explain the difference between kinetic and potential energy
  • Describe endergonic and exergonic reactions
  • Discuss how enzymes function as molecular catalysts

Cells perform the functions of life through various chemical reactions. Anabolic processes build complex molecules out of simpler ones and require energy.

In studying energy, the term system refers to the matter and environment involved in energy transfers. Entropy is a measure of the disorder of a system. The physical laws that describe the transfer of energy are the laws of thermodynamics. The first law states that the total amount of energy in the universe is constant. The second law of thermodynamics states that every energy transfer involves some loss of energy in an unusable form, such as heat energy. Energy comes in different forms: kinetic, potential, and free. The change in free energy of a reaction can be negative (releases energy, exergonic) or positive (consumes energy, endergonic). All reactions require an initial input of energy to proceed, called the activation energy.

Enzymes are chemical catalysts that speed up chemical reactions by lowering their activation energy. Enzymes have an active site with a unique chemical environment that fits particular chemical reactants for that enzyme, called substrates. Enzymes and substrates are thought to bind according to an induced-fit model. Enzyme action is regulated to conserve resources and respond optimally to the environment.

Practice Questions

  1. Look at each of the processes shown in Figure 1, and decide if it is endergonic or exergonic.
  • Does physical exercise to increase muscle mass involve anabolic and/or catabolic processes? Give evidence for your answer.
  • Explain in your own terms the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.
  • With regard to enzymes, why are vitamins and minerals necessary for good health? Give examples.
  • [reveal-answer q=”32093″]Show Answers[/reveal-answer]
    [hidden-answer a=”32093″]

    1. A compost pile decomposing is an exergonic process. A baby developing from a fertilized egg is an endergonic process. Tea dissolving into water is an exergonic process. A ball rolling downhill is an exergonic process.
    2. Physical exercise involves both anabolic and catabolic processes. Body cells break down sugars to provide ATP to do the work necessary for exercise, such as muscle contractions. This is catabolism. Muscle cells also must repair muscle tissue damaged by exercise by building new muscle. This is anabolism.
    3. A spontaneous reaction is one that has a negative ∆G and thus releases energy. However, a spontaneous reaction need not occur quickly or suddenly like an instantaneous reaction. It may occur over long periods of time due to a large energy of activation, which prevents the reaction from occurring quickly.
    4. Most vitamins and minerals act as cofactors and coenzymes for enzyme action. Many enzymes require the binding of certain cofactors or coenzymes to be able to catalyze their reactions. Since enzymes catalyze many important reactions, it is critical to obtain sufficient vitamins and minerals from diet and supplements. Vitamin C (ascorbic acid) is a coenzyme necessary for the action of enzymes that build collagen.[/hidden-answer]

    Summary of Energy and Respiration

    1 Organisms must do work to stay alive. The energy input necessary for this work is
    either light, for photosynthesis, or the chemical potential energy of organic molecules.
    Work includes anabolic reactions, active transport and movement. Some organisms,
    such as mammals and birds, use thermal energy released from metabolic reactions to
    maintain their body temperature.
    2 Reactions that release energy must be harnessed to energy-requiring reactions. Th is
    „harnessing‟ involves an intermediary molecule, ATP. Th is can be synthesised from
    ADP and phosphate using energy, and hydrolysed to ADP and phosphate to release
    energy. ATP therefore acts as an energy currency in all living organisms.
    3 Respiration is the sequence of enzyme-controlled steps by which an organic
    molecule, usually glucose, is broken down so that its chemical potential energy can be
    used to make the energy currency, ATP.
    4 In aerobic respiration, the sequence involves four main stages: glycolysis, the link
    reaction, the Krebs cycle and oxidative phosphorylation.
    5 In glycolysis, glucose is fi rst phosphorylated and then split into two triose phosphate
    molecules. Th ese are further oxidised to pyruvate, giving a small yield of ATP and
    reduced NAD. Glycolysis occurs in the cell cytoplasm.
    6 When oxygen is available (aerobic respiration), the pyruvate passes to the matrix of a
    mitochondrion. There, in the link reaction, pyruvate is decarboxylated and
    dehydrogenated and the remaining 2C acetyl unit combined with coenzyme A to give
    acetyl coenzyme A.
    7 The acetyl coenzyme A enters the Krebs cycle in the mitochondrial matrix and
    donates the acetyl unit to oxaloacetate (4C) to make citrate (6C).
    8 The Krebs cycle decarboxylates and dehydrogenates citrate to oxaloacetate in a
    series of small steps. Th e oxaloacetate can then react with another acetyl coenzyme A
    from the link reaction.

