Each question need 150 word Min./ total of 3 scholarly sources due 1/16
73
The Cell As a City
© Kendall Hunt Publishing Company
3 EssEnTiAls
Theta and Joules are in a clique – Sally is not accepted
The cell is like a city
Primitive cells absorb mitochondria-like organismsA cell with its organelles
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Microvilli PeroxisomeCentrioles
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Flagellum
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Smooth Endoplasmic Reticulum (S.E.R.)
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Microtubules
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Cytosol (Cytoplasmic fluid)
Cytoplasm (Cell contents outside nucleus)
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74 Unit 1: That’s Life
ChECk in
From reading this chapter, you will be able to:
• Explain how differences caused by inherited organelles could have societal implications. • Describe how the characteristics that are valued change from culture to culture and over time. • Outline the cell theory, list and describe types of cells, and explain endosymbiosis. • List and describe the organelles found in a cell, and explain their main functions. • Explain the processes of diffusion, osmosis, facilitated diffusion, active transport, and bulk transport.
The Case of the Meddling houseguest: A Friendship Divided Theta and Joules liked their friend Sally, but when they entered college, they learned that Sally was different. When they were all young, they played together on the block, went to each other’s birthday parties, and had some great sleepovers. “We had a lot of fun with Sally in sixth grade . . . I wish she could join our sorority,” said Theta. Aghast at the thought, Joules replied, “Don’t even say it – you know what that would mean for us. We shouldn’t even admit that we know her.”
“Why can I not hang out with people I like? . . . Am I not allowed to be Sally’s friend because of some test?” thought Theta. “There is no law against me being friends with Sally!” exclaimed Theta, after a long pause. Joules dismissed Theta smugly, “You know you can’t do it. It will never happen.” They were expecting Sally to come into the dorm any minute. Sally was expecting to hang out with them as usual. But on this day, their friendship had to end. On this day, Joules and Theta were going to pledge their new sorority . . . and Sally did not have the mark.
It was an advanced society, in 2113 with all of the comforts – space travel beyond the solar system, teleporting, and no more diseases that the ancients had; instead there were life spans approaching two centuries for the marked people. Humans had it better than ever, and teens had the world in their hands. Everyone with parents that had any sense had a mark on their children to denote their superior genetic lineage. People in the line of descent from genetically modified mitochondria had an “M” on the inside of their ears. Their life expectancy was much higher and their health much better than those without the mark. Finding out about one’s mitochondrial DNA was easy, with tests dating back over 100 years to trace the origin of one’s genes.
Mitochondria are organelles that make energy for a cell; they are inherited from mother to children because they have their own genetic material and divide on their own. Mitochondria are, in fact, separate structures existing within our cells. They were absorbed some 2.5 billion years ago, with their own set of DNA, making them houseg- uests in our bodies.
The genes in the mitochondria stay intact from generation to generation. “This is why the mark was so important – the health benefits,” thought Theta. Mitochondrial DNA with modified genes of a particular line of mitochondria made people much health- ier, free of many diseases in the society of this story. Mitochondria are the meddling houseguests in the title because defects in them cause a range of diseases. For example,
Mitochondria
Is the organelle that makes energy for a cell.
Organelle (subcel- lular structure)
Structures that function within cells in a discrete manner
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Chapter 3: The Cell As a City 75
mitochondrial defects in the 21st century were responsible for many ailments, ranging from heart disease and diabetes to chronic sweating, optic nerve disorders, and epilepsy.
Joules told Theta, “People without the mark are jealous of us because they die earlier and have a worse life with more diseases. You know Sally would never understand us. Sally’s genes are still from the 21st century.” But something still bothered Theta: She liked Sally. Sally came into the dorm and Joules explained that they were leaving for the sorority. Sally knew what that meant and said good-bye. Theta looked deeply at Sally, realizing that their past was gone and that they would not see each other again as friends. Sally and Theta both had a single tear in their eyes and they knew they were part of each other’s youth . . . and that meant something.
But Theta looked back one last time and said thoughtfully to herself, “She’s not one of us.”
Culture, Biology, and social stratification Culture plays an important role in defining what is desirable and valued in society. Often decisions on what it means to be “better” are based on cell biology. Our genetic material makes each of us unique and guides the workings of our cells. We all have the same set of cell structures or organelles, but, as in our story, genetic variations give each per- son unique characteristics. While the opening story is science fiction, its possibilities are real. Gene technology is improving human health and has the potential to “design” human genes and organelles, possibly leading to social issues like those described in the conflict faced by Sally, Joules, and Theta.
Biological differences may lead to social changes based on what a society values at any one time. For example, research shows that certain biological features are used to decide social value of people: symmetry of one’s face, body fat distribution in both genders, and musculature in males; smooth skin, good teeth, and a uniform gait. These are all biologically determined, based on how our cell structures work together. Much as mitochondrial inheritance, described in the story dictates health and organismal func- tioning, all cell structures give living systems their characteristics.
Historically, all cultures have used biology to classify people. Humans are suscepti- ble to group messages, such as the one that influenced Theta’s and Joules’ final decision to abandon their friendship with Sally. The average American is exposed to about 3,000 marketing messages per day. This sets up a value system that requires us to reflect on how biology and society can affect our thinking.
ChECk Up sECTion
The exclusion of people in our futuristic science fiction story reflects a theme in human society and history. As a result of cell differences between Theta and Sally, their friendship ended – each possessed a different type of mitochondrion.
Choose a particular situation in which a social stratification (layering) system is set up in a society, in which one group thinks it is better than another. You may choose a present system or one of the past. Is the stratification system reasonable? Is the system based on cell biology? What are the system’s benefits? What are its drawbacks?
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BOdy Art And Skin BiOlOgy in SOciety
Body alterations in the quest for physical beauty are as old as history. Egyptians used cosmetics in their First Dynasty (3100–2907 BC). Hairstyles, corsets, body- weight goals, and body piercing and tattooing trends have changed through human history. Scars have been viewed as masculine and a mark of courage, and tattoos were drawn and carved in ancient European, Egyptian, and Japanese worlds.
Body art was popular in modern western society among the upper classes in the early 19th century. It lost favor due to stories of disease spreading because of unsanitary tattoo practices. Only the lower classes adopted body art to show group affiliation. Tattooing has recently gained popularity; but body art has been used as a symbol of self-expression and as a social-stratification mechanism in many cultures: Indian tattoos mark caste; Polynesians used marks for showing mar- ital status; the Nazis marked groups from their elite SS to concentration camp pris- oners; and U.S. gangs use it to show group membership. Tattooing has been firmly established in societies and continues to grow in popularity in the United States.
The canvas for tattoos is skin, which is part of the integumentary system and has a variety of functions in humans (Figure 3.1). It • maintains temperature; • stores blood and fat; and • provides a protective layer. We will discuss this important system in a later chapter.
In this chapter, we will look at the structure and function of the eukaryotic cell. We will see that, while there are marked differences between plant and animal cells, the basic processes carried out at the cellular level are remarkably the same, as are those of simple, unicellular organisms. We will compare the organelles (structures) of the cell to functions of a city to emphasize that all parts are needed. Each organelle has its own duties, and the parts work together to make an efficient machine. We begin by looking at the development of the microscope, without which our understanding of cells and how they function would be incomplete.
Figure 3.1 Tattoos and body art. Dyes penetrate into the skin cells of a tattoo.
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Exploring the Cell The Microscope The human body is composed of over 10 trillion cells, and there are over 200 different types of cells in a typical animal body, with an amazing variety in sizes (see Figure 3.2). Despite the variety in size, all of these cells and the structures within them are too small
Figure 3.2 Biological size and cell diversity. When comparing the relatives’ sizes of cells, we use multiples of 10 to show differences. The largest human cell, the female egg, is 100 µm, while the smallest bacterial cell is 1000 times smaller at 100 nm. Most cells are able to be seen with the light microscope. The smallest object a human eye can see is about 1 mm, the size of a human egg cell (or a grain of sand). From Introductory Plant Science, by Cynthia McKenney et al.
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78 Unit 1: That’s Life
to be visible to our naked eyes and can only be identified by using microscopes to magnify them.
There are several types of microscopes; perhaps the one with which you are already familiar is the compound light microscope. The compound light microscope uses two lenses: an ocular and an objective lens. Each of these is a convex lens, meaning that its center is thicker than its ends. Convex lenses bring light to a central, converging point to magnify the specimen. A microscope’s parts are seen in Figure 3.3.
The purpose of a microscope is to magnify subcellular parts. What is magnifica- tion? Magnification is the amount by which an image size is larger than the object’s size. If a hair cell’s image is 10 times bigger than its original object, the magnification is 10 times. If it is 100 times bigger, then the magnification is 100 times. The microscope uses two lenses to magnify the specimen: an ocular (eyepiece), which generally magnifies between 10 and 20 times, and a series of objective lenses (each with higher magnifica- tions). The total magnification of a specimen is equal to the ocular (in this example let’s use10 times) times the magnification of one of the objective lenses.
Most animal cells are only 10–30 µm in width. It would take over 20 cells to span the width of a single millimeter. Recall that a millimeter is only as wide as the wire used to make a paper clip. See Table 3.1 for measurements used for looking at living structures.
How were cells and their smaller components discovered using the microscope? Anton van Leeuwenhoek and Marcello Malpighi built microscopes in the late 1600s. At this time, those instruments were very rudimentary. They consisted of a lens or a com- bination of lenses to magnify smaller objects, including cells. Both scientists used their instruments to observe blood, plants, single-celled animals, and even sperm. Van Leeu- wenhoek’s microscope is shown in Figure 3.4. At about the same time that van Leeuwen- hoek and Malpighi were making their observations, Robert Hooke (1635–1703) coined the term cell, as he peered through a primitive microscope of his own construction. When he viewed tissues of a cork plant, Hooke saw what seemed to be small cavities separated by walls, similar to rooms or “cells” in a monastery (see Figure 3.4). These cells are defined as functioning units separated from the nonliving world.
Although it has progressed in design, materials, and technology, the compound light microscope is based on the same principle as in the 17th century: light bends as it passes through the specimen to create a magnified image. Some amount of light always bends
compound light microscope
Microscope that uses two sets of lenses (an ocular and an objective lens).
Magnification
Is the amount by which an image size is larger than the object’s size.
Figure 3.3 Compound light microscope – its parts and internal lens system.
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when hitting the edges of the lens, causing scattering in a random way. The random scattering of light, called diffraction is bad for getting a clear focus on the image. Dif- fraction also limits the resolution of the image. Resolution is defined as the ability to see two close objects as separate. (Think about looking at two lines on a chalkboard that is very far away; chances are they blur together and look like one messy line.) In fact, the human eye has a resolving power of about 100 µm or 1/10th of a millimeter for close-up images. In other words, two lines on a paper closer than 1/10th of a millimeter apart look blurry to us. The light microscope is limited in the same way by diffraction because the diffracted rays create blurry images.
diffraction
The random scattering of light.
resolution
Is the ability to see two close objects as separate.
Figure 3.4 Hooke’s microscope from the 1600s and van Leeuwenhoek with his microscope. These simple microscopes led to the first descriptions of cells. Van Leeuwenhoek’s microscope consisted of a small sphere of glass in a holder.
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Higher magnification under the microscope leads to greater diffraction. This is the reason a compound light microscope can magnify only up to 1000–1500 times (under oil immersion), after which there is too much diffraction for a clear image to be formed. To overcome the effect of diffraction and achieve clarity at higher magnifications, oil is placed on the slide. However, even with oil immersion, only the large nucleus of a cell can be seen; other organelles appear as small dots or not at all.
So how did the more complex world of even smaller structures within cells get dis- covered? The 1930s saw the development of the electron microscope that allowed for magnifications of over 200,000 times greater than that of the human eye. There are two types of electron microscopes: transmission electron microscope (TEM) and scanning electron microscope (SEM). Transmission electron microscopy allows a resolving power of roughly 0.5 nm (see Table 3.1) that visualizes structures as small as five times the diameter of a hydrogen atom. Electron microscopes use electrons instead of light, which limits diffraction and increases resolution. Magnets instead of lenses focus electrons to create the image. The electrons pass through very thin slices of the specimen and form an image.
A SEM looks at the surfaces of objects in detail, while a TEM magnifies structures within a cell. The SEM has a resolving power slightly less than the TEM, at 10 nm. (A depiction of an electron microscope is shown in Figure 3.5.) Electron microscopy has led to many scientific developments, uncovering subcellular structures to help us under- stand cell biology. Seeing a mitochondrion enables us to better understand diseases and perhaps, if our opening story becomes reality, improve societal health through its use.
Cell Theory Fairly recent advances in microscopy have allowed scientists to learn about the structure and function of even the tiniest components of cells, but the cell theory, which states key ideas about cells, developed a long time ago. We have seen that scientists began study- ing cells in the early 1700s. About a century later, in 1838, a German botanist named Matthias Schleiden (1804–1881) concluded that all plants he observed were composed of cells. In the next year, Theodor Schwann (1810–1882) extended Schleiden’s ideas,
transmission elec- tron microscope (teM)
A type of electron microscope that magnifies structures within a cell.
Scanning electron microscope (SeM)
An electron microscope that looks at the surfaces of objects in detail by focusing a beam of electrons on the surface of the object.
Figure 3.5 a. A researcher sits at a modern electron microscope. b. Apple tree pollen grains on cells, an electron micrograph.
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observing that all animals are also made of cells. But how did these cells come to survive generation after generation? The celebrated pathologist Rudolf Virchow (1821–1902) concluded in 1858 that all cells come from preexisting cells (He wrote this in Latin: “Cellula e cellula”). These scientists contributed, together, to the postulates of the cell theory. The cell theory is a unifying theory in biology that places the cell as the center of life and unifies the many branches of biology under its umbrella. The cell theory states that:
1) All living organisms are composed of cells. 2) The chemical reactions that occur within cells are separate from their
environment. 3) All cells arise from other cells. 4) Cells contain within them hereditary information that is passed down from par-
ent cell to offspring cell.
The cell theory showed not only that cells are the basic unit of life, but that there is continuity from generation to generation. Genetic material is inherited in what we refer today as the cell.
Types of Cells Microscopes allowed researchers to examine differences between organisms that had previously been impossible to determine. A current classification of organisms defines five kingdoms, with organisms in those kingdoms having similar types of cells (There is some debate arguing inclusion of Archaea bacteria as a separate kingdom, and a six- system classification scheme is thus also accepted). Cells of organisms in the five king- doms each have many internal differences, as summarized in Table 3.2. Images of some organisms of each kingdom are given in Figure 3.17 as examples.
Prokaryotes (bacteria) are composed of cells containing no membrane-bound nucleus and no compartments or membranous organelles. They are much smaller than eukaryotes, by almost 10 times. Prokaryotic genetic material is “naked,” without the protection of a membrane and nucleus. They are composed of very few cell parts: a membrane, cytoplasm, and only protein-producing units called ribosomes. Even without most structures found in other organisms, prokaryotes contain genetic material to repro- duce and direct the functions of the chemical reactions occurring within its cytoplasm.
group domain cell type cell number cell Wall component energy Acquisition
Bacteria Bacteria Prokaryotic Unicellular Peptidoglycan Mostly heterotrophic, some are autotrophic
Protists Eukarya Eukaryotic Mostly unicellular, some are simple multicellular
Cellulose, silica; some have no cell wall
Autotrophic, heterotrophic
Plants Eukarya Eukaryotic Multicellular Cellulose Autotrophic
Animals Eukarya Eukaryotic Multicellular No cell wall Heterotrophic
Fungi Eukarya Eukaryotic Mostly multicellular Chitin Heterotrophic
From Introductory Plant Science by Cynthia McKenney et al. Copyright © 2014 by Kendall Hunt Publishing Company. Reprinted by permission.
table 3.2 Differences in Cell Structure within the Five Kingdoms: Plants, Animals and Prokaryotes.
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Prokaryotes have a simple set-up, but all of the needed equipment to carry out life func- tions. Bacteria have a rapid rate of cell division and a faster metabolism than eukaryotes. Most organisms on Earth, in terms of sheer number, are prokaryotes.
• As indicated in Chapter 1, prokaryotes include organisms in the Bacteria and Archae domains. These organisms will be discussed further in Chapter 8.
All other organisms (plants, animals, fungi, and protists) are eukaryotes. Cells of eukaryotes are complex, containing a membrane-bound nucleus that houses genetic material. Eukaryotic cells comprise compartments that form a variety of smaller internal structures, or organelles. Eukaryotic cells are the focus of this chapter, which will give an overview of the primary organelles and their functions (Figure 3.6).
Eukaryotes may be examined by dividing into its four groups: plants, animals, fungi, and protists. Plants contain cells that are surrounded by a cell wall, a rigid structure giv- ing its organisms support. Plant cells contain chloroplasts, which enable plants to carry out photosynthesis, using energy from sunlight to make food.
• Plant cell walls contain cellulose, which gives structure to plants as discussed in Chapter 2. The process of photosynthesis, producing food for plants, will be further discussed in Chapter 4.
Plants also have large vacuoles or storage compartments to hold water and minerals for a plant’s functions. While both plants and animals have a cell membrane, animal cells are
Photosynthesis
The process by which green plants use sunlight to synthesize nutrients from water and carbon dioxide.
Figure 3.6 a. Differences between prokaryotes and eukaryotes. Prokaryotes have a generally simple structure (see top cell in figure above), while eukaryotes (the lower cell in figure above) have multiple organelles and membranes forming complex com- partmentalization. From Biological Perspectives, 3rd ed by BSCS. b. Differences between plants and animals. Plant and animal cells perform different functions, and their subcel- lular structures are also different. Plant cells have chloroplasts to produce sugar and a cell wall to give added strength. The animal cell shown has no cell wall or chloroplasts but possesses centrioles. From Biological Perspectives, 3rd ed by BSCS.
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Figure 3.6 (Continued)
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84 Unit 1: That’s Life
less rigid, surrounded only by a cell membrane and lacking a cell wall for support. Both plants and animals contain membrane-bound organelles, but animals also contain a set of small structures called centrioles, which serve in cell division. Animal cells are also quite complex, as we will see. While lacking certain organelles, such as cell walls and chloroplasts, they have flexible strategies to perform many functions.
Fungi have cell walls but no chloroplasts. They are not able to make their own food and, instead live off of dead and decomposing matter as well as other living organisms,
centriole
Minute cylindrical organelles found in animal cells, which serve in cell division (not given in bold in text).
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Figure 3.6 (Continued)
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Chapter 3: The Cell As a City 85
to obtain energy. Mushrooms and yeasts are familiar types of fungi, which will be dis- cussed in Chapter 7.
Some species of protists are a bit animal-like in that they are able to move; other species are a bit plant-like in that they have chloroplasts. Protists such as Amoeba in Figure 3.7 have varied environments. Amoeba live in freshwater and, in a rare infectious disease, grow and destroy human brain cells. We will discuss protists in more detail in a later chapter.
Figure 3.7 Cells of the five kingdoms. While the cells of organisms in all of the kingdoms perform similar life functions, their individual structures enable differing functions unique to each kingdom. From Biological Perspectives, 3rd ed by BSCS.
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86 Unit 1: That’s Life
The Role of inheritance The stratification system depicted in our opening story is based on the inheritance of cellular components. We know that organelles are structures that carry out functions within a cell. In fact, organelles work in concert with one another, coming together to
Figure 3.7 (Continued)
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Chapter 3: The Cell As a City 87
form a complex, dynamic cell. Mitochondria, so important in the society in our story, are the powerhouses of the cell, providing energy for a cell’s functions.
All organelles are built and controlled by inherited genetic material. Thus, the way a cell works is based upon its genetics. But some organelles are inherited separately from the others. There are three ways to inherit organelles, including mitochondria: 1) maternal inheritance, in which organelles are inherited from mothers; 2) paternal inher- itance, in which organelles are inherited from fathers; and 3) bi-parental inheritance, in which organelles are inherited from both mothers and fathers. The inheritance type varies among species and for different organelles. For example, chloroplasts are pater- nally inherited in the giant redwood Sequoia but maternally inherited in the sunflower Helianthus. Most animal species have maternal transmission of mitochondria, as seen in the story, because female eggs hold most of the mitochondria in their large cytoplasmic cells. Sperm contributes very little cytoplasm or organelles in human species, although there are exceptions among other organisms; for example, green algae Chlamydomonas has paternal transmission of mitochondria.
Endosymbiosis Eukaryotes appeared in Earth’s history about 1.5 billion years after prokaryotes. In their 2.0 billion years on Earth, eukaryotes have evolved into living systems that range from butterflies to beavers, crocodiles to humans. As you progress through this text and the course, you will learn about how this amazing diversity evolved.
Eukaryotes have two types of organelles – those that evolved as membranes and those from other, simpler organisms as precursors, called endosymbionts. Endosymbionts
endosymbionts
Any organism living in the body or cells of another organism.
the Oxygen revOlutiOn
Have you ever tried to imagine the Earth in its early stages? After millions of years during which the Earth was a mass of molten gases, those gases began to cool into layers that became landmasses, while water vapor formed seas. There were as yet no animals and only a few prokaryotic forms living in the waters. One form of bacteria, known as cyanobacteria, is believed to have been among the first organisms to photosynthesize (convert light energy into chemical energy to drive cellular activities). Since a by-product of many forms of photosynthesis is oxygen, over several million years, more and more oxygen was released into the atmosphere – the oxygen revolution.
The oxygen revolution occurred, according to geologists, about 2.5 billion years ago. It led to an availability of oxygen that could be used by cells that evolved to use it. The advantage of using oxygen to yield larger amounts of energy led to an increase in the number of aerobic cells. Aerobic cells, or those cells that use oxygen as the fuel for obtaining energy, contain mitochondria to provide large amounts of energy production. Over the course of many millions of years, single-celled eukaryotes, with more complex cellular processes than prokaryotes, evolved, then multicellular eukaryotes, and eventually organisms became larger and more complex. It is important to realize that oxygen is a key player in the development of organisms since it can be used to produce fast, plentiful energy.
Oxygen revolution
The biologically induced appearance of dioxygen in Earth’s atmosphere 2.5 billions of years ago.
Aerobic
Occurring in the presence of oxygen or require oxygen to live.
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have their own genetic material and are semi-independent within the cells of eukary- otes. Endosymbionts include two types of organelles – mitochondria and chloroplasts. Mitochondria are the energy producers of animal cells, and chloroplasts are solar power transformers of plant cells. Mitochondria divide independently and have an internal environment that is different from that in the rest of the cell.
Evidence tells us that endosymbionts were once independent prokaryotes that were somehow incorporated into eukaryotic cells. Lynn Margulis, a well-known evolution- ary biologist, formulated the endosymbiotic theory, which states that some organelles in eukaryotes were descendants of ancient bacteria that were absorbed by larger cells. (Symbiosis refers to a mutually beneficial relationship of organisms living together; endo means within.) The larger cell gave ancient bacteria absorbed by larger cells a “home.” The bacteria received protection and in return gave their unique set of chemical reactions to the larger host cell.
Analysis of endosymbionts have particular uses in today’s society: Crime scene investigations can test for mitochondrial DNA; ancestry can be traced using mitochon- drial DNA; and research on diseases inherited due to faulty mitochondrial genetic mate- rial may yield medical treatments. In our story, Joules and Theta had a different form of mitochondria than Sally. Joules, Theta, and Sally all inherited their mother’s mitochon- dria, but Sally’s inheritance included a predisposition for a number of diseases.
We have said that mitochondria can be thought of as the power plants of the cell. We say this because mitochondria carry out the series of energy-producing reactions that convert food energy into ATP (defined in Chapter 2; the form of energy used to drive cell functions). This set of reactions is called cell respiration.
Chloroplasts may have originated from cyanobacteria, which were able to transform light energy into usable sugar for energy in the process called photosynthesis – the making of food (glucose) from sunlight, carbon dioxide, and water. Thus, precursor mitochondria provided instant ATP energy for animal host cells, and ancient chloro- plasts made stored glucose available for longer-term use in plants. Modern chloroplasts use sunlight to rearrange carbon to form food in the form of sugars, for cell usage. They are the solar power plants of cells because they trap sunlight and generate ATP energy.
• These energy-obtaining processes, cell respiration, and photosynthesis will be discussed in greater detail in Chapter 4.
Evidence for mitochondria and chloroplasts as endosymbionts is considerable:
1) Mitochondria and chloroplasts are similar in size and shape to bacteria, roughly 7 µm in length.
2) Both contain their own genetic materials and divide in the same way as prokaryotes, through binary fission (splitting in half).
3) Both contain the same type of 70S ribosomes (described below; small organ- elles that make protein) as bacteria, whereas eukaryotes contain 80S ribosomes.
4) Mitochondrial and chloroplast DNA are more related to bacterial than to eukary- otic DNA. See the proposed process on endosymbiosis in Figure 3.8.
We will now look at each of the structures of the cell, using the analogy of a city, with the organelles being structures critical to the smooth functioning of the “city.” Organelles work in concert with each other to carry out life functions. Chloroplasts and mitochondria carry out key energy-producing and releasing functions to drive cell activities. However, there is an intricacy to cell biology akin to the workings of a large and dynamic city.
endosymbiotic theory
The theory that states that some organelles in eukaryotes were descendants of ancient bacteria that were absorbed by larger cells.
cell respiration
A series of energy- producing reactions that convert food energy into ATP.
chloroplast
A part of plant that contains chlorophyll and conducts photosynthesis.
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Figure 3.8 This model shows how mitochondria and chloroplasts wound up in eukaryotic cells. Evidence for endosymbiosis. Chloroplasts and mitochondria are simi- lar in size and shape to bacteria. The ribosomes in bacteria and mitochondria and chlo- roplasts are “70S.” Most bacteria have a size of roughly 7–10 µm and 70S ribosomes, both characteristics are similar to mitochondria and chloroplasts.
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Proteobacterium engulfed by a heterotrophic
eukaryote
Loss of bacterial cell wall and transfer of some genes to the nucleus-endosymbiont becomes mitochondrium
Loss of bacterial cell wall and transfer of some genes to the
nucleus-endosymbiont becomes chloroplasts
Heterotrophic eukaryotes: animals,
fungi and some protists
Autotrophic eukaryotes: plants and algae
Cyanobacterium
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Cell Architecture: The Cell As a City The organelles of a cell work independently but in concert, with its components inter- acting actively, having many thousands of chemical reactions occurring at any one time. In an analogous way, the components of a city work independently but cooperatively to ensure its smooth functioning. The structures in the cell are shown in the cartoon image depicting the cell as a city in Figure 3.9.
A cell membrane or plasma membrane surrounds each cell, which is a wrapping that allows some materials across it. While all cells contain a plasma membrane, some cells have structures surrounding the membrane for protection and support. For example, plant cells have cell walls surrounding their membranes, and fungi have chitin barriers, which are protective polysaccharides.
Plasma membranes are important to living systems because they control the mate- rials entering and leaving them. Plasma membranes are selectively permeable. Selective permeability means that the membrane allows some materials to pass through cells but not others. Within the cell is the cytoplasm, a semisolid liquid that holds organelles suspended within it. Cytoplasm is roughly 60–80% water by volume, and chemical reac- tions occur within its medium. The other 20–40% of cytoplasm is composed of pro- teins and dissolved ions. Cytoplasm is all of the cell material found between the plasma
Plasma (cell) membrane
A biological membrane that separates the cell’s interior from the outside environment.
Selectively permeable
A condition in which the membrane allows some materials to pass through cells but not others.
cytoplasm
A semisolid liquid that holds organelles suspended within it.
Figure 3.9 A cell is like a city. Its parts work together to perform a cell’s function.
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A. Nuclear envelope City Hall
Nuclear pores
Nucleoplasm
Outer and inner membranes
F. Plasma membrane the Border Patrol
B. Endoplasmic reticulum The Subway
C. Golgi body Processing Plant
D. Centrioles MoversE. Mitochondrion Power Plant
Proteins
Phospholipid bilayer
Channel (allows ions and water to move in and out of cell)
Inner and outer membranes
Cristae Microtubules
Maturing face Secretory
vesicles Forming face
Rough E.R.
Smooth E.R.
Fixed ribosomes
Cisternae
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Chapter 3: The Cell As a City 91
membrane and the nucleus. A nucleus, or control organelle, serves as the city hall of the cell, usually centered within cytoplasm. It contains the genetic material that produces and controls the cell’s parts.
A cell’s architecture is very complex, with continual activity within many com- partments of each cell. Compartmentalization of a cell is accomplished by a series of membranes throughout its cytoplasm. These membranes allow for a surface to per- form chemical reactions such as those carried out by enzymes. The layers of mem- branes also separate different chambers of the cell so that conditions may be different within each chamber. An acidic pH in one compartment, for example, may be suitable for cell functioning in one section, while a basic pH might be needed in another section. Throughout the cell, ropes (such as collagen and elastin) and membranes maintain a cell’s organization. Figure 3.9 gives you an idea of the complex architecture of the cell.
plasma Membrane: The “Flexible” Border patrol A border surrounds all cells: in eukaryotic animal cells, the border is the plasma or cell membrane. This dynamic covering is very complex, with parts that continually move to allow certain materials into the cell and keep other substances out. The plasma mem- brane is composed of a phospholipid bilayer in and around which are membrane pro- teins (see Figure 3.10).
• Recall from Chapter 2 that phospholipids are molecules with a phosphate (hydrophilic) head and two tails made of carbon and hydrogen atoms.
The phospholipids are arranged as a bilayer, with the heads facing to the outside and tails to the interior of the cell. The membrane proteins are part of the plasma membrane border, moving in between cholesterol molecules and phospholipids. Although it may seem that they are arranged randomly, in fact, they form a pattern. The membrane is often referred to as a fluid mosaic (see Figure 3.11) because it is made of different pieces that form a pattern and seem to float and move in the watery environment. Note the phospholipid bilayer in Figure 3.11.
There are two types of membrane proteins suspended within the phospholipid bilayer: integral proteins, which span the entire lipid bilayer; and peripheral proteins, which station either inside or outside of the membrane. Integral proteins are also known as transmembrane proteins. These membrane proteins serve to anchor cells to each
nucleus
The central and the most important part of a cell and contains the genetic material.
compartmental- ization
The formation of cellular compartments.
Fluid mosaic model
A model that describes the structure of cell membranes.
integral protein (transmembrane or carrier protein)
A type of membrane protein permanently embedded within the biological membrane (not given in bold in text).
Peripheral protein
Is a protein that adheres only temporarily to the biological membrane with which it is associated.
Figure 3.10 Every living cell is surrounded by a cell membrane. From Biological Perspectives, 3rd ed by BSCS.
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other, move materials, much like a ferry, across the membrane, receive chemicals such as hormones, and transport ions through pores within them. These proteins orient them- selves according to the bonds they make with surrounding chemicals.
Integral proteins in the fluid mosaic model serve four functions:
1) As receptors, allowing chemicals to bind to cell surfaces. Insulin, a hormone- regulating blood sugar, binds to cells to increase the absorption of sugar from blood into cells. Insulin’s special shape matches to the shape on integral pro- teins. The docking of the two elicits chemical reactions within the cell to main- tain sugar balance;
2) As recognition proteins, giving the immune system a code that informs it that a cell is its own and not a foreign substance. Cells with the wrong recognition proteins are rejected, as occurs in cases of organ transplants whose codes do not match up with those of the recipient;
3) As enzyme surfaces, facilitating various chemical reactions occurring on the surface of a cell. Enzymes assist in the digestion of certain nutrients, the first step in allowing nutrients to be absorbed; and
4) As transport proteins, moving material across a membrane.
Transport proteins act as a sort of flexible border-patrol system, which allows some materials to pass through the membrane while keeping others out. This border-patrol system is an important part of the cell as a city. The size, shape, and chemistry of sub- stances attempting to move into and out of cells determine which materials pass through the plasma membrane. Fatty materials pass through the membrane easily. For example, ethanol in alcoholic drinks easily passes through membranes because it also dissolves in fats. It is absorbed through the phospholipid bilayer and quickly giving a “high” to a person after drinking. The interior of the lipid bilayer is hydrophobic, so it avoids water and dissolves other substances that avoid water. Thus, only other hydrophobic, usually fatty or fat-soluble, materials may pass easily through the bilayer.
Integral proteins of the membrane allow certain nonfatty materials, including smaller charged or polar particles to move through the lipid bilayer directly. These include chem- icals such as oxygen (O2), carbon dioxide (CO2), and ammonia (NH3). The quick move- ment of these materials is essential for life functions. For example, NH3, a nitrogenous waste that builds up within all cells, needs to be rapidly removed.
Thus, integral membrane proteins serve to facilitate movement of materials that can- not easily pass across the membrane. Integral proteins move nonfatty materials across the membrane, such as sodium ions Na+ and potassium ions K+. Larger polar molecules, such as amino acids and glucose, also move through polar (or charged) channels within integral proteins. Channels that are polar are thus hydrophilic, allowing materials with a charge to pass through. Some substances use carrier proteins as pumps for transport, as is the case for the very important ions sodium (Na+), potassium (K+), and calcium (Ca+2).In the case of the movement of water (H2 O), the most abundant chemical in living systems, it was recently determined from studies on aquaporins, or integral proteins with channels within them, that water presses its way through the bilayer using integral proteins. Sometimes water moves directly across the lipid bilayer by “wiggling” type motion to press itself through. Physical forces drive this movement.
As shown in Figure 3.11, short carbohydrate chains jut out from the plasma mem- brane. These serve as a recognition code for the immune system of animals. Embedded within the phospholipid bilayer of animal cells, cholesterol binds together molecules, helping to maintain the flexibility and motion of the membrane.
Consider for a moment, how many different pieces make up the plasma mem- brane, and you can see why it is called a mosaic. It is called a fluid mosaic because it
receptor
A structure of the cell’s surface that selectively receives and binds a specific substance.
recognition