The Prokaryotic Cell

Of all the types of cells revealed by the microscope,bacteria have the simplest structure and come closest to showing us life stripped down to its essentials.Indeed,bacteria contain essentially no organelles (recognizable substructures)-not even a nucleus to hold their DNA.This property-the presence or absence of a nucleus–is used as the basis for a simple but fundamental classification of all living things.Organisms whose cells have a nucleus are called eukaryotes (from the Greek words eu,meaning “well”or ‘truly,”and karyon,a kemel”or “nucleus”).Organisms whose cells do not have a nucleus are called prokaryotes (from pro,meaning “before”).The terms “bactenium and “prokaryote are often used interchangeably although we shall see that the category of prokaryotes also includes another class of cells so remotely related to ordinary bacteria that they are given a separate name.

Bacteria are typically spherical,rod-like.or corkscrew-shaped. and small-just a few micrometers long.They often have a tough protective coat,called a cell wall.In the electron microscope the cell interior typically appears as a matrix of varying texture without any obvious organized internal structure.The cells reproduce quickly by dividing in two.Under optimum conditions,when food is plentiful,a prokaryotic cell can duplicate itself in as little as 20 minutes.In less than 11 hours,by repeated divisions:a single prokaryote can give rise to 5 billion progeny.Thanks to their large numbers,rapid growth rates,and ability to exchange bits of genetic material by a process akin to sex,populations of prokaryotic cells can evolve fast,rapidly acquiring an ability to use a new food source or to resist being killed by a new antibiotic.

Most prokaryotes live as single-celled organisms.although some join together to form chains,clusters.or other organized multicellular structures.In shape and structure prokaryotes may seem simple and limited,but in terms of chemistry they are the most diverse and inventive class of cells.These creatures exploit an enormous range of habitats,from hot puddles of volcanic mud to the interiors of other living cells,and they vastly outnumber other living organisms on Earth.Some are aerobic,using oxygen to oxidize food molecules: some are strictly anaerobic and are killed by the slightest exposure to oxygen.

Virtually any organic material,from wood to petroleum,can be used as food by one sort of bacterium or another.Still more remarkable,some prokaryotes can live entirely on inorganic substances:they get their carbon from CO2 in the atmosphere,their nitrogen from atmospheric N2.and their oxygen,hydrogen,sulfur. and phosphorus from air.water.and inorganic minerals.Some of these prokaryotic cells.like plant cells.perform photosynthesis. getting the energy they need for biosynthesis from sunlight:others derive energy from the chemical reactivity of inorganic substances in the environment.In either case,such prokaryotes play a unique and fundamental part in the economy of life on Earth:other living things depend on the organic compounds that these cells generate from inorganic materials.

Traditionally.all prokaryotes have been classified together in one large group.But molecular studies reveal that there is a gulf within the class of prokaryotes,dividing it into two distinct domains,called the eubacteria (or simply bacteria)and the archaea.Remarkably,at a molecular level,the members of these two domains differ as much from one another as either does from the eukaryotes.Most of the prokaryotes familiar from everyday life-the species that live in the soil or make us ill-are eubacteria.Archaea are not only found in these habitats.but also in environments hostile to most other cells: there are species that live in concentrated brine,in hot acid volcanic springs,in the airless depths of marine sediments,in the sludge of sewage treatment plants,in pools beneath the frozen surface of Antarctica,and in the acidic,oxygen-free environment of a cow’s stomach,where they break down cellulose and generate methane gas.Many of these environments resemble the harsh conditions that must have existed on the primitive Earth.where living things first evolved,before the atmosphere became rich in oxygen.

 

 

1

Of all the types of cells revealed by the microscope,bacteria have the simplest structure and come closest to showing us life stripped down to its essentials.Indeed,bacteria contain essentially no organelles (recognizable substructures)-not even a nucleus to hold their DNA.This property-the presence or absence of a nucleus–is used as the basis for a simple but fundamental classification of all living things.Organisms whose cells have a nucleus are called eukaryotes (from the Greek words eu,meaning “well”or ‘truly,”and karyon,a kemel”or “nucleus”).Organisms whose cells do not have a nucleus are called prokaryotes (from pro,meaning “before”).The terms “bactenium and “prokaryote are often used interchangeably although we shall see that the category of prokaryotes also includes another class of cells so remotely related to ordinary bacteria that they are given a separate name.

