Unit 3 Energy Flow in the Life of a Cell

Introduction

Listen and answer the questions.

1. What is caused within a system by any use of energy (according to the second law of thermodynamics)?

2. How does energy flow in chemical reactions?

3. Give an example of an exergonic reaction.

4. How is cellular energy carried between coupled reactions?

Reading

I. Put paragraphs of the following abstract in order, then read and translate it.

Energy Carriers in Cells

1 ATP is admirably suited to carry energy within cells. The bonds joining the last two phosphate groups of ATP to the rest of the molecule (sometimes called high-energy bonds) require a large amount of energy to form, so considerable energy can be trapped from exergonic reactions; it readily releases its energy in the presence of appropriate enzymes. Under most circumstances, only the bond joining the last phosphate group (the one joining phosphate to ADP to form ATP) carries energy from exergonic to endergonic reactions.

2 Energy can also be transported within a cell by other carrier molecules. In some exergonic reactions, including both glucose metabolism and the light-capturing stage of photosynthesis, some energy is transferred to electrons. These energetic electrons (in some cases, along with hydrogen atoms) are captured by electron carriers.

3 The life span of an ATP molecule is very short, because this energy carrier is continuously formed, broken apart to ADP, and resynthesized. More stable molecules, such as sucrose, glycogen, starch, or fat, store energy in our body for hours, days, or months.

4 Common electron carriers include nicotinamide adenine dinucleotide and its relative flavin adenine dinucleotide. Electron-carrier molecules pick up electrons generated by exergonic reactions and hold them in high-energy outer electron shells. Hydrogen atoms are often picked up simultaneously. The electron is then deposited with another molecule to drive an endergonic reaction, typically the synthesis of ATP.

5 The most common energy-carrier molecule in cells is adenosine triphosphate, or ATP. It is a nucleotide composed of the nitrogen-containing base adenine, the sugar ribose, and three phosphate groups. Energy released in cells through glucose breakdown is used to drive the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate. ATP carries this energy to sites in the cell that perform energy-requiring reactions, such as the synthesis of proteins or muscle contraction. The ATP is then broken down to form ADP and inorganic phosphate. During these energy transfers, heat is given off at each stage, and there is a loss of usable energy.

II. Choose the right answer:

ATP is important in cells because:

1. it is assembled into long chains that make up cells membranes.

2. it acts as an enzyme.

3. it accelerates diffusion.

4. it transfers energy from exergonic reactions to endergonic reactions.

5. all of the above.

III. Fill in the blanks with appropriate terms from the text:

1. Some substances in our organism are formed just for seconds, broken apart and then ………….. .

2. If a substance contains no carbon molecules, it is an ………………….. substance.

3. ……………………….. is among common electron carriers.

4. The synthesis of proteins is an …………………… reaction.

5. During exergonic reactions heat is …………………….. .

IV. Using italicized words compose three true sentences and two false ones. Swap the sentences with your partner and try to guess true and false ones.

Language focus 1

Ved Forms. Participle Constructions

1 Verb forms like left, studied, broken, finished are called ‘past participles’, or Ved Forms. Ved Forms can be used with auxiliary verbs be and have to make perfect and passive verb forms.

I’ve completed the work. We’ll be sent for when it’s necessary.

2 Past participles can be used like adjectives.

He’s got a broken heart.

3 Participles can combine with other words into clause-like structures.

Most of the students invited to the party didn’t turn up.

Accepted by all the scientists, the new idea became a breakthrough concept.

4 Ved Forms are often used after nouns in order to define or identify the nouns.

We couldn’t agree on any of the issues discussed. (= … the issues that were discussed.) (NOT … the discussed issues.)

The students questioned gave very different opinions. (= The people who were questioned…) (NOT The questioned students…)

We had the only sample left. (NOT … the only left sample.)

5 A few participles change their meaning according to their position. Compare:

- a concerned expression (= a worried expression)

the people concerned (= the people who are/were affected)

- an involved explanation (= a complicated explanation)

the people involved (= the people concerned)

- an adopted child (= a child who is brought up by people who are not his/her biological parents)

the solution adopted (= the solution that is/was chosen)

I. Find Ved Forms in the following sentences and say what role they play in the sentence.

1. Sugar, produced in photosynthetic organisms, contains far more energy than do the carbon dioxide and water from which it is formed.

2. Many of chemical reactions in a cell are linked in sequences called metabolic pathways.

3. The original reactant molecule undergoes a series of reactions, each catalyzed by a specific enzyme.

4. Under the influence of certain type of poisons the active site of an enzyme remains plugged up, so no substrate molecule can enter.

5. Nuclear reactions in the sun produce concentrated energy (sunlight) along with vast increases in entropy.

6. Living organisms constantly use the energy given off by exergonic reactions to drive essential endergonic reactions, as shown in Figure 4-4b.

