•Cellular respiration is
the process of breaking sugar into a form that the cell can use as energy. This
happens in all forms of life. Cellular respiration takes in food and uses it to
create ATP (Adenosine
triphosphate), a chemical which
the cell uses for energy.
•It
has
four stages known as glycolysis, Link reaction,
the Krebs cycle, and
the electron
transport chain.
This produces ATP which
supplies the energy that cells need to do work.
•When
they don't get enough oxygen, the cells use anaerobic
respiration,
which doesn’t require oxygen. However, this process produces lactic acid, and
is not as efficient as when oxygen is used.
•Aerobic
respiration, produces
much
more energy and doesn’t produce lactic acid. It also produces carbon dioxide as
a waste product, which then enters the circulatory system. The
carbon dioxide is taken to the lungs, where it is exchanged for oxygen.
The
simplified
formula for aerobic cellular respiration is:
The
word equation for this is:
•Aerobic
cellular
respiration has four stages. Each is important, and could not happen without
the one before it. The steps of aerobic cellular respiration are:
Glycolysis
•In
glycolysis,
glucose in the cytoplasm is broken into two molecules of pyruvate.
Ten enzymes are
needed for the ten intermediate compounds in this process.
•At
the end are two pyruvate molecules, plus
•Four
molecules of ATP are made and two NADH molecules.
Both types are energy-rich and used in other cell reactions.
•In
cells which use oxygen, the
pyruvate is used in a second process, the Krebs cycle,
which produces more ATP molecules.
•Biology
textbooks
often state that 38 ATP molecules can be made per oxidised glucose molecule
during cellular respiration (two from glycolysis, two from the Krebs cycle, and
about 34 from the electron transport chain).[1] However,
this maximum yield is never quite reached, mainly because of losses through
leaky membranes.
Estimates are 29 to 30 ATP per glucose.[1]
•Aerobic
metabolism is about 15
times
more efficient than anaerobic metabolism. Anaerobic metabolism yields 2 mol ATP
per 1 mol
glucose. They share the initial pathway of glycolysis but
aerobic metabolism continues with the Krebs cycle and oxidative
phosphorylation. The post glycolytic reactions take place in the mitochondria
in eukaryotic cells, and
in the cytoplasm in prokaryotic cells.
Pyruvate from glycolysis is actively pumped into mitochondria. One carbon dioxide molecule and one hydrogen molecule are removed from the pyruvate (called oxidative decarboxylation) to produce an acetyl group, which joins to an enzyme called CoA to form acetyl CoA. This is essential for the Krebs cycle
Krebs
cycle.
Acetyl CoA joins with oxaloacetate to
form a compound with six carbon atoms.
This is the first step in the ever-repeating Krebs cycle. Because two
acetyl-CoA molecules are produced from each glucose molecule, two
cycles are required per glucose molecule. Therefore, at the end of two cycles,
the products are: two ATP, six NADH, two FADH, and four CO2. The ATP is a
molecule which carries energy in chemical form to be used in other cell
processes.
Electron
transport chain
This is where most of the ATP is made.
All of the hydrogen molecules which have been removed in the steps before
(Krebs cycle, Link reaction) are pumped inside the mitochondria using energy
that electrons release. Eventually, the electrons powering the pumping of
hydrogen into the mitochondria mix with some hydrogen and oxygen to form water
and the hydrogen molecules stop being pumped.
Eventually, the hydrogen flows back into
the cytoplasm of the mitochondria through protein
channels.
As the hydrogen flows, ATP is made from ADP and
phosphate ions.
•Anaerobic respiration is
a form of respiration which
does not use oxygen.
Elements other than oxygen are
used for electron transport.
Common replacements for oxygen are nitrates, iron, manganese, sulfates, sulfur, fumaric acid and carbon dioxide. Escherichia coli uses
nitrates and fumaric acid
for respiration.
•For
the electron
transport chain to
work, there must be a final electron acceptor at the end of the chain.[1]This
allows electrons to pass through the chain. In aerobic organisms, this final
electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and
so it is an excellent acceptor. In anaerobes, other less-oxidizing substances
such as sulphate (SO42−),
nitrate (NO3−),
sulphur (S) are used. These terminal electron acceptors have smaller reduction
potentials than O2, so less energy is released per oxidized
molecule.[2]Anaerobic
respiration is therefore less efficient than aerobic respiration except, of
course, when oxygen pressure is low.
•If
oxygen is not used at all, the process is called fermentation.
Examples of organisms using
fermentation are lactic acid bacteria,
and yeast.
Yeast is a fungi, not
bacteria.
The
equation
for anaerobic respiration is:
C6H12O6 -> 2C3H6O3
in
word equation: glucose ->
lactic acid
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