Chapter 8 – An Introduction to Metabolismhttp://quizlet.com/6248900/ap-bio-ch-8-flash-cards/http://quizlet.com/15497122/textbook-questions-exam-3-flash-cards/

KEY CONCEPTS An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously ATP powers cellular work by coupling exergonic reactions to endergonic reactions Enzymes speed up metabolic reactions by lowering energy barriers Regulation of enzyme activity helps control metabolism

SECTION 8.1 THE ENERGY OF LIFE

Sugars -> Amino Acids -> Proteins o Sugars can be converted to amino acids that are linked together into proteins o Proteins are dismantled into amino acids that can be converted to sugars when food is

digested Cellular Respiration

o Responsible for the cellular economy o Extracts energy stored in sugars and other fuels

Cells apply this energy to do various workso Process of oxidizing food molecules, like glucose, to carbon dioxide and water o C6H12O6 + 6O2 + 6H2O 12H2O + 6CO2

Bioluminescence (and other metabolic activities) is coordinated and controlledo Bioluminescence is the production and emission of light by a living organism

Bioenergetics – the study of how energy flows through living organisms

METABOLISM TRANSFORMS MATTER & ENERGY – Laws of Thermodynamics Metabolism – the totality of an organism’s chemical reactions; “road map of thousands of chemical

reactions that occur” Metabolic Pathway – begins with a specific molecule, which is then altered in a series of defined

steps, resulting in a certain producto Pathways can have more than one starting moleculeo Each step of the pathway is catalyzed by a specific enzyme

oo Catabolic & anabolic pathways are the “downhill” and “uphill” avenues of the metabolic

map Energy released from the downhill reactions of catabolic pathways can be stored

and then used to drive the uphill reactions of anabolic pathwayso Metabolism manages the material and energy resources of the cello Catabolic pathways (breakdown pathways) – release energy by breaking down complex

molecules to simpler compounds Cellular Respiration – sugar glucose and other organic fuels are broken down in the

presence of oxygen, carbon dioxide, and water. o Anabolic Pathways (biosynthetic pathways) – consume energy to build complicated

molecules from simpler ones Ex: synthesis of a protein from amino acids

FORMS OF ENERGY Energy – the capacity to cause change

o Can be used to do worko Ability to rearrange a collection of mattero Kinetic Energy – energy can be associated with the relative motion of objects

Heat/Thermal Energy – kinetic energy associated with the random movement of atoms or molecules

o Light – (do not forget) is also a type of energy that can be used to perform work, such as powering photosynthesis in plants

o Potential Energy – energy that matter possesses because of its location or structure Ex: Water behind a dam possesses energy b/c of its altitude above sea level Molecules have energy b/c of its arrangement of atoms Chemical Energy – potential energy available for release in a chemical reaction

Catabolic pathways release energy by breaking down complex molecules, which are high in chemical energy

o During a catabolic reaction, atoms are rearranged and energy is released, resulting in lower-energy breakdown products

o Ex: Engine of a Car – hydrocarbons of gasoline react violently with oxygen, releasing energy that pushes pistons and produces exhaust

EXAMPLE: Transformations between potential & kinetic energy in divers

o

LAWS OF ENERGY TRANSFORMATION Thermodynamics – the study of the energy transformations that occur in a collection of matter

o System – denotes the matter under study Isolated System – unable to exchange either energy or matter with its surroundings

Ex: liquid in a thermos bottle Open System – energy and matter can be transferred between the system and its

surroundings Ex: Organisms b/c they absorb energy and release heat and metabolic waste

to the surroundingso Surrounding(s) – everything outside the system (universe)

First Law of Thermodynamics (the Principle of Conservation of Energy) – energy can be transferred and transformed, but it cannot be create or destroyed

o The energy of the universe is constant Ex: Electric companies do not make energy, but convert it to a form that is

convenient for use Ex: Plants convert sunlight to chemical energy, but does not produce sunlight for it

to create chemical energy Ex: Consumers convert chemical energy of organic molecules in food to kinetic (and

other forms of energy as it carries out biologic processes) Second Law of Thermodynamics – every energy transfer or transformation increases the disorder