    9 Dehydrogenation provides hydrogen atoms, which are accepted by the carriers NAD
    and FAD. Th ese pass to the inner membrane of the mitochondrial envelope, where they
    are split into protons and electrons.
    10 In the process of oxidative phosphorylation, the electrons are passed along a series
    of carriers. Some of the energy released in this process is used to move protons from
    the mitochondrial matrix to the intermembrane space. This sets up a gradient of protons
    across the inner membrane of the mitochondrial envelope. The protons pass back into
    the matrix, moving down their concentration gradient through protein channels in the
    inner membrane. An enzyme, ATP synthase, is associated with each of these channels.
    ATP synthase uses the electrical potential energy of the proton gradient to
    phosphorylate ADP to ATP.
    11 At the end of the carrier chain, electrons and protons are recombined and reduce
    oxygen to water.
    12 In the absence of oxygen as a hydrogen acceptor (in anaerobic respiration), a small
    yield of ATP is made by dumping hydrogen into other pathways in the cytoplasm which
    produce ethanol or lactate. The lactate pathway can be reversed in mammals when
    oxygen becomes available. The oxygen needed to remove the lactate produced during
    anaerobic respiration is called the oxygen debt.
    13 The energy values of respiratory substrates depend on the number of hydrogen
    atoms per molecule. Lipids have a higher energy density than carbohydrates or
    14 The respiratory quotient (RQ) is the ratio of the volumes of oxygen absorbed and
    carbon dioxide given off in respiration. The RQ reveals the nature of the substrate being
    respired. Carbohydrate has an RQ of 1.0, lipid 0.7 and protein 0.9.
    15 Oxygen uptake, and hence RQ, can be measured using a respirometer.

    1. End-of-chapter questions
    1. What does not occur in the conversion of glucose to two molecules of pyruvate?
    A hydrolysis of ATP
    B phosphorylation of ATP
    C phosphorylation of triose (3C) sugar
    D reduction of NAD
    2 Wheredoes each stage of aerobic respiration occur in a eukaryotic cell?

    Metabolism And Its Integration (With Diagram)

    Hundreds of reactions simultaneously take place in a living cell, in a well-organized and integrated manner. The entire spectrum of chemical reactions, occurring in the living system, is collectively referred to as metabolism.

    A metabolic pathway (or metabolic map) constitutes a series of enzymatic reactions to produce specific products. The term metabolite is applied to a substrate or an intermediate or a product in the metabolic reactions.

    Introduction to Metabolism:

    Metabolism is broadly divided into two categories (Fig. 67.1).

    The degradative processes concerned with the breakdown of complex molecules to simpler ones, with a concomitant release of energy.

    The biosynthetic reactions involving the formation of complex molecules from simple precursors. A clear demarcation between catabolism and anabolism is rather difficult, since there are several intermediates common to both the processes.


    The very purpose of catabolism is to trap the energy of the biomolecules in the form of ATP and to generate the substances (precursors) required for the synthesis of complex molecules. Catabolism occurs in three stages (Fig. 67.2).

    1. Conversion of complex molecules into their building blocks:

    Polysaccharides are broken down to monosaccharide’s, lipids to free fatty acids and glycerol, and proteins to amino acids.

    2. Formation of simple intermediates:

    The building blocks produced in stage (1) are degraded to simple intermediates such as pyruvate and acetyl CoA. These intermediates are not readily identifiable as carbohydrates, lipids or proteins. A small quantity of energy (as ATP) is captured in stage 2.

    3. Final oxidation of acetyl CoA:

    Acetyl CoA is completely oxidized to CO2, liberating NADH and FADH2 that finally get oxidized to release large quantity of energy (as ATP). Krebs cycle (or citric acid cycle) is the common metabolic pathway involved in the final oxidation of all energy-rich molecules. This pathway accepts the carbon compounds (pyruvate, succinate etc.) derived from carbohydrates, lipids or proteins.