According to paragraph 1,which of the following comparisons betwveen bacteria and eukaryotes is true?

AEukaryotes lack the nucleus that bacteria have.

BEukaryotes are more complex than bacteria.

CThere are more types of eukaryotes than there are of bacteria.

DEukaryotes evolved earier than did bacteria.

 

2

Bacteria are typically spherical,rod-like.or corkscrew-shaped. and small-just a few micrometers long.They often have a tough protective coat,called a cell wall.In the electron microscope the cell interior typically appears as a matrix of varying texture without any obvious organized internal structure.The cells reproduce quickly by dividing in two.Under optimum conditions,when food is plentiful,a prokaryotic cell can duplicate itself in as little as 20 minutes.In less than 11 hours,by repeated divisions:a single prokaryote can give rise to 5 billion progeny.Thanks to their large numbers,rapid growth rates,and ability to exchange bits of genetic material by a process akin to sex,populations of prokaryotic cells can evolve fast,rapidly acquiring an ability to use a new food source or to resist being killed by a new antibiotic.

The word“optimum’’ in the passage is closest in meaning to

Acertain

Bsafe

Cthe most common

Dthe most favorable

 

3

Bacteria are typically spherical,rod-like.or corkscrew-shaped. and small-just a few micrometers long.They often have a tough protective coat,called a cell wall.In the electron microscope the cell interior typically appears as a matrix of varying texture without any obvious organized internal structure.The cells reproduce quickly by dividing in two.Under optimum conditions,when food is plentiful,a prokaryotic cell can duplicate itself in as little as 20 minutes.In less than 11 hours,by repeated divisions:a single prokaryote can give rise to 5 billion progeny.Thanks to their large numbers,rapid growth rates,and ability to exchange bits of genetic material by a process akin to sex,populations of prokaryotic cells can evolve fast,rapidly acquiring an ability to use a new food source or to resist being killed by a new antibiotic.

Paragraph 2 specifically mentions all of the following as contributing to the ability of populations of prokaryotic cells to evolve rapidly EXCEPT

Atheir rapid growth rates

Btheir large numbers

Ctheir ability to resist being killed by a new antibiotic

Dtheir ability to exchange bits of genetic material

 

4

Most prokaryotes live as single-celled organisms.although some join together to form chains,clusters.or other organized multicellular structures.In shape and structure prokaryotes may seem simple and limited,but in terms of chemistry they are the most diverse and inventive class of cells.These creatures exploit an enormous range of habitats,from hot puddles of volcanic mud to the interiors of other living cells,and they vastly outnumber other living organisms on Earth.Some are aerobic,using oxygen to oxidize food molecules: some are strictly anaerobic and are killed by the slightest exposure to oxygen.

Why does the author mention that some cells “join together to form chains,clusters.or other organized multicellular structures”?

ATo suggest that prokaryotes are evolving to become more complex organisms

BTo contrast simple prokaryotes with more complex organisms

CTo explain how prokaryotes can exploit such an enomous range of habitats

DTo point out some exceptions to the most common pattem of prokaryote life

 

5

Most prokaryotes live as single-celled organisms.although some join together to form chains,clusters.or other organized multicellular structures.In shape and structure prokaryotes may seem simple and limited,but in terms of chemistry they are the most diverse and inventive class of cells.These creatures exploit an enormous range of habitats,from hot puddles of volcanic mud to the interiors of other living cells,and they vastly outnumber other living organisms on Earth.Some are aerobic,using oxygen to oxidize food molecules: some are strictly anaerobic and are killed by the slightest exposure to oxygen.

Paragraph 3 suggests that prokaryotic cells living in radically different habitats differ primarily with respect to their

Ashape

Bstructure

Corganization

Dchemistry

 

6

Virtually any organic material,from wood to petroleum,can be used as food by one sort of bacterium or another.Still more remarkable,some prokaryotes can live entirely on inorganic substances:they get their carbon from CO2 in the atmosphere,their nitrogen from atmospheric N2.and their oxygen,hydrogen,sulfur. and phosphorus from air.water.and inorganic minerals.Some of these prokaryotic cells.like plant cells.perform photosynthesis. getting the energy they need for biosynthesis from sunlight:others derive energy from the chemical reactivity of inorganic substances in the environment.In either case,such prokaryotes play a unique and fundamental part in the economy of life on Earth:other living things depend on the organic compounds that these cells generate from inorganic materials.

