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Cell Respiration
All living cells, including the cells in our body to the cells of microorganisms, need energy for
all cellular processes. ATP is a special molecule which provides energy in a form that cells
can use for cellular processes. Cellular respiration is the process that cells use to transfer
energy from the organic molecules in food to ATP
The following equation summarizes the chemical changes that occur in cellular respiration
of the monosaccharide glucose when oxygen is available.
C6H12O6 + 6 O2
6 CO2 + 6 H2O + ATP
Cellular respiration involves multiple steps that allow the cell to use the energy from each
glucose molecule efficiently in order to make as many ATP molecules as possible. Cellular
respiration that uses O2 is called aerobic respiration. Most of the time, the cells in our bodies use
aerobic respiration:
We can divide aerobic cellular respiration into three metabolic processes and each of
these occurs in a specific region of the cell.

Glycolysis (occurs in the cytosol)

Krebs cycle (takes place in the matrix of mitochondria)

Oxidative phosphorylation (occurs in the inner mitochondrial membrane)
Glycolysis
Glycolysis literally means "splitting sugars." During glycolysis, the 6-carbon sugar, glucose, is
broken down into two molecules of a 3-carbon molecule called pyruvate. In the process of
glyoclysis, two molecules of ATP, two molecules of pyruvic acid and two "high energy"
electron carrying molecules of NADH are produced. Glycolysis can occur with or without
oxygen.
Krebs Cycle
Krebs Cycle also known as the tricarboxylic acid cycle and the Citric acid Cycle, begins after
the two molecules of the three carbon sugar produced in glycolysis are converted to a
slightly different compound (acetyl CoA). Through a series of intermediate steps, several
compounds capable of storing "high energy" electrons are produced along with two ATP
molecules. These compounds, known as nicotinamide adenine dinucleotide (NAD) and
flavin adenine dinucleotide (FAD), are reduced in the process. These reduced forms carry
the "high energy" electrons to the next stage. The citric acid cycle occurs only when oxygen
is present but it doesn't use oxygen directly.
Electron Transport and Oxidative Phosphorylation
Electron transport also requires oxygen directly. The electron transport chain is a series of
electron carriers in the membrane of the mitochondria in eukaryotic cells. Through a series
of reactions, the "high energy" electrons are passed to oxygen. In the process, a gradient is
formed, and ultimately ATP is produced by oxidative phosphorylation. The enzyme ATP
synthase uses the energy produced by the electron transport chain for the phosphorylation
of ADP to ATP.
When oxygen is not available, cells use anaerobic processes to produce ATP. (The "an" in
front of aerobic means "not aerobic".)
Under anaerobic conditions, many cells use a process called fermentation to make ATP. As
shown in the figure above, there are two types of fermentation:

Lactate fermentation (e.g. in muscles when an animal exercises hard)

Alcoholic fermentation (e.g. by yeast to make bread, wine and beer)
The Process of Alcohol Fermentation
The basic equation for alcohol fermentation shows that yeast starts with glucose, a type of
sugar, and finishes with carbon dioxide and ethanol. However, to better understand the
process, we need to take a look at some of the steps that take us from glucose to the final
products.
The process of alcohol fermentation can be divided into two parts. In the first part, the yeast
breaks down glucose to form 2 pyruvate molecules. This part is known as glycolysis. In the
second part, the 2 pyruvate molecules are converted into 2 carbon dioxide molecules and 2
molecules of ethanol, otherwise known as alcohol. This second part is called fermentation.