Ground Rules of Metabolism

Chapter 6

6.1 What Is Energy?

• Capacity to do work

• Forms of energy– Potential energy– Kinetic energy– Chemical energy

What Can Cells Do with Energy?

• Energy inputs become coupled to

energy-requiring processes

• Cells use energy for:

– Chemical work

– Mechanical work

– Electrochemical work

First Law of Thermodynamics

• The total amount of energy in the universe remains constant

• Energy can undergo conversions from one form to another, but it cannot be created or destroyed

One-Way Flow of Energy

• The sun is life’s primary energy source

• Producers trap energy from the sun and convert it into chemical bond energy

• All organisms use the energy stored in the bonds of organic compounds to do work

Second Law of Thermodynamics

• No energy conversion is ever 100

percent efficient

• The total amount of energy is flowing

from high-energy forms to forms

lower in energy

Entropy

• Measure of degree of disorder in a system

• The world of life can resist the flow toward maximum entropy only because it is resupplied with energy from the sun

6.2 Energy Changes & Cellular Work

Energy changes in cells tend to run spontaneously in the direction that results in a decrease in usable energy

Endergonic Reaction

+ 602 and 6H2O

Energy in

energy-poorstarting substances 6 12

glucose - a product with more energy

Figure 6.5a,bPage 100

Exergonic Reaction

Energy out

glucose - energy-rich starting

substance

+602

products with less energy

6 6

Figure 6.5a,bPage 100

Structure of ATP

three phosphate groups

sugar (ribose)

nucleotide base (adenine)

Figure 6.6bPage 101

ATP: Main Energy Carrier

• ATP couples energy inputs and outputs

• ATP/ADP cycle regenerates ATP

energyinput

ADP + Pi

ATP

energy output

Electron Transfers

• Oxidation - lose electron

• Reduction - gain electron

• Central to the formation of ATP during

photosynthesis and aerobic respiration

6.3 Participants in Metabolic Pathways

• Energy Carriers

• Enzymes

• Cofactors

• Reactants

• Intermediates

• Products

Degradative and Anabolic Pathways

ATP

BIOSYNTHETIC PATHWAYS(ANABOLIC)

ENERGY INPUT

DEGRADATIVE PATHWAYS

(CATABOLIC)

energy-poor products

large energy-rich molecules

simple organic compounds

ADP + Pi

Types of Reaction Sequences

BRANCHING PATHWAY

LINEAR PATHWAY CYCLIC PATHWAY

A B C D EF

K J I G

N M L H

Figure 6.8Page 102

Which Way Will a Reaction Run?

• Nearly all chemical reactions are

reversible

• Direction reaction runs depends upon

– Energy content of participants

– Reactant-to-product ratio

Chemical Equilibrium

HIGHLYSPONTANEOUS

EQUILIBRIUM

HIGHLYSPONTANEOUS

RELATIVE CONCENTRATION

OF PRODUCT

RELATIVE CONCENTRATION

OF REACTANT

Figure 6.9Page 103

Chemical Equilibrium

• Energy in the reactants equals that in the products

• Product and reactant molecules usually differ in energy content

• Therefore, at equilibrium, the amount of reactant almost never equals the amount of product

No Vanishing Atoms

• Law of conservation of mass

• Reactions rearrange atoms, but they

never destroy them

• As many atoms of each element in all

the products as there were in all the

reactants

6.4 Electron Transfer Chains

• Arrangement of enzymes, coenzymes,

at cell membrane

• As one molecule is oxidized, next is

reduced

• Create H+ concentration and electric gradients that are used for making ATP

6.5 Enzyme Structure and Function

Enzymes are catalytic molecules

They speed the rate at which reactions approach equilibrium

Four Features of Enzymes

1) Enzymes do not make anything happen that could not happen on its own. They just make it happen much faster.

2) Reactions do not alter or use up enzyme molecules.

Four Features of Enzymes

3) The same enzyme usually works for both the forward and reverse reactions.

4) Each type of enzyme recognizes and binds to only certain substrates.

Activation Energy

• For a reaction to occur, an energy barrier must be surmounted

• Enzymes make the energy barrier smaller

activation energywithout enzyme

activation energywith enzyme

energyreleased

by thereaction

products

starting substance

Figure 6.12aPage 105

Activation Energy

Used to:

• Align reactive chemical groups

• Briefly destabilize electric charges

• Rearrange, create, and break bonds

6.6 Transition State

• Point when a reaction can easily run in

either direction, to product or back to a

reactant

• Substrate is bound most tightly to an

enzyme in this state

Mechanisms of Bringing about Transition State

• Helping substrates get together

• Orienting substrates in positions

favoring reaction

• Shutting out water

• Inducing changes in enzyme shape

Factors Influencing Enzyme Activity

Coenzymes and cofactors

Allosteric regulators

Temperature

pH

Salt concentration

6.7 How Is Enzyme Activity Controlled?

• Allosteric Activation

• Allosteric Inhibition

• Feedback Inhibition

Allosteric Activation

allosteric activator

vacantallosteric binding site

active site altered, can bind substrate

active site cannot bind substrate

enzyme active site

Figure 6.15aPage 108

Allosteric Inhibition

allosteric inhibitor

allosteric binding site vacant; active site can bind substrate

active site altered, can’t bind substrate

Figure 6.15bPage 108

Feedback Inhibition

enzyme 2 enzyme 3 enzyme 4 enzyme 5

enzyme 1

SUBSTRATE

END PRODUCT

(tryptophan)

A cellular change, caused by a specific activity, shuts down the activity that brought it about

Figure 6.16Page 108

Effect of Temperature

• Small increase in temperature increases molecular collisions, reaction rates

• High temperatures disrupt bonds and destroy the shape of active site

Figure 6.17bPage 109

Effect of pH

Figure 6.17cPage 109