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Cellular Respiration
Chapter 7
Energy
All life requires energy
Energy is required for:
- growth
- active transport
- reproduction
- synthesis of carbohydrates,
lipids, proteins etc
- movement
The source of energy for cells is
energy stored in the chemical bonds
of organic molecules
Ex. Food molecules, especially
carbohydrates (also lipids)
Most common source of energy is
glucose
Cellular respiration
the process by which the chemical energy of
"food" molecules is released and partially
captured in the form of ATP.
Carbohydrates, fats, and proteins can all be
used as fuels in cellular respiration
glucose is often used as an example to
examine the reactions and pathways
involved.
Overview of Respiration
We can divide cellular respiration into
three metabolic processes:
1. Glycolysis
2. the Krebs cycle
3. oxidative phosphorylation.
Each of these occurs in a specific region
of the cell.
Where?
1. Glycolysis occurs in the cytosol.
2. The Krebs cycle takes place in the
matrix of the mitochondria.
3. Oxidative phosphorylation via the
electon transport chain is carried out on
the inner mitochondrial membrane.
Not enough oxygen to run 5
miles?
In the absence of oxygen, respiration
consists of two metabolic pathways:
1. Glycolysis
2. Fermentation
Both of these occur in the cytosol.
Adenosine triphosphate (ATP)
Cells store energy in chemical bonds
of sugar, but cannot use it directly
To use energy, the cell must transfer
energy in its sugar molecules to ATP
ATP is the energy currency of cells
ATP (contd.)
ATP = base (adenine) + sugar (ribose)
+ 3 phosphate groups
Phosphate groups are connected by
energy rich bonds
ATP is like a rechargeable battery
If a cell needs energy, it breaks the
last phosphate bond of the ATP to
release energy
ATP
ADP + Pi + energy (12 kcal)
Where does ATP come from?
What is a kcal?
Kcal is an abbreviation for kilocalorie
measurement for energy
(for example for the amount of energy
obtained from burning food).
1 kcal is the same as 1 Calorie
www.emulsifiers.org/ViewDocument.asp
Cellular Respiration
The process by which cells break the
energy containing bonds of glucose to
produce ATP
Occurs in the mitochondria
Summary equation:
Glucose
Oxygen
Carbon
dioxide
Water
Energy
Cell Respiration vs. “Respiration”
In Respiration (as in “respiratory
system” or breathing) the exchange
of gases is between blood and the
atmosphere (oxygen in, carbon
dioxide out)
In Cellular Respiration (aerobic*) the
exchange of gases is between blood
and the cell (oxygen in, carbon
dioxide out).
* means in the presence of oxygen
Breathing
Lungs
Muscle
cells
Cellular
respiration
Digestion vs. Cellular Respiration
Digestion: Enzymes break down large
organic molecules into smaller ones
Cellular Respiration: Smaller organic
molecules (namely glucose) are broken
down to make ATP
Cellular Respiration vs. Photosynthesis
Photosynthesis:
6CO2 + 6H2O + SOLAR ENERGY
C6H12O6 + 6O2
Cellular Respiration:
C6H12O6 + 6O2  6CO2 + 6H2O + ATP
Recycling Energy
Cellular Respiration vs.
Photosynthesis continued
Photosynthesis: takes place in
autotrophs (plants, algae, cyanobacteria)
Cellular Respiration: takes place in ALL
organisms
Glucose: C6H12O6
Most foods contain a variety of
carbohydrates, proteins, and lipids.
We will examine cellular respiration
by following the breakdown of
glucose, one of the most abundant
organic molecules. But remember
that cellular respiration happens with
other organic compounds too.
What is a “biochemical
pathway”?