(entropy) of the universe; for a process to occur spontaneously, it must increase the entropy of the universe

o During every energy transfer/transformation, some energy becomes unusable energy, unavailable to do work

o More usable forms of energy are at least partly converted to heat (energy associated with the random motion of atoms/molecules)

o Ex: Consumption of Food A small fraction of the chemical energy from food is transformed into usable energy Most of the chemical energy is lost as heat

o Systems can put heat to work when there are temperature differences that result in heat flowing from warmer to cooler locations

Temperature is uniform – (as it is in a living cell) heat energy during chemical reactions warm a body of matter, such as the organism

EXAMPLE: A cheetah eating a deero First Law of Thermodynamics – energy can be transferred or transformed but not created

nor destroyed The chemical (potential) energy in food will be converted to the kinetic energy of

the cheetah’s movement o Second Law of Thermodynamics – every energy transfer/transformation increases the

disorder (entropy) of the universe Disorder is added to the cheetah’s surroundings in the form of heat and the small

molecules that are the by-products of metabolism, such as carbon dioxide Entropy – measurement of disorder or randomness

o The more randomly arranged a collection of matter is, the greater its entropyo Increased entropy is evident in the physical disintegration of a system’s organized

structure Ex: Gradual decay of an unmaintained building

o For a process to occur on its own, without outside help (an input of energy), it must increase the entropy of the universe

o Spontaneous – a process that can occur without an input of energy (the process could occur instantaneously or slowly)

o Nonspontaneous – a process that cannot occur on its own and can only occur if energy is added to the system

o Ex: Water flows downhill spontaneously, but moves uphill only with an input of energy

BIOLOGICAL ORDER & DISORDER Cells create ordered structures from less organized starting materials

o Ex: Amino acids are ordered into the specific sequences of polypeptide chains Organisms also takes in organized forms of matter and energy from the surroundings and replaces

them with less ordered forms o Ex: Animal obtains starch, proteins, and other complex molecules from the food it eats. As

catabolic pathways break these molecules down, the animal releases CO2 and O2 (molecules that possess less chemical energy than the food eaten)

Energy flows into an ecosystem in the form of light and exists in the form of heat The entropy of a particular system (such as an organism) may decrease as long as the total

entropy of the universe increases

CONCEPT CHECK 8.11. How does the second law of thermodynamics help explain the diffusion of a substance across a

membrane?a. When a substance diffuses across a membrane, it goes from a high concentration to a low

concentration, or high entropy to low entropy. Since this action releases energy, this helps to increase the entropy of the universe. Diffusion of a substance to a region where it is initially less concentrated increases entropy, making it an energetically favorable (spontaneous) process as described by the second law.

2. Describe the forms of energy found in an apple as it grows on a tree, then falls, and is digested by someone who eats it.

a. As an apple forms and grows, it has chemical energy and as it hangs from the tree, it has potential energy. If it falls onto the ground, it has kinetic energy. After it is eaten, its molecules are broken down, and some of its chemical energy is used to do work through kinetic energy, saved as potential energy, and lost as thermal energy.

3. What if you place a teaspoon of sugar in the bottom of a glass of water, it will dissolve completely over time. Left longer, eventually the water will disappear and the sugar crystals will reappear. Explain these observations in terms of entropy.

a. The sugar crystals become less ordered (entropy increases) as they dissolve and become randomly spread out in the water. Over time, the water evaporates, and the crystals form again because the water volume is insufficient to keep them in solution. While the reappearance of sugar crystals may represent a “spontaneous” increase in order (decrease in entropy), it is balanced by the decrease in order (increase of entropy) of the water molecules, which changed from a relatively compact arrangement as liquid water to much more dispersed/disordered form as water vapor.

SECTION 8.2 FREE ENERGY CHANGE OF REACTIONS DETERMINE SPONTANEITY Universe is equivalent to “the system” plus “the surroundings.”

