Energy and Metabolism

Chapter 4

Part 1

4.1 Impacts/Issues

A Toast to Alcohol Dehydrogenase

Metabolic processes break down organic

molecules such as ethanol and other toxins –

binge drinking is currently the most serious drug

problem on college campuses

Video: Alcohol, enzymes, and your liver

Fig. 4-1a, p. 62

Fig. 4-1b, p. 62

alcohol dehydrogenase

Video: ABC News: The wine of life

4.2 Life Runs on Energy

Energy

• The capacity to do work

Energy can be converted from one form to

another, but cannot be created or destroyed –

energy disperses spontaneously

Laws of Thermodynamics

First law of thermodynamics

• Energy cannot be created or destroyed

• It can be converted from one form to another and

thus transferred between objects or systems

Second law of thermodynamics

• Energy tends to disperse spontaneously

• A bit disperses at each energy transfer, usually in

the form of heat

One-Way Flow of Energy

Living things maintain their organization by

harvesting energy from someplace else

Energy flows in one direction through the

biosphere (starting mainly from the sun) then

into and out of ecosystems

Material Recycle

Energy inputs drive a cycling of materials among

producers and consumers

Producers and then consumers use energy to

assemble, rearrange, and break down organic

molecules that cycle among organisms

throughout ecosystems

One-Way Flow of Energy

Fig. 4-2, p. 63

Light energy radiating from

the sun reaches Earth.

Producers capture some of

it by converting it to

chemical energy. They and

all other organisms use

chemical energy to drive

cellular work.

ENERGY IN

PRODUCERSplants and other self-

feeding organisms

nutrient

cycling

CONSUMERSanimals, most fungi,

many protists, bacteria

ENERGY OUT

With each conversion, there is a one-

way flow of a bit of energy back to the

environment, mainly in the form of heat.

Active Figure: Matter recycling and

energy flow

Animation: One-way energy flow and

materials cycling

4.3 Energy in the Molecules of Life

Cells store and retrieve energy by making and

breaking chemical bonds in metabolic reactions

Some reactions require a net input of energy –

others end with a net release of energy

Chemical Reactions

Reaction

• Process of chemical change

Reactant

• Molecule that enters a reaction

Product

• A molecule remaining at the end of a reaction

A Chemical Reaction

p. 64

Reactants Products

2 H2

(hydrogen)

2 H2O

(water)

4 hydrogen atoms

+ 2 oxygen atoms

4 hydrogen atoms

+ 2 oxygen atoms

O2

(oxygen)

+

Energy Inputs and Outputs

in Chemical Reactions

Chemical bonds hold energy – the amount

depends on which elements take part in the bond

Cells store energy in chemical bonds by running

energy-requiring reactions, and access energy by

running energy-releasing reactions

Energy Inputs and Outputs

in Chemical Reactions

Fig. 4-3, p. 64

2 H2 + O2

En

erg

y

energy in

energy out

2 H2O

Why the World Doesn’t Go Up in Flames

Molecules of life release energy when combined

with oxygen – but not spontaneously – energy is

required to start even energy-releasing reactions

Activation energy

• Minimum amount of energy required to start a

reaction

Activation Energy

Fig. 4-4, p. 65

Reactants:

2 H2 + O2

Activation energy

En

erg

y

Difference in

energy between

reactants and products

Products: 2 H2O

Time

Fig. 4-4, p. 65

Reactants:

2 H2 + O2

Activation energy

En

erg

y

Difference in

energy between

reactants and products

Products: 2 H2O

TimeStepped Art

Animation: Chemical equilibrium

ATP – The Cell’s Energy Currency

Energy carriers accept energy from energy-

releasing reactions and deliver energy to

energy-requiring reactions

ATP (Adenosine triphosphate)

• Main energy carrier between reaction sites in

cells

Phosphorylation

Phosphate-group transfers (phosphorylation)

to and from ATP couple energy-releasing

reactions with energy-requiring ones

p. 65

ATP forms in energy-

requiring reactions

ATP drives energy-

requiring reactions

ADP + phosphate

ATP: The Energy Currency of Cells

Fig. 4-5, p. 65

adenine

three phosphate

groups

ribose

adenine AMP ADP ATP

ribose P P P

Animation: Structure of ATP

Animation: Activation energy

Animation: Energy changes in chemical

work

Animation: Total energy remains

constant

4.4 How Enzymes Work

Enzymes make chemical reactions proceed

much faster than they would on their own

Enzyme

• Protein or RNA that speeds a reaction without

being changed by it

Substrates

An enzyme’s particular substrates bind at its

active site

Substrate

• A reactant molecule that is specifically acted upon

by an enzyme

Active Sites

Active site

• Pocket in an enzyme where substrates bind and

a reaction occurs

p. 66

active site reactant(s)

enzyme product(s)

