Metabolism is the sum of all chemical reactions within an
organism.
The chemical processes in a living cell or organism.
In metabolism some substances are broken down to yield
energy while others are synthesized which requires energy.
All metabolisms have some common features. These features
include a requirement for the nutrient to get into the cell and
that the nutrient be available to be metabolized.
The complementary process of catabolism and anabolism create
energy and building materials. This energy and building
materials are used in biosynthesis, assemble, polymerization and
general cell survival.
The process of catabolism and anabolism may utilize
different pathways depending on the cell, nutrient and current
environmental pressures the cell is currently experiencing.
Energy Production
Nutrient molecules have energy electrons associated with
their chemical bonds.
Catabolic processes oxidize the electrons (remove electrons)
and store them in other accessible high energy bonds like ATP.
ATP (adenosine triphosphate) is an energy form that is
retrievable by the cell.
ATP and ADP (adenosine diphosphate) have high energy
phosphate bonds with a negative fee energy of around 7 Kcal per
mole.
Oxidation and reduction reactions are couples (redox).
Oxidation is the removal of electrons from a molecule.
Reduction is the acquisition of electrons.
Oxidation reaction release energy. In the cell oxidation
usually is accompanied by the loss of hydrogen atoms from a
complex substrate molecule in the process electrons are usually
lost as well. When a molecule is oxidized it gives up energy.
Reduction reactions grab or trap chemical energy. The
substrate or complex bimolecular gains electrons and or
hydrogens. A molecule that has been reduced (e.g. accepted an
electron) is energy rich.
Substrate level phosphorylation is the transfer of phosphate
to ADP from another phosphorylated organic compound.
Oxidative phosphorylation is the process in which energy
from redox reactions of respiration is used to attach an
inorganic phosphate to ADP.
Photophosphorylation is the process in which light energy is
used to phosphorylate ADP with inorganic phosphate.
Nicotinamide adenine dinucleotide (NAD+) and nicotinamide
adenine dinucleotide phosphate (NADP+) are cofactors in cells
and act as electron carriers. NAD+ is reduced to NADH and NADPH
is reduced to NADP+.
Flavin adenine dinucleotide FADH and FADH2 are reduced forms
of FAD. FADH2 is produced in the citric acid cycle.
Metabolic Cycles
Glycolysis: a single molecule of glucose yields two
molecules of pyruvic acid (pyruvate). There are 4 ATPs formed
and two used in the cycle. Therefore there is a net of 2 ATPs.
Two molecules of NAD+ are reduced to NADH.
Krebs Cycle: also known as TCA Cycle (tricarboxylic
acid cycle). Pyruvic acid is decarboxylated to acetate. The
acetate is attached to coenzyme A in the process a molecule of
NAD+ is reduced to NADH. For every two molecules of acetyl-CoA
in the Krebs cycle two molecules of ATP are generated. In redox
reactions for every two molecules of acetyl-CoA entering Krebs
cycle six molecules of NADH and two of FADH2 are made. The NADH
and FADH2 are the most important products of the Krebs cycle.
Electron Transport: The largest amounts of ATP
produced are in the Electron Transport chain. This is a series
of membrane bound carrier molecules that pass or transfer
electrons from one to the next and ultimately to a final
electron acceptor. The electrons� energy is used to pump
protons across the membrane. Electron transport chain in
eukaryotes are in the inner mitochondrial membranes in
prokaryotes it is in the cytoplasmic membrane. Electron carrier
molecules are diverse and in prokaryotes can change depending on
environmental factors. These carriers include: flavoproteins,
ubiquinones, metal containing proteins and cytochomes.
Electrons carried by NADH enter the chain at a flavoprotein,
FADH2 are introduced by a ubiquinone. Final electron acceptors
include oxygen atoms which can make H2O upon the addition of
hydrogen ions. The later is aerobic respiration. In anaerobic
respiration use other inorganic molecules (rarely organic
molecules) instead of oxygen as the final electron acceptor. In
aerobic electron transport chain 34 ATP are produced.
Fermentation: the partial oxidation of metabolites
releasing energy using an organic molecule as an electron
acceptor instead of the electron transport chain. Fermentation
oxidizes NADH to NAD+ while reducing organic molecules which are
the final electron acceptors.
Anabolic Processes: are reactions that synthesize or
make something and require energy. The energy is provided by
ATP. There are many anabolic pathways that are catabolic
pathways running in revere. Example of anabolic process is the
production of cell membrane and cell wall components such as
cellulose and peptidoglycan.
