Biology 12

56 MHR • Unit 1 Metabolic Processes

the exothermic reaction of ATP hydrolysis occurs.

This energy value was determined in a laboratory.

The high-energy bonds of ATP do not require a

large quantity of activation energy to break, so little

energy is required to initiate hydrolysis. In the

hydrolysis of ATP, a typical metabolic reaction, only

the outer high-energy bond is broken, releasing the

end phosphate group. Figure 2.17 shows that the

products of this reaction are ADP (adenosine

diphosphate), Pi(an inorganic phosphate

molecule), and the release of 31 kJ/mol of energy.

Figure 2.16Diagram of the ATP molecule (A) and its

chemical structure (B). ATP is composed of a ribose sugar,

adenine, and a triphosphate group. High-energy bonds link

the three phosphate groups. Energy is released from the

molecule when these bonds are broken. AMP is adenosine

monophosphate.

When cells require energy, they use ATP. Much

ATP is needed as fuel for cellular work. A person

who consumes 10.46 MJ (2500 Calories or 2500 kcal)

per day manufactures and uses about 180 kg of ATP

per day. ATP is constantly being generated from

ADP and Pi. Consequently, cells do not keep a

stockpile of ATP; it is more efficient for them to

make ATP as the need arises.

Figure 2.17The ATP/ADP +Picycle

ATP Working Inside

and Outside Cells

There are at least three distinct uses for ATP

in cells:

1. Chemical work ATP supplies the energy
   needed to synthesize the macromolecules
   that comprise the cell.


2. Mechanical work ATP supplies the energy
   needed to permit muscles to contract, cilia
   and flagella to beat, chromosomes to move,
   and other functions.


3. Transport work ATP supplies the energy
   cells need to pump substances across the
   cell membrane.

Recent research indicates the molecule adenosine can

induce sleep. Adenosine binds to specific receptors on the

surface of nerve cells in the brain that promote sleep. The

action of adenosine can be interrupted by caffeine, a

stimulant found in foods, which blocks receptors on the

nerve cells. As a result, a drink containing caffeine can

make you feel more alert. Some researchers are investigating

if sleep is influenced by ATP molecules, which are

composed of an adenosine molecule and phosphate groups.

BIO FACT

31 kJ

of energy

released

energy

added

ATP

ADP+Pi

phosphate high-energy bonds

phosphate

phosphate

ribose

adenine

B

A

CH O

OH OH

2

NH 2

O

HH

HH

OOP

O

PO

PO

adenine

high-energy

bonds

triphosphate

group

ribose

AMP

core

O−

O−

O−

O−

N N

N N

ATP is formed by bonding

an inorganic phosphate

(Pi) onto ADP. This

requires an input

of energy.

A Removal of the

phosphate group from

ATP results in a release

of 31 kJ/mol of energy.

B