Chapter 7
Cellular Respiration
Section 7.1
Glycolysis and Fermentation
• All cells break down complex organic
compounds into simpler molecules
• As the compounds are broken down, cells
release energy
• Some of the energy is used to make ATP from
ADP
Harvesting Chemical Energy
• ATP is the main energy currency of cells
• Cellular respiration- the complex process in
which cells make ATP by breaking down
organic compounds
Cellular Respiration
• Begins with glycolysis
• Glycolysis yields a small amount of ATP
• Two possible pathways depending on the
presence or absence of oxygen
Aerobic vs. Anaerobic
• Aerobic: oxygen is present (pathway will lead
to aerobic respiration)
• Anaerobic: oxygen is absent (pathway leads to
fermentation)
Glycolysis
• Takes place in the cytosol of the cell
• Involves four main steps
• One 6-Carbon molecule of glucose is oxidized
to produce two three-Carbon molecules of
pyruvic acid
• Two ATP molecules are used in step 1
• Four ATP molecules are produced in step 4
• Glycolysis has a net yield of 2 ATP
Fermentation
• Occurs in the absence of oxygen
• Converts pyruvic acid into other compounds
• These pathways regenerate NAD+
Lactic Acid Fermentation
• An enzyme converts pyruvic acid into another
3-carbon compound called lactic acid
• Two hydrogen atoms are transferred to
pyruvic acid
Uses of Lactic Acid
• Microorganisms are used for lactic acid
fermentation in cheese and yogurt
• Also occurs in muscle cells during strenuous
exercise (muscle cells use up oxygen faster
than it can be delivered)
Alcoholic Fermentation
• Plant cells and unicellular organisms, such as
yeast, convert pyruvic acid into ethyl alcohol
• Used in bread, wine, beer, etc.
Energy Yield
• Energy is measured in units of kilocalories
(kcal)
• One kilocalorie equals 1000 calories (cal)
• Oxidation of a standard amount of glucose
releases 686 kcal
Efficiency of Glycolysis
• Efficiency of = energy required to make ATP
glycolysis
energy released by oxidation
of glucose
= 2 X 12 kcal X 100%
686 kcal
= 3.5%
• Anaerobic pathways probably evolved very early in
the history of life on Earth
• For more than a billion years, this was the only
pathway available for harvesting chemical energy
• The first organisms to use anaerobic pathways were
bacteria
• Larger organisms have greater energy
requirements that cannot be met by only
anaerobic pathways
• Must use aerobic respiration
Chapter 7
Section 7.2
Aerobic Respiration
• Produces 20 times as much ATP as produced
by glycolysis alone
• Has two major stages: the Krebs cycle and the
electron transport chain
Energy Yield
• Maximum of 38 ATP molecules can be produced
from 1 molecule of glucose
• Efficiency = Energy required to make ATP
Energy released by oxidation of glucose
= 38 X 12 kcal X 100% = 66%
686 kcal
• Aerobic respiration is nearly 20 times more
efficient than glycolysis alone
• Glucose  glycolysis  pyruvic acid 
acetyl CoA  Krebs cycle
Summarizing Cellular Respiration
• C6H12O6 + 6O2  6 CO2 + 6H2O + energy
• Cellular respiration provides ATP that all cells
need to support the activities of life
• Also provides carbon skeletons that can be
built up into larger molecules
• Cells need specific organic compounds to
build macromolecules