Cellular respiration
Adenosine Triphosphate (ATP)
ATP acts as an immediate source of
chemical energy that is used to
power cellular processes
When ATP is hydrolysed to release the
outermost phosphate, the energy
stored in the phosphate bond is
released to be used by the cell
The presence of adenine and ribose
provides additional sites for
attachment to enzymes (allowing ATP
to fuel enzymatic activities)
ATP is a ribonucleotide consisting of an
adenine base and three phosphate groups
attached to the central ribose sugar
Energy Transfer
When ATP is hydrolysed, the terminal phosphate is released, and it is converted to its
lower energy form - ADP (adenosine diphosphate)
The chemical energy released by ATP hydrolysis is used by an enzyme to catalyse a metabolic
reaction within the cell
Chemical energy from other molecules can then be utilised to reform ATP from ADP and
inorganic phosphate
Using ATP
There are a wide range of biochemical processes that require the use of
ATP as an energy source:
Biosynthesis
The anabolic assembly of organic macromolecules requires ATP hydrolysis
Active transport
ATP is required to move material against a concentration gradient such as the
sodium potassium pump
Vesicular transport (endocytosis / exocytosis) requires ATP to break and reform
membranes
Movement
The movement of cell components or the whole cell is dependent on ATP, such as
moving chromosomes during mitosis and meiosis, and the contraction of muscle
cells
Coenzymes
Coenzymes are non-protein organic
compounds that facilitate enzyme reactions
by cycling between a high energy and low
energy form
ATP is a coenzyme that transfers chemical
energy to enzymes and enables the
activation energy threshold to be reached
(triggering catalysis)
Other important coenzymes are involved in
respiration, such as Acetyl Coenzyme A and
Nicotinamide Adenine Dinucleotide (NAD+)
Cellular Respiration
Cellular respiration primarily uses glucose to
produce ATP
This can be done with and without and
oxygen, and produces energy, carbon dioxide,
and water
Cellular respiration takes place in the cell
cytoplasm and the mitochondria
The general equation for cellular respiration is:
Glucose + Oxygen ➙ Carbon dioxide + Water + Energy
C6H12O6 + 6O2 ➙ 6CO2 + 6H2O + Energy
Anaerobic and Aerobic Respiration
Respiration can occur via two
pathways:
Anaerobic respiration (or fermentation)
which does not require oxygen and occurs
in the cytoplasm
Aerobic respiration which requires oxygen
and the use of the mitochondria
Both pathways begin with glycolysis,
which occurs in the cytoplasm Note: mitochondria have a double membrane
structure, with the inner membrane containing
numerous in-folds called cristae. The central,
fluid filled section is called the matrix.
Anaerobic and Aerobic Respiration
Redox Reactions
As well as using ATP, cells can transfer energy through the
transfer of electrons in what are called oxidation-
reduction reactions.
Oxidation is the chemical process in which a substance loses
electrons
Reduction is the chemical process in which a substance gains
electrons
Because oxidation and reduction reactions occur simultaneously,
they are called redox reactions
In cells, redox reactions usually involve transfer of a hydrogen
atom rather than just an electron
In cellular respiration, redox reactions release the energy
stored in food molecules so that it can be used to
synthesize ATP
Electron Carriers
Hydrogen carriers (or electron carriers) transfer high energy electrons
(plus protons) between molecules via redox reactions, essentially
functioning like chemical delivery systems
Glycolysis
Glycolysis is the first step of cellular
respiration
It converts glucose into pyruvate
It occurs in the cytoplasm and is an
anaerobic process
Glycolysis involves:
Production of ATP
Production of coenzyme energy carriers (NADH)
Production of pyruvate for use in the Krebs cycle
of aerobic respiration
Glycolysis
Note – Glycolysis produces 4 ATP in total, but the first step requires the use of
2 ATP which means there is net production of 2 ATP per glucose molecule
Anaerobic Respiration
All organisms can metabolize glucose
anaerobically (without oxygen) using glycolysis
Fermentation pathways for glucose metabolism
do not use oxygen as a final electron acceptor
The energy yield from fermentation is low, and
few organisms can obtain sufficient energy for
their needs this way
The purpose of anaerobic respiration is to
restore stocks of NAD+ as this molecule is
needed for glycolysis to continue producing ATP
Lactic Acid Fermentation
Glycolysis can continue in the absence of oxygen
by converting pyruvate to lactic acid (also
known as lactate)
It is a reversible process
It does not produce carbon dioxide
Lactic acid can be harmful if not metabolised, and its
production when exercising anaerobically can impact
a muscle cells ability to continue functioning