    For the synthesis of a large variety of complex molecules, the starting materials are relatively few. These include pyruvate, acetyl CoA and the intermediates of citric acid cycle. Besides the availability of precursors, the anabolic reactions are dependent on the supply of energy (as ATP or GTP) and reducing equivalents (as NADPH + H + ).

    The anabolic and catabolic pathways are not reversible and operate independently. As such, the metabolic pathways occur in specific cellular locations (mitochondria, microsomes etc.) and are controlled by different regulatory signals.

    The terms—intermediary metabolism and energy metabolism—are also in use. Intermediary metabo­lism refers to the entire range of catabolic and anabo­lic reactions, not involving nucleic acids. Energy metabolism deals with the metabolic pathways concerned with the storage and liberation of energy.

    Types of Metabolic Reactions:

    The biochemical reactions are mainly of four types:

    3. Rearrangement and isomerization.

    4. Make and break of carbon-carbon bonds.

    These reactions are catalysed by specific enzymes—more than 2,000 known so far.

    Methods Employed to Study Metabolism:

    The metabolic reactions do not occur in isolation. They are interdependent and integrated into specific series that constitute metabolic pathways. It is, therefore, not an easy task to study metabolisms. Fortunately, the basic metabolic pathways in most organisms are essentially identical. Several methods are employed to elucidate biochemical reactions and the metabolic pathways.

    These experimental approaches may be broadly divided into 3 categories:

    1. Use of whole organisms or its components.

    2. Utility of metabolic probes.

    3. Application of isotopes.

    The actual methods employed may be either in vivo (in the living system) or in vitro (in the test tube) or, more frequently, both.

    1. Use of whole organism or its components:

    (a) Whole organisms: Glucose tolerance test (GTT).

    (b) Isolated organs, tissue slices, whole cells, subcellular organelles etc., to elucidate biochemical reactions and metabolic pathways.

    2. Utility of metabolic probes:

    Two types of metabolic probes are commonly used to trace out biochemical pathways. These are metabolic inhibitors and mutations.

    3. Application of isotopes.

    Integration of Metabolism:

    Metabolism is a continuous process, with thousands of reactions, simultaneously occurring in the living cell. However, biochemists prefer to present metabolism in the form of reactions and metabolic pathways. This is done for the sake of convenience in presentation and understanding. We have learnt the metabolism of carbohydrates, lipids and amino acids. We shall now consider the orga­nism as a whole and integrate the metabolism with particular reference to energy demands of the body organism.

    Energy Demand and Supply:

    The organisms possess variable energy demands hence the supply (input) is also equally variable. The consumed metabolic fuel may be burnt (oxidized to CO2 and H2O) or stored to meet the energy requirements as per the body needs. ATP serves as the energy currency of the cell in this process (Fig. 67.21).

    The humans possess enormous capacity for food consumption. It is estimated that one can consume as much as 100 times his/her basal requirements! Obesity, a disorder of over nutrition mostly prevalent in affluent societies, is primarily a consequence of overconsumption.

    Integration of Major Metabolic Pathways of Energy Metabolism:

    An overview of the interrelationship between the important metabolic pathways, concerned with fuel metabolism depicted in Fig. 67.22, is briefly described here.

    The degradation of glucose to pyruvate (lactate under anaerobic condition) generates 8 ATP. Pyruvate is converted to acetyl CoA.

    2. Fatty acid oxidation:

    Fatty acids undergo sequential degradation with a release of 2-carbon fragment, namely acetyl CoA. The energy is trapped in the form of NADH and FADH2.

    3. Degradation of amino acids:

    Amino acids, particularly when consumed in excess than required for protein synthesis, are degraded and utilized to meet the fuel demands of the body. The glucogenic amino acids can serve as precursors for the synthesis of glucose via the formation of pyruvate or intermediates of citric acid cycle. The ketogenic amino acids are the precursors for acetyl CoA.

    Acetyl CoA is the key and common metabolite, produced from different fuel sources (carbohydrates, lipids, amino acids). Acetyl CoA enters citric acid cycle and gets oxidized to CO2. Thus, citric acid cycle is the final common metabolic pathway for the oxidation of all foodstuffs. Most of the energy is trapped in the form of NADH and FADH2.

