Biology 12

26 MHR • Unit 1 Metabolic Processes

Cellular respiration is an important example

of a redox reaction that takes place in biological

systems. The overall reaction is:

In cellular respiration, high-energy electrons are

removed from food molecules, which oxidizes

them. These high-energy electrons are transferred

to increasingly electronegative atoms, and help the

cell manufacture energy-rich molecules used by

cells to do work.

Hydrolysis and

Condensation Reactions

Macromolecules in living systems are built and

broken down by hydrolysis and condensation

reactions (see Figure 1.17). In condensation(or

dehydration synthesis), the components of a water

molecule are removed to bond two molecules

together. Because the organic molecule formed is

bigger than the two organic molecules that reacted,

condensation is an anabolic process. In the process

of hydrolysis, the components of a water molecule

are added to a molecule to break it into two

molecules. Because the organic molecules

produced are smaller than the organic molecule

that reacted, hydrolysis is a catabolic process. Read

on to see how hydrolysis and condensation work to

break down and build carbohydrates, nucleic acids,

proteins, and lipids.

Making and Breaking

Carbohydrates

Carbohydrates are important macromolecules

because they store energy in all organisms.

Carbohydrates are groupings of C, H, and O atoms,

usually in a 1:2:1ratio. Often, carbohydrates are

represented by the chemical formula (CH 2 O)n,

where nis the number of carbon atoms in the

carbohydrate.

Carbohydrates can be simple, such as the

monomer glucose. Glucose is a hexose (six-carbon)

sugar with seven energy-storing C-H bonds. If

the number of carbon atoms in a carbohydrate

molecule is low (from three to seven), then it is a

monosaccharide. Greek prefixes for the numbers

three through seven are used to name these sugars.

For example a five-carbon sugar is a pentose, and a

six-carbon sugar is a hexose. The glucose, fructose,

and galactose isomers you studied in the previous

section are all hexoses. Glucose is the primary

source of energy used by cells.

Two monosaccharides can bond to form a

disaccharide. For example, two glucose molecules

can join to form the disaccharide maltose, as

shown in Figure 1.18.

Organisms store energy in molecules known as

polysaccharides. Polysaccharide molecules, such as

starch and glycogen, are polymers made up of chains

of linked monosaccharides. The long chains of

glucose molecules, which make up starch, glycogen,

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + energy

glucose

HO

HO H

HHHO

HO H

HO H

HO H

condensation hydrolysis

HO 2 HO 2

Figure 1.17Condensation

and hydrolysis

++

H

O

C 6 H 12 O 6

CH 2 OH CH 2 OH CH 2 OH CH 2 OH

glucose glucose

HO 2

HO

H

O

C 6 H 12 O 6 C 12 H 22 O 11

O

O

maltose

condensation

synthesis

hydrolysis

water

O

OH

Figure 1.18Maltose is a disaccharide. During condensation synthesis of maltose,

a bond forms between the two glucose molecules and the components of water

are removed. During hydrolysis, the components of water are added, and this

bond is broken.