Cellular Respiration

ENERGY EXTRACTION
Photosynthesis provides plants and other photosynthetic organisms (autotrophs) with a longterm energy supply that they can draw on even when sunlight is not available. Plants store
carbohydrates in their tissues or in special structures such as tubers in potatoes.
Other organisms (heterotrophs) rely on these energy-rich carbohydrates for their
nourishment. They break down these complex polysaccharides through digestive processes
into a simple compound called glucose whose high-energy bonds can be broken to release
useful energy. The extraction of energy from a molecule of glucose involves complex enzymecontrolled steps that occur in the cell cytoplasm and inside the mitochondrion, the energy
factory of cells.

An overview of glycolysis and cellular respiration

In most organisms, the extraction of energy from glucose involves two processes. The first,
called glycolysis, takes place in the cytoplasm of the cell in the absence of oxygen. The
second process, called aerobic respiration or cellular respiration, occurs inside the
mitochondria in the presence of oxygen. Glycolysis and cellular respiration are catabolic
reactions in which a molecule of glucose is broken down gradually with the aid of enzymes to

yield energy needed to drive the activities of organisms. The overall reaction is represented
by the following formula, which is essentially a reverse of the formula for photosynthesis.

Glycolysis
Glycolysis or glucose splitting is the first step in the catabolic degradation of the glucose
molecule to release energy contained in its bonds. In nine enzyme-catalyzed reactions, one
molecule of glucose is split into two molecules of a 3-carbon compound called pyruvic acid.
The process produces two molecules of ATP by substrate-level phosphorylation ( substratelevel phosphorylation is the production of ATP using energy from other high-energy compounds
but without the use of the electron transport system in the mitochondria) and reduces two
molecules of nicotinamide adenine dinucleotide (NAD+) to two molecules of nicotinamide
adenine dinucleotide hydrogen (NADH). Both ATP and NADH are rich-energy storage
molecules. The cell can use the energy in ATP immediately but not that of NADH because
electrons from NADH must first pass down the respiratory transport electron chain before
they can yield ATP, the energy currency of cells.

(Energy yield from glycolysis: 2ATP and 2NADH).

Fermentation
The fate of pyruvic acid depends on the environment of the organisms in which the reactions
are taking place. In organisms that live in environments that lack oxygen, called anaerobes,
such as yeast and some chemoautotrophic bacteria that dwell in deep-sea thermal vents, the
pyruvic acid is broken down to alcohol with the release of CO2. This is known as alcoholic
fermentation. Other anaerobic bacteria and muscle cells deprived of oxygen especially after a
strenuous exercise, covert pyruvic acid to lactic acid in another fermentative process called
lactic acid fermentation. Fermentation is not a very effective way of extracting energy.

Aerobic Respiration
The fact that glycolysis exists today in all organisms, supports the view that it was probably
the first process by which the original inhabitants of the earth used to extract useful energy.
Glycolysis must have evolved at a time when there was not much oxygen in the atmosphere.
Aerobic respiration became important in energy extraction from carbohydrates after
photosynthetic plants had appeared.
Although glycolysis occurs in all organisms, it releases only a fraction of the total energy
contained in glucose. The rest of the energy remains trapped in the two molecules of pyruvic
acid. Aerobic respiration is by far a more efficient method of dismantling the bonds of glucose
than glycolysis.

Formation of Acetyl-coA
Glycolysis ends where the formation of acetylCoA starts. In the presence of oxygen, the
pyruvic acid produced in glycolysis enters the 2mitochondrion and once inside each molecule
of pyruvic acid loses a carbon dioxide, converts NAD+ to NADH and attaches an enzyme called
coenzyme A to form a new molecule called acetyl CoA.

Formation of acetylCoA

Acetyl CoA is a 2-carbon molecule important in metabolism. In addition to being produced

from pyruvic acid, it is part of the catabolic pathways of fats and amino acids. The formation
of acetyl CoA is a bridge between glycolysis and aerobic or respiration.
Briefly, let us recap what has happened so far. One molecule of glucose has been converted to
two molecules of pyruvic acid, each of which has been converted to a molecule of acetyl CoA
and two molecules of CO2 have been released.

The Krebs cycle

Acetyl CoA now enters another pathway, the Krebs cycle, also known as the citric acid cycle.
Energy is released as hydrogen atoms and CO2 are removed from the intermediates of the
Krebs cycle. The Krebs cycle results in the production of 1 ATP molecule and 3 NADH
molecules. I molecule of another electron carrier, flavin adenine dinucleotide (FAD), is
reduced to FADH2. However, the energy from each glucose molecule is twice this because
each glucose molecule sends 2 molecules of acetyl CoA through the Krebs cycle.

The Krebs cycle

(Energy yield of Krebs cycle: 2ATP, 6NADH, 2FADH2).

