Chapter 1: Cells: The Fundamental Units of Life

Question 1

1. What are four characteristics of all life? (There’s more but pick 4 general characteristics)

Answer 1

Organization: All living organisms are highly organized, and usually has patterns.

Reproduction: All living organisms must be able to successfully grow and produce offspring.

Homeostasis: All living organisms must be able to maintin their internal environment from their external surrounding.

Metabolism: All living things must be able to convert chemical processes into usable energy that will allow them to maintain basic life functioning.

Response to stimuli: All living things should be able to react to their environment.

Growth and Development: All living things should be capable of to grow in numbers and develop traits that will aid in fitness of reproduction.

Question 2

2. What are two main differences between Prokaryotic Cells and Eukaryotic Cells?

Answer 2

Prokaryotic Cells:

• Are much smaller in size compared to eukaryotes (0.1-5.0µm)
• Does not have a nucleus or membrane-bound organelles

Eukaryotes:

• Are much larger in comparison to prokaryotes (10-100µm)
• have a nucleus surrounded by a nuclear envelope that consists of two lipid membranes

Question 3

3. What is Cell Theory? (Three main points)

Answer 3

1. All living organisms are composed of one or more cells. 2. Cells are the basic units of life. 3. Living organisms do not arise spontaneously but can be generated only from pre-existing organisms.

Question 4

4. What is the Endosymbiont Theory? Explain how scientists think that this happened.

Answer 4

The Endosymbiont Theory explains that primitive eukaryotic cell, with a nucleus and cytoskeleton, engulfed the free-living, oxygen-consuming bacteria that were the likely ancestors of the mitochondria (see Figure 1–19). This partnership is thought to have been established 1.5 billion years ago, when the Earth’s atmosphere first became rich in oxygen. A subset of these cells later acquired chloroplasts by engulfing photosynthetic bacteria

Question 5

5. What is a Model Organism? Why do scientists use Model Organisms?

Answer 5

A model organism is a living thing that is selected for intensive research or study under represenative of a large group of species. Popular choices for studying are:

Saccharomyces cerevisiae: a budding yeast which are cheap and reproduce rapidly.

Arabidopsis thaliana: a comman wall cress weed can be grown indoors produce thousands of offspring within 8-10 weeks, genes found in Arabidopsis have counterparts in agricultural species.

Drosophila melanogaster: a small fruit fly has shown us how the genetic instructions encoded in DNA molecules direct the development of a fertilized egg cell.

Question 6

6. What types of organisms are these model organisms?

C. elegans : Nematode worm

• S.cervisiae :*
• E.coli :*
• Arabidopsis thaliana :*
• Mus musculus :*
• Danio rerio :*
• Drosophila melanogaster :*
• Xenopus laevis :*

Answer 6

MODEL ORGANISM

SHORT DESCRIPTION

• C. elegans* : Nematode worm
• S.cervisiae :* Brewyer’s Yeast (Fungi)
• E.coli :* Bacterium
• Arabidopsis thaliana :* Common wall cress (Plant)
• Mus musculus :* Mouse (Animal)
• Danio rerio :* Zebra Fish (Animal)
• Drosophila melanogaster :* Fruit Fly (Animal)
• Xenopus laevis :* African clawed frog (Animal)

Question 7

7. Explain the experiment that compares homology of yeast and human CDC2. (Challenge Question 2 Part*Long Anwser)

Answer 7

1. Cdc genes identified, called Cdc2, was required to trigger several key events in the cell-division cycle. When that gene was inactivated by a mutation, the yeast cells would not divide. And when the cells were provided with a normal copy of the gene, their ability to reproduce was restored.

2.S.pombe cells that contained a temperaturesensitive mutation in the Cdc2 gene that kept the cells from dividing when the heat was turned up. And they found that some of the mutant S. pombe cells regained the ability to proliferate at the elevated temperature. If spread onto a culture plate containing a growth medium, the rescued cells could divide again and again to form visible colonies, each containing millions of individual yeast cells (Figure 136). Upon closer examination, the researchers discovered that these “rescued” yeast cells had received a fragment of DNA that contained the S. cerevisiae version of Cdc2—a gene

Question 8

Cytosol

Answer 8

Cytosol: Liquid inside the cell the intracellular fluid at which organelles, proteins, and cell structure stay alfoat along with an abundance of nutrients.

Question 9

Cytoplasm

Answer 9

Cytoplasm: Contents of a cell that are contained within its plasma membrane but, in the case of eukaryotic cells, outside the nucleus.

Question 10

Plasma Membrane

Answer 10

The protein-containing lipid bilayer that surrounds a living cell.

