The Tobacco Plant Nicotiana Tabacum Biology Essay

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The tobacco plant, Nicotiana Tabacum was first introduced into western society in the 16th century. Nicotine is a psychoactive chemical that occurs as a natural alkaloid. An alkaloid is any group of nitrogen containing compounds produced by plants. Alkaloids in general have a stimulating effect on humans. Nicotine is synthesized and stored in the tobacco plant. A tobacco plant yields approximately 2-5% nicotine of the total weight of the plant.Type equation here.

A cigarette on average contains between 8-20 mg nicotine but the human body will only absorb about 1mg per cigarette. Nicotine is a poisonous drug and can be fatal if ingested in large amounts or have serious side effects which include nausea, vomiting and muscle paralysis. The lethal dose of nicotine is about 60mg.


Nicotine can be absorbed into the body in a number of ways, the most popular method being smoking. There is an estimated 1.2 billion people worldwide who smoke. The alveoli in our lungs are specialized respiratory cells which facilitate the process of respiration. When a smoker takes a puff of a cigarette, these cells provide the large surface area in which nicotine and other chemicals are able to pass through into the respiratory capillaries and into the cardiovascular system, where it affects the brain 8-10 seconds after inhalation.

Tar is the sticky residue that is left behind when smoke is exhaled. It sticks to the lung tissue and is a cancer causing compound. A tobacco leaf gives off more than 4000 chemicals, 400 of which are believed to be poisonous to humans and a further 43 of these have been proven to cause cancer. A few examples of these dangerous chemicals which a lit cigarette releases include carbon monoxide, ammonia, hydrogen cyanide and benzene. These dangerous chemicals also contribute as to why second hand smoke is also so dangerous.

Nicotine can also be absorbed by the mucous membranes lining the oral cavity and this is done by chewing on the tobacco. Tobacco can also be sniffed and absorbed by the mucous membranes in the nasal cavity. Nicotine users who chew or sniff tobacco are at high risk of developing oral cancer and gum diseases. Nicotine may also be absorbed by the skin with the help of nicotine patches and are often used by smokers who are trying to quit in attempt to maintain their bodies nicotine levels so as not to suffer from the withdrawal symptoms.

Effects of Nicotine on the Central Nervous System and the Peripheral Nervous System

Nicotine has both a stimulating and depressing effect. Upon absorption, nicotine has numerous effects, it first travels to the brain where it acts upon cholinergic neurons. Cholinergic neurons release Acetylcholine (ACh) which is the most widespread neurotransmitter in the Central Nervous System (CNS) and Peripheral Nervous System (PNS). In the CNS, ACh is released by presynaptic neurons and in the PNS cholinergic synapses are located at the neuromuscular junctions, preganglionic synapses of ANS and neuroglandular junctions of the parasympathetic division. ACh is believed to be involved in memory, learning and mood and this is why depression and irritability are experienced by smokers who are trying to quit.

ACh has two known receptors, Nicotinic and Muscarinic. Nicotinic receptors are stimulated by nicotine and ACh whereas Muscarinic receptors are stimulated by muscarine and ACh. Nicotinic receptors are located at the synapses between preganglionic and postganglionic neurons of both the parasympathetic and sympathetic nervous systems as well as at neuromuscular junctions of the PNS.

ACh is synthesized and stored in the vesicles of presynaptic neurons and is released via exocytosis into the synaptic cleft after the arrival of the action potential that causes depolarization. Upon attachment of ACh onto its receptors, the post synaptic membrane releases acetylcholinesterase (AChE) which hydrolyzes the ACh into choline and acetate which is reabsorbed and used to synthesize new ACh. If the ACh is not removed from the synaptic cleft, it will stimulate the receptor a number of times, which happens in the presence of nicotine. Nicotine is not a chemical that is regulated by our bodies and our neurons do not possess an enzyme to break nicotine down in our synaptic cleft. In the presence of nicotine at the neurons, it will stimulate the postsynaptic neuron a number of times and this is the reason as to why smokers suffer from muscle tremors.

Dopamine released in the hypothalamus stimulates the release of epinephrine (E). Epinephrine is the “fight or flightâ€Â hormone and upon release increases blood pressure, respiration, and heart rate. E also stimulates the breakdown of glycogen into glucose into the bloodstream. Nicotine has inhibitory effects on the secretion of insulin which makes a smoker hyperglycemic. Nicotine cannot undergo hydrolysis and be reabsorbed like ACh, this also contributes to the addictive property of nicotine. Nicotine stimulates the release of cortisol, the hormone that relieves stress and this is why smokers say they feel relaxed.

Nicotine activates Dopamine centers in the brain. Dopamine is a neurotransmitter released by the limbic system which is the pleasure circuit of the brain and gives us feelings of enjoyment. Dopamine released prior to a behavior is proportional to its potential for providing pleasure. Dopamine helps to shape our behavior by reinforcing rewarding activities. Nicotine stimulates the Nucleus Accumbens and the neurons of the Ventral Tegmental Area (VTA) to release dopamine which sends a signal that tobacco use is rewarding. Nicotine also stimulates the release of beta-endorphins which also add to the pleasurable effects smoking gives its user. Nicotine users have a higher than normal levels of dopamine in their brains. Prolonged use of nicotine desensitizes the dopamine receptors so despite having elevated dopamine levels in the brain the nicotine user will still crave another cigarette in attempt to replenish the good feeling. In periods where the user abstains from using nicotine for example an overnight period, the dopamine receptors regain some of their sensitivity and because there are lower levels of dopamine, the smoker will crave another cigarette in order to increase the levels of nicotine in the bloodstream.

Dopamine is removed from the synaptic clefts back into the cytosol by means of a dopamine transporter. A dopamine transporter is a membrane protein that deposits the dopamine from the synaptic cleft into surrounding cells, much like AChE gets rid of ACh to terminate its effect. Nicotine is also believed to inhibit the take up of dopamine.

Nicotine is one of the most addictive drugs and therefore also one of the hardest drugs to quit. Smokers who are trying to quit will experience withdrawal symptoms such as irritability, sleep disturbances, anxiety, depression and headaches. Research has shown that this is mainly due to the higher levels of dopamine that a smoker will possess and without smoking to replace the dopamine, the smoker will experience negative symptoms.


Nicotine is excreted mainly by enzymes in the liver but the lungs and kidneys play a small role in excretion too. The liver breaks down 80% of the nicotine into cotinine. In the lungs, nicotine is broken down to cotinine and nicotine oxide. Nicotine has a half life of about 60 minutes. Cotinine, a byproduct of nicotine has a half life of about 24 hours. It is excreted in the urine along with other metabolites and is the metabolite tested for in a nicotine drug test.

Studies have shown that some people possess a mutant gene in producing the enzymes that breakdown nicotine in the liver. People who possess this gene breakdown nicotine at a slower pace and as a result the nicotine level in the blood will stay elevated for a longer period of time and as a result are likely to smoke less.

Besides nicotine’s addictive properties, it has many other dangers too. Cigarettes are believed to be the leading cause of death, killing an estimated 4.2 billion people a year. Smoking damages the inner lining of our blood vessels so smokers are more susceptible to developing atherosclerosis and other coronary heart diseases. Besides having a negative impact on the cardiovascular system, smoking has also been associated with cancer or the oral cavity, lungs, esophagus, stomach and liver. Smoking damages the alveoli of the lungs and can cause chronic bronchitis and emphysema.

It is never too late to quit smoking. Smokers who have quit, decrease their chances of developing coronary heart disease and lung cancer amongst other things. Ex smokers life spans have also proven to be lengthened after quitting.