Page 979 - Fundamentals of anatomy physiology

966 Unit 5 Environmental Exchange

Figure 25–3 Glycolysis. Glycolysis breaks down a 6-carbon glucose molecule into two 3-carbon molecules of

pyruvate through a series of enzymatic steps. Further catabolism of pyruvate begins with its entry into a mitochondrion.

Glucose INTERSTITIAL
FLUID

1 ATP CYTOSOL
P ADP

Steps in Glycolysis 2 Glucose-6-phosphate

1 As soon as a glucose molecule P ATP
enters the cytosol, a phosphate 3 ADP
group is attached to the molecule.
P Fructose-1,6-bisphosphate
2 A second phosphate group is
attached. Together, steps 1 and 2 Glyceraldehyde P P Dihydroxyacetone
cost the cell 2 ATP molecules. 3-phosphate P phosphate

3 The 6-carbon chain is split 2P 2 NAD From mitochondria
into two 3-carbon molecules, 4 2 NADH To mitochondria
each of which then follows the
rest of this pathway. PP Energy Summary
1,3-Bisphosphoglycerate Steps 1 & 2: –2 ATP
4 Another phosphate group is
attached to each molecule, and PP Step 5: +2 ATP
NADH is generated from NAD. 5 2 ADP Step 7: +2 ATP
2 ATP
5 One ATP molecule is formed for P NET GAIN: +2 ATP
each molecule processed. Step 5 3-Phosphoglycerate
produces 2 ATP molecules. P
6 2 H2O
6 The atoms in each molecule are P
rearranged, releasing a
molecule of water. Phosphoenolpyruvate
P
7 A second ATP molecule is formed
for each molecule processed. 7 2 ADP
Step 7 produces 2 ATP molecules. 2 ATP
Pyruvate
25

To mitochondria

called the citric acid cycle. It is also known as the tricarboxylic complete revolution or “turn” of the citric acid cycle removes
(trı.-kar-bok-SIL-ik) acid cycle (TCA cycle) and the Krebs cycle. 2 carbon atoms, regenerating the 4-carbon chain. (This is why
Because citric acid is the first substrate of the cycle, we’ll use the reaction sequence is called a cycle.) We can summarize the
citric acid cycle as the preferred term. fate of the atoms in the acetyl group as follows:

The function of the citric acid cycle is to remove hydrogen The 2 carbon atoms are removed in enzymatic reactions
atoms from organic molecules and transfer them to coenzymes.
The overall pattern of the citric acid cycle is shown in Figure 25–4. that incorporate 4 oxygen atoms and form 2 molecules of
carbon dioxide, a waste product.
At the start of the citric acid cycle, the 2-carbon acetyl group
carried by CoA is transferred to a 4-carbon oxaloacetic acid mol- The hydrogen atoms are removed by the coenzyme NAD or
ecule to make the 6-carbon compound citric acid. Coenzyme
A is released intact and can thus bind another acetyl group. A a related coenzyme called FAD (flavin adenine dinucleotide)
(Figure 25–4).