Biology: An Interactive Tour
Biology: An Interactive Tour

Biology: An Interactive Tour

Lead Author(s): Robert Pozos

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Biology: An Interactive Tour is for a non-majors audience in technology-enhanced learning and makes the complex world of biological science approachable and relatable.

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Top Hat has reimagined the textbook – one that is designed to improve student readership through interactivity, is updated by a community of collaborating professors with the newest information, and accessed online from anywhere, at anytime.


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Key features in this textbook

Biology: An Interactive Tour introduces the content in a much more approachable way than traditional texts.

Includes homework sets with 30+ questions per chapter.

Embedded videos that apply biology concepts to the real world!

Comparison of Introduction to Biology Textbooks

Consider adding Biology: An Interactive Tour to your upcoming course. We’ve put together a textbook comparison to make it easy for you in your upcoming evaluation.

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Pricing

Average price of textbook across most common format

Up to 40-60% more affordable

Lifetime access on any device

$63

E-book

$126

Hardcover print text only

$117

E-book

$166.95

Hardcover print text only

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

In-Book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Only available with supplementary resources at additional cost

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Built-In Interactive Assessment Questions

Assessment questions with feedback embedded throughout textbook

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Pricing

Average price of textbook across most common format

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Up to 40-60% more affordable

Lifetime access on any device

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

$63

E-book

$126

Hardcover print text only

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

$117

E-book

$166.95

Hardcover print text only

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Only available with supplementary resources at additional cost

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

About this textbook

Lead Author

Robert PozosUniversity of Minnesota-Duluth School of Medicine

Robert Pozos has extensive experience studying human response to environments resulting in hypothermia and hyperthermia. He established the hypothermia laboratory at University of Minnesota-Duluth School of Medicine and was a part of the chief civilian scientists at Naval Health Research Center in San Diego where he evaluated the thermal effectiveness of military garb for combat operations.

Contributing Authors

Christina AlevrasUniversity of Saint Joseph

Marion McClaryFairleigh Dickinson University

Explore this textbook

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Chapter 4: Cells

Figure 4.1: An aircraft carrier (USS Cell) is similar to a cell in that is surrounded by water and has numerous mechanisms to survive. It generates its energy, but it obviously cannot reproduce. [1]

Your entire body is composed of cells that are floating in salt water. These cells make up your tissue and organs. This image shows the USS Cell ship as a "cell" with other "cells" approaching it. Each cell is an organized entity similar to a ship that is in contact with other ships. These "cells" may be invading bacteria that USS Cells must kill, or they may be a group of cells that are going to transmit some product required by the USS Cells.

In this chapter, you will learn about cells. We will discuss how cells form tissues, organs, and organisms; how the nucleus controls reproduction and all chemical reactions; how the mitochondria make energy-rich molecules; and how the transport system is required to feed the cell.  

Concept Map

This chapter is divided into the four different sections as shown in the concept map below.

Cells: This concept map provides you with a tour of the four concepts relating to this topic.​

4.1 Three Major Cell Types

Concept 4.1: Cells are divided into three major groups.​

The cell is the unit of all life forms. Whether it be a single cell or an organism with billions of cells, the function and dysfunction of the cell are fundamental to understanding the diversity of biological systems. The cell can be considered to be a carbon-based, saline-filled balloon enclosed by a sophisticated membrane that has various kinds of internal structures, as well as external ones composed of proteins, lipids, and carbohydrates. Inside cells, there are a number of key structures, called organelles, that are responsible for regulating multiple chemical reactions that maintain and trigger various functions of the cell. All cells utilize energy in the form of adenosine triphosphate or ATP. A master molecule called deoxyribonucleic acid, or DNA, in the nucleus, controls nearly every aspect of cell function. In the analogy of the fleet above, imagine one boat (in this case a red blood cell) being responsible for delivering oxygen and food to the other ships (or cells) to make ATP, and for removing carbon dioxide and other metabolic by-products from the cells. This is a large task that requires communication since animals have billions of cells that need to be supplied with food and removal of waste products.