FREE-ENERGY CHANGE, ΔG 1878 J. Willard Gibbs

o Free-Energy – denoted by the letter “G”, is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system (Ex. Living Cell)

o The change in free-energy, denoted by ΔG, can be calculated for a chemical reaction with the formula:

ΔG = ΔH – T Δ S ΔH = the change in the system’s enthalpy (equivalent to total energy) ΔS = the change in the system’s entropy ΔT = the absolute temperature in Kelvin (K) units (K = °C + 273)

change in free energy = change in enthalpy – (absolute temp.)*(change in entropy)o Knowing ΔG will help predict whether the process will be spontaneous (whether it will

occur without an input of an outside energy source) Negative ΔG are spontaneous

o Spontaneous Processes – system must either: Give up enthalpy (H must decrease) Give up order (TS must increase) Or both, give up enthalpy and give up order

o When ΔH and TΔS are tallied, ΔG must have a negative value (ΔG < 0) for a spontaneous reaction to occur

o Every spontaneous process decreases the system’s free energy Processes with a positive or zero ΔG are never spontaneous

FREE ENERGY, STABILITY, & EQUILIBRIUM ΔG represents the difference between the free energy of the final state and the free energy of the

initial state:ΔG = G final state – G initial state

ΔG can be negative only when process involves a loss of free energy during the change from initial state to final state.

o With less free energy, the system in its final state is less likely to change and is more stable than before.

Free energy – measure of a system’s instability, its tendency to change to a more stable state. o Unstable Systems (higher G) tend to change so that they become more stable (lower G)

Ex. Diver on top of a platform is less stable (more likely to fall) than when floating in the water – system moves towards greater stability (diver falls)

Ex. A drop of concentrated dye is less stable (more likely to disperse) than when the dye is spread randomly through the liquid – system moves towards greater stability (solution becomes uniformly colored)

Ex. Sugar molecule is less stable (more likely to break down) than the simpler molecules into which is can be split – system moves towards greater stability (sugar molecule breaks down)

Equilibrium – describes a state of maximum stabilityo Note: chemical reactions are reversible and proceed to a point at which the forward and

backward reactions occur at the same rate; said to be at chemical equilibrium, and there is no further net change in the relative concentration of products & reactants

o Free energy (of the mixture of reactions & products) decrease when a reaction tends toward equilibrium

Free energy decreases as reactions tend towards equilibriumo Free energy (of the mixture of reactions & products) increases when a reaction is moves

away from equilibrium Free energy increases as reactions move away from equilibrium

o For a system at equilibrium, G (free energy) is at its lowest possible value in that system Any change from the equilibrium position will have a positive ΔG and will not be

spontaneous Process is spontaneous and can perform work only when it is moving toward

equilibrium The relationship of free energy to stability, work capacity, and spontaneous change:

Unstable systems (top row) are rich in free energy and have a tendency to change

spontaneously to a more stable state (bottom row) Able to harness this “downhill” change to perform work

FREE ENERGY & METABOLISM Exergonic (“energy outward”) & Endergonic (“energy inward”) Reactions in Metabolism

o Exergonic Reaction – proceeds with a net release of free energy The lost of free energy (G decreases), ΔG is negative for an exergonic reaction Exergonic Reaction = -ΔG Exergonic reactions occur spontaneously Represents the amount of work the reaction can perform

The greater the decrease in free energy, the greater the amount of work that can be done

More –ΔG = More Work

Ex: Cellular Respiration

Each mole (180 g) of glucose broken down by cellular respiration under “standard conditions” (1 M of each reaction and product, at 25°C and pH 7), 686 kcal (2,870 kJ) of energy are made available for work

o Conservation of energy dictates that chemical products (in cell. resp.) store 686 kcal less free energy per mole than the reactants

“The products are the spent exhaust of a process that tapped the free energy stored in the sugar molecules.”

o Endergonic Reaction – absorbs free energy from its surroundings and stores free energy in molecules (G increases)

Positive G Endergonic Reaction = +ΔG Nonspontaneous Magnitude of ∆G is the quantity of energy required to drive the reaction(s) If a chemical process is exergonic (downhill), releasing energy in one direction then

the reverse process must be endergonic (uphill), using energy Reversible processes cannot be downhill in both directions Ex: Cellular Respiration

If ∆G = -686 kcal/mol., then the reverse process (the conversion of carbon dioxide and water to sugar and oxygen) must be strongly endergonic, with ∆G = +686 kcal/mol.

