Chapter 6 – Metabolism: Energy and Enzymes

Energy laws, metabolic pathways, and production and use of ATP in cells are covered in this chapter.

I. Section 6.1: Energy

A. Energy

1. capacity to do work; cells must continually use energy to do biological

work

2. kinetic energy is energy of motion; all moving objects have kinetic energy

3. potential energy is stored energy

a. energy within an atom lies in arrangement of its atoms in molecule; glucose has more energy than its breakdown components, CO₂ and H₂O

B. Two Laws of Thermodynamics

1. first law: law of conservation of E

a. E cannot be created or destroyed, but can be changed from one

form to another

b. heat E converts to kinetic E

c. chemical E in food converted to chemical E in ATP, then

converted to mechanical E of muscle contraction

2. second law

a. E cannot be changed from one form to another without loss of usable E (e.g only 25% of chemical E in gasoline used to move car, rest lost as heat)

b. when muscles convert chemical E in ATP for mechanical E, some lost as heat

c. heat is form of E, but quickly dissipates into environment; because heat dissipates, can never be converted back to form of potential E

C. Entropy

1. measure of randomness or disorder

2. organized/usable forms of E have low entropy; unorganized/less stable

forms have high E

3. energy conversions results in heat, therefore entropy is always increasing

4. takes constant input of usable E from food to keep you organized

II. Section 6.2: Metabolic Reactions and Energy Transformation

A. Metabolism

1. sum of all biochemical reactions in cell

2. A + B à C + D, then A and B are reactants and C and D are products

3. free E (∆G) is amount of E free to do work after a chemical reaction

4. change in free E is noted as ∆G; negative ∆G means that products have

less free E than reactants; reaction occurs spontaneously

5. exergonic reactions have a negative ∆G and E is released

6. endergonic reactions have a positive ∆G; products have more E than

reacants; such reactions can only occur with an input of E

7. reversible reactions have a free E difference near zero; such a reaction is

at equilibrium

8. cells use product of first reaction as reactant in second reaction; such process pulls first reaction in one direction

B. Coupled reactions

1. occur when E released by an exergonic reaction used to drive an

endergonic reaction

2. E released from ATP à ADP + P used to fuel many biological reactions

3. ATP breakdown is coupled to reaction that requires E; both reactions take place at same time in same place

4. when ATP breaks down to drive reaction, some E is lost as heat; overall reaction becomes exergonic

C. ATP: Energy for cells

1. ATP (adenosine triphosphate) is E currency for cells; when cells

require E, they “spend” ATP

2. great demand for ATP requires body to constantly produce ATP

3. small amount of ATP is constantly recycled from ADP and P –

continually made, broken down, and remade in cells

4. E released from ATP à ADP + P is just enough for most biological reactions

D. Function of ATP

1. chemical work: supplies E to synthesize macromolecules that make up cell

2. transport work: supplies E to pump substances across PM

3. mechanical work: supplies E to move muscles, cilia, flagella, chromosomes, etc.

E. Structure of ATP

1. nucleotide made of base adenine, sugar ribose, and three phosphate

groups

2. high E compound because a P group is easily removed

3. in cells, ~7.3 kcal/mole released when ATP hydrolyzed to ADP + P

III. Section 6.3: Metabolic Pathways and Enzymes

A. Reactions in cells are orderly

1. orderly sequence of chemical reactions; each step catalyzed by a

specific enzyme

2. begin with particular reactant, end with end product and have many

intermediate steps

3. one pathway leads to next; can be several others

4. E captured more easily; released in small increments

5. reactant is substance that participates in reaction; product formed by

reaction

6. each step is series of chemical reactions assisted by enzyme

7. enzymes are catalysts that speed chemical reactions without enzyme

changing

8. every enzyme is specific in its action and catalyzes only one reaction or one type of reaction

9. substrate is reactant in an enzymatic reaction

B. Energy of activation

1. for metabolic reactions to occur in cells, enzyme must usually be present

2. without enzymes, activation is achieved by heating reaction to increase molecule collisions

3. energy of activation (E_a) is E that must be added to cause molecules to react

C. Enzyme-substrate complexes

1. enzymes speed reactions by lowering the E_a by forming complex with

their substrate(s) at the active site

a. active site is small region on surface of enzyme where

substrate(s) bind

b. when substrate binds to enzyme, active site undergoes a slight

change in shape that facilitates reaction—called the induced-fit model

2. only small amount of enzyme is needed in cell because enzyme not used up

3. some enzymes actually participate in reaction

4. particular reactants may produce more than one type of product

a. presence or absence of enzyme determines which reaction takes

place

b. if reactants can form more than one product, enzymes present

determine product produced

5. every cell reaction requires its specific enzyme; enzymes are named for

substrates by adding “-ase.”