4.1.1 War Between Cells

Figure 4.2: E. coli is a bacteria found in sewage that causes human disease. [2]

One day, a public warning is sent out urging citizens on the island of Maui to stay away from certain beaches. The beaches have a high level of E. coli bacteria. Sewage with a high concentration of E. coli and other bacteria has been swept into the various waterways of the island. Joe Smith is out late at night and gets into a fight with another person and is pushed into the water. He swallows some of the contaminated water. After a day at home, he notices that he is feeling sick, has diarrhea and a fever. He goes into the emergency room.

E. coli bacteria can be found in fecal material and may cause intestinal problems In rare cases; it can cause death when we eat contaminated food. It is a “simpler form of life” since it belongs to a domain of organisms called Bacteria that have no membrane around their nucleus, and interact with animal cells from a different domain called Eukarya.

The cause of Joe's sickness is bacteria, E. coli, that entered his body as he swallowed the contaminated water. The bacterium is millions of single cells that, like all cells, require energy and must reproduce. Bacteria are found in many environments ranging from sewage to meat left out in the head to long, or the human body. If bacteria are reproducing within our body, we get an infection. Our body fights against this bacterial infection with its own group of specialized cells found in the immune system. A person’s body, composed of millions of cells, is being attacked by millions of bacteria smaller than the width of a strand of your hair. Who wins? The story of biology, in this case, is the interaction of various cell types from different domains. Biological life is divided into three major categories called domains. They are Bacteria, Archaea, and Eukaryota. E. coli are from the domain bacteria whereas humans are from the eukaryote. How did these different domains originate?

Although we have been describing the animal cell in detail, there are other cells as well. The entire issue of different kinds of life can ultimately be asked is the origin of cells and whether there are different kinds of cells. 

4.1.2 Origin of the Cells

The origin of the first cells is a controversial issue. The consensus today is that after millions of years, a cell type originated from the hydrothermal vents in the sea. The vents were an ideal environment for different chemicals to combine, as well as for these chemical reactions to be enclosed in membranes. The primitive ocean was an organic soup from which the chemical structures that formed primitive cells originated. This process of combining molecules from the ocean, membrane building, and energy production took billions of years. This complex process began 3.3 billion years ago and is continuing. From this beginning, life diversified. There are other explanations for the beginning of cells which could gather energy and reproduce. The one presented, dealing with hydrothermal vents as the source of the beginning of life, is the one that is the most currently accepted. The evolution of single cells to multiorganisms took place over billions of years.

Ch4_Cells Timeline  figure 4.3.png
Figure 4.3. The evolution of current biological life forms demonstrates how old bacteria and archaea are relative to the eukaryotes.



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Presently, different forms of life are divided into Bacteria, Archaea, and Eukaryota. We know that since they share some common biochemical features that more than likely they sprung from a common ancestor. 

Figure 4.4: Living systems are divided into three domains: Bacteria, Archaea, and Eukaryota.

For example, a component of the protein-making system of the cell called the ribosome is found to be common for bacteria, archaea, and eukarya. Viruses do not have any ribosomes and are considered to be parasites since they cannot generate any of their own energy. Bacteria, archaea, and eukaryote split millions of years ago. The archaea, also called extremophiles, are those living forms that can stand extreme temperatures over 100°C(!). Others can survive in extremely alkaline or acid environments. They are remarkable in that some can survive in anoxic environments and thrive in petroleum deposits! A hypersaline environment is not a deterrent for these remarkable one cellular entities. Life in or on other planets may be similar to the archaea. 


Question 4.02

Which of the following is not considered to be a living system?

A

Viruses

B

Bacteria

C

Archaea

D

Eukaryotes


4.1.3 How Did These Cells Evolve? Where Did They and We Come From? 

Figure 4.5: Bacteria (B), Archae (A), and Eukaryota (E) are derived from the last universal common ancestor (LUCA). The lines demonstrate that they shared common molecular features.

Based on genetic studies, archaea, and eukaryotes are from the same genetic origin and bacteria are not. However, all of them share common features. The age of the eukaryote is 1.45 billion years in the fossil records, and they had mitochondria. Thus mitochondria are found in all groups of eukaryotes and not just in animal cells. LUCA refers to last universal common ancestor. Scientists continue to group living systems into different categories, based on their similarity of function or their genetic profile. Another grouping considers bacteria and archaea to be prokaryote (before cell), while the eukaryotes (true cell) comprise all other living systems. The controversy is far from over.

