Nitrogen in the air enters and leaves living systems by way of the nitrogen cycle. Describe the events which take place when atmospheric nitrogen molecules move along a food chain (involving legumes and herbivorous mammals) and eventually return to the atmosphere. In your answer, describe the uptake, processing and utilization of nitrogen in the bodies of these organisms

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Atmospheric nitrogen in air enters by diffusion or gaseous exchange and leaves the bodies of most organisms without biotransformation. Yet nitrogen can enter into the living system by the following processes: a) Passage of nitrogen into legumes from N2 1) Nitrogen fixation (1/2) : nitrogen to ammonia (1/2) – biological nitrogen fixation of nitrogen in soil air by free living bacteria / microorganism / blue green algae in the soil (1/2) and mutualistic bacteria in the root nodules of legumes (1).

Ammonium compounds in root nodules can be used by legumes to form plant protein (1/2) – lightning / electrical discharge to fix atmospheric nitrogen (1/2) 2) Nitrification (1/2) : by bacteria, oxidation of ammonia or ammonium compounds in soil into nitrite (1/2), nitrite into nitrate (1/2) 3) Absorption (1/2) : nitrate absorption into root hairs of a legume by diffusion (1/2) and active uptake (1/2) ) Uptake and processing of organic nitrogen by herbivorous mammal from legumes – Feeding along a roof chain, organic nitrogen in legume is taken in as food by a herbivorous mammal (1/2) and become transformed into various biomoelcules before excretion 1) Mastication – In the oral cavity, mechanical breakdown of food by teeth into small pieces (1/2) before entry to the stomach and duodenum (1/2) for digestion by protease / pepsin / protein-digesting enzyme (1/2) into amino acids / peptides (1/2) 2) Absorption in the ileum / small intestine (1/2) – by active uptake (1/2) and diffusion (1/2) into the blood stream 3) In the liver (1/2), amino acids may be converted into glucose, glycogen and fat (1/2). Some amino acids will be deaminated (1/2) to form ammonia which will eventually form urea (1/2) and be released into the blood stream.

Most of the urea will be excreted via the kidney (1/2). c) Return of nitrogen to the air 1) Dead bodies / excreta (1/2) : nitrogenous molecules are decomposed and turned into ammonium compounds (1/2) by bacteria / saprophytes / putrefying organisms / decomposers (1/2) 2) Nitrification of ammonium compounds to nitrites (1/2) then to nitrates (1/2) by nitrifying bacteria (1/2) 3) Denitrification into gaseous nitrogen by denitrifying bacteria from nitrates (1) ) Utilization of nitrogenous compounds by legumes and mammals: 1) From amino acids to proteins / peptides (1/2) Types of proteins (1/2) x 3 Two examples (any 2 for each type, 1/2 each, max 3) Structural proteins actin, myosin etc. Functional proteins enzymes, hormones, haemoglobin etc Defence molecules in mammals antibodies, lysozyme etc 2) From amino acids to other organic compounds: DNA, RNA, chlorophyll, ATP etc (any two examples, 1/2 each. If answer is nucleic acids, 1/2 only) 3) Transformation (1/2): to 3C, 4C and 5C compounds and metabolized to liberate energy (1/2)

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