RESPIRATION
Cellular Respiration is the process by which cells obtain ATP (energy) from the
substrate glucose using enzymes [CoA], (although fats and proteins can also be
used as a substrate for the enzymatic process). Respiration is important
because ATP is required in muscle contraction, DNA replication, vesicle
transport, cell signalling, protein synthesis and active transport. It involves
oxidation and reduction – a redox reaction.
There are two different types of respiration: Aerobic and Anaerobic.
Aerobic Respiration is the process of decomposing glucose using enzymes.
There are three steps: Glycolysis, the Krebs cycle, the Electron Transport Chain
and chemiosmosis.
Glycolysis is the splitting or breakdown of glucose. It occurs in the cytoplasm of
a cell and the glucose molecule (C₆H₁₂O₆) splits into 2 molecules of pyruvic acid
(C₃H₄O₃) through ten steps. This occurs anaerobically because it occurs
whether or not there are O₂ molecules present or not (apart from those in the
glucose). There is a net gain of 2 ATP molecules per glucose molecule that has
undergone glycolysis. The extra hydrogen removed join the hydrogen carrier
NAD making NADH. Therefore there is a net yield of 2 NADH molecules.
Although some ATP is used for the reaction, there is an overall net gain of 2
pyruvate molecules, 2 ATP molecules and 2 NADH or redNAD molecules.
2 NAD⁺ 2 NADH
(Glucose)
C₆H₁₂O₆ C₃H₄O₃ (Pyruvate)
2 ATP 2 ADP 2 ADP 2 ATP 2 ADP 2 ATP
Next there is a sub-step which is the formation of Acetyl coenzyme A or, the
Link reaction. This is when the pyruvate enters the mitochondrion via active
transport and is converted into Acetyl CoA. This process gains 2 NADH
molecules.
Pyruvic Acid → Acetic Acid + CO₂ + NADH⁺
Acetic Acid + coenzyme A → Acetyl CoA
The Krebs cycle releases the energy in the form of ATP.
Mitochondria
Pyruvate Acetyl CoA Krebs Cycle
CO₂
Acetyl CoA
4C CoA
NADH
6C
NAD⁺ CO₂
NAD⁺
FADH₂ NADH
FAD⁺ 5C
CO₂
ATP
4C NAD⁺
ADP + P₁
NADH
The Krebs cycle occurs in the mitochondrial matrix and starts when Acetyl CoA
(2 Carbon molecule) joins with a 4 Carbon molecule, Oxalo Acetic Acid (OAA),
to form Citric Acid (6 Carbon molecule). In the presence of O₂ (aerobic phase),
the Acetyl CoA is stripped of all the H₂ ions to gain electrons (reduction) for the
production of ATP. The remaining protons, or H⁺ ions are absorbed into 2
compounds: NAD → NADH and FAD → FADH₂. Each citrate molecule that goes
through the series of reactions that is the Krebs cycle produces 1 CO₂
molecule, 1 ATP molecule, 1 FADH₂ molecule, 3 NADH molecules and a
regenerated OAA molecule.
The final step which releases ATP is the Electron Transport Chain and
Chemiosmosis. This is when oxidative phosphorylation takes place and it
occurs in the folded membranes of the mitochondria known as the cristae in
aerobic conditions. All the leftover protons or hydrogen ions produced from
the Krebs cycle and glycolysis are transported to the cytochrome system which
consists of a series of electron carrying enzymes in the inner mitochondrial
membrane. The protons are kept in the inter-membrane space. The protons
flow through ATP synthase enzyme molecules and are carried by mobile
cytochrome C carrier and Quinone carriers (Quinone also carries protons). The
NADH molecules are the electron donors and the O₂ molecules are the
electron acceptors which when added to the protons that reduced the oxygen
molecules make water (H₂O).
2 H⁺ + 2 e⁻ + ½ O₂ → H₂O
The transfer of an electron from one protein to another releases energy and
60% of this energy is released as heat whereas the remainder 40% is used to
make ATP (usually 3 ATP per NADH molecule). Also the osmotic gradient allows
ATP synthase to use its energy to produce ATP. The total yield of ATP in this
process of Electron Transport Chain is 32 ATP per glucose molecule. This is
added to the two made in glycolysis and the two made in the Krebs cycle
making a total yield of 36 ATP molecules per glucose molecule.
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + 36 ATP (energy)
Anaerobic Respiration, also known as fermentation, occurs when there is not
enough O₂ for aerobic respiration to occur. It is the breakdown of glucose in
the absence of O₂. It is common in most organisms however in some organisms
such as eukaryotes (bacteria) and fungi, it is used exclusively because these
organisms do not have the necessary enzymes to use oxygen. The enzymes for
this process are kept in the cytoplasm which is also the site of the reaction.
There are two outcomes for the reaction depending on which organism is
respiring. There are two types of organisms that use anaerobic respiration;
obligate anaerobes like yeast that cannot produce ATP any other way and
facultative anaerobes that use it as an alternative in the absence of O₂. The
outcome products are Lactic Acid or Ethanol (Alcohol) and CO₂.
C₆H₁₂O₆ → 2 CO₂ + 2 CH₃CH₂OH + 2 ATP (Alcoholic Fermentation)
C₆H₁₂O₆ → 2 CH₃CHOHCOOH + 2 ATP (Lactic Acid Fermentation)
The Lactic Acid outcome occurs in bacteria and humans (during muscle
fatigue). The bacteria that respire anaerobically help make cheese, yoghurt and
buttermilk.
The Alcohol and CO₂ outcome occurs in fungi, for example, yeast and there
anaerobic respiration is used in bakery and brewery.
Lactic Acid fermentation uses the following pathway:
Glucose → Pyruvate → Lactate
In anaerobic respiration, glycolysis proceeds as normal in anaerobic conditions
and the biochemical reaction catalysed by enzymes refers to the conversion of
sugars to alcohol or acid due to the excess of H⁺ ions. Two pyruvate molecules
are reduced and NAD⁺ ions required for the initiation of glycolysis is recycled
and two ATP molecules are produced. Cells do not deplete their store NAD,
they produce 2 ATP molecules worth of energy and as a by-product of
fermentation, Ethanol or Lactic Acid is produced as a waste product.