The Respiratory Electron Transport Chain

As is now familiar to us, glucose is an energy rich compound and its

breakdown releases energy that is stored as ATP, which is the energy currency of cells. Our
examination of the first three stages of glucose catabolism shows a net gain of only 4 ATP
molecules (2 in glycolysis and 2 in Krebs cycle). These represent a fraction of the energy
originally present in a molecule of glucose. Of the remainder, much is liberated during the
first 3 stages (mostly as heat, which is necessary for the reaction to proceed); the rest is
stored in the high-energy intermediaries, NADH and FADH.
In the final stage of extraction, the energy contained in NADH and FADH2 is used to convert
ADP to ATP. This takes place through a series of electron accepting enzymes that are
embedded in the inner mitochondrial membrane forming a respiratory chain. Electrons are
transferred from NADH and FADH2 to electron carrier molecules in the respiratory chain,
losing energy as they move from one carrier molecule to the next in the chain. Some of the
released energy is captured to make ATP. Each molecule of NADH is used to convert 3
molecules of ADP to ATP, while the energy from 1 molecule of FADH 2 is used to convert 2
molecules of ADP to ATP. Oxygen, the final acceptor molecule of the respiratory chain,
combines with hydrogen to form water.
Summary of the Energy generated from 1 molecule of glucose
Cellular respiration

location

step
Glycolysis

Number of ATP

Function

produced
cytoplasm

4 gross, net 2 ATP

Begins catabolism of

glucose; reduces
glucose to ATP &
NADH
Krebs cycle

matrix

2 ATP

Completes the
degradation of
glucose; converts
glucose energy to
ATP, NADH and FADH2

Electron transport

cristae

32 ATP

Converts energy
stored in NADH and
FADH2 into ATP

The ATP output from the Krebs cycle is therefore 2+18+4, or 24. A total ATP accounting (6
from glycolysis, 6 from acetyl CoA production, and 24 from the Krebs cycle) indicates that 36
ATP are generated from the complete breakdown of 1 glucose molecule.
Yeast is used in bread baking and in wine making. When it is added to bread dough, yeast is in an aerobic environment. As the yeast

cells grow and multiply, they use up oxygen and give off carbon dioxide. The carbon dioxide bubbles cause the bread to rise. When the
bread is baked, the heat kills yeast cells.
In wine making, yeast is added to solution in sealed vats. The yeast cells then consume all the available oxygen as they multiply making
the environment in the vats increasingly anaerobic. The yeast cells then turn to the sugars in the wine solution for energy. The sugars are
broken down to yield energy, carbon dioxide and alcohol. This type of energy pathway is called alcoholic fermentation. The yeast cells
are killed when the alcohol content of the wine reaches 12-16 % due poisoning by their own wastes.
Some anaerobic bacteria and animal cells that are temporarily deprived of oxygen, convert the pyruvic acid of glycolysis to a threecarbon compound called lactic acid. Lactic acid fermentation, as this pathway is called, occurs in human muscle cells when they are
exercising so strenuously that the consumption of pyruvic acid exceeds their supply of oxygen. In this condition of oxygen debt, the cells
revert to anaerobic respiration causing an accumulation of lactic acid in the muscles. This lowers the pH and causes muscle fatigue and
soreness. When the oxygen supply is restored, lactic acid is converted to pyruvic acid by the liver.
The mitochondrion
Mitochondria are rod-shaped structures that are enclosed within two membranes - the outer membrane and the inner membrane. The
outer membrane is smooth and completely permeable to nutrient molecules, ions, ATP and ADP. The inner membrane is more complex
than the outer membrane as it contains the complexes of electron transport chain and ATP synthetase complex. The mitochondrion is
permeable to oxygen, CO2 and water and consists of a large number of proteins that take part in the production of ATP. The inner
membrane has infoldings called the cristae that increase the surface area for the complexes and proteins that aid in the production of
ATP. The matrix is a complex mixture of enzymes that are important for synthesis of ATP molecules,
2

After glycolysis occurs, the two pyruvic acid molecules enter the mitochondria and diffuse to the matrix where the remaining hydrogen
atoms and their energy rich electron are removed. One turn of the Krebs cycle yields one ATP, one FADH2 and 3 NADH with the release
of 2 CO2 . There are 2 molecules of pyruvic acid. Therefore, there are 2 turns of the Krebs cycle, yielding 2 ATP, 2 FADH 2, and 6 NADH with
4 molecules of carbon dioxide released.
The FADH2 and 3 NADH proceed to the cristae where they provide energy for Electron Transport (also known as oxidative
phosphorylation).The energy provided by all the NADH and FADH 2 is used to pump H ions into the outer compartment. This creates a
charge imbalance across the cristae (high potential energy) and as H ions diffuse back to the matrix, they pass through a protein channel
with an enzyme, ATP synthase, which takes the energy released by the H ions and uses it to create ATP (from ADP + P). Oxygen bonds with
2 hydrogen ions (removing them so aerobic respiration can continue) to form water.