Chapter 5 Gases

Chapter 5
Ground Rules of Metabolism
(Sections 5.1 - 5.4)

5.1 A Toast to
Alcohol Dehydrogenase

Metabolic processes build and break down organic molecules such as ethanol and other toxins

Alcohol breakdown directly damages liver cells, and interferes with normal processes of metabolism

Currently the most serious drug problem on college campuses is binge drinking

Alcohol Metabolism

The enzyme alcohol dehydrogenase helps the liver break down toxic alcohols (ethanol)

Figure 5.1 Alcohol metabolism. Alcohol dehydrogenase helps the body break down toxic alcohols such as ethanol. This enzyme makes it possible for humans to drink beer, wine, and other alcoholic beverages

5.2 Energy and the World of Life

There are many forms of energy:

Kinetic energy, potential energy

Light, heat, electricity, motion

Energy cannot be created or destroyed (first law of thermodynamics)

Energy can be converted from one form to another and thus transferred between objects or systems

Energy Disperses

Energy tends to disperse spontaneously (second law of thermodynamics)

A bit disperses at each energy transfer, usually as heat

Entropy is a measure of how dispersed the energy of a system has become

Key Terms

energy

The capacity to do work

kinetic energy

The energy of motion

entropy

Measure of how much the energy of a system is dispersed

Key Terms

first law of thermodynamics

Energy cannot be created or destroyed

second law of thermodynamics

Energy tends to disperse spontaneously

Kinetic Energy

Figure 5.2 Demonstration of a familiar type of energy: motion, or kinetic energy.

Entropy

Entropy tends to increase, but the total amount of energy in any system always stays the same

Figure 5.3 Entropy. Entropy tends to increase, but the total amount of energy in any system always stays the same

Fig. 5.3, p. 76

Entropy

Time

heat energy

Stepped Art

Entropy

Figure 5.3 Entropy. Entropy tends to increase, but the total amount of energy in any system always stays the same

Work

Work occurs as a result of an energy transfer

A plant converts light energy to chemical energy in photosynthesis

Most other cellular work occurs by transfer of chemical energy from one molecule to another (such as transferring chemical energy from ATP to other molecules)

Energys One-Way Flow

Living things maintain their organization only as long as they harvest energy from someplace else

Energy flows in one direction through the biosphere, starting mainly from the sun, then into and out of ecosystems

Producers and then consumers use energy to assemble, rearrange, and break down organic molecules that cycle among organisms throughout ecosystems

Energy Conversion

It takes 10,000 pounds of feed to raise a 1,000-pound steer

About 15% of energy in food builds body mass; the rest is lost as heat during energy conversions

Figure 5.4 It takes more than 10,000 pounds of soybeans and corn to raise a 1,000-pound steer. Where do the other 9,000 pounds go? About half of the steers food is indigestible. The animals body breaks down molecules in the remaining half to access energy stored in chemical bonds. Only about 15% of that energy goes toward building body mass. The rest is lost during energy conversions, as heat.

Energy Flow

Energy flows from the environment into living organisms, and back to the environment

Materials cycle among producers and consumers

Fig. 5.5, p. 77

Consumers animals, most fungi, many protists, bacteria

nutrient cycling

Producers plants and other self-feeding organisms

sunlight energy

Energy Flow

Figure 5.5 Energy flows from the environment into living organisms, and then back to the environment. The flow drives a cycling of materials among producers and consumers.

Animation: One-Way Energy Flow and Materials Cycling

Potential Energy

Energys spontaneous dispersal is resisted by chemical bonds

Energy in chemical bonds is a type of potential energy, because it can be stored

potential energy

Stored energy

Key Concepts

Energy Flow

Organisms maintain their organization only by continually harvesting energy from their environment

ATP couples reactions that release usable energy with reactions that require it

Animation: Energy Changes in Chemical Work

5.3 Energy in the Molecules of Life

Every chemical bond holds energy the amount of energy depends on which elements are taking part in the bond

Cells store and retrieve free energy by making and breaking chemical bonds in metabolic reactions, in which reactants are converted to products

Key Terms

reaction

Process of chemical change

reactant

Molecule that enters a reaction

product

A molecule that remains at the end of a reaction

Chemical Bookkeeping

In equations that represent chemical reactions, reactants are written to the left of an arrow that points to the products

A number before a formula indicates the number of molecules

The same number of atoms that enter a reaction remain at the reactions end

Chemical Bookkeeping

2H2O(water)

Fig. 5.6, p. 78

Stepped Art

Reactants

4 hydrogen atoms+ 2 oxygen atoms

Products

4 hydrogen atoms+ 2 oxygen atoms

2H2(hydrogen)

O2(oxygen)

Chemical Bookkeeping

Figure 5.6 Chemical bookkeeping. In equations that represent chemical reactions, reactants are written to the left of an arrow that points to the products. A number before a formula indicates the number of molecules. Atoms shuffle around in a reaction, but they never disappear: The same number of atoms that enter a reaction remain at the reactions end.

