Biology 12

the fact that water can act as a solvent for so

many substances in living and non-living

systems. When ionic and polar substances

dissolve in water, they may form hydrogen

bonds to water molecules. Hydrogen bonds can

also bind the components of complex biological

substances, such as the chains of nucleic acids

that make up a DNA molecule. These two long

intertwined chains are held to each other by

thousands of hydrogen bonds. Proteins, such as

enzymes, also require hydrogen bonding to

maintain their shapes and their functions.

Aerobic Cellular Respiration

Aerobic cellular respiration involves the

breakdown of glucose molecules (food) to form

energy-rich molecules called ATP, adenosine

triphosphate. Molecules of ATP are used to fuel

many other chemical reactions in cells. ATP

molecules are formed in several systems in

aerobic cellular respiration. The system that

produces the most ATP is called the electron

transport chain (see Figure A6.5). In the

electron transport chain, electrons (e−) flow

along the chain from one protein molecule to

another. As electrons move along the chain,

they release energy. This energy is used to

make ATP molecules. Once electrons reach the

end of the chain, however, they need to be

removed from the system. Removal of the

electrons is necessary in order to allow new

electrons to enter at the start of the chain. The

element that accepts the electrons at the end

of the electron transport chain is oxygen. As

you have learned, oxygen is one of the most

electronegative elements in the periodic table.

Oxygen accepts the electrons and combines

with hydrogen ions (H+), obtained from the

breakdown of glucose, to form water molecules.

Without oxygen to accept the electrons, the

electron transport chain could not function

and, as a result, could not produce ATP

molecules. This is why oxygen is so important

in cellular processes.

Figure A6.5The electron transport chain

2H+ + 2e−

2H+

2e− 1

2 O^2

H 2 O

2H

Electron transport chain

Energy for

synthesis of

(from food)

AT P

566 MHR • Appendix 6