Question 4.03

You are on a mission to discover how cells live in hot environments and low acidity. This data will help your company see if life can exist in harsh environments such as Mars, Saturn, etc. Which group of cells would you study to see how they survive these difficult environments?

A

Archaea

B

Bacteria

C

Eukaryota: animal cells

D

Eukaryota: plant cells


4.2 Eukaryote Cells

Concept 4.2. Eukaryote cells have distinct anatomical and functional characteristics from prokaryotes

All living systems have some common structures and chemistry. In this chapter, we will be focusing more on the eukaryotes, which include plants and animals. However, as the example of the person who is fighting a bacterial infection demonstrates, the bacterial domain is also important. In addition, the recent findings that there are billions of bacteria in the human gastrointestinal tract which may contribute to our overall functioning suggests that they may have evolved with eukaryotes.

Figure 4.6: Prokaryotes (bacteria, algae) and eukaryotes (plants and animals) share common features.

All cells are not alike. The cell presented is a model for either plants or animals. But cells vary in shape, size, and function. The science of histology is devoted to studying the differences between cells. Each kind of cell is a chemical factory that, in many cases, also moves. Cells are dynamic and are constantly dying and being replaced


Question 4.04

Which of the following do prokaryotes lack, but are found in eukaryotes?

A

DNA

B

Organelles

C

Cellular Membrane

D

Ribosomes

The organelles of eukaryotes are presented in a table format to orient the reader. Each component is described briefly, except for the chloroplast which is described in the photosynthesis chapter. 

Figure 4.7: Shared Features Between Plants and Animals


Question 4.05

Match the organelle with its correct definition.

Premise
Response
1

Cell Wall

A

Organelle studded with ribosomes that produce proteins

2

Nucleus

B

A protective barrier that maintains the shape of the cell.

3

Ribosomes

C

Contains and protects the cell's DNA

4

Cytoplasm

D

Gel-like liquid that fills the cell

5

Lysosomes

E

Smallest organelles that turn RNA into protein through protein synthesis

6

Smooth ER

F

Membrane-bound organelles that are filled with digestive enzymes to break down biomolecules

7

Rough ER

G

Produces and metabolizes fats


4.2.1 The Cell Membrane

Cell membranes are a key component in the communication between the internal environment of the cell and the external environment. They are dynamic structures. Some organelles such as the nucleus are also enclosed with membranes. All membranes are composed of glycerophospholipid (GPL) molecules composed of glycerol, a phosphate group, and two fatty acid chains. Glycerol is a three-carbon molecule that is the backbone of the membrane lipids. In addition to lipids, membranes have proteins that account for roughly half of the mass of the membrane. These proteins are embedded in the membrane, and some proteins can stick out of both sides of the membrane. 

Figure 4.8: The cell membrane has glycerophospholipids and multiple embedded proteins such as transporters, receptors, enzymes, and, anchors. In addition, it is electrically polarized with a positive charge on the extracellular side and a negative charge on the intracellular side.

A cell membrane plays four essential roles to ensure the cell’s survival:

  • Gatekeeper (Transporter): A Cell structure that regulates which molecules go in and out of the cell. It has various kinds of openings called pores, and different kinds of carriers which are proteins (enzyme) that transfer chemicals either into or out of the cell. 
  • Receptor: A protein which, when activated by an extracellular molecule, activates an intracellular process. The membrane-bound protein is called a receptor. 
  • Anchor: it has proteins that physically link intracellular structures with extracellular structures. 
  • Identifier: The cell membrane has groups of protein that identify it. These proteins are called major histocompatibility complex (MHC) which identify a cell as belonging to the organism or being an invader. This will be discussed in the immunity chapter. 


Question 4.06

Which of the following properties of a membrane are responsible for receiving molecular signals from outside (extracellular) space.

A

Gatekeeper

B

Receptor

C

Anchor

D

Identifier


Figure 4.9: Cross section of a cell showing the major organelles each of which has a major function. The organelles are floating in a saline solution allowing them to chemically communicate with each other.

4.2.2 Cell Membranes: First Line of Communication with the Outside World

Similar to a ship, the cell monitors its outside world and protects itself with an external barrier. This barrier, called the plasma membrane, is made up of a phospholipid bilayer that supports an electrical charge difference. Due to the nature of the membrane and its ability to separate different ions, a cell normally has a negative electrical charge on the inside and a positive electrical charge on the outside. 

