HUMAN BIOLOGY

Cells and how they work 61

hoW does cellular respiration make atp?

• ATP forms as the stages of cellular respiration take place in the
  cell cytoplasm and mitochondria.


• Glycolysis occurs in the cytoplasm and does not require oxygen.
  Glycolysis breaks down a carbohydrate such as glucose, with a
  net yield of two ATP molecules.


• A second set of reaction steps, now in mitochondria, require
  oxygen. Enzymes acting on pyruvate molecules from glycolysis
  strip away carbon atoms that end up in carbon dioxide. The rest
  of the molecule enters the Krebs cycle, which produces two
  more ATP.


• Much more ATP forms in mitochondria as electrons and H+ move
  through transport systems in which enzymes add a phosphate
  group to ADP.

taKe-hoMe Message

This substance enters the Krebs cycle. For each turn of the

cycle, six carbons, three from each pyruvate, enter and six

also leave, in the form of carbon dioxide. The bloodstream

then transports this CO 2 to the lungs where it is exhaled.

Reactions in mitochondria before and during the Krebs

cycle have three important functions. First, they produce

two molecules of ATP. Second, they regenerate intermedi-

ate compounds required to keep the Krebs cycle going.

And in a third, crucial step, a large number of the coen-

zymes called NAD^1 and FAD pick up H^1 and electrons,

in the process becoming NADH and FADH 2. Loaded with

energy, NADH and FADH 2 will now move to the site of the

third and final stage of reactions that make ATP.

step 3: electron transport produces

many atp molecules

ATP production increases during the last stage of cel-

lular respiration. Now, chains of reactions capture and

use energy released by electrons. Each chain is called an

electron transport system. It includes enzymes inside

the membrane that divides the mitochondrion into two

compartments (Figure 3.26). As electrons flow through the

system, each step transfers a small amount of energy to

a molecule that briefly stores it. This gradual releasing of

energy reduces the amount of energy that is lost (as heat)

while a cell is making ATP.

As shown at the lower left of Figure 3.26, an electron

transport system uses electrons and hydrogen ions that

are provided by NADH and FADH 2. The electrons are

transferred from one molecule of the transport system to

the next in line. The yellow “bouncing” line in Figure 3.26

represents this process. When molecules in the chain accept

electrons and then donate them, they also pick up hydro-

gen ions in the inner compartment, then release them to

the outer compart ment. At the end of an electron transport

system, oxygen accepts electrons in a reaction that forms

water (H 2 O).

As the system moves hydrogen ions into the outer com-

partment of a mitochondrion, an H^1 concentration gradient

develops. As the ions become more concentrated in the

outer compartment, they follow the gradient back into the

inner compartment, crossing the inner membrane through

the interior of enzymes that can catalyze the formation of

ATP from ADP and phosphate (Pi). This step is shown at

the far right of Figure 3.26.

Figure 3.26 Animated! Electron transport forms large amounts of ATP.

GLUCOSE

NET ENERGY YIELD: 2

INTERMEDIATES DONATE

PHOSPHATE TO ADP, MAKING 4

PGAL:

ADP

ADP

P

P P

P P

Energy

in

(2 ATP)

Pyruvate

To second

set of

reactions

ATP

ATP

ATP

ATP

H+ H+ H+ H+ H+ H+ H+ H+

H+

HH H+

e–

ADP + Pi

oxygen (O 2 )

e–

NADH

FADH 2

O

O

O

ATP

Electron Transport Chain

outer membrane

cytoplasm

inner compartment

inner membrane

outer compartment

e^2 5 electron

H^1 5 hydrogen ion

Pi 5 phosphate Sir Hans Krebs discovered

© Cengage Learningthe Krebs cycle.

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