Chapter 6
How Cells Harvest
Chemical Energy
Overview:
•Cellular Respiration
•Reactions involved in cellular respiration
•Glycolysis
•Krebs Cycle
•Electron Transport
•Fermentation
•Food used to produce ATP
•ATP used to produce food
INTRODUCTION TO CELLULAR
RESPIRATION
Photosynthesis and cellular respiration
provide energy for life
• Nearly all the cells in our body break down
sugars for ATP production
• Cellular respiration occurs in mitochondria
– Cellular respiration is a chemical process that
harvests energy from organic molecules
– Cellular respiration yields CO2, H2O, and a large
amount of ATP
• The ingredients for photosynthesis are carbon
dioxide and water
– CO2 is obtained from the air by a plant’s leaves
– H2O is obtained from the damp soil by a plant’s roots
• Chloroplasts rearrange the atoms of these
ingredients to produce sugars (glucose) and
other organic molecules
– Oxygen gas is a by-product of photosynthesis
The Relationship Between Cellular
Respiration and Breathing
• Cellular respiration and breathing are closely
related
– Cellular respiration requires a cell to exchange gases
with its surroundings
– Breathing exchanges these gases between the blood
and outside air
Cellular respiration banks (stores)
energy in ATP molecules
• Cellular respiration breaks down glucose
molecules and banks their energy in ATP
– The process uses O2 and releases CO2 and H2O
Glucose
Oxygen gas
Carbon
dioxide
Water
Energy
Connection: The human body uses
energy from ATP for all its activities
• ATP powers almost
all cell and body
activities
• A calorie is the
amount of energy that
raises the temperature
of 1 gram of water by
1 degree Celsius
Cells tap energy from electrons transferred
from organic fuels to oxygen
• Glucose gives up energy as it is oxidized
Loss of hydrogen atoms
Energy
Glucose
Gain of hydrogen atoms
Redox Reactions
• Chemical reactions that transfer electrons from
one substance to another are called oxidationreduction reactions
– Redox reactions for short
• The loss of electrons during a redox reaction is
called oxidation
• The acceptance of electrons during a redox
reaction is called reduction
Oxidation
[Glucose loses electrons (and hydrogens)]
Glucose
Oxygen
Carbon
dioxide
Reduction
[Oxygen gains electrons (and hydrogens)]
Water
• Why does electron
transfer to oxygen
release energy?
– When electrons move
from glucose to oxygen,
it is as though they were
falling
– This “fall” of electrons
releases energy during
cellular respiration
NADH and Electron Transport Chains
• The path that
electrons take on
their way down
from glucose to
oxygen involves
many stops
An enzyme called
dehydrogenase and a
coenzyme called NAD+
(nicotinamide adenine
dinucleotide) play important
role in oxidizing glucose.
• The first stop is an electron acceptor called
NAD+
– The transfer of electrons from organic fuel to NAD+
reduces it to NADH
• The rest of the path consists of an electron
transport chain
– This chain involves a series of redox reactions
– These lead ultimately to the production of large
amounts of ATP
•All of the reactions involved in cellular
respiration can be grouped into three main
stages
–Glycolysis
–The Krebs cycle
–Electron transport
Stage 1: Glycolysis
• A molecule of glucose is split into two molecules
of pyruvic acid
• Glycolysis breaks a six-carbon glucose into two
three-carbon molecules
– These molecules then donate high energy electrons to
NAD+, forming NADH
•Glycolysis occurs in the cytoplasm
• Glycolysis makes some ATP directly when
enzymes transfer phosphate groups from fuel
molecules to ADP (This process is called
substrate-level phosphorylation)
Enzyme
Stage 2: The Krebs Cycle
• The Krebs cycle completes the breakdown of
sugar
• In the Krebs cycle, pyruvic acid from glycolysis
is first “prepped” into a usable form, Acetyl-CoA
• The Krebs cycle extracts the energy of sugar by
breaking the acetic acid molecules all the way
down to CO2
– The cycle uses some of this energy to make ATP
– The cycle also forms NADH and FADH2
•The Krebs cycle and the electron transport
chain occur in the mitochondria
Stage 3: Electron Transport
• Electron transport releases the energy your cells
need to make the most of their ATP
• The molecules of electron transport chains are
built into the inner membranes of mitochondria
– The chain functions as a chemical machine that uses
energy released by the “fall” of electrons to pump
hydrogen ions across the inner mitochondrial
membrane
– These ions store potential energy
• When the hydrogen ions flow back through the
membrane, they release energy
– The ions flow through ATP synthase
– ATP synthase takes the energy from this flow and
synthesizes ATP
•
•
•
The electrons from NADH and FADH2 travel down the electron transport chain to
oxygen
Energy released by the electrons is used to pump H+ into the space between the
mitochondrial membranes
In chemiosmosis, the H+ ions diffuse back through the inner membrane through ATP
synthase complexes, which capture the energy to make ATP
Connection: Certain poisons interrupt
critical events in cellular respiration
Rotenone
Cyanide,
carbon monoxide
Oligomycin
Fermentation is an anaerobic alternative
to aerobic respiration
• Some of your cells can actually work for short
periods without oxygen
– For example, muscle cells can produce ATP under
anaerobic conditions
• Fermentation
– The anaerobic harvest of food energy
•Under anaerobic conditions, many kinds of cells
can use glycolysis alone to produce small amounts
of ATP
• Aerobic metabolism
– When enough oxygen reaches cells to support energy
needs
• Anaerobic metabolism
– When the demand for oxygen outstrips the body’s
ability to deliver it
• Anaerobic metabolism
– Without enough oxygen, muscle cells break down
glucose to produce lactic acid
– Lactic acid is associated with the “burn” associated
with heavy exercise
– If too much lactic acid builds up, your muscles give
out
Fermentation in Human Muscle Cells
• Human muscle cells can make ATP with and
without oxygen
– They have enough ATP to support activities such as
quick sprinting for about 5 seconds
– A secondary supply of energy (creatine phosphate)
can keep muscle cells going for another 10 seconds
– To keep running, your muscles must generate ATP by
the anaerobic process of fermentation
Fermentation in Microorganisms
• Various types of microorganisms perform
fermentation
– Yeast cells carry out a slightly different type of
fermentation pathway
– This pathway produces CO2 and ethyl alcohol
• The food
industry uses
yeast to produce
various food
products
• In alcoholic fermentation, pyruvic acid is
converted to CO2 and ethanol
– This recycles NAD+ to keep glycolysis working
• In lactic acid fermentation, pyruvic acid is
converted to lactic acid
– As in alcoholic fermentation, NAD+ is recycled
• Lactic acid fermentation is used to make cheese
and yogurt
INTERCONNECTIONS BETWEEN MOLECULAR
BREAKDOWN AND SYNTHESIS
Cells use many kinds of organic molecules as fuel for
cellular respiration
• Polysaccharides can be hydrolyzed to
monosaccharides and then converted to glucose
for glycolysis
• Proteins can be digested to amino acids, which
are chemically altered and then used in the Krebs
cycle
• Fats are broken up and fed into glycolysis and
the Krebs cycle
Food molecules provide raw materials
for biosynthesis
• In addition to energy, cells need raw materials
for growth and repair
– Some are obtained directly from food
– Others are made from intermediates in glycolysis
and the Krebs cycle
• Biosynthesis consumes ATP
• Biosynthesis of macromolecules from
intermediates in cellular respiration
The fuel for respiration ultimately comes
from photosynthesis
• All organisms have the
ability to harvest energy
from organic molecules
– Plants, but not animals,
can also make these
molecules from inorganic
sources by the process of
photosynthesis