TEXT 2. BIOCHEMICAL REACTIONS

TEXT 1. BIOCHEMISTRY

Biochemistry can be described as chemistry of living objects (cells and organisms). Living objects differ from inanimate ones basically in their ability (a) to metabolize, (b) to reproduce (transmit genetic information). Still, they are an integral part of nature and, therefore, governed by all the principal laws of nature, such as the law of conservation of mass and energy and the laws of thermodynamics.

Living objects represent open systems (thermodynamically speaking) or relatively isolated systems (cybernetically speaking), both of which means that they can communicate with the surroundings. Such communication is mediated by substrates (sources of free energy) and by information (resulting in decreased entropy and increased structuredness of living systems). This is mainly achieved on the basis of Le Chatelier's principle and results in a steady state that can be characterized as a dynamic state of a system which, in a given time interval, receives from the surroundings the same amount of material and energy as it returns to it so that concentrations inside the system remain unchanged. This is one of the features in which living objects differ from inanimate isolated systems where a time-independent, equilibrium, state obtains. In such systems all quantities remain constant and all processes come to a stop.

Reactions in living systems are thus seen to proceed in time and space. According to the degree of their development they differ in the complexity of their structures.

Living structures are usually composed of simple inorganic and organic compounds, which possess a certain configuration in space that cannot be visualized from a structural formula at first sight. This is important to bear in mind, especially in connection with the reactivity of some compounds in situations where others are nonreactive. The relatively simple compounds evolved into macromolecules and ultimately into supramolecular structures which form the basis for the construction of the fundamental building block of living systems – the cell and its organelles. The molecules have their definite dimensions and relative orientation which underlies their function.

The function of living systems is based on biochemical reactions which take place on the cellular and sub-cellular structure mentioned above as well as in the soluble milieu of the cytoplasm or of extracellular fluids.

Notes on the text:

1. inanimate – неодушевленный, неживой

2. integral – неделимый, неотъемлемый

3. surroundings – окружающая среда

4. to mediate – осуществлять

5. result in – приводит к

6. visualize – отчетливо представлять себе

7. milieu – окружающая среда

8. to underlie – лежать в основе, определять

Задания к тексту:

1) Просмотрите текст, найдите в нем следующие слова и словосочетания, дайте их русские эквиваленты и запомните их:

to describe, cell, to differ from, ability, to reproduce, to transmit, the principal law of nature, the law of conservation of mass and energy, to represent, this is mainly achieved, a steady state, the same amount, inside the system, equilibrium, to come to a stop, to proceed, the complexity of the structure, at first sight, to bear in mind, in connection with, relatively simple, definite dimensions, to evolve, cellular.

2) Прочтите 1 абзац текста и ответьте на вопрос: “What is the difference between living objects and inanimate ones?”

3) Прочтите 2 абзац текста и ответьте на вопрос: “How can a steady state be characterized?”

4) Прочтите текст до конца и ответьте на вопрос: “What are living structures composed of?”

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TEXT 2. BIOCHEMICAL REACTIONS

Biochemical reactions take place within a relatively narrow range of physical and chemical parameters. Beside the limited range of temperatures and pressures we must consider the concentration or activity of hydrogen ions (pH). This concentration is maintained at the required level by buffer systems obeying the Henderson-Hasselbalch equation, the relative constancy of pH being important for preventing dissociation of biologically active compounds which may result in changes of protein shape and reactivity (structural stability or enzyme activity). Some biochemical reactions proceed at the optimum rate only at a given osmotic pressure and ionic strength in a medium where ions are present at fairly constant ratios. All these factors affect the properties and possibly the function of dispersed molecules and, depending on the nature of the solvent and of the size of solute particles, we can distinguish between true solutions, colloid solutions, and suspensions.

Biochemical reactions can proceed only if certain energy requirements are met. The primary source of energy on our planet has been solar radiation. A part of this energy is now stored as chemical energy in the bonds of various compounds. Under the present conditions which are highly aerobic most of the energy required for maintaining the structure of living systems derives from oxidation-reduction processes (in particular from the oxidation of hydrogen by atmospheric oxygen). The individual partial reactions are either exergonic (they take place spontaneously) or endergonic (they require a supply of energy). Many endergonic reactions can proceed because they are coupled with exergonic reactions. The most common mediator of such energy transfer is the molecule of adenosine triphosphate (ATP).

Biochemical reactions proceed at a rate that depends on the concentration of the reacting molecules and on the rate constants characterizing the given type of reaction. This rate can be markedly altered (usually increased) in living systems by the presence of catalysts (enzymes). Noxious effects of the environment usually interfere at the level of enzyme-catalyzed reactions by inhibiting their progress.

The individual reactions taking place in living objects must be controlled in a very definite way. This is achieved partly by their spatial orientation (changes in entropy of living systems), partly by alterations in the rates of reactions.

Notes on the text:

1. constancy – постоянство

2. to meet the requirements – отвечать требованиям

3. noxious effects – вредное воздействие

4. oxidation-reduction processes – окислительно – восстановительные процессы

5. spatial orientation – пространственная ориентация

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