Human Physiology, 14th edition (2016)

Enzymes and Energy 97

Energy may be defined as the ability to do work (exert a force

that acts over a distance). Bioenergetics refers to the flow of

energy in living systems. Organisms maintain their highly

ordered structure and life-sustaining activities through the con-

stant expenditure of energy obtained ultimately from the envi-

ronment. The energy flow in living systems obeys the first and

second laws of a branch of physics known as thermodynamics.

According to the first law of thermodynamics, energy can

be transformed (changed from one form to another), but it can

neither be created nor destroyed. This is sometimes called the law

of conservation of energy. For example, the mechanical energy of

a waterfall can be transformed into the electrical energy produced

by a hydroelectric plant; the chemical bond energy in gasoline

can be transformed into the mechanical energy of turning gears;

and (in a hybrid car), mechanical energy can be transformed into

electrical energy. Figure 4.12 shows a more biological example;

indeed, this is the energy transformation upon which all animal

and plant life depends: the transformation of light energy into the

chemical bond energy in glucose molecules.

However, in all energy transformations, you can never get

out what you put in; the transformation is never 100% efficient

4.3 Bioenergetics

Living organisms require the constant expenditure of

energy to maintain their complex structures and pro-

cesses. Central to life processes are chemical reactions

that are coupled, so that the energy released by one reac-

tion is incorporated into the products of another reaction.

| CHECKPOINT

3. Draw graphs to represent the effects of changes
   in temperature, pH, and enzyme and substrate
   concentration on the rate of enzymatic reactions.
   Explain the mechanisms responsible for the effects
   you have graphed.


4. Describe a reversible reaction and explain how the
   law of mass action affects this reaction.
   5a. Using arrows and letters of the alphabet, draw a flowchart
   of a metabolic pathway with one branch point.
   5b. Define end-product inhibition and use your diagram
   of a branched metabolic pathway to explain how
   this process will affect the concentrations of different
   intermediates.
   5c. Because of an inborn error of metabolism, suppose
   that the enzyme that catalyzed the third reaction in
   your pathway (see no. 5a) was defective. Describe
   the effects this would have on the concentrations of
   the intermediates in your pathway.

LEARNING OUTCOMES

After studying this section, you should be able to:

6. Distinguish between endergonic and exergonic
   reactions, and explain how ATP functions as a
   universal energy carrier.


7. Distinguish between oxidation and reduction
   reactions, and explain the functions of NAD and FAD.

Table 4.4 | Examples of Inborn Errors in the Metabolism of Amino Acids,
Carbohydrates, and Lipids

Metabolic Defect Disease Abnormality Clinical Result

Amino acid metabolism Phenylketonuria (PKU) Increase in phenylpyruvic acid Mental retardation, epilepsy

Albinism Lack of melanin Susceptibility to skin cancer

Maple-syrup disease Increase in leucine, isoleucine,

and valine

Degeneration of brain, early death

Homocystinuria Accumulation of homocystine Mental retardation, eye problems

Carbohydrate metabolism Lactose intolerance Lactose not utilized Diarrhea

Glucose 6-phosphatase

deficiency (Gierke’s disease)

Accumulation of glycogen in liver Liver enlargement, hypoglycemia

Glycogen phosphorylase

deficiency

Accumulation of glycogen in muscle Muscle fatigue and pain

Lipid metabolism Gaucher’s disease Lipid accumulation (glucocerebroside) Liver and spleen enlargement,

brain degeneration

Tay-Sachs disease Lipid accumulation (ganglioside GM2) Brain degeneration, death by age five

Hypercholestremia High blood cholesterol Atherosclerosis of coronary and

large arteries