Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

14.4

METALS IN BIOLOGY Many main group elements, including Na, K,

Ca, Mg, Se, and Si, and transition metals

such as V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu,

and Zn, are essential for biological function.

These functions are both structural and metabolic. For example, the structural role of Ca

2+

in teeth and bone is well recognized. In additi

on, proteins use metal ions for the formation

and stabilization of their three-dimensional structures and to catalyze reactions that are not feasible in their absence. Two dramatic examples of the importance of metal ions in biology are the complementary processes of carbohydrate production in plants (photosynthesis) and the combustion of carbohydrates in animals (respiration), the topics of the this section. PHOTOSYNTHESIS AND RESPIRATION Plants extract energy from the sun to synthesize carbohydrates in a process called photosynthesis

(synthesis using light), while animals burn the carbohydrates and extract

the stored energy from exothermic co

mbustion reactions in a process known as

respiration

. The reversible process, shown in Reac

tion 14.1a, is an electron transfer

reaction, with each half-reaction (Reactions

14.1b and 14.1c) involving a four-electron

transfer for each carbon atom in the carboh

ydrate molecule consumed or produced.

respiration

n2n

2

2

2

photosynthesis

C (H O) + nO

nCO + nH O + Energy

ZZZZZZZZZZ

X

YZZZZZZZZZ

Z

Rxn 14.1a

1-

1+

22

nO + 4ne + 4nH

2nH O

U

Rxn 14.1b

1+

1-

n2n

2

2

C (H O) + nH O

nCO + 4nH + 4ne

U

Rxn 14.1c

Metals play key roles in both processes. F

our-electron transfer reactions involve high

activation energies, so nature uses metals to cat

alyze the reactions.* Plants use a cluster of

four

manganese

ions coordinated to proteins within the plant cells to break up the reaction

into four one-electron processes, while animals use

iron

and

copper

ions to do the same

for the reverse process. Chlorophyll uses a

magnesium

ion in the center of a porphyrin

derivative to change the electronic structure of

the porphyrin so that it absorbs red light,

giving porphyrin its green color. The plant uses

the energy of the red photons to carry out

the photosynthesis reaction shown in Reaction

14.1a. Photosynthesis is the source not only

of carbohydrates; it is also the source of all oxygen in the atmosphere today.

Without the

light harvesting role of Mg

2+

and the catalytic activity of the Mn cluster, animals such as

humans would not exist

.

* As discussed in Section 9.8, catalysts decrease the activation

energy by changing the nature of the transition state.

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State

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