© 2012 pearson education, inc. bioenergetics chapter 5

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BIOENERGETICSChapter 5

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Bioenergetics: The Flow of Energy in the Cell

• Every cell has four essential needs

– Molecular building blocks

– Chemical catalysts (enzymes)

– Information to guide activities

– Energy to drive reactions and processes essential to life

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Figure 5-1

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The Importance of Energy

• All living systems require an ongoing supply of energy

• Energy can be thought of as the capacity to cause specific chemical or physical changes

• Capacity to do work

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Cells Need Energy to Drive Six Different Kinds of Changes

• Six categories of change require energy

– Synthetic work– Mechanical work– Concentration work– Electrical work– Generation of heat– Generation of light

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Figure 5-2

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1. Synthetic Work: Changes in Chemical Bonds

• The work of biosynthesis results in the formation of new chemical bonds and the synthesis of new molecules

• Biosynthesis is required for growth and maintenance of cells and cellular structures

• Energy that cells require for biosynthetic work is used to make energy-rich organic molecules and incorporate them into macromolecules

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Figure 5-2A

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2. Mechanical Work: Changes in the Location or Orientation of a Cell or a Subcellular Structure

• Mechanical work involves a physical change in the position or orientation of a cell or some part of it

• The movement of a cell relative to its environment often requires one or more appendages, such as cilia or flagella

• These require energy to move the cell

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Figure 5-2B

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Figure 5-3

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Other examples of mechanical work

• A large number of muscle cells work together in muscle contraction

• Chromosomes move along spindle fibers during mitosis – Where do the spindle fibers originate from?

• Cytoplasmic streaming and movement of organelles and vesicles along microtubules

• Ribosomes move along a strand of mRNA

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3. Concentration Work: Moving Molecules Across a Membrane Against a Concentration Gradient

• Concentration work accumulates substances within a cell or organelle or removes toxic by-products of cellular activity

• Requires energy so what type of process?• Examples include the concentration of specific

molecules and enzymes in organelles, digestive enzymes in secretory vesicles, and the import of sugars and amino acids into cells

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Figure 5-2C

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4. Electrical Work: Moving Ions Across a Membrane Against an Electrochemical Gradient

• During electrical work, ions are transported across a membrane, resulting in differences in both concentration and electrical potential (or membrane potential)

• Every cellular membrane has a characteristic electrical potential

• In the case of mitochondria or chloroplasts the difference is essential in production of ATP

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Electrical work in neurotransmission

• Electrical work is important in transmission of nerve impulses

• In this case a membrane potential is generated by pumping Na+ ions into and K+ ions out of the cell

• The electric eel (Electrophorus electricus) uses energy to generate a membrane potential of 150 mV per cell and several hundred volts for the entire organism

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Figure 5-2D

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• http://video.nationalgeographic.com/video/animals/reptiles-animals/alligators-crocodiles/eels_electric/

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5. Heat: An Increase in Temperature That Is Useful to Warm-Blooded Animals

• Living organisms do not use heat as a form of energy as a steam engine does

• However, producing heat is a major use of energy in all homeotherms

• Homeotherms: animals that regulate their body temperature independent of the environment

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Figure 5-2E

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6. Bioluminescence: The Production of Light

• Bioluminescence, the production of light, is important in a number of organisms such as fireflies, certain jellyfish, and luminous toadstools

• Bioluminescence is generated by the reaction of ATP with luminescent compounds

• Green fluorescent protein (GFP; from the jellyfish Aequorea victoria) and its variants are very useful to cell biologists

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Figure 5-2F

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Figure 5-4

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Organisms Obtain Energy from Sunlight or from the Oxidation of Organic Compounds• Nearly all life on Earth is directly or indirectly sustained

from sunlight

• Organisms may be divided into

– Phototrophs or chemotrophs, depending on the source of energy they use

– Autotrophs or heterotrophs, depending on their source of carbon

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Phototrophs

• Phototrophs capture light energy from the sun and transform it into chemical energy, stored as ATP

• Photoautotrophs use solar energy to produce all the carbon compounds they need from CO2 (photosynthesis)

• Photoautotrophs include plants, algae, cyanobacteria and photosynthetic bacteria

• Some bacteria are photoheterotrophs, that harvest solar energy for some cellular activities but rely on intake of organic molecules as a source of carbon

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Chemotrophs

• Chemotrophs obtain energy by oxidizing chemical bonds in molecules (organic or inorganic)

• A few bacteria are chemoautotrophs, which oxidize inorganic compounds such as H2S, H2, or inorganic ions for energy, and use CO2 as a carbon source

• Chemoheterotrophs ingest and use chemical compounds (carbohydrates, fats and proteins) to provide both energy and carbon for cellular needs

• All animals, protozoa, fungi, and many bacteria are chemoheterotrophs

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Energy Flows Through the Biosphere Continuously

• Oxidation is the removal of electrons from a substance, usually hydrogen atoms (H+ plus one electron)