A chemical reaction that occurs
in sequences where the product
of one reaction becomes the
reactant for the next reaction
Summary equation for cellular
respiration:
C6H12O6 + 6O2  6CO2 + 6H2O
+ ATP
In an explosion, fuel is burned in
one step
Energy released as heat and light
In cellular respiration, glucose is
broken down in multiple steps
Glucose
C6H12O6
+ 6O2
NAD+ NADH FAD FADH2 NAD+ NADH + H+
+ H+
ADP
+ Pi
6CO2 + 6H2O
ATP
ADP
+ Pi
ATP
NAD: Nicotinamide Adenine Dinucleotide
FAD: Flavin Adenine Dinucleotide
Two stages of cellular respiration
Anaerobic stage – without oxygen
- Strict anaerobes; lack the
enzymes needed to break down energy
rich molecules with O2
- Facultative anaerobes; can use
O2 to break down energy rich
molecules when it is available
Aerobic stage – with oxygen
CELLULAR RESPIRATION: OVERVIEW
Organic Compounds (Glucose)
Glycolysis
O2 absent
Anaerobic
Respiration
2 ATP
O2 present
Aerobic
Respiration
ATP
1. Glycolysis (anaerobic stage)
Occurs in the cytoplasm
Glucose (6C) splits up into
2 pyruvic acid (3C)
Two molecules of ATP are required to
split glucose
Splitting of glucose releases enough
energy to produce 4 molecules of ATP
Net gain of 2 ATP
More about Glycolysis
During conversion of glucose to pyruvic acid,
hydrogen is released
This hydrogen is picked up by a coenzyme,
nicotinamide adenine dinucleotide (NAD)
When it accepts hydrogen, it becomes NADH
NADH is an energy rich compound that will be
used in the aerobic stage of respiration
Glycolysis: Harvests energy by
splitting glucose to pyruvic acid
2 Pyruvic acid
Glucose
Summary of Glycolysis
Cytoplasm
Glucose
Pyruvic
acid
Fermentation
Fermentation = breakdown of
glucose, yielding ATP, without O2
Many types of bacteria and other
single celled organisms still use
anaerobic processes to convert energy
Types of Fermentation
2 kinds:
Alcoholic fermentation:
occurs in micro-organisms such as yeast
Lactic acid fermentation:
occurs in bacteria and animal cells
More Fermentation
The first living organisms were single
cells that existed without O2
Anaerobic
Lack the enzymes needed to break down
energy molecules with O2
Fermentation in the Cytosol
Fermentation occurs
in the cytosol
It produces lactic
acid or alcohol
Fermentation begins
with the process of
glycolysis, which is
also part of aerobic
respiration.
Alcoholic 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
Yeast
Yeast are a tiny form of fungi or plant-like
microorganism that exist in or on all living
matter i.e. water, soil, plants, air, animals, etc.
A common
example of a
yeast is the
bloom (fuzz)
we can
observe on
grapes.
http://home.earthlink.net/~ggda/biology_of_yeast_cells_simplified.htm
As a living
organism yeast
needs sugars,
water and
warmth to stay
alive. In addition,
albumen or
nitrogenous
material are also
necessary for
yeast to thrive.
Alcoholic fermentation
2ADP
+
2
2 CO2 released
Glycolysis
2NAD
Glucose
2 Pyruvic
acid
2NAD + 2H
2 Ethyl
alcohol
In alcoholic fermentation, pyruvic acid is
converted to CO2 and ethanol
This recycles NAD+ to keep glycolysis working
Simple Sugar → Ethyl Alcohol + Carbon Dioxide
C6 H12 O6 → 2C H3 CH2 OH + 2CO2
The food industry uses yeast to
produce various food products
Lactic Acid Fermentation
– Pyruvic acid is converted to lactic acid
– Lactic acid fermentation is used to
make cheese and yogurt
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
Lactic acid fermentation
2ADP+
2
Glycolysis
2 NAD
Glucose
2 NAD + 2 H
2 Pyruvic
acid
2 Lactic
acid
Uses of Lactic Acid Fermentation
During strenuous exercise glycolysis occurs
at a high rate
Pyruvic acid is produced rapidly
Muscle cells may not receive enough O2 to
process pyruvic acid through aerobic
respiration
Therefore muscles produce lactic acid which
permits glycolysis to continue to supply ATP
to your muscles
What happens when you don’t have
enough oxygen to undergo
glycolysis?
When lactic acid builds up, your muscles
ache
O2 you take in from heavy breathing helps
convert lactic acid back to pyruvic acid
http://www.scientificamerican.com/article.cf
m?id=why-does-lactic-acid-buil
Cellular Respiration
Most cells produce ATP by breaking the
energy containing bonds of glucose in the
presence of oxygen
Production of ATP this way = Respiration
Uses O2 to break sugars down to CO2 & H2O
Not the same as breathing
breathing provides O2, but otherwise quite
different
Aerobic respiration occurs in the many
mitochondria of each cell
Breathing
Lungs
Muscle
cells
Cellular
respiration
The Process of Cellular Respiration
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy
(sugar)
Two stages of Cellular Respiration:
Anaerobic
without oxygen
Aerobic
with oxygen
(ATP)
Cellular Respiration Overview
Anaerobic Stage: Glycolysis
The anaerobic stage of cellular respiration is
glycolysis, the same pathway used in
fermentation
This part of cellular respiration occurs in the
cytoplasm
Recall the energy budget for glycolysis:
One molecule of glucose is split into two molecules
of a three carbon compound called pyruvic acid
2 molecules of ATP provide the energy to split the
glucose molecule
When glucose splits, it releases enough energy to
form 4 molecules of ATP from ADP + P
Therefore 2 molecules of ATP are gained
Energy of Glycolysis
Aerobic Stage
After glycolysis, the chemical bonds of
pyruvic acid are broken down in a series of
chemical reactions
These occur in the mitochondria and
require O2
The aerobic stage has two main parts:
- The Citric Acid Cycle (Krebs cycle)
The Electron Transport Chain
These two stages are preceded by an
intermediate step in which pyruvic acid is
converted to acetyl-CoA
Pyruvate Forms Acetyl CoA
in the Intermediate Step
Citric Acid (Krebs ) Cycle
In aerobic organisms, the citric acid cycle is a
metabolic pathway that forms part of the
breakdown of carbohydrates, fats and
proteins into carbon dioxide and water in
order to generate energy.
It is one of three metabolic pathways that are
involved in fuel molecule catabolism and ATP
production
the other two being glycolysis and oxidative
phosphorylation.
The citric acid cycle also provides
precursors for many compounds, such
as certain amino acids, and some of its
reactions are important in cells
performing fermentation reactions in the
absence of oxygen.
The citric acid cycle is also known as
the Krebs Cycle in honor of Sir Hans
Adolf Krebs (1900 - 1981), who
proposed the key elements of this
pathway in 1937, and was awarded the
Nobel Prize in Medicine for its discovery
in 1953.
http://www.newworldencyclopedia.org/entry/Citric_aci
d_cycle
The Citric Acid (Krebs) Cycle
Steps to break down pyruvic acid:
In the presence of O2, pyruvic acid breaks
down to acetic acid and CO2
CO2 is released as waste
Acetic acid combines with coenzyme A 
acetyl CoA
This step also forms NADH2 from NAD
Acetyl CoA enters the citric acid cycle and
combines with a 4 carbon compound to
produce citric acid
As the cycle continues, citric acid is broken
down in a series of steps, back to the original
4 carbon compound
Energy from the Citric Acid Cycle
For each molecule of
acetyl CoA that enters
the cycle, 8 atoms of H
are released.
These hydrogen atoms
are trapped by NAD
and FAD, forming
NADH and FADH2
Therefore, each turn
of the cycle yields
3NADH and one FADH2
Intermediate
step
The Electron Transport Chain
NADH and FADH2 releases the hydrogen
atoms trapped during glycolysis & the citric
acid cycle
Thus, NADH/FADH2 becomes NAD/FAD again
Electrons contained in the H atoms pass
through a series of coenzymes which are
electron acceptors.
Each time an electron moves from one
acceptor to another, energy is released
The energy released is used by chemiosmosis
to form molecules of ATP from ADP + Pi
This whole process = electron transport chain
Oxygen & the Electron Transport Chain
The last part of
the chain is the
electron acceptor,
oxygen
Electrons combine
with oxygen &
hydrogen to form
H2O, which is
released as a
byproduct
Picturing Chemiosmosis
FADH2 FAD
+ 2H+
Chemiosmosis
The process of ATP formation from the ETC
of aerobic respiration when a pH gradient
forms across the membrane of the cristae in
the mitochondria = chemiosmosis
Steps:
H+ ions from the matrix are pumped into the space
between the cristae and the outer membrane
A H+ gradient develops between the inside and
outside of the cristae
This pH differential creates free energy
H+ pass back across the membrane through ATP
synthase, producing ATP from ADP + Pi
O2 is the final H+/electron acceptor producing H2O
Cellular Respiration Summary
Thus for every molecule of glucose that is
broken down by glycolysis and respiration, 38
molecules of ATP are formed