Animation: Chemical Bookkeeping

Energy In, Energy Out

In most reactions, free energy of reactants differs from free energy of products

Reactions in which reactants have less free energy than products are endergonic they will not proceed without a net energy input

Reactions in which reactants have greater free energy than products are exergonic they end with a net release of free energy

Key Terms

endergonic

Energy in

Reaction that converts molecules with lower energy to molecules with higher energy

Requires net input of free energy to proceed

exergonic

Energy out

Reaction that converts molecules with higher energy to molecules with lower energy

Ends with a net release of free energy

Energy In, Energy Out

Fig. 5.7, p. 78

Free energy

energy out

energy in

2H2O

O2

2H2

1

2

2H2O

Energy In, Energy Out

Figure 5.7 Energy inputs and outputs in chemical reactions. 1 Endergonic reactions convert molecules with lower energy to molecules with higher energy, so they require a net energy input in order to proceed. 2 Exergonic reactions convert molecules with higher energy to molecules with lower energy, so they end with a net energy output.

Why Earth Does Not Go Up in Flames

Earth is rich in oxygenand in potential exergonic reactions; why doesnt it burst into flames?

Luckily, energy is required to break chemical bonds of reactants, even in an exergonic reaction

activation energy

Minimum amount of energy required to start a reaction

Keeps exergonic reactions from starting spontaneously

Activation Energy

Fig. 5.8, p. 79

O2

Free energy

2H2

Activation energy

Products: 2H2O

Difference between free energy of reactants and products

Reactants:

Activation Energy

Figure 5.8 Activation energy. Most reactions will not begin without an input of activation energy, which is shown here as a bump in an energy hill. In this example, the reactants have more energy than the products. Activation energy keeps this and other exergonic reactions from starting spontaneously.

Animation: Activation Energy

ATPThe Cells Energy Currency

ATP is the main currency in a cells energy economy

ATP (Adenosine triphosphate)

Nucleotide with three phosphate groups linked by high-energy bonds

An energy carrier that couples endergonic with exergonic reactions in cells

ATP

Fig. 5.9a, p. 79

A Structure of ATP.

ribose

adenine

three phosphate groups

ATP

Figure 5.9 ATP, the energy currency of cells.

Phosphorylation

When a phosphate group is transferred from ATP to another molecule, energy is transferred along with the phosphate

Phosphate-group transfers (phosphorylations) to and from ATP couple exergonic reactions with endergonic ones

phosphorylation

Addition of a phosphate group to a molecule

Occurs by the transfer of a phosphate group from a donor molecule such as ATP

ATP and ADP

Fig. 5.9b, p. 79

B After ATP loses one phosphate group, the nucleotide is ADP (adenosine diphosphate); after losing two phosphate groups, it is AMP (adenosine monophosphate)

ribose

adenine

AMP

ATP

ADP

ATP and ADP

Figure 5.9 ATP, the energy currency of cells.

ATP/ADP Cycle

Cells constantly use up ATP to drive endergonic reactions, so they constantly replenish it by the ATP/ADP cycle

ATP/ADP cycle

Process by which cells regenerate ATP

ADP forms when ATP loses a phosphate group, then ATP forms again as ADP gains a phosphate group

ATP/ADP Cycle

Fig. 5.9c, p. 79

energy out

ADP + phosphate

energy in

C ATP forms by endergonic reactions. ADP forms again when ATP energy is transferred to another molecule along with a phosphate group. Energy from such transfers drives cellular work.

ATP/ADP Cycle

Figure 5.9 ATP, the energy currency of cells.

Animation: Mitochondrial Chemiosmosis

5.4 How Enzymes Work

Enzymes makes a reaction run much faster than it would on its own, without being changed by the reaction

catalysis

The acceleration of a reaction rate by a molecule that is unchanged by participating in the reaction

Most enzymes are proteins, but some are RNAs

Substrates

Each enzyme recognizes specific reactants, or substrates, and alters them in a specific way

substrate

A molecule that is specifically acted upon by an enzyme

Active Sites

Enzyme specificity occurs because an enzymes polypeptide chains fold up into one or more active sites

An active site is complementary in shape, size, polarity, and charge to the enzymes substrate

active site

Pocket in an enzyme where substrates bind and a reaction occurs

An Active Site

Fig. 5.10a, p. 80

An Active Site

Figure 5.10 An example of an active site. This one is in a hexokinase, an enzyme that phosphorylates glucose and other six-carbon sugars.