• Oxidation reactions release energy

• Reduction, the addition of electrons to a substance through addition of hydrogen atoms (and a loss of oxygen atoms), requires an input of energy

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Examples of oxidation

• Glucose oxidation

• C6H12O6 + 6O2 → CO2 + 6H2O + energy

• Methane oxidation

• CH4 + 2O2 → CO2 + 2H2O + energy

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Example of reduction

• Carbon dioxide reduction

• energy + 6CO2 + 6H2O → C6H12O6 + 6O2

• Note that this is the opposite of glucose oxidation

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Producers and consumers

• Phototrophs, the producers, use sunlight energy to produce more reduced cellular compounds through photosynthesis

• These compounds are converted to all the materials needed for survival

• Chemotrophs, the consumers, take in reduced compounds and oxidize them to release their stored energy

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Efficiency of biological processes

• No process in biological systems is 100% efficient; some energy is inevitably released (lost) as heat, usually dissipated into the environment

• Some of this heat is used

– in warm-blooded animals to maintain body temperature

– in certain plants to melt snow around themselves

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Figure 5-6

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Flow of energy drives all life processes

• There is a massive, unidirectional flow of energy from the source in the fusion reactions in the sun to the sink, the environment

• The flow begins with the production of high-energy, reduced compounds by photosynthesis

• Energy is then released as needed (by all organisms) in oxidation reactions

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Figure 5-5

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The Flow of Energy Through the Biosphere Is Accompanied by a Flow of Matter• Energy enters the biosphere as photons and

leaves as heat, both without matter

• However, while passing through the biosphere, energy exists primarily in the form of chemical bond energies

• So, the flow of energy is coupled to a corresponding flow of matter

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Cyclic flow of matter

• Matter cycles between phototrophs and chemotrophs

• Carbon, oxygen, nitrogen, and water all cycle continuously

• They enter the chemotrophic sphere as reduced, energy-rich compounds, and leave it as oxidized, energy-poor forms

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Bioenergetics

• Energy flow is governed by the principles of thermodynamics (energy change)

• Thermodynamics concerns the laws governing energy transactions that accompany most physical and chemical processes

• Bioenergetics (applied thermodynamics) applies principles of thermodynamics to the biological world

• What would happen to an environment without energy????

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To Understand Energy Flow, We Need to Understand Systems, Heat, and Work

• Energy can be defined as the ability to cause change or capacity to do work

• Though energy is distributed throughout the universe, the energy under consideration in any particular case is called the system, and the rest of the universe is called the surroundings

• The boundary between the system and surroundings may be real or hypothetical

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Systems can be open or closed

• A closed system is sealed from its environment and cannot take in nor release energy

• An open system can have energy added to it or removed from it

• Organisms are open systems, capable of uptake and release of energy

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Figure 5-7

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The state of a system

• We have to specify the state of the system??• A system is in a specific state if each of its

variable properties is held at a specified value (temperature, pressure, and volume)– In this situation, the total energy of the system

has a unique value– If the state changes, the total energy change

is determined only by the initial and final states of the system

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Biological systems

• Three of the most important variables are constant during biological reactions

• This is because reactions occur in dilute solutions within cells at approximately the same temperature, pressure, and volume during the entire reaction

• These environmental conditions are slow to change

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Heat and work

• Exchange of energy between a system and its surroundings occurs as either heat or work

• Heat is not a very useful energy source for cells because many biological systems are isothermal (at a fixed temperature)– Heat is useful for things like machines

• Work is the use of energy to drive a process other than heat flow- muscle contraction, etc.

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Quantifying energy change

• The units for quantifying the energy changes during chemical reactions are calories (cal).

• calorie: the amount of energy required to raise one gram of water by one degree centigrade at one atmosphere of pressure

• One kilocalorie (kcal) = 1000 calories. What is Calorie with a “big” C.

• Physicists prefer the joule (J); 1J = 0.239 cal

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The First Law of Thermodynamics Tells Us That Energy Is Conserved

• The first law of thermodynamics is called the law of conservation of energy

• It states that in every physical or chemical change, the total amount of energy in the universe remains constant

• Energy may be converted from one form to another but cannot be created or destroyed

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Conservation of energy in biological systems

• In biological systems the energy that leaves a system must equal that which entered it plus the amount remaining (stored) in the system

• Total energy stored within a system is called internal energy, or E– Why are we not concerned with actual values?

• E is the change in internal energy that occurs during some process

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Calculating E

• E is the difference in internal energy of a system before a process (E1) and after it (E2)

E = E2 - E1

• For a chemical reaction, this can be written as

E = Eproducts - Ereactants

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Enthalpy

• We are usually most interested in change in enthalpy (H) or heat content, which is related to E, dependent on pressure and volume

• Because pressure and volume change little or not at all in biological reactions

H = E, and H = Hproducts - Hreactants

H may be either positive or negative

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H

• If H is negative, a reaction is exothermic

• In exothermic reactions, energy is released (e.g., the burning of gasoline in a car)

• If H is positive, a reaction is endothermic

• In endothermic reactions, energy is absorbed (e.g., the melting of an ice cube)

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The Second Law of Thermodynamics Tells Us That Reactions Have Directionality• A thermodynamically spontaneous reaction is one that

is a favorable reaction.

