Metabolism of the Cell
Energy Production
Metabolism
• Refers to “all chemical reactions necessary to
maintain life”.
• Anabolic processes (anabolism) build from smaller
molecules
– for example, building of proteins from amino acids
– anabolic processes generally require energy input
• Catabolic processes (catabolism) break down larger
molecules into smaller ones.
– for example, breakdown of glucose into carbon dioxide
and water
– catabolic processes generally release energy
Cellular Respiration
• Describes the series of reactions that break
down glucose to release ATP.
• Includes glycolysis, Krebs cycle, and
oxidative phosphorylation within the electron
transport system.
Major Stages of Metabolism
I. Glycolysis
II. Krebs Cycle (a.k.a., tricarboxylic acid cycle,
TCA cycle, citric acid cycle)
III. Oxidative Phosphorylation
Electron Transport System and Chemiosmosis
• Digestion, the breakdown of food into
usable molecules such as glucose, is
required before metabolism can occur
Metabolism Terms
• ATP – Adenosine triphosphate - simplest storage
form of cellular energy
• Glucose – main substrate for ATP production; a
monosaccharide (simple sugar).
• NAD+ – coenzyme that accepts hydrogen; derived
from niacin
– NADH (more accurately – NADH+H+) reduced form
of the coenzyme; includes two additional electrons and
one hydrogen.
• FAD+ – another coenzyme that accepts hydrogen;
derived from riboflavin.
– FADH2 – reduced form including two hydrogens and
their electrons
Metabolism Terms(cont.)
• Oxidation - reactions within cellular respiration that
occurs due to a loss of electrons (usually seen as
hydrogen) or a gain in oxygen.
• Oxidation-reduction (redox) reactions – coupled
reactions in which one substance donates (loses)
electrons (i.e., is oxidized) and another then gains
those electrons (i.e., is reduced).
• Reduction – reaction in which a substrate gains
electrons (usually seen as hydrogen)
Metabolism Terms(cont.)
• Phosphorylation – addition of phosphate group to
a molecule resulting in addition of energy to the
molecule; e.g., ADP + Pi ATP
– substrate-level phosphorylation – direct transfer from
one molecule to another
•e.g., bisphophoglycerate + ADP  ATP +
phosphoglycerate
– moves phosphate from bisphosphoglycerate
(which has 2 phosphates) to ADP to make ATP
and phosphoglycerate (which has one
phosphate)
– oxidative phosphorylation – more complex method of
ATP production involving electron transport chain
(series of redox reactions)
Glycolysis
Glycolysis
• Breakdown of glucose into pyruvic acid
– Glucose comes from the food that we eat
• Occurs in the cytoplasm of the cells
• Results in a net production of 2 ATP and 2
reduced electron carriers (NADH)
Glycolysis - Overview
3 Phases:
•sugar activation
•sugar cleavage
•sugar oxidation
and formation of
ATP
Fig. 25.6, p. 966
Phase 1 – Sugar Activation
Glucose (6-carbon) gains
phosphate (and energy) from
each of two (2) ATP
molecules and becomes an
unstable 6-carbon molecule
(fructose-1,6-diphosphate)
• Adenosine triphosphate (ATP)
becomes adenosine diphosphate
(ADP)
Fig. 25.6, p. 966
Phase Two – Sugar Cleavage
DHAP
G3P
Unstable 6-carbon molecule,
(fructose-1,6-diphosphate),
is broken into 2 3-carbon
molecules (DHAP and
G3P)
• no additional energy
required
Fig. 25.6, p. 966
Phase Three – Sugar Oxidation
2 3-carbon molecules
(DHAP and G3P) are each
oxidized resulting in release
of energy used to make 4
ATP (2 from each) and
formation of 2 pyruvate (1
from each)
• ATP produced by substratelevel phosphorylation
Fig. 25.6, p. 966
Phase Three – Sugar Oxidation (con’t)
• oxidation of these 2 3-carbon molecules (DHAP
and G3P) also results in formation of 2 molecules
of NADH (one from each 3-carbon molecule), the
energy from which will be used in the mitochondria
during oxidative phosphorylation
• when oxygen is present, pyruvate diffuses into the
mitochondria for the next steps of cellular
respiration
• when oxygen is not present, pyruvate is reduced
using the NADH and becomes lactic acid
Glycolysis Energy Summary
• Net ATP production = 2
– 4 produced – 2 used
•two (2) ATP are utilized in phase 1, so 2 are
subtracted from the total produced
• 2 NADH produced total (one per G3P)
– will be used in electron transport chain /
oxidative phosphorylation in mitochondria when
oxygen is present
From Glycolysis to Krebs
• Pyruvate created in glycolysis diffuses from
the cell cytoplasm into the matrix of the
mitochondria to be further broken down
• Going from glycolysis to Krebs involves an
intermediate step in which pyruvate is
reduced and reworked into a 2-carbon
molecule called acetyl-CoA by removing one
CO2 group and addition coenzyme A; this
also produces 1 molecule of NADH for
each pyruvate
Krebs Cycle
Krebs Cycle
• Begins when the acetyl group of acetyl-CoA
combines with oxaloacetate (4-carbon
molecule) to form citrate (6-carbon
molecule) and release the CoA.