    5. Oxidative phosphorylation:

    The NADH and FADH2, produced in different metabolic pathways, are finally oxidized in the electron transport chain (ETC). The ETC is coupled with oxidative phosphorylation to generate ATP.

    6. Hexose monophosphate shunt:

    This pathway is primarily concerned with the liberation of NADPH and ribose sugar. NADPH is utilized for the biosynthesis of several compounds, including fatty acids. Ribose is an essential component of nucleotides and nucleic acids (note—DNA contains deoxyribose).

    The synthesis of glucose from non-carbohydrate sources constitutes gluconeogenesis. Several compounds (e.g. pyruvate, glycerol, amino acids) can serve as precursors for gluconeogenesis.

    8. Glycogen metabolism:

    Glycogen is the storage form of glucose, mostly found in liver and muscle. It is degraded (glycogenolysis) and synthesized (glycogenesis) by independent pathways. Glycogen effectively serves as a fuel reserve to meet body needs, for a brief period (between meals).

    Regulation of Metabolic Pathways:

    The metabolic pathways, in general, are controlled by four different mechanisms:

    1. The availability of substrates

    2. Covalent modification of enzymes

    4. Regulation of enzyme synthesis.

    The details of these regulatory processes are discussed under the individual metabolic pathways.

    1. Phosphagen System

    During short-term, intense activities, a large amount of power needs to be produced by the muscles, creating a high demand for ATP. The phosphagen system (also called the ATP-CP system) is the quickest way to resynthesize ATP (Robergs & Roberts 1997).

    Creatine phosphate (CP), which is stored in skeletal muscles, donates a phosphate to ADP to produce ATP: ADP + CP = ATP + C. No carbohydrate or fat is used in this process the regeneration of ATP comes solely from stored CP. Since this process does not need oxygen to resynthesize ATP, it is anaerobic, or oxygen-independent. As the fastest way to resynthesize ATP, the phosphagen system is the predominant metabolic energy system used for all-out exercise lasting up to about 10 seconds. However, since there is a limited amount of stored CP and ATP in skeletal muscles, fatigue occurs rapidly.


    Basal Metabolic Rate

    The basal metabolic rate (BMR) is the rate of energy expenditure of a person at rest it eliminates the variable effect of physical activity. The BMR accounts for approximately 60% of the daily energy expenditure. Thus it includes energy used for normal body cellular homeostasis, cardiac function, brain and other nerve function, and so on. It is related to body weight by the calculation:

    A passive increase in energy expenditure occurs during digestion of food. This is referred to as the thermic effect or, in the older literature, specific dynamic action of food it accounts for about 10% of the daily energy expenditure.

    The total daily energy expenditure is calculated from knowledge of the BMR and a physical activity factor. The physical activity factor is a function of the type of activity for an individual (e.g., 1.3 for sedentary, 1.5 for moderately active, and 1.7 for extremely active). When multiplied by the BMR, an estimate of the daily energy expenditure is obtained.

    Example: A 220 lb (220/2.2 = 100 kg) person with moderate energy expenditure (e.g., a cabinet maker):

    Glycolysis and Cellular Respiration - Metabolism and Respiration Overview

    All organisms need energy to live. Humans like to sit down to three square meals a day (even if they are on round plates), but other living things have drastically different ways of dealing with their energy needs. Some creepy crawlies get their energy from eating dirt, which is probably not the tastiest meal on the planet. And plants sunbathe, make their own food, and then chow down. The point is we all have to eat.

    But energy and food are not the same thing when we're discussing the cellular level. Metabolism is the way we get to the good stuff, the chemical energy, in the molecules of the food we eat. Chemical energy is boss when it comes to getting things done: it powers every process needed for life.

    Cellular respiration overview

    Cellular respiration is also called aerobic respiration because it takes place when oxygen is present. The purpose of cellular respiration is to make usable energy for the cell. Instead of Red Bull or Monster Energy, cellular energy takes the form of a compound called ATP (short for adenosine triphosphate). ATP is often called the energy currency of the cell.

    The end product of cellular respiration is exactly 38 molecules of ATP. That is a pretty good payout for one molecule of glucose.