Such a reaction would never happen by itself Plants, on the other hand, require energy from the environment by

capturing light and converting its energy to chemical energy

Equilibrium and Metabolism

o Reactions in isolated systems eventual reach equilibrium and can then do no worko Systems are equilibrium are at a minimum of G and can do no more work (cell has reached

metabolic equilibrium and is dead) Living cells are never in equilibrium b/c there is a constant flow of materials in and

out of the cell Ex: Cellular Respiration

Glucose is broken down in a series of exergonic reactions that power the work of the cell

The product of each of these reactions become the reactants for the next reaction

Therefore, no reaction reaches equilibrium The overall sequence of reactions is continuous by the huge free-energy

difference between glucose and oxygen at the top of the energy “hill” and carbon dioxide and water at the “downhill” end.

CONCEPT CHECK 8.21. Cellular respiration uses glucose and oxygen, which have high levels of free energy, and releases

carbon dioxide and water, which have low levels of free energy. Is respiration spontaneous or not? Is it exergonic or endergonic? What happens to the energy released from glucose?

a. Cellular respiration is a spontaneous exergonic process and the energy released from glucose is used to either do work in the cell or is lost as heat.

2. A key process in metabolism is the transport of hydrogen ions (H+) across a membrane to create a concentration gradient. Other processes can result in an equal concentration of hydrogen ions on each side. Which arrangement of hydrogen ions allows the H+ to perform work in this system?

a. When the H+ concentrations are the same, the system is at equilibrium and can do no work. H+ can perform work only if their concentrations on each side of a membrane are different (when a gradient is present). The energy input (provided by ATP hydrolysis) is required to establish the concentration gradient (the H+ gradient) that can in turn perform work.

3. At nightly celebrations, revelers can sometimes be seen wearing glow-in-the-dark necklaces. They start glowing once they are “activated,” in a way that allows two chemicals to react and emit light in the form of “chemiluminescence.” Is this exergonic or endergonic?

a. The reaction is exergonic because it releases energy in the form of light.

SECTION 8.3 ATP POWERS CELLULAR WORK BY COUPLING EXERGONIC REACTIONS TO ENDERGONIC REACTIONS

A cell has three main kinds of jobs:o Chemical Work – the pushing of endergonic reactions, which would not occur

spontaneously, such as the synthesis of polymers from monomers. o Transport Work – the pumping of substances across membranes against the direction of

spontaneous movement o Mechanical Work – such as the beating of cilia, the contraction of muscle cells, and the

movement of chromosomes during cellular reproduction Energy coupling – the use of an exergonic process to drive an endergonic process

o ATP is responsible for mediating most energy coupling in cells o ATP acts as the immediate source of energy that powers cellular work

Structure and Hydrolysis of ATP Adenosine triphosphate (ATP) – contains the sugar ribose, with the nitrogenous adenine base, and

a chain of three phosphate groups bonded to it

oo Note: Aside from its role in energy coupling, ATP is also one of the nucleoside

triphosphates used to make ribonucleic acid (RNA) o Bonds between the phosphate groups in ATP can be broken by hydrolysis

When a terminal phosphate bond is broken, a molecule of inorganic phosphate leaves the ATP and becomes adenosine diphosphate (ADP)

Reaction is exergonic and releases 7.3 kcal of energy per mole of ATP hydrolyzed

Above is the free-energy change measured under standard conditions In the cell, conditions do not conform to standard conditions b/c reaction

and product concentrations differ from 1 M. Since hydrolysis releases energy, the phosphate bonds of ATP are sometimes

referred to as high-energy phosphate bonds (but the bonds are not unusually strong bonds)

The reactants (ATP and water) have high energy relative to the energy of the products (ADP and the inorganic phosphate (P))

o Hydrolysis of ATP – the reaction of ATP and water yields inorganic phosphate, ADP, and energy