Question 11

Nucleus

Answer 11

In biology, refers to the prominent, rounded structure that contains the DNA of a eukaryotic cell. In chemistry, refers to the dense, positively charged center of an atom.​

Question 12

Nuclear Envelope/Pores

Answer 12

is made up of two lipid bilayer membranes which in eukaryotic cells surrounds the nucleus, which encases the genetic material. The nuclear envelope consists of two lipid bilayer membranes, an inner nuclear membrane, and an outer nuclear membrane.

Question 13

Lysosomes

Answer 13

is a membrane-bound cell organelle that contains digestive enzymes that break down excess or worn-out cell parts. They may be used to destroy invading viruses and bacteria

Question 14

Chromosomes

Answer 14

Long, threadlike structure composed of DNA and proteins that carries the genetic information of an organism; becomes visible as a distinct entity when a plant or animal cell prepares to divide.

Question 15

Mitochondria

Answer 15

Membrane-enclosed organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eukaryotic cells.

Question 16

Chloroplasts

Answer 16

Specialized organelle in algae and plants that contains chlorophyll and serves as the site for photosynthesis.

Question 17

Rough ER

Answer 17

Region of the endoplasmic reticulum associated with ribosomes and involved in the synthesis of secreted and membrane-bound proteins.

Question 18

Peroxisomes

Answer 18

membrane-bound organelle occurring in the cytoplasm of eukaryotic cells. Peroxisomes play a key role in the oxidation of specific biomolecules. They also contribute to the biosynthesis of membrane lipids known as plasmalogens.

Question 19

Smooth ER

Answer 19

The main function of the smooth ER is to make cellular products like hormones and lipids

Question 20

Golgi Apparatus

Answer 20

Membrane-enclosed organelle in eukaryotic cells that modifies the proteins and lipids in endoplasmic reticulum and sorts them for transport to other sites.

Question 21

Cytoskeleton

Answer 21

System of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and the capacity for directed movement. Its most abundant components are actin filaments, microtubules, and intermediate filaments.

Question 22

Ribosome

Answer 22

Large macromolecular complex, composed of RNAs and proteins, that translates a messenger RNA into a polypeptide chain.

Question 23

What do eukaryotic cells have that prokaryotes lack?

Answer 23

a nucleus and membrane-bound organelles

*All cells have a means of converting energy from one form to another, whether from food, inorganic minerals, or directly from sunlight. Additionally, all cells contain nucleic acids (DNA as their genome) and ribosomes (for creating protein), and some eukaryotic cells like plants and fungi have cell walls. However, only eukaryotic cells have cell nuclei and other membrane-bound organelles; these features are not observed in prokaryotic cells. DNA is contained in the eukaryotic cell nucleus and each membrane-bound organelle carries out a specific cellular function.*

Question 24

What is a drawback to using light microscopy?

Answer 24

It cannot be used to view structures smaller than a bacterium.

*One drawback of light microscopy is that it cannot be used to view structures smaller than a bacterium. Although the limit of resolution for light microscopy is about 0.2 µm, in practice structures smaller than bacteria or mitochondria (which are about 0.5 µm wide) are difficult to resolve without the use of special stains or fluorescent dyes. Unlike electron microscopy, light microscopy can be used to view living specimens. For example, in an introductory biology class, students commonly watch amoebas, paramecia, and other unicellular organisms swim in a sample of pond water. Light microscopy is ideal for viewing individual cells and single-celled organisms like these on slides that are called a “wet mount.” Although samples viewed with a light microscope must be thin enough for light to pass through them, many unicellular organisms can be viewed without any more advanced preparation. Techniques such as confocal microscopy can be used to view the three-dimensional architecture of a cell or tissue that is too thick for light microscopy. *

Question 25

Which is not a function of proteins?

Answer 25

encoding genetic information

*Encoding genetic information is not a cellular function of proteins; rather, nucleic acids perform this function. A cell’s genome—that is, the entire sequence of nucleotides in an organism’s DNA—provides the information that instructs a cell how to produce all of the proteins needed to function. However, DNA codes for proteins indirectly, with messenger RNA molecules serving as the link between the DNA sequence of a gene and the amino acid sequence of the protein. Proteins carry out a wide range of tasks and the function of a cell is determined, in large part, by which proteins it produces.*

Question 26

Which organelle’s ancestor was likely engulfed by primitive eukaryotes to help the cell survive in an oxygen-rich atmosphere?

Answer 26

mitochondrion

*The mitochondrion is an organelle that contains a double membrane—that is, two lipid bilayers that surround it. This organelle’s ancestor was likely engulfed by primitive eukaryotes to help the cell survive in an oxygen-rich atmosphere. Thus, the inner mitochondrial membrane derived from the plasma membrane of the engulfed cell, and the outer mitochondrial membrane derived from the evolving eukaryotic cell’s plasma membrane. The Golgi apparatus, endoplasmic reticulum, lysosome, and peroxisome are not surrounded by a double membrane.*

Question 27

What is the name of the process by which eukaryotic cells engulf material captured from an external medium?