    Cellular respiration takes place in three steps:

    1. Glycolysis
    2. Citric acid cycle, aka the Krebs cycle
    3. Oxidative phosphorylation

    Prepare to Be Reduced…or Oxidized

    To follow along during our behind-the-scenes tour of cellular respiration, it helps to be familiar with oxidation and reduction reactions. Redox reactions (as they're known) are responsible for many of the changes that occur during cellular respiration. Redox reactions involve either losing or gaining electrons.

    • In oxidation, an atom loses electrons.
    • In reduction, an atom gains electrons.

    This can be confusing, because why would you call something reduced if it's actually gaining electrons? Good question.

    The answer lies in the charge of the atom. Since electrons are negatively charged, gaining an electron also means gaining a -1 charge, reducing the overall charge of the atom.

    You can remember oxidation and reduction with a simple trick: LEO the lion says GER.LEO stands for Loss of Electrons is Oxidation, andGER stands for Gain of Electrons is Reduction.

    Electron donors are oxidized, and electron acceptors are reduced.

    Redox reactions are an important source of energy and happen during all kinds of combustion reactions, such as the burning of methane to heat a stove or the heating of gasoline to make a car run. In biology, redox reactions are common and extremely important, such as during cellular respiration.

    In fact, cellular respiration is one of the processes that allows us to stay warm and cozy in our skin. Heat released from these funky reactions is what keeps our bodies at a constant temperature, even when it's chilly outside.

    In cellular respiration, glucose (C6H12O6) is oxidized in a series of steps that releases energy little by little. The electrons that glucose loses as it's oxidized are picked up by NAD + or FAD 2+ molecules that act as electron carriers. In redox terms, glucose is oxidized, and the NAD + and FAD 2+ molecules are reduced to form NADH and FADH2.

    During oxidative phosphorylation, the last molecule in the chain to accept an electron is called the terminal electron acceptor. In organisms that breathe oxygen, such as our lovely human selves, oxygen is the terminal electron acceptor therefore, it is reduced in the whole process.

    Brain Snack

    For a summary of cellular respiration set to a catchy tune, check out this video.


    The process of glycogen synthesis from glucose residues is called glycogenesis. Before studying the steps involved in its synthesis, it is important to first understand the general structure of glycogen.


    Glycogen is a branched polymer of alpha-glucose. The glucose molecules are linked together via alpha 1-4 glycosidic linkages in the linear chains while the residue at the branch points are linked via alpha 1-6 glycosidic linkages. The glycogen molecule shows extensive branching, with one branch point occurring after every 8 to 12 glucose residues in the linear chain. It has a protein at its core, called the glycogenin protein. The glycogen molecule appears as branches of tree emerging from the glycogenin core.


    Glycogen is synthesized from the molecules of alpha D-glucose. The process takes place in the cytoplasm and uses energy in the form of ATP as well as UTP. It involves the following steps.

    Synthesis of Glucose-1-Phosphate

    First of all, the molecules of glucose are phosphorylated to form glucose-6-phosphate. This reaction is catalyzed by glucokinase enzymes. The phosphate is provided by the ATP molecules.

    These glucose-6-phosphate molecules are later converted to glucose-1-phosphate via phosphoglucomutase enzyme. During this conversion, glucose-1,6-bisphosphate is also generated that is an obligatory intermediate of the reaction.

    Synthesis of UDP-glucose

    All the glucose residues found in glycogen are provided by UDP-glucose. The UDP-glucose molecules are synthesized from glucose-1-phosphate and UTP via UDP-glucose phosphorylase enzyme.

    A molecule of pyrophosphate (PPi) is also produced during this process. This pyrophosphate is hydrolyzed to release two inorganic phosphates along with energy. This exergonic reaction makes sure that the UDP-glucose synthesis reaction always proceeds in the forward direction.

    Glycogen Synthase Enzyme

    Once the UDP-glucose molecules are formed, they are utilized by the glycogen synthase enzyme to form a linear chain of alpha D-glucose. An important feature of this enzyme is that it can only elongate already existing chains of glycogen. It cannot begin the synthesis of a new chain starting from the first residue. A primer is always needed by the glycogen synthase enzyme to begin its process.

    However, if some pre-existing chains of glycogen are present in the cell, the glycogen synthase enzyme can use these fragments as a primer and continue its process of making glycogen.