The release of energy comes from the chemical change to a state of lower free

energy, not from the phosphate bondso ATP is useful b/c the energy it releases on losing a phosphate group is somewhat greater

than the energy most other molecules could delivero Hydrolysis releases so much energy

All three phosphate groups in the ATP molecule are negatively charged Negatively charges crowd together and their mutual repulsion contributes to the

instability of this region of the ATP molecule

How ATP Performs Work Shivering in humans – uses ATP hydrolysis

during muscle contraction to generate heat and warm the body

In most cases in the cell, the generation of heat alone would be an inefficient (and potentially dangerous) use of a valuable energy resource

o Instead, cell’s proteins use the energy released during ATP hydrolysis in several ways to perform chemical, transport, and mechanical works.

Ex: Energy coupling using ATP hydrolysis (right)

o The exergonic process of ATP hydrolysis is used to drive an endergonic process

Cellular synthesis of the amino acid glutamine from glutamic acid and ammonia

Cells can use the energy released by ATP hydrolysis (with the help of specific enzymes) to drive chemical reactions that, by themselves, are endergonic

o If ∆G of an endergonic reaction is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that the coupled reactions are exergonic.

Involves the transfer of a phosphate group from ATP to some other molecule (reactant)

Phosphorylated – recipient of the phosphate group Key to coupling exergonic and endergonic reactions is the formation of this

phosphorylated intermediate, which is more reactive and less stable than the original unphosphorylated molecule

Transport & Mechanical works in the cell are powered by the hydrolysis of ATP o ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind another

molecule (sometimes occurs through a phosphorylated intermediated)o Mechanical work involving motor proteins “walking” along cytoskeletal elements

ATP is first bound noncovalently to the motor protein ATP is then hydrolyzed, releasing ADP and inorganic phosphate Another ATP molecule can then bind At each stage, the motor protein changes its shape and ability to bind the

cytoskeleton, resulting in movement of the protein along the cytoskeletal tracko Quick Summary:

ATP hydrolysis causes changes in the shapes & binding affinities of proteins, which can occur either:

(A) DIRECTLY – by phosphorylation, as shown for the membrane proteins involved in active transport of solutes

(B) INDIRECTLY – by noncovalent binding of ATP and its hydrolytic products, as is the case for motor proteins that move vesicles (and organelles) along cytoskeletal “tracks” in the cell

The Regeneration of ATP ATP is a renewable resource that can be regenerated by the addition of phosphate to ADP

o The free energy required to add a phosphate group to ADP comes from exergonic breakdown reactions (catabolism) in the cell, called the ATP cycle

o ATP cycle couples the cell’s energy-yielding (exergonic) process to the energy-consuming (endergonic) ones

ATP-Cycle

o Energy released by breakdown reactions (catabolism) in the cell is used to phosphorylate ADP, regenerating ATP.

o Chemical potential energy stored in ATP drives most cellular worko Both directions of a reversible process cannot go downhill, the regeneration of ATP from

ADP and (P) is endergonic

o ATP from ADP and (P) is not spontaneous

Free energy must be spent to make ATP-Cycle work Catabolic (exergonic) pathways provide the energy for the endergonic process of

making ATP Plants use light energy to produce ATP

o ATP-Cycle is a turnstile through which energy passes during its transfer from catabolic to anabolic pathways.

Note: Catabolic & anabolic pathways are the “downhill” and “uphill” avenues of the metabolic map

Energy released from the downhill reactions of catabolic pathways can be stored and then used to drive the uphill reactions of anabolic pathways

CONCEPT CHECK 8.31. In most cases, how does ATP transfer energy from exergonic to endergonic reactions in the cell?

a. ATP usually transfers energy to endergonic processes by phosphorylating (adding phosphate groups to) other molecules. Exergonic processes phosphorylate ADP in order to regenerate ATP.

2. Which of the following combinations has more free energy: a. glutamic acid + ammonia + ATP b. glutamine + ADP + (P)c. A set of coupled reactions can transform the first combination into the second. Since this is

an exergonic process overall, ∆G is negative and the first combination must have more free energy.