• Thermodynamic spontaneity is a measure of whether or not a reaction or process can occur (not will occur)

• Reactions have directionality, that is, they can only proceed spontaneously in one direction (e.g., drops of dye diffusing in water, burning paper)

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The second law of thermodynamics

• The second law of thermodynamics is the law of thermodynamic spontaneity

• It says that in every physical or chemical change, the universe tends toward greater disorder or randomness (entropy)

• It allows us to predict what direction a reaction will proceed under specific conditions, how much energy will be released, and how changes in conditions will affect it

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Entropy and Free Energy Are Two Alternative Means of Assessing Thermodynamic Spontaneity

• No process or reaction disobeys the second law of thermodynamics

• Whether or not a reaction can proceed can be measured by changes in entropy or free energy

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Entropy

• Entropy (S) is a measure of randomness or disorder– For any system, the change in entropy (delta S),

represents a change in degree of randomness or disorder

– Entropy increases when a system becomes less ordered (e.g., when ice melts or a solvent is allowed to evaporate, combustion of paper)

– Entropy decreases when a system becomes more ordered (e.g., when ice forms from water)

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Entropy Change as a Measure of Thermodynamic Spontaneity

• Suniverse is positive for every spontaneous process or reaction (increases the entropy of the universe)

– Second law pertains to the universe as a whole– May not apply to specific system under

consideration• Expressing the second law in terms of S is not

very useful in predicting the spontaneity of biological processes

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Free Energy

• A measure of spontaneity for a system alone is called free energy (G)– Most useful thermodynamic concepts in cell biology

• G (the free energy change) = Gproducts - Greactants

• What is constant?

• G is related to enthalpy and entropy of a reaction

• G = H - T S (T= temperature of the system in degrees Kelvin, or oC +273)

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Free Energy Change as a Measure of Thermodynamic Spontaneity

• Free energy is a readily measurable indicator of spontaneity

• Every spontaneous reaction is characterized by a decrease in free energy of the system

• So, if G < 0, the reaction is thermodynamically spontaneous. What if delta G is = O? What if delta G is >0

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Exergonic and endergonic reactions

• Exergonic reactions are energy-yielding, and occur spontaneously (G < 0)

• Endergonic reactions are energy-requiring and do not occur spontaneously under the conditions specified (G > 0)

• The values for both H and -TS influence the value of G

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Figure 5-9

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Free Energy Change and Thermodynamic Spontaneity: A Biological Example

• Consider the oxidation of glucose (a highly exergonic process)

• C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

• Under standard conditions, H = -673 kcal /mole glucose and -TS = -13 kcal/mole

• G = -686 kcal/mole

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Figure 5-10A

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The reverse reaction:

• The reverse reaction is endergonic

• 6CO2 + 6H2O + energy → C6H12O6 + 6O2

• Compared to the oxidation of glucose, H and -TS have the same magnitude but the opposite sign

• G = + 686 kcal/mole

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Figure 5-10B

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The Meaning of Spontaneity

• The term spontaneous tells us that a reaction can take place, not that it will

• Whether an exergonic reaction will proceed depends on a favorable (negative) G but also the availability of a mechanism

• Usually an input of activation energy is required as well

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Understanding G

• We need to be able to calculate G and determine whether it is positive or negative under the specified conditions

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The Equilibrium Constant is a Measure of Directionality

• The equilibrium constant Keq, the ratio of product concentrations to reactant concentration at equilibrium

• At equilibrium there is no net change in the concentrations of reactants or products

• A B Keq = [B]eq / [A]eq

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The equilibrium constant

• If you know the equilibrium constant for a reaction, you can tell whether a particular mixture of products and reactants is in equilibrium

• The tendency toward equilibrium provides the driving force for every chemical reaction

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Concentration ratio

• A concentration ratio (products to reactants) less than Keq means that the reaction will proceed to the right to generate more product

• A concentration ratio greater than Keq means that the reaction will proceed to the left

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Figure 5-11

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G′

• For real life situations, G′ is the most useful measure of thermodynamic spontaneity

• G′ provides a measure of how far from equilibrium a reaction is, under the conditions in a cell

• Under the special case where G′ = 0, the reaction is in equilibrium; however, reactions in living cells are rarely in equilibrium

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Table 5-1

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Life and the Steady State: Reactions That Move Toward Equilibrium Without Ever Getting There

• At equilibrium the forward and reverse rates of a reaction are the same, so there is no net flow of matter, nor energy produced

• Living cells are characterized by continuous reactions and maintain themselves in states far from equilibrium

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Steady state

• Cells maintain a steady state in which reactants, products, and intermediates are kept at levels far from equilibrium

• This is possible because cells are open systems that receive large amounts of energy from the environment