Exercise 2. Read the following facts about caffeine and coffee trees to check your answers in Exercise 1.

What is caffeine? An alkaloid found in coffee, cocoa beans, tea, kola nuts and guarana. Also added to many fizzy drinks, energy drinks, pep pills and cold and flu remedies. For a single portion of espresso, 50 to 55 roasted coffee beans are required; a single imperfect bean will taint the whole sufficiently to be noticeable. This is because human olfaction and taste senses originated as defense mechanisms that protected our ancestors from rotten—hence, unhealthy—foods.

What does caffeine do? A stimulant of the central nervous system. Pure caffeine is a moderately powerful drug and is sometimes passed off as amphetamine. In small doses, such as the 150 milligrams in a typical cup of filter coffee, it increases alertness and promotes wakefulness. Caffeine also raises heart and respiration rate and promotes urine production. Higher doses induce jitteriness and anxiety. The fatal dose is about 10 grams.

How does caffeine work? Caffeine blocks receptors for the neurotransmitter adenosine, which is generally inhibitory and associated with the onset of sleep. Also raises dopamine levels, and stimulates the release of the fight-or-flight hormone adrenalin (From Newscientist.com).

What is coffee? Raw coffee beans are the seeds of plants belonging to the Rubiaceae family, which comprises at least 66 species of the genus Coffea. The two species that are commercially exploited are Coffea arabica, which accounts for two thirds of world production, and C. canephora, often called robusta coffee, with one third of global output. Robusta coffee plants and all wild coffee species have 22 chromosomes, whereas arabica has 44. Therefore, arabica and other coffee species cannot be crossed to produce a hybrid plant.

Robusta is a high-yielding and disease-resistant tree standing up to 12 meters tall that grows best in warm, humid climates. It produces a cup featuring substantial body, a relatively harsh, earthy aroma, and an elevated caffeine content that ranges from 2.4 to 2.8 percent by weight. Although robusta is sold by many purveyors, it does not give rise to the highest-quality coffee.

Arabica, which originated in the Ethiopian highlands, is a medium- to low-yielding, rather delicate tree from five to six meters tall that requires a temperate climate and considerable growing care. Commercially grown coffee bushes are pruned to a height of 1.5 to 2.0 meters. Coffee made from Arabica beans has an intense, intricate aroma that can be reminiscent of flowers, fruit, honey, chocolate, caramel or toasted bread. Its caffeine content never exceeds 1.5 percent by weight. Because of its superior quality and taste, arabica sells for a higher price than its hardy, rougher cousin.

A good rainfall induces coffee plants to blossom, and some 210 days afterward red or yellow fruit called cherries appear. Each cherry contains two oblong seeds—the coffee beans. The ultimate quality of the resulting coffee beans depends on the genetics of the plant, the soil in which it grows and the microclimate, which encompasses factors such as altitude, the amount of rainfall and sunlight, and daily temperature fluctuations. Along with the roasting processes that are applied, these agricultural and geographical considerations are responsible for the taste differences among the many varieties of coffee beans that suppliers combine to produce the various distinctive blends one can purchase (From Scientific American, June 2002).

Exercise 3. Now read detailed explanation of the effects of caffeine provided by biologist Neal J. Smatresk, Dean of the College of Science at the University of Texas at Arlington, and find answers to these questions:

1. What is a neurotransmitter? What is a second messenger?

2. How does caffeine affect heart?

3. Does caffeine affect all animals?

How does caffeine affect the body?

Caffeine--the drug that gives coffee and cola its kick--has a number of physiological effects. At the cellular level, caffeine blocks the action of a chemical called phosphodiesterase (PDE). Inside cells, PDE normally breaks down the second chemical messenger cyclic adenosine monophosphate (cAMP). Many hormones and neurotransmitters cannot cross the cell membrane, and so they exert their actions indirectly via such second messengers; when they bind to a receptor on the surface of a cell, it initiates a chemical chain reaction called an enzyme cascade that results in the formation of second messenger chemicals.

Historically, cAMP was the first second messenger ever described. Now, however, scientists have identified several major classes of second messengers, which are generally formed in similar ways through a set of molecules called G proteins. The advantage of such a complex system is that an extracellular signal can be greatly amplified in the process, and so have a massive intracellular effect.

Thus, when caffeine stops the breakdown of cAMP, its effects are prolonged, and the response throughout the body is effectively amplified. In the heart, this response prompts norepinephrine--also called noradrenalin--and a related neurotransmitter, epinephrine, to increase the rate and force of the muscle's contractions. Although the two act in concert, norepinephrine is released by sympathetic nerves near the pacemaker tissue of the heart, whereas epinephrine is released primarily by the adrenal glands. These chemical messages lead to "fight or flight" behavior. During stressful or emergency conditions, they raise the rate and force of the heart, thereby increasing the blood pressure and delivering more oxygen to the brain and other tissues.

Caffeine would be expected to have this effect on any animals that used these neurotransmitters to regulate their heartbeat. Generally speaking, the effects of caffeine are most pronounced in birds and mammals. Reptiles have some response, and lower vertebrates and invertebrates have rather small or no responses. From an evolutionary perspective, fish and amphibians don't show as strong a response to epinephrine and norepinephrine as the higher vertebrates, and they lack a well-developed sympathetic (that is, stimulatory) enervation to heart.

Exercise 4. What positive and negative effects produced by coffee do you know? Choose from the list below:

1. Coffee boosts attention, concentration and alertness.
2. Coffee improves mental performance.
3. Coffee increases life expectancy.
4. Coffee helps to fight infections.
5. Coffee prevents aging.
6. Coffee increases the risk of cardiovascular diseases.
7. Coffee can lead to a stroke.
8. Coffee protects against cancer.

Дата добавления: 2015-11-14