|Lecture Notes: 24 January
© R. Paselk 2006
Cells and Organelles, cont.
Compartmentation in Eukaryotes
As mentioned earlier we will be focusing on eukaryotes in the rest of this course. Eukaryotes differ from prokaryotes in having a nucleus and cell organelles (their cells are physically compartmentalized). As a point of reference, an E. coli cell is about the size of a typical mammalian mitochondria.
Let's look at where different major metabolic pathways occur in a "typical" liver cell. [overhead-Animal cell]
public domain image via Wikipedia Creative Commons
- Nucleolus: localized region of the nucleus in which ribosomal RNA's are synthesized and processed.
- Nucleus: DNA replication, synthesis and processing of messenger RNA's.
- Ribosome: Ribonucleoprotein machines for translating RNA information into proteins.
- Vesicle:small membrane enclosed vessels used in transporting proteins between different organelles (e.g. sER and Golgi etc.)
- Rough Endoplasmic Reticulum: Biosynthesis and modification of membrane and export proteins.
- Golgi Complex: Further modification of membrane and export proteins.
- Cytoskeleton: multi-subunit fibrous proteins such as actin and microtubulin providing support for cell shape
- Smooth Endoplasmic Reticulum: Lipid Synthesis; Steroid synthesis; Phase one detoxification reactions.
- Mitochondria: Kreb's Citric Acid Cycle; Electron transport system and Oxidative Phosphorylation; Fatty acid oxidation; Amino acid catabolism; Interconversion of carbon skeletons.
- Lysosomes: Hydrolytic (digestive) enzyme localization.
- Cytosol: Glycolysis and most of gluconeogenesis; Pentose Phosphate shunt; Fatty acid biosynthesis.
- Peroxisomes: Amino acid oxidases, catalase-oxidative degradation reactions.
- Centrioles within Centrosome: Centrioles are barrel-shaped structures composed of nine microtular triplet-strands. A pair of associated centrioles in amorphous stuff makes up the centrosome which is involved in organizing the mitotic spindle in cell division in animal and most fungal cells (they are not present in plants). It is also essential for flagella and celia formation.
- Plasma Membrane: Active and passive transport systems; receptors and signal processing systems (synthesis of various second messengers etc.).
Not shown - Glycogen Granules: enzymes of glycogen synthesis and
breakdown. including branching and debranching.
Let's look at a "typical" plant cell for a
moment. All of the organelles we saw in animals are here as well,
but with a few additions:
- Chloroplasts: Light capturing processes and electron
transport & oxidative phosphorylation for photosynthesis;
Calvin cycle (dark reactions of photosynthesis).
- Glyoxisomes: location of glyoxalate cycle.
- Cell wall: made up of cellulose glued together with
lignin (a plastic like polymer) - maintains cell integrity against
high osmotic pressure, gives cell rigidity.
- Vacuole: storage of dilute aqueous solutions, provides
fluid for osmotic pressure.
Introduction to Vitamins and Cofactors
- Cofactor: a co-catalyst required for enzyme activity.
- coenzyme - a dissociable cofactor.
- prosthetic group - non-dissociable cofactor.
- metal ion - a metal cofactor.
- Vitamin: a required micro-nutrient (organism cannot synthesize adequate quantities for normal health - may vary during life-cycle).
- water soluble - not stored, generally no problem with overdose.
- lipid soluble - stored, often toxic with overdose.
Let's start with a brief overview of the names, structures and physiological functions of the common vitamins as shown on the overhead: Niacin, Riboflavin [B2], Thiamine, Pantothenic acid, Biotin, Pyridoxal [B6], Folic acid, Lipoic acid, Cobalamin [B12], L-Ascorbic acid [C], and the lipid soluble vitamins A, D, E, and K). Fun facts:
- Niacin deficiency gives rise to pellagra (swollen tongue, dermatitis, neurologic and gastro-intestinal disturbances).
- Thiamine deficiency gives rise to berberi (extensive damage to nervous and circulatory systems, muscle wasting and edema - once common in Asia due to the comsumption of polished rice, which is low in nutrients, as a staple.).
- Folate and Cobalamin deficiency both give rise to megaloblastic anemia, however cobalamin deficiency also results in irreversible CNS damage.
- In contrast to most water soluble vitamins, the lipid soluble vitamins can be taken in excess, with toxic results:
- Excess vitamin D (actually a hormone) causes excess Ca2+ buildup with precipitation to form kidney stones and precipitates in tissues. Deficiency results in rickets.
Note the relationships of the various cofactors to their vitamin precursors. Chapter 7 of your text has images of the various cofactors and vitamins, as well as example chemistries. You should keep it in mind to refer back too during our studies.
Nucleotide Functions: Most involve use of the nucleotide as a recognition molecule, e.g. ATP
Nicotinamide Adenine Dinucleotide (NAD+) uses ADP (bolded) as a recognition "handle." (Note the two nitrogenous bases each attached to a ribose and linked through a phosphoric acid anhydride linkage:
Similarly adenosine with a modified ribose (reduced to the alcohol - ribitol) is used in Flavin Adenine Dinucleotide = FAD (not truly a dinucleotide since ribitol instead of ribose!):
Coenzyme A has an ADP attached to an arm of pantothenic acid, which in turn is attached to beta-mercaptoethylamine. Acetyl groups can be carried on the sulfhydryl group:
Three vitamins give cofactors with long "arms" which enable the cofactors to shift an attached substrate between adjacent active sites on a single enzyme. (Note attachment of biotin and lipoate to lysine side-chain to give 10 atom arms.)
Last modified 27 January 2010