Answer 27

Endocytosis

*Endocytosis is the name of the process by which eukaryotic cells engulf material captured from an external medium. This is accomplished when a portion of the plasma membrane invaginates and then pinches off, forming a transport vesicle. In the reverse process, termed exocytosis, a transport vesicle fuses with the plasma membrane and in so doing, releases its contents outside the cell. Together, endocytosis and exocytosis represent an important import and export mechanism for the cell, especially for large sized or large quantities of materials.*

Question 28

Which cellular component separates the DNA of eukaryotic cells from the cytoplasm?

Answer 28

nuclear membrane

*The DNA of eukaryotic cells is contained within the cell nucleus, and the nucleus is separated from the cytoplasm by the nuclear envelope. The nucleus is enclosed within two concentric membranes that form the nuclear envelope, and it contains molecules of DNA: extremely long polymers that encode the genetic information of the organism. All eukaryotic cells contain an endoplasmic reticulum and plasma membrane, but these are not the barriers between eukaryotic DNA and the cytoplasm.*

Question 29

Which of the following is a stack of flattened membrane-enclosed sacs that receives molecules made in the endoplasmic reticulum, often chemically modifies them, and then directs them to the exterior of the cell or to various locations inside the cell?

Answer 29

Golgi apparatus

*The Golgi apparatus functions to modify and package molecules and appears like a stack of large, flattened vesicles with smaller vesicles that have budded from the Golgi stack nearby. These vesicles contain molecules that are being transported to other parts of the cell or are destined to be excreted. The Golgi apparatus is one of a group of membrane-bound organelles including the endoplasmic reticulum, lysosomes, and peroxisomes, which all work together in a dynamic fashion to synthesize, break down, modify, and transport molecules within the cell.*

Question 30

Which of the following organelles is an irregular maze of interconnected spaces enclosed by a membrane and functions as the site where most cell membrane components, as well as materials destined for export from the cell, are made?

Answer 30

endoplasmic reticulum

*The endoplasmic reticulum (ER), which is continuous with the nuclear envelope, is an irregular maze of interconnected spaces enclosed by a membrane and is the major site of protein and lipid synthesis. The portion of the ER that is responsible for making proteins is studded with numerous ribosomes and is called the rough ER due to its appearance. In contrast, the smooth ER lacks bound ribosomes and is the site of lipid synthesis in the cell. Molecules synthesized in the ER move via transport vesicles to the Golgi apparatus for further modification and/or secretion out of the cell. Transport vesicles also interconnect other membrane-bound organelles, including lysosomes and peroxisomes, resulting in a dynamic and continuous movement of materials within the cell.*

Question 31

Which of the following is a model plant used by scientists?

Answer 31

Arabidopsis thaliana

*Arabidopsis thaliana, the common wall cress, is a model plant that is favored by plant biologists for a number of reasons: it can be grown indoors and reproduces quickly, with one plant giving rise to thousands of offspring in just 8–10 weeks, and it is related to plants of agricultural value, making research with it relevant to the production of crops that feed humans. A. thaliana, like S. cerevisiae, C. elegans, and other model organisms, has its strengths and limitations, but all are indispensable in helping researchers understand the cellular and molecular processes that govern life.*

Question 32

Consider this image of a unicellular free-living eukaryote.

Which model organism is represented in this microscopic image?

Answer 32

Saccharomyces cerevisiae

*The brewer’s yeast, Saccharomyces cerevisiae, is an important model system that has been instrumental in understanding basic aspects of processes, including cell division and cell-cycle control. As a single-celled organism, S. cerevisiae can be grown more quickly and easily than multicellular organisms. Because it is a eukaryote, many of its cellular mechanisms are more closely related to humans than bacterial model systems like Escherichia coli.*

Question 33

In what way are all cells alike?

Answer 33

They store their genetic instructions in DNA.

*All cells store their genetic instructions as DNA. Though all cells use multiple forms of RNA during the processes of gene expression, all genetic information itself is encoded as DNA for long-term storage. Cells come in a variety of shapes; plant cells are boxlike due to their rigid cell walls, while the bacterium that causes syphilis is shaped like a corkscrew. And cells vary enormously in size. Many of these cells, all of which are surrounded by a plasma membrane made of lipids, are free-living, single-celled organisms that can survive and proliferate on their own.*

Question 34

Antibiotics tend to target features that are unique to bacterial cells and absent from eukaryotic cells such as our own. Which of the following would present a safe target for a new antibiotic?