Examples of organisms that utilise this process include
animals and some bacteria
In animals, it allows energy capacity to exceed oxygen
levels for a short period of time
Alcoholic Fermentation
Glycolysis can also continue in the absence of
oxygen by converting pyruvate to ethanol
There is an intermediate product called acetaldehyde
(also known as ethanal)
It is not a reversible process
It does produce carbon dioxide
As ethanol is harmful, it must be converted to
respiratory intermediates and respired aerobically to
reduce its levels
Examples of organisms that utilise this process include yeasts,
plants, and some bacteria
Fermentation
Anaerobic fermentation has many practical applications:
Lactic acid producing bacteria can modify milk proteins to make certain types of
yogurts and cheese
Fermentation of organic matter by bacteria, yeasts or fungi is part of the
production of biofuels such as bioethanol
Carbon dioxide from yeast fermentation can help proof/rise dough during bread
making
Yeast fermentation produces the ethanol found in beer
Aerobic Respiration
When there is sufficient oxygen present, respiration will occur aerobically
This produces much higher yields of ATP and does not produce any significant by-
products that are problematic for the cells or tissues
It involves further steps after glycolysis that occur in the mitochondria
Link Reaction
In the link reaction, the pyruvate molecules
produced as the end-product of glycolysis
are converted to Acetyl Coenzyme A
(also called Acetyl CoA)
This occurs in the matrix of the mitochondria
During the reaction, a carbon is removed and
forms carbon dioxide
This step does not produce any ATP, but it does
form NADH
Link Reaction
The Krebs Cycle
Acetyl CoA is the feed molecule for the Krebs
cycle, which occurs in the matrix
Acetyl CoA is split, releasing the 2-carbon acetyl
group from the CoA, the acetyl group combines with
a 4-carbon molecule to form a 6-carbon molecule
ATP, NADH, FADH2 and CO2 are formed as the 6-
carbon molecule is converted back to the 4-carbon
molecule in order to restart the cycle
The Krebs cycle yields a small amount of ATP and a lot
of potential energy in the form of NADH and FADH2
Two turns are required to completely oxidize one
glucose molecule as there are two Acetyl CoA molecules
The Krebs Cycle
Note: you are not expected to know the names of any intermediate molecules in the Krebs Cycle
Electron Transport Chain
Hydrogen carriers, such as NADH and FADH2, produced by prior
reactions (glycolysis, link reaction, Krebs cycle) are transported to the
folded mitochondrial cristae (the inner mitochondrial membrane)
The cristae are the locations for the electron transport chains and the
enzyme ATP synthase
Electron Transport Chain
The electron transport chain is a series of membrane proteins that
release the energy stored within the reduced hydrogen carriers in
order to synthesise ATP
This is called oxidative phosphorylation, as the energy to synthesise
ATP is derived from the oxidation of hydrogen carriers
Oxidative phosphorylation occurs over a number of distinct steps:
Step One: Proton pumps create an electrochemical gradient (proton motive
force)
Step Two: ATP synthase uses the subsequent diffusion of protons
(chemiosmosis) to synthesise ATP
Step Three: Oxygen accepts electrons and protons to form water
Electron Transport Chain
Electron Transport Chain
Electron Transport Chain
ATP Production
Unlike oxidative ATP production, substrate
level ATP production uses an enzyme for
direct synthesis and is not coupled to the
oxidation of other molecules such as NADH
Summary of Aerobic Respiration
A theoretical maximum of 38 molecules
of ATP can be produced from one
glucose molecule
4 ATP come from direct phosphorylation
during glycolysis and the Krebs Cycle
34 ATP come from oxidative
phosphorylation in the ETC
Each NADH can power the production of 3
ATP molecules (10 NADH = 30 ATP)
Each FADH2 can power the production of
2 ATP molecules (2 FADH2 = 4 ATP)
Respiration Substrates
Glucose is the most common substrate for respiration, but other molecules
can be utilised:
Fructose is part of the third step of the glycolysis pathway so can easily be utilised
for respiration
Many different amino acids can be converted into molecules involved in the various
steps of respiration
For example, glutamate can be converted into one of the intermediate molecules of the
Krebs Cycle, and several different amino acids can be converted to Acetyl CoA
Fatty acids can be converted to Acetyl CoA via a process called beta-oxidation,
so fat can be digested aerobically and bypass glycolysis
When fats are being used as the primary energy source such as in starvation, exercise,
fasting or untreated diabetes, an excess amount of Acetyl CoA is produced and is
converted into acetone and ketone bodies (which can also be used as fuel)