Respiration
Glycolysis
Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms
and does not require oxygen. The process converts one molecule of glucose into two
molecules of pyruvate, and makes energy in the form of two net molecules of ATP.
Glucose → 2 pyruvate + 2 ATP
In glycolysis glucose is partially oxidized and broken down into two 3 carbon molecules called
pyruvate or pyruvic acid. In the process, glycolysis produced 4 ATP for a net gain of two ATP
and two molecules of NADH. Each NADH is carrying two energy rich electrons away from the
glucose and these electrons can be used by the cell to do work. Note that glycolysis itself is
anaerobic, in that oxygen is not required.
In eukaryotic organisms aerobic respiration is compartmentalized. Glycolysis takes place in the
cytoplasm and the Kreb's cycle and electron transport taking place in the mitochondrion.
Oxidative decarboxylation of pyruvate
Produces acetyl-CoA from pyruvate inside the mitochondrial matrix. This oxidation reaction
also releases carbon dioxide as a product. In the process one molecule of NADH is formed per
pyruvate oxidized.
Krebs cycle/Citric Acid cycle
When oxygen is present, acetyl-CoA enters the citric acid cycle inside the mitochondrial
matrix, and gets oxidized to CO2 while at the same time reducing NAD to NADH. NADH can
be used by the electron transport chain to create further ATP as part of oxidative
phosphorylation. To fully oxidize the equivalent of one glucose molecule two acetyl-CoA must
be metabolized by the Krebs cycle. Two waste products, H2O and CO2 are created during
this cycle.
The Citric Acid Cycle The Kreb's cycle produces 8 more NADH molecules and two molecules
of FADH2. Again both of these are carrying energy rich electrons.
Each of the 3 carbon atoms present in the pyruvate that entered the mitochondrion leaves as a
molecule of carbon dioxide (CO2).
At 4 steps, a pair of electrons (2e-) is removed and transferred to NAD+ reducing it to NADH +
H+.
At one step, a pair of electrons is removed from succinic acid and reduces FAD to FADH2.
Oxidative phosphorylation
In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It
comprises the electron transport chain that establishes a proton gradient across the
inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is
synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to
drive the phosphorylation of ADP.
The electrons of NADH and FADH2 are transferred to the electron transport chain.
The electron transport chain accomplishes:
the stepwise transfer of electrons from NADH (and FADH2) to oxygen molecules to form (with
the aid of protons) water molecules (H2O);
Mitochondria are important cell organelles as they are involved in the electron transport chain
forming water and 32 ATP
The electron transport chain is a system of electron carriers embedded into the inner membrane
of a mitochondrion. As electrons are passed from one compound to the next in the chain, their
energy is harvested and stored by forming ATP. For each molecule of NADH which puts its two
electrons in, approximately three molecules of ATP are formed, and for each molecule of
FADH2, about two molecules of ATP are formed.
1. Glycolysis occurs in the cytosol.
2. The Krebs cycle takes place in the matrix of the mitochondria.
3. Oxidative phosphorylation via the electon transport chain is carried out on the inner
mitochondrial membrane.
In the absence of oxygen, respiration consists of two metabolic pathways: glycolysis and
fermentation. Both of these occur in the cytosol.
In glycolysis, the 6-carbon sugar, glucose, is broken down into two molecules of a 3-carbon
molecule called pyruvate. This change is accompanied by a net gain of 2 ATP molecules and 2
NADH molecules
Krebs Cycle
The Krebs cycle occurs in the mitochondrial matrix and generates a pool of chemical energy
(ATP, NADH, and FADH2) from the oxidation of pyruvate, the end product of glycolysis.
Pyruvate is transported into the mitochondria and loses carbon dioxide to form acetyl-CoA, a
2-carbon molecule. When acetyl-CoA is oxidized to carbon dioxide in the Krebs cycle,
chemical energy is released and captured in the form of NADH, FADH2, and ATP.
Oxidative Phosphorylation via the Electron Transport
Chain
The electron transport chain allows the release of the
large amount of chemical energy stored in reduced NAD+
(NADH) and reduced FAD (FADH2). The energy
released is captured in the form of ATP (3 ATP per
NADH and 2 ATP per FADH2).
NADH + H+ + 3 ADP + 3 Pi + 1/2 O2 → NAD+ + H2O + 3 ATP
FADH2 + 2 ADP + 2 Pi + 1/2 O2 → FAD+ + H2O + 2 ATP
The electron transport chain (ETC) consists of a series of molecules, mostly proteins, embedded
in the inner mitochondrial membrane.
Fermentation
All cells are able to synthesize ATP via the process of glycolysis. In many cells, if oxygen is not
present, pyruvate is metabolized in a process called fermentation.
Fermentation complements glycolysis and makes it possible for ATP to be continually produced
in the absence of oxygen. By oxidizing the NADH produced in glycolysis, fermentation
regenerates NAD+, which can take part in glycolysis once again to produce more ATP. Each
molecule of glucose can generate 36-38 molecules of ATP in aerobic respiration but only 2 ATP
molecules in respiration without oxygen (through glycolysis and fermentation).
http://www.phschool.com/science/biology_place/biocoach/cellresp/glucose.html