    Synthesis of Primer

    In the case when glycogen fragments are not present, a protein called glycogenin serves as a primer. The hydroxyl group present in the side-chain of a tyrosine residue in glycogenin acts as the acceptor of the first glucosyl residue from UDP-glucose. The reaction is called autoglucosylation as it is catalyzed by the glycogenin itself. The protein keeps adding few more glucosyl residues via alpha 1-4 glycosidic linkages until a short chain is formed. This short-chain of glucose residues then serves as a primer for glycogen synthase enzyme.

    Elongation of Chain

    Once the primer has been formed, it can be acted upon by the glycogen synthase enzyme. This enzyme elongates the glycogen chain by adding new glucosyl residues to the non-reducing end of the chain. The glucose residues are provided by UDP-glucose molecules. the non-reducing end of the chain is the one having free anomeric carbon, carbon of the aldehydic functional group. During the process of chain elongation, the hydroxyl group at the fourth carbon of the new glycosyl residue reacts with the aldehydic group of the residue present at the non-reducing end, forming an alpha 1-4 glycosidic bond.

    During this process, a molecule of UDP is released with each glucosyl residue added to the chain. This UDP is converted back to UTP by nucleoside diphosphate kinase, using ATP as a source of energy as well as the provider of inorganic phosphate.

    The linear chain alpha 1-4 glucosyl residues formed by the glycogen synthase enzyme resembles the amylase starch found in plants. On the other hand, glycogen is a highly branched polymer of alpha 1-4 glucosyl residues.

    The next step in the glycogenesis is the process of making branching so that a highly branched molecule is formed. This is carried out by a separate enzyme called branching enzyme.

    The branching enzyme is called amylo-alpha(1-4) to alpha(1-6) transglucosidase. A branch is made in two steps:

    • In the first step, the branching enzyme removes a short-chain of six to eight glucosyl residues from the non-reducing end of the linear chain by breaking an alpha 1-4 glycosidic linkage.
    • In the next step, the branching enzyme inserts this short linear branch at a non-reducing residue of the chain via an alpha 1-6 glycosidic bond. The first residue at the branch point is attached via an alpha 1-6 glycosidic bond while the rest of residues in the chain have the same alpha 1-4 glycosidic linkages.

    Once the branch has been formed, both the chains can be further elongated by the glycogen synthase enzyme. In addition, more branches can also be added by the branching enzyme.

    The ultimate result is the formation of a large molecule having extensive tree-like branches with one branch occurring every eight to twelve residues in the chain.

    The glycogenin protein that was used to make the primer remains a part of the molecule and forming the core of glycogen granules found in the cells.


    Metabolism (pronounced: meh-TAB-uh-liz-um) is the chemical reactions in the body's cells that change food into energy. Our bodies need this energy to do everything from moving to thinking to growing.

    Specific proteins in the body control the chemical reactions of metabolism. Thousands of metabolic reactions happen at the same time &mdash all regulated by the body &mdash to keep our cells healthy and working.

    How Does Metabolism Work?

    After we eat food, the digestive system uses enzymes to:

    • break proteins down into amino acids
    • turn fats into fatty acids
    • turn carbohydrates into simple sugars (for example, glucose)

    The body can use sugar, amino acids, and fatty acids as energy sources when needed. These compounds are absorbed into the blood, which carries them to the cells.

    After they enter the cells, other enzymes act to speed up or regulate the chemical reactions involved with "metabolizing" these compounds. During these processes, the energy from these compounds can be released for use by the body or stored in body tissues, especially the liver, muscles, and body fat.

    Metabolism is a balancing act involving two kinds of activities that go on at the same time:

    • building up body tissues and energy stores (called anabolism)
    • breaking down body tissues and energy stores to get more fuel for body functions (called catabolism)

    Anabolism (pronounced: uh-NAB-uh-liz-um), or constructive metabolism, is all about building and storing. It supports the growth of new cells, the maintenance of body tissues, and the storage of energy for future use. In anabolism, small molecules change into larger, more complex molecules of carbohydrates, protein, and fat.

    Catabolism (pronounced: kuh-TAB-uh-liz-um), or destructive metabolism, is the process that produces the energy needed for all activity in the cells. Cells break down large molecules (mostly carbs and fats) to release energy. This provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move.