Answer 34

cell wall

*Antibiotics tend to target features that are unique to bacterial cells and absent from eukaryotic cells such as our own. This means that the drug will ideally inhibit prokaryotic cell biology but not eukaryotic cell biology. Bacteria do not have nuclear envelopes, an endoplasmic reticulum, or intermediate filaments, so an antibiotic that targeted these features would harm only the infected person’s cells, making it a poor choice of drug. Both human cells and bacterial cells have a plasma membrane, so an antibiotic that targeted this structure would also harm human cells. Bacteria have cell walls made of a chemical polymer called peptidoglycan. Human cells do not have cell walls at all, much less the ability to produce peptidoglycan. Thus, an antibiotic that targets peptidoglycan cell walls, much like penicillin and the cephalosporins, would be a great candidate because it would have selective toxicity: it would kill bacteria and not human cells.*

Question 35

The fruit fly, Drosophila melanogaster (D. melanogaster) is an excellent model for studying fundamentals of development because

Answer 35

many of the genes involved in the development of the fruit fly are also found in humans.

*Fruit flies have been studied for over a century, and their complex body plan can be manipulated genetically to uncover which genes govern development of which parts. Many of these genes have direct counterparts in the human genome.*

Question 36

If a gene sequence in one organism is highly similar to the sequence of another gene in another organism it is called

Answer 36

homologous.

*Gene sequences from one organism can be compared to another organism to see if they are homologous. Homology between genes (and thus also their protein products) strongly suggests that they evolved from a common ancestral gene.*

Question 37

Which statement represents the cell theory?

Answer 37

All cells are formed by the growth and division of existing cells.

*Observations made with the light microscope in the nineteenth century by biologists, including the seminal work by Matthias Schleiden and Theodor Schwann, documented that all plant and animal tissues were composed of cells. With repeated examination, a concept slowly emerged that all living organisms are composed of cells and that new cells are formed by the division of existing cells. Later experiments by Louis Pasteur helped confirm the cell theory and dispel the competing idea of spontaneous generation.*

Question 38

How long ago is it estimated that the common ancestor for all life existed?

Answer 38

between 3.5 and 3.8 billion years ago

*Our entire universe is only about 14 billion years old. Dinosaurs were already extinct by 3.5 to 3.8 million years ago and human ancestors were already using fire as a tool between 350,000 and 380,000 years ago. Somewhere between 3.5 and 3.8 billion years ago is the time frame by which it is estimated that the common ancestor for all cellular life existed. This would represent the advent of the first cell. Through a very long process of mutation and natural selection, the descendants of this ancestral cell have gradually diverged to fill every habitat on Earth with organisms that exploit the potential of the machinery in a seemingly endless variety of ways.*

Question 39

Consider this evolutionary tree and then answer the questions. Scientists study model organisms within these groups to gain insight into these types of organisms and their genetic heritage.

Which of these statements is consistent with the data presented in the evolutionary tree?

Pt.1
A. Saccharomyces cerevisiae do not contain chloroplasts.

B. Drosophila melanogaster do not contain mitochondria.

C. Caenorhabditis elegans contain chloroplasts.

D. All of these choices are correct regarding these organelles.

Pt.2

**Is the following statement true, false, or impossible to determine?

Photosynthetic bacteria contain chloroplasts.

A. true**

B. false

C. impossible to determine

Answer 39

A. Saccharomyces cerevisiae do not contain chloroplasts.

*The yeast Saccharomyces cerevisiae is a model eukaryote. These organisms are unicellular fungi, and according to the evolutionary tree, fungi are not derived from the genetic lineage from where the chloroplast developed. Chloroplasts almost certainly evolved from engulfed photosynthetic bacteria. The bacteria are thought to have been taken up by early eukaryotic cells that already contained mitochondria, but this happened along a different evolutionary path from which fungi had already separated. Drosophila melanogaster is an animal and all animal cells have mitochondria. Like D. melanogaster, Caenorhabditis elegans is an animal and animal cells do not contain chloroplasts.*

B. false

*Photosynthetic bacteria do not contain chloroplasts. Photosynthetic bacteria are prokaryotic, and prokaryotic cells do not contain any membrane-bound organelles. The chloroplast is a membrane-bound organelle that is part of some eukaryotic cells. For photosynthetic bacteria, the photosystem complexes that absorb light energy are embedded in the plasma membrane of the cell; this functions analagously to the inner membrane of the chloroplast of eukaryotic cells. *

Question 40

Which is not evidence for the endosymbiotic origins of mitochondria and chloroplasts?

Answer 40

Mitochondria and chloroplasts have similar DNA.