Factors That Influence Enzyme Activity

Each enzyme works best within a characteristic

range of temperature, pH, and salt concentration

When conditions break hydrogen bonds, an

enzyme changes its characteristic shape

(denatures), and stops working

Enzymes, Temperature, and pH

Fig. 4-6a, p. 66

Fig. 4-6a, p. 66

normal

tyrosinase

temperature-

sensitive

tyrosinase

En

zy

me a

cti

vit

y

20°C (68°F) 30°C

(86°F)

40°C

(104°F)A Temperature

Fig. 4-6b, p. 66

Fig. 4-6b, p. 66

glycogen

phosphorylase

trypsin

pepsin

En

zy

me

ac

tivit

y

1 2 3 4 5 6 7 8 9 10 11

B pH

Cofactors and Coenzymes

Cofactor

• A metal ion or a coenzyme that associates with

an enzyme and is necessary for its function

Coenzyme

• An organic cofactor

• Unlike enzymes, may be modified by a reaction

Organized, Enzyme-Mediated Reactions

Cells concentrate, convert, and dispose of most

substances in enzyme-mediated reaction

sequences

Metabolic pathway

• Series of enzyme-mediated reactions by which

cells build, remodel, or break down an organic

molecule

Linear and Cyclic Metabolic Pathways

Control of Metabolic Pathways

Various controls over enzymes allow cells to

conserve energy and resources by producing

only what they require

• Concentrations of reactants and products

• Feedback inhibition

• Allosteric sites

Control of Metabolic Pathways

Feedback inhibition

• Mechanism by which a change that results from

some activity decreases or stops the activity

Allosteric site

• A region of an enzyme, other than the active site,

that can bind regulatory molecules

Feedback Inhibition

Fig. 4-7, p. 67

Stepped Art

Animation: Feedback inhibition

Allosteric Sites

Fig. 4-8, p. 67

regulatory

molecules

active site substrate in

active site

A inactive form B active form

Animation: Allosteric activation

Animation: Allosteric inhibition

Electron Transfers

Electron transfer chains allow cells to harvest

energy in manageable increments

Electron transfer chain

• An array of membrane-bound enzymes and other

molecules that accept and give up electrons in

sequence

Uncontrolled and Controlled

Energy Release

Fig. 4-9, p. 68

carbon dioxide

glucose carbon dioxide glucose e–

1 Energy input splits glucose

into carbon dioxide, electrons,

and hydrogen ions (H+).

++oxygen water oxygen H+

2 Electrons lose energy as

they move through an electron

transfer chain.spark

3 Energy released by electrons

is harnessed for cellular work.

e–

4 Electrons, hydrogen ions, and

oxygen combine to form water.water

A Glucose and oxygen

react (burn) when exposed

to a spark. Energy is

released all at once as light

and heat when CO2 and

water form.

B The same overall reaction

occurs in small steps with an

electron transfer chain. Energy

is released in amounts that

cells can harness for cellular

work.

Fig. 4-9, p. 68

glucose

++oxygen water

spark

A Glucose and oxygen

react (burn) when exposed

to a spark. Energy is

released all at once as light

and heat when CO2 and

water form.

carbon dioxide

carbon dioxide

glucose e–

oxygen H+

e–

water

B The same overall reaction

occurs in small steps with an

electron transfer chain. Energy

is released in amounts that

cells can harness for cellular

work.

1 Energy input splits glucose

into carbon dioxide, electrons,

and hydrogen ions (H+).

2 Electrons lose energy as

they move through an electron

transfer chain.

3 Energy released by electrons

is harnessed for cellular work.

4 Electrons, hydrogen ions, and

oxygen combine to form water.

Stepped Art

Animation: Controlling energy release

Animation: Enzymes and their role in

lowering activation energy

Animation: Enzymes and temperature

Animation: How catalase works

Animation: Induced fit model

Video: Diffusion of dye in water