Biology Now, 2e

Rock Eaters ■ 83

the noonday sun, the cells in their green leaves

capture energy to make glucose from carbon

dioxide; a complex molecule is assembled from

simpler ones. Catabolism, unsurprisingly, refers

to the opposite: metabolic pathways that release

chemical energy in the process of breaking down

complex molecules. Plant cells rely on cellular

respiration, a catabolic process, to break down

the glucose made by photosynthesis.

All of this metabolic activity requires energy, so

cells need energy carriers to deliver usable “on-

demand” energy. Every living cell uses ATP (adeno-

sine triphosphate), a small, energy-rich organic

molecule, to carry energy from one part of the cell

to another. ATP powers almost all activities in the

cell, such as moving molecules and ions in and

out of the cell, sending nerve impulses, trigger-

ing muscle contractions, and moving organelles

around inside the cell. In addition to powering a

cell’s activities, ATP fuels metabolic reactions and

most other enzymatic reactions. If a cell exhausts

its ATP supply, it will die.

that electrons couldn’t cross the cell membrane,

the discovery was a big surprise. For decades,

however, the way the electron transfer worked

remained a mystery. Then, in 2006, a scientific

team identified a trio of proteins that form a

bridge in the bacterial cell membrane, allowing

the transfer of electrons from the inside to the

outside of the cell.

Inspired by these “rock breathers” that expelled

electrons onto a mineral surface, scientists began

looking for “rock eaters.” If nature produced

microbes that spit out electrons, why not micro-

bes that directly ingest electrons from minerals?

All living cells have two main types of metab-

olism: anabolism and catabolism (Figure 5.3).

Anabolism refers to metabolic pathways that

create complex molecules from simpler com-

pounds—a process used by all cells to make

the basic building blocks of the cell. These

processes often require an energy input. Photo-

synthesis is an example of anabolism where the

energy is provided by sunlight: as plants sit in

Polymers

Energy

invested

Energy

released

Monomers

Anabolism:

Energy-requiring

chemical reactions that

build complex molecules

from simpler ones.

Catabolism:

Energy-releasing

reactions involved

in the breakdown

of complex

molecules into

Anabolism Catabolism simpler ones.

Figure 5.3

Anabolism builds biomolecules; catabolism breaks them down

Metabolic reactions either build or break down molecules. Molecule-building reactions (anabolism) cost

energy, and molecule-breakdown reactions (catabolism) release energy.

Q1: What source of energy would plants use for anabolic reactions? Would an animal use the

same kind of energy?

Q2: What source of energy would plants release in catabolic reactions? Would an animal

release the same kind of energy?

Q3: Create a mnemonic or jingle that helps you remember the difference between anabolism

and catabolism.