According to paragraph 4,prokaryotic cells are very important to other living things because they

Acan remove more CO2 from the atmosphere than other types of organisms can

Bcan get most of the inorganic substances they need from air and water

Ccan generate necessary organic compounds from inorganic materials

Dcan help other organisms get energy for biosynthesis from sunlight

 

7

Traditionally.all prokaryotes have been classified together in one large group.But molecular studies reveal that there is a gulf within the class of prokaryotes,dividing it into two distinct domains,called the eubacteria (or simply bacteria)and the archaea.Remarkably,at a molecular level,the members of these two domains differ as much from one another as either does from the eukaryotes.Most of the prokaryotes familiar from everyday life-the species that live in the soil or make us ill-are eubacteria.Archaea are not only found in these habitats.but also in environments hostile to most other cells: there are species that live in concentrated brine,in hot acid volcanic springs,in the airless depths of marine sediments,in the sludge of sewage treatment plants,in pools beneath the frozen surface of Antarctica,and in the acidic,oxygen-free environment of a cow’s stomach,where they break down cellulose and generate methane gas.Many of these environments resemble the harsh conditions that must have existed on the primitive Earth.where living things first evolved,before the atmosphere became rich in oxygen.

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?Incorrect choices change the meaning in important ways or leave out essential information.

AIn addition to these habitats,archaea are found in a wide veriety of extreme habitats where most other cells cannot survive.

BArchaea are not found in these habitats but instead in extremely cold habitats as well as in acidic and oxygen-free environments.

CArchaea serve various functions in these habitats,including breaking down cellulose in the stomachs of cows and consuming sludge in sewage treatment plants.

DDifferent species of archaea are found performing a wide variety of functions in environments that differ from these habitats.

 

8

Traditionally.all prokaryotes have been classified together in one large group.But molecular studies reveal that there is a gulf within the class of prokaryotes,dividing it into two distinct domains,called the eubacteria (or simply bacteria)and the archaea.Remarkably,at a molecular level,the members of these two domains differ as much from one another as either does from the eukaryotes.Most of the prokaryotes familiar from everyday life-the species that live in the soil or make us ill-are eubacteria.Archaea are not only found in these habitats.but also in environments hostile to most other cells: there are species that live in concentrated brine,in hot acid volcanic springs,in the airless depths of marine sediments,in the sludge of sewage treatment plants,in pools beneath the frozen surface of Antarctica,and in the acidic,oxygen-free environment of a cow’s stomach,where they break down cellulose and generate methane gas.Many of these environments resemble the harsh conditions that must have existed on the primitive Earth.where living things first evolved,before the atmosphere became rich in oxygen.

Paragraph 5 suggests that archaea

Aevolve much more slowly than eubacteria do

Bare likely to have existed on Earth earlier than eubacteria

Care all chemically much more similar to each other than eubacteria are

Dinclude more of the organisms that make us ill than eubacteria do

 

9

Look at the four squaresthat indicate where the following sentence could be added to the passage

lt is this that determines the cell’s shape.

Where would the sentence best fit？Click on a square  sentence to the passage.

10

Prokaryotes and eukaryotes are distinguished by the absence or presence of a nucleus.

ACells with spherical,rod-like,or corkscrew shapes can be identified as bacteria,while those with simpler and more limited shapes are generally prokaryotes.

BBacteria can eat nearly any kind of organic material,and some prokaryotes can also get energy from sunlight or from the chemical reactivity of inorganic substances.

CBecause their chemical makeup is simple,some prokaryotes are killed by exposure to oxygen,and they have to live in harsh environments like volcanic mud and the interiors of other living cells.

DProkaryotes are small,rapidly reproduce by division,evolve fast, exploit an enormous range of habitats,and vastly outnumber all other organisms.

ETraditionally,it had been assumed that only plant cells could perform photosynthesis,but scientists now recognize that most prokaryotic cells can also get the energy they need for biosynthesis from sunlight.

FProkaryotes are divided into two distinct domains,eubacteria and archaea,that are as different from one another as eubacteria or archaea are from eukaryotes.

 

 

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