*Mitochondria and chloroplasts do not have similar DNA. The endosymbiotic origins of mitochondria and chloroplasts suggest that these organelles arose from separate instances of the developing/evolving primitive eukaryotic cell engulfing a smaller bacterial cell. The inner membrane would be derived from the engulfed cell membrane and the outer membrane would be derived from the primitive eukaryotic cell membrane. Because these are separate instances of endosymbiosis, there would be no expectation that the mitochondria and the chloroplast would have similar DNA. Mitochondria and chloroplasts resemble bacteria and have DNA that has evolved from those very different forms of bacteria—thus, they have unique DNA. *

Question 41

Mitochondria are essentially the same in all eukaryotes, including plants, animals, and fungi. Based on this observation, how were mitochondria most likely acquired?

Answer 41

by an ancestral eukaryotic cell before the lines that led to animal cells, plant cells, and fungi diverged

*Currently, the most favored explanation for the appearance of mitochondria in eukaryotic cells is the establishment of an endosymbiotic relationship between a primitive eukaryotic cell and a free-living aerobic bacterium. The proto-eukaryote, which gave rise to plants, animals, and fungi, would have engulfed the bacterial cell whole, allowing it to persist inside itself where it eventually became what we now call the mitochondrion. Several lines of evidence support this idea, including the presence of predatory protozoans such as Didinium, which are capable of engulfing other cells whole, and the presence of ribosomes and mitochondrial DNA inside of mitochondria, which highlights their potential prokaryotic origins.*

Question 42

Which statement is not true of mitochondria?

Answer 42

Mitochondria are not present in plant cells.

*The statement that mitochondria are not present in plant cells is false. Plant cells undergo cell respiration just like animal cells, and thus they have mitochondria where bioenergetic reactions of the citric acid cycle and electron transport chains take place. All mitochondria have both an inner and an outer membrane. The proteins of the electron transport chain are embedded in the inner mitochondrial membrane and this double-membrane system is integral to the function of this organelle. Mitochondria contain their own DNA as well. This genome is referred to as mitochondrial DNA, which is circular just like most types of prokaryotic DNA. Mitochondria are thought to have originated from bacteria. The endosymbiotic theory of organelle development suggests that the evolving eukaryotic cell engulfed a smaller bacterial cell, and coevolution allowed these to rely on one another to sustain life. This evidently created a symbiotic relationship in which the host eukaryote and the engulfed bacterium helped each other to survive and reproduce.*

Question 43

**Is the following statement true or false, and why?

Within a developed multicellular organism, all cells possess the ability to divide and do so regularly.**

Answer 43

It is false, because some cells lose the ability to replicate their DNA and divide.

*The ability to reproduce themselves is the hallmark of a living organism. However, in a multicellular organism, this does not necessarily mean that all cells in that organism undergo continual cell division. Multicellular organisms have many different types of specialized cells, each performing a specific function. In certain cases, such as epithelial cells that line body cavities, that function requires continual cell division and replacement. However, in other cases—including red blood cells, nerve cells, skeletal muscle cells, and gametes—the cells are no longer capable of further cell division. Such cells are said to be “terminally differentiated.”*

Question 44

Genetic changes followed by selection are best described as the fundamentals of what process?

Answer 44

evolution

*Genetic changes followed by selection are the fundamentals of evolution, and this process is responsible for the stunning variety of organisms on this planet. With a deeper understanding of cells, we can begin to tackle the grand historical problems of life on Earth: its mysterious origins, its stunning diversity produced by billions of years of evolution. All living things on Earth share a common cell ancestor that lived on Earth at least 3.5 billion years ago. Over the course of time, driven by evolution, this ancestral cell gave rise to the great diversity of life we see around us today, yet it also explains why all living things are united by the cell as the fundamental unit of life.*

Question 45

When comparing liver cells and kidney cells within an organism, many differences can be observed and documented. Which of following is not a difference between liver cells and kidney cells in the same animal?

Answer 45

The different cells have different DNA.

*The statement that these different cells have different DNA is NOT true about liver cells and kidney cells in the same animal. Liver cells and kidney cells have the same genome, but they use the encoded information differently, expressing different genes and producing different proteins. The different cells have different roles in the body and are generated during the animal’s embryological development. *

Question 46

Scientists were studying yeast cells in which a mutation inactivated a gene required for cell division. Without this gene—and the protein it encoded—these mutant yeast cells were unable to divide normally. But the scientists discovered that introducing a related protein from human cells could “rescue” these mutant yeast cells, allowing the cells to resume normal division. Based on this finding, what is the most likely conclusion you could make?

Answer 46

The proteins that control cell division in yeast and humans are functionally equivalent and have been conserved, almost unchanged, for more than a billion years.

*The experimental finding presented in this question leads to the conclusion that the human and yeast proteins are functionally equivalent. The most likely explanation for two very different species to have genes with such similar function is that this gene was found in the common ancestor cell from which these organisms diverged. In the case of humans and yeast, this common ancestor cell existed approximately 1.5 billion years ago. That the human and yeast genes and the proteins that they encode remain functionally equivalent is a testament to how fundamentally important this gene is in the cell-division process in all eukaryotic cells. There has been strict selective pressure against mutations in this gene and they have therefore retained a high degree of sequence similarity over the millennia.

More than a hypothetical experiment, this question highlights a Nobel prize-winning experiment conducted by Paul Nurse, Lee Hartwell, and colleagues that examined the ability of a human cell-cycle gene, Cdc2, to rescue cell cycle mutant yeast. Human Cdc2, did indeed rescue the mutant yeast and examining the amino sequence of Cdc2 and the yeast equivalents revealed, as predicted, that they have very similar sequences, as shown below. *

Question 47

Which organism would be the most useful for studying how mutations that cause sudden death in young athletes can affect the development of the heart?

Answer 47

zebrafish

*While zebrafish and humans might seem quite different from one another, compared to the other model organisms listed in this question, the zebrafish is the most closely related to humans as both species are vertebrates. Additionally, the zebrafish has distinct advantages for studying aspects of development, including their quick gestation time and the fact that they are transparent for the first two weeks of life, making observation of cells and tissues easier. This makes zebrafish the most useful organism among the choices listed for studying how mutations that cause sudden death in young athletes can affect the development of the heart.*

Question 48

Which of the following statements are correct? Explain your answers.

The hereditary information of a cell is passed on by its proteins.

Answer 48

False. The hereditary information is encoded in the cell’s DNA, which in turn specifies its proteins (via RNA).

Question 49

Which of the following statements are correct? Explain your answers.

Bacterial DNA is found in the cytoplasm.

Answer 49

True. Bacteria do not have a nucleus.

Question 50

Which of the following statements are correct? Explain your answers.

Plants are composed of prokaryotic cell.

Answer 50

False. Plants, like animals, are composed of eukaryotic cells, but unlike animal cells, they contain chloroplasts as cytoplasmic organelles. The chloroplasts are thought to be evolutionarily derived from engulfed photosynthetic bacteria.

Question 51

Which of the following statements are correct? Explain your answers.

With the exception of egg and sperm cell, all of the nucleated cells within a single multicellular organism have the same number of chromosomes.

Answer 51

True. The number of chromosomes varies from one organism to another, but is constant in all nucleated cells (except germ cells) within the same multicellular organism.

Question 52

Which of the following statements are correct? Explain your answers.

The cytosol includes membrane-enclosed organelles such as lysosomes.

Answer 52

False. The cytosol is the cytoplasm excluding all membrane-enclosed organelles.

Question 53

Which of the following statements are correct? Explain your answers.

The nucleus and a mitochondrion are each surrounded by a double membrane.

Answer 53

True. The nuclear envelope is a double membrane, and mitochondria are surrounded by both an inner and an outer membrane.

Question 54

Which of the following statements are correct? Explain your answers.​

Protozoans are complex organisms with a set of specialized cells that form tissues such as flagella, mouthparts, stinging darts, and leglike appendages.

Answer 54

False. Protozoans are single-celled organisms and therefore do not have different tissues or cell types. They have a complex structure, however, that has highly specialized parts.

Question 55

Which of the following statements are correct? Explain your answers.

Lysosomes and peroxisomes are the sites of degradation of unwanted materials.

Answer 55

Somewhat true. Peroxisomes and lysosomes contain enzymes that catalyze the breakdown of substances produced in the cytosol or taken up by the cell. One can argue, however, that many of these substances are degraded to generate food molecules, and as such are certainly not “unwanted.”

Question 56

Identify the different organelles indicated with letters in the electron micrograph of a plant cell shown below. Estimate the length of the scale bar in the figure.

Answer 56

In this plant cell, A is the nucleus, B is a vacuole, C is the cell wall, and D is a chloroplast. The scale bar is about 10 μm, the width of the nucleus.

Question 57

There are three major classes of protein filaments that make up the cytoskeleton of a typical animal cell. What are they, and what are the differences in their functions? Which cytoskeletal filaments would be most plentiful in a muscle cell or in an epidermal cell making up the outer layer of the skin? Explain your answers.

Answer 57

The three major filaments are actin filaments, intermediate filaments, and microtubules. Actin filaments are involved in rapid cell movement, and are the most abundant filaments in a muscle cell; intermediate filaments provide mechanical stability and are the most abundant filaments in epidermal cells of the skin; and microtubules function as “railroad tracks” for many intracellular movements and are responsible for the separation of chromosomes during cell division.

Question 58

Natural selection is such a powerful force in evolution because organisms or cells with even a small reproductive advantage will eventually outnumber their competitors. To illustrate how quickly this process can occur, consider a cell culture that contains 1 million bacterial cells that double every 20 minutes. A single cell in this culture acquires a mutation that allows it to divide faster, with a generation time of only 15 minutes. Assuming that there is an unlimited food supply and no cell death, how long would it take before the progeny of the mutated cell became predominant in the culture? (Before you go through the calculation, make a guess: do you think it would take about a day, a week, a month, or a year?) How many cells of either type are present in the culture at this time? (The number of cells N in the culture at time t is described by the equation N = N0 × 2t/G, where N0 is the number of cells at zero time and G is the generation time.)

Answer 58

It takes only 20 hours (i.e., less than a day) before mutant cells become more abundant in the culture. Using the equation provided in the question, we see that the number of the original (“wild-type”) bacterial cells at time t minutes after the mutation occurred is 10^6× 2^t/20. The number of mutant cells at time t is 1 × 2t/15. To find out when the mutant cells “overtake” the wild-type cells, we simply have to make these two numbers equal to each other (i.e., 10^6× 2^t/20 = 2^t/15). Taking the logarithm to base 10 of both sides of this equation and solving it for t results in t = 1200 minutes (or 20 hours). At this time, bthe culture contains 2 × 10^24 cells (10^6× 2^60 + 1 × 2^80). Incidentally, 2 × 10^24 bacterial cells, each weighing 10^–12 g, would weigh 2 × 10^12 g (= 2 × 10^9 kg, or 2 million tons!). This can only have been a thought experiment.

Question 59

When bacteria are cultured under adverse conditions—for example, in the presence of a poison such as an antibiotic—most cells grow and divide slowly. But it is not uncommon to find that the rate of proliferation is restored to normal after a few days. Suggest why this may be the case.

Answer 59

Bacteria continually acquire mutations in their DNA. In the population of cells exposed to the poison, one or a few cells may already harbor a mutation that makes them resistant to the action of the poison. Antibiotics that are poisonous to bacteria because they bind to certain bacterial proteins, for example, would not work if the proteins have a slightly changed surface so that binding occurs more weakly or not at all. These mutant bacteria would continue dividing rapidly while their cousins are slowed down. The antibiotic-resistant bacteria would soon become the predominant species in the culture.

Question 60

Apply the principle of exponential growth of a population of cells in a culture (as described in Question 1–12) to the cells in a multicellular organism, such as yourself. There are about 10^13 cells in your body. Assume that one cell has acquired mutations that allow it to divide in an uncontrolled manner to become a cancer cell. Some cancer cells can proliferate with a generation time of about 24 hours. If none of the cancer cells died, how long would it take before 10^13 cells in your body would be cancer cells? (Use the equation N = N0 × 2^t/G, with t the time and G the generation time. Hint: 10^13 ≈ 243.)

Answer 60

10^13 = 2^(t/1). Therefore, it would take only 43 days [t = 13/log(2)]. This explains why some cancers can progress extremely rapidly. Many cancer cells divide much more slowly, however, and many die because of their internal abnormalities or because they do not have a sufficient blood supply, and so the actual progression of cancer is usually slower.

Question 61

“The structure and function of a living cell are dictated by the laws of chemistry, physics, and thermodynamics.” Provide examples that support (or refute) this claim.

Answer 61

Living cells evolved from nonliving matter, but they grow and replicate. Like the material they originated from, they are governed by the laws of physics, thermodynamics, and chemistry. Thus, for example, they cannot create energy de novo or build ordered structures without the expenditure of free energy. We can understand virtually all cellular events, such as metabolism, catalysis, membrane assembly, and DNA replication, as complicated chemical reactions that can be experimentally reproduced, manipulated, and studied in test tubes.

Despite this fundamental reducibility, a living cell is more than the sum of its parts. We cannot randomly mix

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proteins, nucleic acids, and other chemicals together in a test tube, for example, and make a cell. The cell functions by virtue of its organized structure, and this is a product of its evolutionary history. Cells always come from preexisting cells, and the division of a mother cell passes both chemical constituents and structures to its daughters. The plasma membrane, for example, never has to form de novo, but grows by expansion of a preexisting membrane; there will always be a ribosome, in part made up of proteins, whose function it is to make more proteins, including those that build more ribosomes.

Question 62

What, if any, are the advantages in being multicellular?

Answer 62

In a multicellular organism, different cells take on specialized functions and cooperate with one another, so that any one cell type does not have to perform all activities for itself. Through such division of labor, multicellular organisms are able to exploit food sources that are inaccessible to single-celled organisms. A plant, for example, can reach the soil with its roots to take up water and nutrients, while at the same time, its leaves above ground can harvest light energy and CO2 from the air. By protecting its reproductive cells with other specialized cells, the multicellular organism can develop new ways to survive in harsh environments or to fight off predators. When food runs out, it may be able to preserve its reproductive cells by allowing them to draw upon resources stored by their companions—or even to cannibalize relatives (a common process, in fact).