• Remaining reactions involve oxidizing the
molecule and regenerating oxaloacetate
– reactions also produce carbon dioxide (CO2),
which will be released from the cell; reduced
electron carriers, which will be used in the next
stage; and GTP, which can be used to produce
an equivalent amount of ATP
Fig. 25.7, p. 968
Krebs Cycle (con’t)
• Each citrate is
rearranged during
this cycle to
produce two (2)
CO2 molecules,
three (3) NADH,
one (1) FADH2,
and one (1) ATP.
Fig. 25.7, p. 968
Acetyl-CoA and Krebs Cycle
Energy Summary
• ATP Production – One (1) per cycle of
Krebs.
• NADH Production
– one (1) NADH created with the formation of
Acetyl CoA
– three (3) NADH created during Krebs cycle
• FADH2 Production – One per cycle of
Krebs
• REMEMBER: Krebs goes through 2 times
for each glucose that started the process
Acetyl-CoA and Krebs Cycle
Energy Summary
• REMEMBER: Krebs goes through 2 times
for each glucose that started the process,
therefore, the total is:
– 2 ATP created during the Krebs cycle
– 2 NADH created with the formation of Acetyl
CoA
– 6 NADH created during Krebs cycle
– 2 FADH2 created during Krebs cycle
Total Energy Summary, Through
Krebs Cycle
From one (1) molecule of glucose:
• 4 ATP – 2 (Glycolysis) + 2 (Krebs)
• 10 NADH – 2 (Glycolysis) + 2 (Acetyl CoA
formation) + 6 (Krebs)
• 2 FADH2 – 2 (Krebs)
Oxidative
Phosphorylation
Electron Transport System (ETS)
and Chemiosmosis
Electron Transport System
• Utilizes the NADH and FADH2 produced
in Glycolysis and Krebs.
• Occurs in the inner membrane of the
mitochondria.
• Cannot occur without oxygen
Electron Transport System Step 1
Molecules within
the inner
membrane of the
mitochondria take
the two (2)
electrons from
NADH and the two
(2) from FADH2
and pass them from
one to another.
(i.e., redox
reactions)
Fig. 25.8, p. 969
Electron Transport System Step 2
• The transfer of electrons moves hydrogen atoms (H+) into
the intermembrane compartment of the mitochondrion.
Fig.Fig.
25.8,
25.8,
p. p.
969969
Electron Transport System Step 3
• When enough H+ atoms collect in the compartment, they
travel down their concentration gradient into the
mitochrondrial matrix in a process called chemiosmosis.
• Movement occurs through ATP synthase enzyme that uses
the energy of H+ movement to create ATP from ADP
Fig. 25.8, p. 969
Electron Transport System Step 4
• Once the electrons have moved to the end of the
molecules in this chain, two(2) electrons combine
with one (1) oxygen atom (formed from the
breakdown of molecular oxygen, or O2) and two
(2) hydrogen atoms to make water.
• Oxygen is a KEY component to this process.
– If oxygen is not present to combine and make water,
then the electron transport system backs up, no H+
atoms are released and ATP cannot form.
– Krebs cycle cannot be run because NAD and FAD are
not regenerated
Electron Transport System ATP
Totals
• Each pair of H+ moved results in formation of 1
ATP
• ATP from NADH – each NADH moves 3 pairs of
H +:
– 6 NADH from Krebs  18 ATP
– 2 NADH from Acetyl CoA  6 ATP
– 2 NADH from Glycolysis  6 ATP
• BUT there is a cost to move the NADH in from the
cytoplasm at 1 ATP each, so the net total in most cells is 4
ATP.
• This loss DOES NOT OCCUR IN HEART CELLS OR
LIVER CELLS. In these cells movement is more
efficient, and the ATP production is 6 ATP!
Electron Transport System ATP
Totals
• ATP from FADH2
– FADH2 transfers its H+ pairs at a different point,
resulting in only 2 ATP per molecule
– 2 FADH2 from Krebs  4 ATP.
• Total ATP:
– ATP from NADH = 18 + 6 + (4 or 6) = 28 (or 30)
– ATP from FADH2 = 4
– TOTAL from ETS = 32 (or 34) ATP
Total ATP Production through
the Entire System
2 ATP (Glycolysis) + 2 ATP (Krebs) + 32-34 ATP (from ETS) = 36-38 ATP
Fig. 25.10, p. 972