Metabolism

Metabolism refers to all the chemical reactions that take place in a cell.
Anabolism - the building up of large molecules from smaller molecules. Usually stores energy (in chemical bonds)

Catabolism- the breaking down of large molecules into smaller molecules, usually with the release of energy.

Cellular Respiration in a nutshell:

C₆H₁₂O₆ +   6 O₂ → 6CO₂      +          6H₂O
glucose   + oxygen → carbon dioxide + water
(reactants)                        (products)

Cellular respiration can be broken down into four parts:

    1)Glycolysis
    2) formation of acetyl - CoA
    3) the citric acid or Krebs cycle
    4) oxidative phosphorylation or the electron transport chain

Oxidation-Reduction (Redox) reactions
Oxidation is the removal of electrons from a molecule and results in a decrease in the energy content of the molecule.

Reduction is the addition of electrons to a molecule, and results in an increase in the energy content of that molecule.

Within a cell, oxidation and reduction reactions are always coupled; that is, whenever one substance is oxidized, another is almost simultaneously reduced.

Coenzymes: - accept electrons given off by other reactions.
NAD+ is reduced to NADH by accepting two electrons
(and a hydrogen ion)

FAD is reduced to FADH by accepting two electrons
(and two hydrogen ions)

Glycolysis must renew the electron receiving coenzymes in order to continue producing ATP. In the absence of oxygen, the electrons are transferred to the end product of glycolysis, pyruvic acid, to form lactic acid. (Remember the tennis balls on the stairs)

This is said to be a state of "Oxygen debt" because oxygen is needed to convert the lactic acid back into pyruvic acid so that it may enter the citric acid cycle.

Pyruvic acid is then broken down to a 2 carbon acetyl group and is combined with Coenzyme A to form acetyl-CoA. This generates 2 NADH.

Acetyl-CoA enters the citric acid cycle, and the acetyl group is added to a 4 carbon compound to make citric acid, which has 6 carbons. During the citric acid cycle, some NADH, and FADH₂ are formed, and 2 CO₂ and 2 ATP are produced. Oxaloacetic acid is regenerated to accept more acetyl groups.

The electron transport chain or oxidative phosphorylation, is composed of several iron containing proteins called cytochromes. Electrons release energy each time they are passed from one cytochrome to another. This energy is used to transport H⁺ across the membrane of the mitochondrion into a region of high H⁺ concentration. This forms a combined chemical and electrical gradient. H⁺s are allowed to fall down this gradient through special channels in the membrane, and the energy released is captured by an enzyme (ATP synthetase) which adds a phosphate group to ADP. This process is called Chemiosmosis.

Sugars, lipids, proteins and nucleotides can all enter these pathways and be broken down for energy.

Protein Synthesis

A segment of DNA that carries the instructions for making a protein is called a gene.

This information is stored in the DNA in three-base units called codons.

Transcription is the process of transferring the information in DNA to a molecule of messenger RNA (mRNA).

RNA polymerase- enzyme which forms mRNA

Pre-mRNA contains :

Expressed sequences or Exons - RNA which codes for protein

Intervening sequences or Introns - "Junk" RNA

Exons are spliced together to form mature mRNA

Ribosomes:

1) act as readers of the mRNA

2) catalyze the formation of peptide bonds

Transfer RNA or tRNA binds a specific amino acid and has a specific anticodon.

Initiation - mRNA binds to the ribosome

Elongation - the next amino acid is put in position

- a peptide bond is formed

- the ribosome moves to the next codon

Termination - the polypeptide is cut from the last tRNA when a stop codon is read.

Polysome - several ribosomes translating the same mRNA at the same time.

Transcription factors are proteins which control the rate at which mRNA is copied from DNA.