Biology Now, 2e

Rock Eaters ■ 85

Catalyzing Reactions

In about 2014, Spormann’s team began studying

an “electric” microbe that lives in water, Meth-

anococcus maripaludis, to see whether it was

capable of directly ingesting electrons. It wasn’t.

The researchers found that the microbe actually

excretes an enzyme onto the surface of an electrode

to do its dirty work. Nearly all metabolic reactions

are facilitated by enzymes. Enzymes are biolog-

ical catalysts—molecules that speed up chemical

reactions. Without the action of enzymes—most

of which are proteins— metabolism would be

extremely slow, and life as we know it could not

exist. Enzymes work by positioning substrates—

molecules that will react to form new products—in

for metabolism; in fact, some are even poisoned

by it.

Oxygen plays an important role in photo-

synthesis, considered by many to be the most

important life process on Earth; it is the way

our planet stores energy from the sun and

produces oxygen for animals to breathe. In the

cells of algae and all plants, photosynthesis

takes place inside chloroplasts, organelles that

look like green, oval gumballs when viewed

under a light microscope (Figure 5.6). Chlo-

roplasts contain an extensive network of struc-

tures called thylakoids, piled up like stacks

of pancakes, that contain enzymes needed

for photosynthesis. Also embedded in those

membranes is a green pigment, called chlo-

rophyll, that is specialized for absorbing light

energy.

Photosynthesis takes place in two princi-

pal stages: the light reactions and the light-

independent reactions, or the Calvin cycle

(Figure 5.7). During the light reactions, chlo-

rophyll molecules absorb energy from sunlight

and use that energy to split water (Figure 5.7,

left). The splitting of water produces oxygen

gas (O 2 )—the oxygen that we breathe—as a

by-product that is released into the atmo-

sphere. More impor tant for photosynthesis,

electrons and protons (H+) from the light reac-

tions are handed over to other molecules via the

electron transport chain, an elaborate chain

of chemical events that ultimately generates

ATP and NADPH. The light reactions depend

on protein complexes embedded in the chloro-

plast membrane, including photosystems I and

II, and ATP synthase.

In the next stage, the light-independent

reactions, or Calvin cycle—a series of chem-

ical reactions—convert carbon dioxide (CO 2 )

into sugar, using energy delivered by ATP,

and electrons and hydrogen ions donated by

NADPH (Figure 5.7, right). Enzymes cata-

lyze these reactions at each step; the enzyme

needed in the first step—and the most abun-

dant enzyme on the planet—is rubisco. This

process is also known as carbon fixation. By

capturing inorganic carbon atoms from CO 2

gas and converting them into glucose, the

Calvin cycle reactions make carbon from the

nonliving world available to the photosynthetic

organisms and eventually to other living organ-

isms, including us.

Chloroplasts are packed with

thylakoids, pancake-shaped

structures that have chlorophyll

molecules in their membranes.

Chloroplast

Thylakoids

Figure 5.6

Chloroplasts are the site of photosynthesis in eukaryotes

Most plants and algae have multiple chloroplasts to contain their chlorophyll.

Photosynthetic bacteria embed chlorophyll directly into the plasma

membrane.

Q1: Is chlorophyll found only within chloroplasts?

Q2: What could be an advantage of concentrating chlorophyll

molecules in the membranes of chloroplasts?

Q3: What is the advantage of having multiple chloroplasts per cell?