BioPHYSICAL chemistry

Buffering in the cardiovascular system

In animals with lungs, bicarbonate is an effective

buffer found in the blood and maintains the pH near

7.4. The pH of blood is dependent upon two coupled

reactions. First, the amount of gaseous carbon dioxide

dissolved in the blood will equilibrate with water and

produce carbonic acid (Figure 5.9). Second, there will

be an equilibrium between carbonic acid and bicarbon-

ate. The amount of carbon dioxide in the blood is

coupled to the amount present in the lungs.

Vigorous exercise will produce lactic acid and

increase the amount of H+. In turn, the concentration

of carbon dioxide in the blood increases, thus increasing the pressure of

carbon dioxide in the lungs, causing the extra carbon dioxide to be exhaled.

Conversely, when the pH of blood plasma is raised by a metabolic pro-

cess the equilibrium shifts, causing more carbon dioxide to dissolve in

the blood. Since the lungs have a large capacity for carbon dioxide, changes

in breathing rapidly lead to compensations for changes in pH. Shifts of

the pH of blood can lead to a condition known as acidosis, when the

pH drops from the normal pH of 7.4 to 7.1, or alkalosis, when the pH

rises to 7.6. Such conditions can arise for different reasons, such as severe

diarrhea.

Research direction: proton-coupled electron transfer and pathways

Proton and electron transfer are two fundamental chemical processes in

biological systems. In many cases, proton transfer is often coupled with

electron transfer to maintain an overall balance in the charge. Three

possible pathways are possible for proton-coupled electron transfer. First,

the electron transfer can precede the proton transfer; second, the proton

transfer can precede the electron transfer; or third, the two transfers can

occur simultaneously or in a concerted fashion. Although the outcomes

of the three pathways are identical, the dependencies of these three path-

ways on biochemical factors vary tremendously.

As an example, consider the properties of quinones as electron acceptors

in proteins. Quinone molecules are six-membered carbon rings with differ-

ent possible substitutents attached, the simplest molecule being benzo-

quinone (Figure 5.10). In photosynthetic organisms (see Chapter 20), quinone

serves as an electron carrier between protein complexes, with light driving

the reduction to quinol by coupling electron transfer with proton transfer:

Q +2e−+2H+↔QH 2 (5.34)

108 PARTI THERMODYNAMICS AND KINETICS

H  HCO 3 

H 2 CO 3

H 2 O H 2 O

Aqueous phase

(blood in capillaries)

Gas phase

(lung air space)

reaction 1

reaction 2

reaction 3

CO 2 (d)

CO 2 (g)

Figure 5.9Carbon
dioxide (CO 2 ) in the
lungs can exchange
with carbon dioxide
dissolved in the
bloodstream, which
is in equilibrium
with carbonic acid
(H 2 CO 3 ) and
bicarbonate (HCO 3 −).

Figure 5.10
Structure of
benzoquinone.

O

O