Question 1

List and provide a brief discussion of the seven categories of toxic responses that a biological system may have.
. There are seven categories of toxic responses:
• Acute toxicity. This category is related to the immediate toxic responses that every biological system may have. This exposure of toxic substances for shorter duration and the diverse effect is high
• Sub-chronic toxicity. This group results from exposure that is recurring numerous times. It is an ordinary human exposure type that occurs due to some pharmaceuticals or environmental agents.
• Chronic toxicity. This relates to exposure of toxic substances for longer duration and the diverse effects are low. For instance, chronic bronchitis for tobacco addict and cirrhosis for alcohol addict
• Carcinogenicity. This group refers to a complex abnormal process in cells which leads to cancer.
• Developmental toxicity. This group refers to toxic responses to the developing embryo. This occurs due to the exposure of toxic substances to your partners prior to the act of conception. This leads to the death of cells of the fetus.
• Genetic toxicity. The major cause of this kind of toxicity is a severe damage of DNA. This process is known as mutagenesis. It leads to cancer, or death of the cell.
• Organ toxicity. This group is associated to toxic responses to different organs. It involves blood and cardiovascular toxicity, eye toxicity, dermal toxicity, reproductive toxicity, respiratory toxicity and various forms. (Hodgson, 2004, p.275)

References
Hodgson, E. (2004) A Textbook of Modern Toxicology. Wiley-Interscience; 3-rd edition.

Question 2 List and provide a brief explanation of the types of Phase I reactions of metabolism of foreign substances.
The types of Phase 1 reactions are:
Oxidation- This is the additional of oxygen or removal of hydrogen to a substrate will make the compound more polar and therefore more water-soluble. The cytochrome P450 is single-oxygenase, situated in the soft endoplasmic reticulum of the cell. Cytochrome P450 is a group of enzymes and is a preferred CYP. CYP are three families that carry out the bulk of oxidation reactions on xenobiotics: CYP 1, CYP 2 and CYP 3. The CYP enzymes display genetic polymorphisms and demonstrate diverse proportions of isozymes between unlike tissues of the similar animal, between different group of creature and different sexes of a creature.
Reduction - Reduction reactions typically reduce aldehyde, ketone, and epoxide and double bond groups contained by molecules. This process is catalyzed by microtonal as well as gut bacteria, which acquire reductase enzymes. Reductive dehalogenation is a vital metabolic route for halide holding molecules like halothane and carbon tetrachloride etc.
Hydrolysis –This involves addition of water with the breakdown of molecules. This catalyzed by esterases and amidases regularly found in the cytosol of cells.
Hydration - The epoxide useful group in this process can be steady metabolic in-between and are reactive with biomolecules. Epoxide hydrolase is an enzyme that put in H2O to the epoxide resulting in a less reactive diol.

Reference Sources:
Introduction to Toxicology", John Timbrell, 3rd ed., informa healthcare. "Casarett & Doull's: Toxicology - The Basic Science of Poisons", Curtis D. Klaassen, 7th ed., McGraw Hill-Medical

Question 3 What is methylation and how may it affect the metal mercury. Support your answer with an additional reference.
Methylation is the process in which a chemical component called methyl group is added to the various constituents of proteins, DNA and other molecules or substitution of atoms or other groups by methyl group to change their solubility. All these are required in human body for good working conditions. This process is vital when it is applied in the conversion of homocysteine which is a by product of amino acid and dangerous when in excess methionine.
Metal mercury that mostly present in the natural water bodies is converted to methylmercury by methylating bacteria in anoxic environments. Methylation effect on metal mercury leads to uptake of mercury by the organism living in water which is an active process and depends on its chemical forms in which it’s consume. It affects the immune system, modifies genetic and damages nervous system of the organism. The process of methylating mercury by organism is damaging to the developing embryo which are more sensitive to fetus than the adult human should it find its way to human being by eating contaminated food such as fish and other wild life that are at aquatic food chain. Ingestion of the methylmercury may result to kidney failure

Reference
Schaefer, J.K., Rocks, W.Zeng, and Liang, B.Gu. (2011) Active transport, substrate specificity and methylation of Hg (11) in anaerobic bacteria. PNAS, 108(21) 8714-8719