D. Factors affecting enzymatic speed

1. enzymatic reactions are rapid

a. to achieve maximum product, need enough substrate to fill

active sites

b. optimal temp and pH increases rate of enzymatic reactions

2. temperature and pH

a. as temp rises, enzyme activity increases due to increased

molecular collisions

b. enzyme activity declines rapidly when enzyme is denatured at a

certain temp; results in change in shape of enzyme

c. each enzyme has optimal pH that maintains normal

configuration

d. change in pH alters ionization of side chains, resulting in

denaturation

3. enzyme concentration

a. regulated by cell

b. some enzymes regulated by phosphorylation; kinase cascade

activates enzymes by phosphorylating them

4. enzyme inhibition

a. inhibition is common means for cells to regulate enzymes

b. competitive inhibition, another molecule similar to substrate

competes for enzyme’s active site, resulting in decreased product formation

c. noncompetitive inhibition, molecule binds to allosteric site—site other than active site—changing 3D structure of enzyme and its ability to bind substrate

d. feedback inhibition regulates activity of most enzymes; products produced by an enzyme binds enzyme’s active site

1) when product abundant, active sites full and activity drops

2) when product used up, inhibition reduced and more product produced

3) concentration of products kept within narrow range

4) pathways regulated by feedback inhibition; end product

of pathway binds allosteric site on first enzyme in pathway, shutting it down

e. cyanide inhibits essential enzyme (cytochrome oxidase) in all

cells

5. enzyme cofactors

a. many enzymes require inorganic ion or non-protein cofactor to

function

b. ions are metals; organic cofactors are coenzymes (e.g. vitamins)

that assist enzymes or accept or contribute atoms to reaction

c. vitamins required in trace amounts for synthesis of coenzymes;

become part of coenzyme’s molecular structure; vitamin deficiency causes lack of coenzyme and lack of enzymatic action

IV. Section 6.4: Metabolic Pathways and Oxidation-Reduction

A. Oxidation-reduction

1. electrons pass from one molecule to another

2. oxidation is loss of electrons

3. reduction is gain of electrons

4. both reactions occur at same time (one molecule loses one molecule

gains)

B. photosynthesis

1. uses E to combine CO₂ and H₂O to produce glucose:

6CO₂ + 6H₂O + E à C₆H₁₂O₆ + 6O₂

2. H₂O is oxidized and CO₂ reduced

3. E needed to produce high-E glucose molecule

4. chloroplasts capture solar E and convert it by electron transport to

chemical E of ATP

5. ATP used along with H atoms to reduce glucose; when NADP⁺

(nicotinamide adenine dinucleotide phosphate) donates H⁺ + e^- to

substrate during photosynthesis, substrate has accepted e^- and is reduced

6. reaction that reduces NADP⁺ is:

NADP⁺ + 2e^- + H⁺ à NADPH

C. cellular respiration

1. overall equation for anaerobic respiration is opposite photosynthesis

C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O + E

2. when NAD removes H⁺ + e^- during respiration, substrate has lost e^-

and is oxidized

NAD⁺ + 2e^- + H⁺ à NADH

3. at end of cellular respiration, glucose has been oxidized to CO₂and

H₂O and ATP produced

D. electron transport system

1. both photosynthesis and respiration are metabolic pathways that use

electron transport system consisting of membrane-bound carries to pass electrons from one carrier to another

2. high-E electrons are delivered to system and low-E electrons leave

3. each time electrons transfer to new carrier, E is released and used to

produce ATP

E. ATP production

1. ATP synthesis coupled to electron transport

2. in mitochondria and chloroplasts, carriers of electron transport systems

are located within a membrane

3. H⁺ ions collect on one side of membrane because they are pumped there by certain carriers

4. electrochemical gradient across membrane is used to provide E for ATP production

5. ATP synthase spans membrane, forming channels that allow H⁺ to flow down gradient

6. flow of H⁺ thru channels provides E to drive ADP + P à ATP

7. as solar E collected by plants and converted to ATP, thylakoid membrane in chloroplasts acts as dam to maintain E gradient