    As complex chemical units break down into more simple substances, the body releases the waste products through the skin, kidneys, lungs, and intestines.

    What Controls Metabolism?

    Several hormones of the endocrine system help control the rate and direction of metabolism. Thyroxine, a hormone made and released by the thyroid gland, plays a key role in determining how fast or slow the chemical reactions of metabolism go in a person's body.

    Another gland, the pancreas, secretes hormones that help determine whether the body's main metabolic activity at any one time are anabolic (pronounced: an-uh-BOL-ik) or catabolic (pronounced: kat-uh-BOL-ik). For example, more anabolic activity usually happens after you eat a meal. That's because eating increases the blood's level of glucose &mdash the body's most important fuel. The pancreas senses this increased glucose level and releases the hormone insulin, which signals cells to increase their anabolic activities.

    Metabolism is a complicated chemical process. So it's not surprising that many people think of it in its simplest sense: as something that influences how easily our bodies gain or lose weight. That's where calories come in. A calorie is a unit that measures how much energy a particular food provides to the body. A chocolate bar has more calories than an apple, so it provides the body with more energy &mdash and sometimes that can be too much of a good thing. Just as a car stores gas in the gas tank until it is needed to fuel the engine, the body stores calories &mdash primarily as fat. If you overfill a car's gas tank, it spills over onto the pavement. Likewise, if a person eats too many calories, they "spill over" in the form of excess body fat.

    The number of calories someone burns in a day is affected by how much that person exercises, the amount of fat and muscle in his or her body, and the person's basal metabolic rate (BMR). BMR is a measure of the rate at which a person's body "burns" energy, in the form of calories, while at rest.

    The BMR can play a role in a person's tendency to gain weight. For example, someone with a low BMR (who therefore burns fewer calories while at rest or sleeping) will tend to gain more pounds of body fat over time than a similar-sized person with an average BMR who eats the same amount of food and gets the same amount of exercise.

    BMR can be affected by a person's genes and by some health problems. It's also influenced by body composition &mdash people with more muscle and less fat generally have higher BMRs. But people can change their BMR in certain ways. For example, a person who exercises more not only burns more calories, but becomes more physically fit, which increases his or her BMR.

    The Role of Cells

    Your body is comprised of an incalculable number of cells, and many different types of cells at that. Brain cells are different from blood cells, which are different from bone cells, which are different from skin cells, and so on. Every cell in your body is uniquely suited for a specific purpose. Others send electrochemical messages to the brain. [7,8] Different cells have different features and may even have structural differences depending on their purpose. [9]

    Metabolic Pathways

    Consider the metabolism of sugar. This is a classic example of one of the many cellular processes that use and produce energy. Living things consume sugars as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. For the most part, photosynthesizing organisms like plants produce these sugars. During photosynthesis, plants use energy (originally from sunlight) to convert carbon dioxide gas (CO2) into sugar molecules (like glucose: C6H12O6). They consume carbon dioxide and produce oxygen as a waste product. This reaction is summarized as:

    Because this process involves synthesizing an energy-storing molecule, it requires energy input to proceed. During the light reactions of photosynthesis, energy is provided by a molecule called adenosine triphosphate (ATP), which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. In contrast, energy-storage molecules such as glucose are consumed only to be broken down to use their energy. The reaction that harvests the energy of a sugar molecule in cells requiring oxygen to survive can be summarized by the reverse reaction to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide is released as a waste product. The reaction is summarized as:

    Both of these reactions involve many steps.

    The processes of making and breaking down sugar molecules illustrate two examples of metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a starting molecule and modifies it, step-by-step, through a series of metabolic intermediates, eventually yielding a final product. In the example of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. These two opposite processes—the first requiring energy and the second producing energy—are referred to as anabolic pathways (building polymers) and catabolic pathways (breaking down polymers into their monomers), respectively. Consequently, metabolism is composed of synthesis (anabolism) and degradation (catabolism) (Figure 3).

    It is important to know that the chemical reactions of metabolic pathways do not take place on their own. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

    Figure 3 Catabolic pathways are those that generate energy by breaking down larger molecules. Anabolic pathways are those that require energy to synthesize larger molecules. Both types of pathways are required for maintaining the cell’s energy balance.