Question 63

Draw to scale the outline of two spherical cells, one a bacterium with a diameter of 1 μm, the other an animal cell with a diameter of 15 μm. Calculate the volume, surface area, and surface-to-volume ratio for each cell. How would the latter ratio change if you included the internal membranes of the animal cell in the calculation of surface area (assume internal membranes have 15 times the area of the plasma membrane)? (The volume of a sphere is given by 4πr3/3 and its surface by 4πr2, where r is its radius.) Discuss the following hypothesis: “Internal membranes allowed bigger cells to evolve.”

Answer 63

he volume and the surface area are 5.24 × 10^–19 m^3 and 3.14 × 10^–12 m^2 for the bacterial cell, and 1.77 × 10^–15 m^3 and 7.07 × 10^–10 m^2 for the animal cell, respectively. From these numbers, the surface-to-volume ratios are 6 × 10^6 m^–1 and 4 × 10^5 m^–1, respectively. In other words, although the animal cell has a 3375-fold larger volume, its membrane surface is increased only 225-fold. If internal membranes are included in the calculation, however, the surface-to-volume ratios of both cells are about equal. Thus, because of their internal membranes, eukaryotic cells can grow bigger and still maintain a sufficiently large area of membrane, which—as we discuss in more detail in later chapters—is required for many essential cell functions.

Question 64

What are the arguments that all living cells evolved from a common ancestor cell? Imagine the very “early days” of evolution of life on Earth. Would you assume that the primordial ancestor cell was the first and only cell to form?

Answer 64

There are many lines of evidence for a common ancestor cell. Analyses of modern-day living cells show an amazing degree of similarity in the basic components that make up the inner workings of otherwise vastly different cells. Many metabolic pathways, for example, are conserved from one cell type to another, and the organic compounds that make up polynucleotides (DNA and RNA) and proteins are the same in all living cells, even though it is easy to imagine that a different choice of compounds (e.g., amino acids with different side chains) would have worked just as well. Similarly, it is not uncommon to find that important proteins have closely similar detailed structures in prokaryotic and eukaryotic cells. Theoretically, there would be many different ways to build proteins that could perform the same functions. The evidence overwhelmingly shows that most important processes were “invented” only once and then became fine-tuned during evolution to suit the particular needs of specialized cells and specific organisms.

It seems highly unlikely, however, that the first cell survived to become the primordial founder cell of today’s living world. As evolution is not a directed process with purposeful progression, it is more likely that there were a vast number of unsuccessful trial cells that replicated for a while and then became extinct because they could not adapt to changes in the environment or could not survive in competition with other trial cells. We can therefore speculate that the primordial ancestor cell was a “lucky” cell that ended up in a relatively stable environment in which it had a chance to replicate and evolve.

Question 65

Looking at some pond water with a light microscope, you notice an unfamiliar rod-shaped cell about 200 μm long. Knowing that some exceptional bacteria can be as big as this or even bigger, you wonder whether your cell is a bacterium or a eukaryote. How will you decide? If it is not a eukaryote, how will you discover whether it is a bacterium or an archaeon?

Answer 65

A quick inspection might reveal the characteristic beating of cilia on the cell surface; their presence would tell you that the cell was eukaryotic (prokaryote flagella have entirely different structures and motions compared to eukaryote cilia and flagella). If you don’t see them—and you are quite likely not to—you will have to look for other distinguishing features. If you are lucky, you might see the cell divide. Watch it then with the right optics, and you might be able to see condensed mitotic chromosomes, which again would tell you that it was a eukaryote. Fix the cell and stain it with a dye for DNA: if the DNA is contained in a well-defined nucleus, the cell is a eukaryote; if you cannot see a well-defined nucleus, the cell may be a prokaryote. Alternatively, stain it with fluorescent antibodies that bind actin or tubulin (proteins that are highly conserved in eukaryotes but absent in bacteria). Embed it, section it, and look with an electron microscope: can you see organelles such as mitochondria inside your cell? Try staining it with Gram stain, which is specific for molecules in the cell wall of some classes of bacteria. But all these tests might fail, leaving you still uncertain. For a definitive answer, you could attempt to analyze the sequences of the DNA and RNA molecules that it contains, using the sophisticated methods we describe more fully in Chapter 10. If the nucleic acid sequences encode molecules that are highly conserved in eukaryotes, such as those that form the core components of the nuclear pore, you can be sure your cell is a eukaryote. If there are no eukaryote-specific sequences, you should still be able to distinguish whether you are looking at a bacterium or an archaeon. If you can’t detect any DNA or RNA, you are probably looking not at a cell but at a piece of dirt.