7. The energy stored is used later in various life processes.

II. Say whether the information contained in the participle constructions is necessary to the meaning of the sentence, or it can be omitted.

1. Energy released in cells through glucose breakdown is used to drive the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate.

2. Each enzyme has a dimple or groove, called the active site, into which reactant molecules, called substrates, can enter.

3. Muscle movement is an endergonic reaction coupled to the exergonic reaction of ATP breakdown.

4. Some enzymes also require the presence of other molecules called coenzymes, typically derived from water-soluble vitamins, in order to function.

5. Far lower temperatures than those required to fry an egg can still be too hot for normal enzyme function.

Now formulate the rule for putting commas. Do we need commas when the information contained in the participle construction can be omitted?

III. Compose your own sentences using the participle constructions given below. Pay attention to the punctuation!

1. … directed by enzymes … 2. …, not usually spoken of, …

3. … kept in secret … 4. … believed to be true by many …

5. …, meant to draw our attention to the idea of Justice, …

Language focus 2

Absolute Participle Construction

A participle construction can have its own subject. This happens often in a rather formal style, for example, in scientific texts:

The active site changed, the reaction continues.

The subject is often introduced by with. It can have different meanings:

time (when?); cause (why?); circumstances (how?); place (where?).

I. Translate the following sentences into Russian paying attention to the function of the participle construction.

1. The breakdown or synthesis of a molecule within a cell normally occurs in many discrete steps, each catalyzed by a different enzyme.

2. Most enzymes function optimally at a pH between 6 and 8, the level found in most body fluids and maintained within cells.

3. With the process initiated, the reaction lasts until the needed quantity of the substance is achieved.

4. The active site plugged up, the substrate molecules cannot enter and the reaction is stopped.

5. The energy released as heat, the amount of entropy increases.

II. Think of your own phrases with absolute participle construction using words:

… found … … broken … … closed …

… rejected … … catalyzed …

Language focus 3

Participles of Durative and Terminative Verbs

In English one can find verbs of terminative and non-terminative (durative) lexical character.

Terminative verbs denote an action implying a certain limit beyond which it cannot go: to come, to bring, to build, to give, to take, to receive, to find, to fall, to kill, to die, to become, to stand up, to sit down, to come to.

Durative verbs denote a certain action which does not imply any limit: to live, to exist, to sleep, to love, to be, to have, to possess, to work, to speak, to respect, to hope, to sit, etc.

I. Match the participle with its translation. Pay attention to whether the action expressed has reached a point after which it can no longer continue. Some verbs can have both durable and terminative meanings.

shown освобождаемый, освобожденный

liberated называемый

oxygen-starved разбитый

called выпускаемый, выпущенный

derived (from) испытывающий нехватку кислорода

broken показываемый, показанный

released произошедший, полученный (из)

II. Say whether the participles in the following sentences have a durative or a terminative meaning. Translate the sentences.

A. Potential energy, or stored energy, includes chemical energy stored in the bonds that hold atoms together in molecules, electrical energy stored in a battery, and positional energy stored in a diver poised to jump.

B. A chemical reaction converts one set of substances, called the reactants, into another set, the

products.

С. In most cases, an enzyme catalyzes a single reaction that involves one or two specific molecules but leaves even quite similar molecules untouched.

D. Enzymes are biological catalysts, normally proteins synthesized by living organisms.

E. Temperature also affects the rate of enzyme-catalyzed reactions.

Words, Words, Words

I. Look through the sentences in the exercises of this unit. Make a classification of new words and phrases according to any parameter chosen by you.

II. Write an abstract describing the flow of energy in the Earth’s living systems and the functioning of enzymes.

Render in English.

Большая часть химических реакций, протекающих в организме, регулируется ферментами, то есть белковыми молекулами, выполняющими функцию катализаторов. Катализатором называют вещество, ускоряющее химическую реакцию, которая без него протекает медленно. Сам катализатор во время этой реакции стойких изменений не претерпевает.

Каждый из ферментов катализирует только те реакции, в которых участвуют молекулы только какого-нибудь одного или нескольких видов. Причина этого в том, что ферменты связываются со своими субстратами, т. е. теми веществами, на которые они действуют. У фермента имеется активный центр. Форма и химическое строение этого активного центра таковы, что с ним могут связываться только определенные субстраты. Катализируя реакцию, фермент тесно сближает молекулы своих субстратов, так что те части молекул, которым предстоит прореагировать, оказываются друг подле друга. Субстрат, присоединившись к ферменту, несколько изменяется. Фермент может, например, притягивать электроны, вследствие чего в некоторых связях молекулы субстрата будет возникать напряжение. Это, в свою очередь, может повышать реакционную способность молекулы.

Предполагается, что именно таким путем фермент и ускоряет реакцию. Активность ферментов находится в зависимости от температуры и кислотности (acidity), а также от концентрации молекул субстрата, самих ферментов и коферментов (витаминов и других веществ).