Fig. 5.10a, p. 80

active site

enzyme

A Like other enzymes, hexokinases active sites bind and alter specific substrates. A model of the whole enzyme is shown to the left.

An Active Site

Figure 5.10 An example of an active site. This one is in a hexokinase, an enzyme that phosphorylates glucose and other six-carbon sugars.

Fig. 5.10b, p. 80

An Active Site

Figure 5.10 An example of an active site. This one is in a hexokinase, an enzyme that phosphorylates glucose and other six-carbon sugars.

Fig. 5.10b, p. 80

reactant(s)

B A close-up shows glucose and phosphate meeting inside the enzymes active site. The microenvironment of the site favors a reaction between the two substrate molecules.

An Active Site

Figure 5.10 An example of an active site. This one is in a hexokinase, an enzyme that phosphorylates glucose and other six-carbon sugars.

Fig. 5.10c, p. 80

An Active Site

Figure 5.10 An example of an active site. This one is in a hexokinase, an enzyme that phosphorylates glucose and other six-carbon sugars.

Fig. 5.10c, p. 80

product(s)

C Here, the glucose has bonded with the phosphate. The product of this reaction, glucose-6-phosphate, is shown leaving the active site.

An Active Site

Figure 5.10 An example of an active site. This one is in a hexokinase, an enzyme that phosphorylates glucose and other six-carbon sugars.

Lowering Activation Energy

Enzymes lower activation energy in four ways:

Bringing substrates closer together

Orienting substrates in positions that favor reaction

Inducing the fit between a substrate and the enzymes active site (induced-fit model)

Shutting out water molecules

induced-fit model

Substrate binding to an active site improves the fit between the two

Lowering Activation Energy

Fig. 5.11, p. 80

Free energy

Reactants

Products

Transition state

Activation energy with enzyme

Activation energy without enzyme

Time

Lowering Activation Energy

Figure 5.11 An enzyme enhances the rate of a reaction by lowering its activation energy.

Animation: Enzymes and Activation Energy

Effects of Temperature, pH, and Salinity

Each type of enzyme works best within a characteristic range of temperature, pH, and salt concentration:

Adding heat energy boosts free energy, increasing reaction rate (within a given range)

Most human enzymes have an optimal pH between 6 and 8 (e.g. pepsin functions only in stomach fluid, pH 2)

Too much or too little salt disrupts hydrogen bonding that holds an enzyme in its three-dimensional shape

Enzymes and Temperature

Fig. 5.12, p. 81

Temperature

Enzyme activity

temperature- sensitivetyrosinase

normal tyrosinase

40C (104F)

30C (86F)

20C (68F)

Enzymes and Temperature

Figure 5.12 Enzymes and temperature. Tyrosinase is involved in the production of melanin, a black pigment in skin cells. The form of this enzyme in Siamese cats is inactive above about 30C (86F), so the warmer parts of the cats body end up with less melanin, and lighter fur.

Animation: Enzymes and Temperature

Enzymes and pH

Fig. 5.13, p. 81

pH

trypsin

glycogen phosphorylase

pepsin

Enzyme activity

1 2 3 4 5 6 7 8 9 10 11

Enzymes and pH

Figure 5.13 Enzymes and pH. Left, how pH affects three enzymes. Right, carnivorous plants of the genus Nepenthes grow in nitrogen-poor habitats. They secrete acids and protein-digesting enzymes into a fluidfilled cup that consists of a modified leaf. The enzymes release nitrogen from insects that are attracted to odors from the fluid and then drown in it. One of these enzymes functions best at pH 2.6.

Help From Cofactors

Most enzymes require cofactors, which are metal ions or organic coenzymes in order to function

cofactor

A metal ion or a coenzyme that associates with an enzyme and is necessary for its function

coenzyme

An organic molecule that is a cofactor

Coenzymes and Cofactors

Coenzymes may be modified by taking part in a reaction

Example: NAD+ becomes NADH by accepting electrons and a hydrogen atom in a reaction

Cofactors are metal ions

Example: The iron atom at the center of each heme

In the enzyme catalase, iron pulls on the substrates electrons, which brings on the transition state

Antioxidants

Cofactors in some antioxidants help them stop reactions with oxygen that produce free radicals (harmful atoms or molecules with unpaired electrons)

Example: Catalase is an antioxidant

antioxidant

Substance that prevents molecules from reacting with oxygen

Key Concepts

How Enzymes Work

Enzymes tremendously increase the rate of metabolic reactions

Cofactors assist enzymes, and environmental factors such as temperature, salt, and pH can influence enzyme function

Animation: How Catalase Works

Albia Dugger Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr