Pp 69 – 73 & 217 - 237
Define cell respiration
 Cell respiration is the
controlled release of
energy from organic
compounds in cells to
form ATP
 Glucose is the major
substrate for respiration
 Adenosine triphosphates
(ATP) is the molecule
which directly fuels the
majority of biological
reactions.
Why cell respiration?
 Cells require a
constant source
of energy to
perform various
tasks e.g.
 Movement
 Transport
 Division
Types of Respiration:
(i) Anaerobic Respiration
 Occurs in the absence of Oxygen
(ii) Aerobic Respiration
 Occurs in presence of Oxygen
 Occurs in the cells’ cytoplasm
 Yields small amount of ATP (2
molecules) per molecule of
glucose
 Involves fermentation of
pyruvate to lactate in
muscles/CO2 & ethanol in plant
& yeast
 Occurs in the cells’
mitochondria
 Yields large amount of ATP
(38 molecules) per molecule
of glucose
 Does not involve
fermentation
Types of Respiration
(i) Anaerobic Respiration
(ii) Aerobic Respiration
Occurs in the absence of Oxygen
Occurs in presence of Oxygen
Occurs in the cells’ cytoplasm
Occurs in the cells’ mitochondria
Yields small amount of ATP (2
molecules) per molecule of glucose
Yields large amount of ATP (38
molecules) per molecule of glucose
Involves fermentation of pyruvate to
lactate in muscles/CO2 & ethanol in
plant & yeast
Does not involve fermentation
Comparison between Aerobic & Anaerobic Respiration
Adenosine triphosphate (ATP):
 ATP the molecule which directly fuels the majority of
biological reactions
 About 1025 ATP molecules are hydrolysed to ADP and Pi daily
 ADP is reduced back to ATP using the free energy from the
oxidation of organic molecules
ATP Cycle
 in cell respiration, glucose in the
cytoplasm is broken down by glycolysis
into pyruvate, with a small yield of ATP
Glycolysis and Cell Respiration:
Glycolysis occurs in the




cytoplasm of the cell
1 glucose molecules is broken
down into 2 pyruvate
molecules
There is a net production of 2
ATP molecules
Glycolysis does not require
oxygen
Fate of pyruvate depends on
presence or absence of oxygen
Anaerobic Cell Respiration:
Summary Equation
 The summary equation for cellular respiration is:
C6H12O6 + O2  CO2 + H2O +
ATP
Glucose + oxygen  carbon dioxide + water + ATP
Key players in this process
 Glucose: source of fuel
 NAD+: electron carrier
 Enzymes: mediate entire process
 Mitochondria: site of aerobic respiration
 ATP: principal end product
 Protons/Electrons: sources of potential
energy
 Oxygen: final electron acceptor
Redox reactions
Reduction:
reducing overall
positive charge
by gaining
electrons
Oxidation: loss
of electrons
NAD+: an electron carrier
 In order for electrons to be
passed from one
compound to another, an
electron carrier is needed
 NAD+ is reduced to
NADH when picking up
electrons
 It is oxidized back to
NAD+ when losing them
Where do the electrons come
from?
 Remember all those
hydrogen atoms that make
up glucose?
 Hydrogens are a part of fats,
too.
 Hydrogen = 1e-, so here, H =
e-
Respiration is a controlled
release of energy
 It’s a highly
exergonic, but wellcontrolled process
 Mediated by
enzymes, electron
carriers
 Otherwise, it would
be like an explosion
 Not compatible with
life!
Phosphorylation
Addition of a phosphate group to
a molecule; in this case, to ADP,
forming ATP
Substrate level phosphorylation
vs. oxidative phosphorylation
Substrate-level phosphorylation
 An enzyme transfers
a phosphate group
from a substrate to
ADP
 Ineffective in
generating large
amounts of ATP
Oxidative phosphorylation
 Refers to phosphorylation
that occurs due to redox
reactions transferring
electrons from food to
oxygen
 Happens on electron
transport chains
Mitochondrion Structure
Outer membrane
Inner membrane
Intermembrane
space
DNA (mtDNA)
Cristae (folds)
Matrix (liquid)
Three stages of respiration
 Stage 1: Glycolysis (energy investment)
 Some ATP is made, some is used
 Stage 2: Krebs Cycle (oxidation of
pyruvate)
 Generation of CO2
 Stage 3: Oxidative Phosphorylation
 Generation of most ATP
Three stages of respiration
Stage 1: Glycolysis
 Where
 Cell’s cytoplasm
 Why
 To break glucose
down into pyruvate,
which feeds into the
Krebs Cycle
 To regenerate NAD,
an electron carrier
Glycolysis: How
Glucose is
phosphorylated.
-1 ATP
An intermediate is
phosphorylated.
-1 ATP
This diphosphate
compound is unstable
and breaks into 2 PGAL.
The PGAL molecules
generate ATP through
substrate-level
phosphorylation.
+4 ATP
A series of enzymes
produces intermediate
products.
NAD is reduced to NADH,
1 each per PGAL
+2 NADH + H+
Summary:
2 pyruvate produced
2 NADH + H+ produced
Net 2 ATP produced
Stage 2: Krebs cycle
 Where:
 Matrix of mitochondria, but
only if O2 present
 Why:
 To oxidize pyruvate to CO2
 To build up a H+ ion gradient
used in electron transport
Krebs Cycle: How
Pyruvate is
decarboxylated.
-1 CO2
Acetyl group (2C) has
Coenzyme A added
1 Acetyl co-A: link reaction
NAD oxidizes a 4C to
form original 4C
molecule: +1 NADH
+ H+
To regenerate the
original 4C base,
ATP is generated
and FAD oxidizes an
intermediate
+1 ATP
+1 FADH2
Resulting acetic acid (2C) is oxidized by
NAD.
+1 NADH + H+
Acetyl co-A (2C) is
added to a 4C base
molecule, forming a
6C intermediate
NAD oxidizes
these
intermediates
CO2 is given off
as a byproduct
+2 NADH + H+
-1 CO2
Krebs cycle summary
 Per pyruvate that enters:
 1 ATP made
 3 CO2 given off
 4 NADH produced
 1 FADH2 produced
 Think: how many pyruvates entered the cycle?
 How many times must this cycle happen to break down ONE
glucose?
Stage 3: Oxidative
phosphorylation
 Where:
 Inner membrane of
mitochondria (on
cristae)
 Why:
 To produce ATP
from H+ ion gradient
generated during
Krebs cycle
 Requires oxygen!
Oxidative Phosphorylation: How
H+ ions accumulate in
the matrix as a result of
NADH picking them up
during the Krebs cycle.
cytochromes
Intermembrane space
matrix
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
Oxidative Phosphorylation: How
e-
Intermembrane space
cytochromes
e-
matrix
H+ +
H
H+
H+
+
+H
+
HH
e-
e-
ee-
The H+ ions diffuse out
of the matrix through
protein channels into
the intermembrane
space where they split
into H protons and
electrons.
Oxidative Phosphorylation: How
Intermembrane space
H+
H+
H+
H+
H+
e-
e-
e-
eee-
cytochromes
The accumulated H+
ions then move
through a pump called
ATP synthase to
produce ATP. What
powers the pump is the
electrochemical
gradient produced.
H+
matrix
ADP
ATP
Oxidative Phosphorylation: How
Intermembrane space
e-
e-
e-
e-
e-
H+
eH+
H+
H+
H+
cytochromes
While the H+
protons move
through ATP
synthase,
electrons
carried by
NADH are
passed along
electron
transport chains
composed of
cytochromes.
H+
matrix
ADP
ATP
Oxidative Phosphorylation: How
Now the H+ atoms are
in the matrix.
H+
H+
eH+
O
H+
H+
H+
cytochromes
The hydrogen atoms
and electrons combine
with oxygen, the final
electron acceptor of
oxidative
phosphorylation, to
form water.
H+
H+
e-
e-
eeeADP
matrix
34 ATP
Summary: Oxidative
Phosphorylation
 34 ATP made
 H2O generated
 NADH oxidized back to NAD
 Very efficient process! Produces a lot of energy.
Remember, the first living organisms lived in an
anaerobic environment…
Without oxygen…
 NADH cannot be oxidized back to NAD+
 In order for aerobic respiration to occur, NADH must
be oxidized and some intermediate compound must
be reduced
Fermentation
 Includes glycolysis
 Also side reactions
that allow for NADH
to be oxidized back
to NAD+ by shuttling
electrons to
intermediate
products such as
ethanol and lactate
Alcoholic Fermentation
 Glycolysis happens
 Pyruvate is then
converted to
acetaldehyde, CO2 is
released
 Acetaldehyde is reduced
by NADH to ethanol
 No additional ATP is
made
 Occurs in yeasts, some
bacteria
Lactic acid fermentation
 Glycolysis happens
 Pyruvate is reduced
by NADH and forms
lactate (lactic acid)
 No CO2 is released
 No additional ATP is
formed
 Done by bacteria,
muscle cells
•Other molecules
can be used in
respiration
•Proteins: must
be deaminated,
then converted to
pyruvate
•Fats: undergo
beta-oxidation
•Cells prefer
carbs