Chapter 8

An Introduction to Metabolism

Overview: The Energy of Life

• The living cell is a miniature chemical factory where thousands of reactions occur

• The cell extracts energy and applies energy to perform work

• Some organisms even convert energy to light, as in bioluminescence

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Metabolism

• Metabolism is the entire collection of an organism’s chemical reactions

• A metabolic pathway begins with a specific molecule and ends with a product

• Each step is catalyzed by a specific enzyme

Enzyme 1 Enzyme 2 Enzyme 3

DCBAReaction 1 Reaction 3Reaction 2

Startingmolecule

Product

Two Types of Metabolism

• Catabolic pathways release energy by breaking down complex molecules into simpler compounds

GLUCOSE CO2 + H2O + ATP

• Anabolic pathways consume energy to build complex molecules from simpler ones

Amino acids Proteins

Energy

• Energy is the capacity to cause change• Types of energy

– Kinetic energy is energy associated with motion

– Potential energy is energy that matter possesses because of its location or structure

• Energy can be converted from one form to another

Fig. 8-2

Climbing up converts the kineticenergy of muscle movementto potential energy.

A diver has less potentialenergy in the waterthan on the platform.

Diving convertspotential energy tokinetic energy.

A diver has more potentialenergy on the platformthan in the water.

The Laws of Energy Transformation

• Thermodynamics is the study of energy transformations

• First law of thermodynamics

– Energy can be transferred and transformed, but it cannot be created or destroyed

• Second law of thermodynamics

– During every energy transfer or transformation, some energy is unusable, and is often lost as heat (entropy)

Free Energy• Free Energy (∆G) - measures a system’s energy

that is available to perform work.

∆G = ∆H – T∆S

• Spontaneous

– a process that can occur without the input of energy; ∆G must be a negative value

• Nonspontaneous

– a process that requires the input of energy; ∆G will be a positive value or zero

As Biologists, we want to know which rxns of life are spontaneous. These rxns canbe harnessed to do work (to drive the nonspontaneous rxns.

When we apply the free energy concept to processes of life and with respect to metabolism, we describe rxns as exergonic or endergonic.

• An exergonic reaction proceeds with a net release of free energy and is spontaneous

Glucose + O2 CO2 + H2O ∆G = -686 kcal/mol

• An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous

CO2 + H2O Glucose + O2 ∆G = +686 kcal/mol

3 Main Types of Cellular Work

• Mechanical

• Transport

• Chemical

Fig. 8-8

Phosphate groupsRibose

Adenine

Fig. 8-9

Inorganic phosphate

Energy

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

P P

P P P

P ++

H2O

i

Fig. 8-11

(b) Mechanical work: ATP binds noncovalently to motor proteins, then is hydrolyzed

Membrane protein

Pi

ADP+

P

Solute Solute transported

Pi

Vesicle Cytoskeletal track

Motor protein Protein moved

(a) Transport work: ATP phosphorylates transport proteins

ATP

ATP

Fig. 8-12

P iADP +

Energy fromcatabolism (exergonic,energy-releasingprocesses)

Energy for cellularwork (endergonic,energy-consumingprocesses)

ATP + H2O

Fig. 8-14

Progress of the reaction

Products

Reactants

∆G < O

Transition state

Fre

e en

erg

y EA

DC

BA

D

D

C

C

B

B

A

A

Enzyme Inhibitors