Sensors called receptors are embedded in the plasma membrane that can detect various incoming signals, much as a submarine can monitor its surroundings using fancy equipment. These sensors are all chemical structures such as proteins or protein-carbohydrate groups (glycoproteins) that, when perturbed, will trigger various cellular reactions. These receptors respond to chemical signals that are transmitted in the blood. For example, the cigarette that you smoke contains nicotine, which travels in the blood that subsequently attaches to a “nicotine” receptor on the membrane of the brain, heart, and intestinal cells. This consequently triggers a different chemical reaction.

In addition to detecting various products, the cell membrane also controls the entry and expulsion of various chemicals depending on the chemical/electrical interactions between the components of the cell membrane and the structure that is entering or leaving. For example, the movement of salt composed of sodium and chloride in and out of cells will depend on their concentration differences and electrical charge differences across the cell membrane. These two forces combine to form the electrochemical gradient that any substance wishing to cross the cell membrane must follow.

The mechanisms of transport in and out the cell vary depending upon the electrochemical gradient and the proteins present for a particular substance. A substance, such as water, enters by specific protein channels called aquaporins. A molecule like glucose requires special protein carriers on the membrane to take it inside the cell. Movement of substances into the cell through the membrane are categorized as either active or passive transport. Active transport requires ATP, whereas passive does not. Active transport requires energy since the cell is importing a substance against a concentration gradient. Normally, without chemical energy, ions will not move against a concentration gradient. Passive transport does not require energy since the ions are flowing with the concentration gradient. This phenomenon will be discussed in more detail in other modules as we study different organs.

Figure 4.10: Nicotine in cigarettes activates specific membrane receptors in nerve cells [3]

The membranes of different cells vary. Thus a heart cell membrane will respond to nicotine, which attaches to nicotine receptors on the surface, but fat cells will not respond since they do not have nicotine receptors. 


Question 4.07

As you inhale your first cigarette for the day, you notice that your heart rate increased. This is due to which of the following?

A

Nicotine stimulates the nucleus of the heart

B

Nicotine stimulates the mitochondria of the heart

C

Nicotine stimulates the nicotinic receptors on the membrane of the heart cells

D

Nicotine activates the plasma membrane directly.


4.2.3 Cytoskeleton: The Support for the Cell

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division and allows the cell to move. The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments, and microtubules. Their function is as follows: 

Figure 4.11: The cell has molecular scaffolding to support its internal and external structure as well as its movements.

Microfilaments are small protein fibers that are composed of the contractile protein, actin. Microfilaments of actin in conjunction with myosin are responsible for muscle contraction. In addition, they are responsible for cellular movements such as gliding, contraction and the separation and division of dividing cells. They are called microfilaments since they are 3-6 nanometers in diameter relative to the size of microtubules.

Microtubules are hollow tubes that function as the scaffolding of the cell. They determine the cell shape and provide an internal transport network on which cell organelles and vesicles move. Microtubules are the largest component of the cytoskeleton measuring 20-25 nanometers in diameter and are composed of tubulin. Their length varies from 200 nanometers to 25 micrometers. They compose the cilia and flagella of cells with a characteristic 9 microtubule doubles +2 central single microtubules. Spermatozoa propel themselves towards the egg by the whipping motion of the tail composed of microtubules. 

Intermediate Filaments provide tensile strength for the cell so that it does not split apart. Intermediate filaments are in between microfilaments and microtubules measuring 10 nanometers in diameter. 


Question 4.08

Which of the following plays a major role in supporting the movement of cells?

A

Microtubule

B

Microfilaments

C

Intermediate filaments

D

None of the above since cells do not move

The solution inside the cell excluding the nucleus is called cytoplasm. It is a gel-like solution containing water and proteins, and it is an ideal medium in which chemical reactions can occur. 

4.2.4 The Nucleus: The Command Center

The control center of animal and plant cells is the nucleus. This is the largest structure of the cell and has its own double-layered membrane with pores which allow the exit and entry of molecules and ions. The nucleus contains the nucleolus, which houses the chromosomes, and is composed of DNA that dictate what the cell will do through the production of proteins. The chromosomes are identical in all cells within an organism. but only certain parts of them, the genes, are expressed in different parts of the body. For example, you have skin cells, intestinal cells, bone cells, hair cells, etc. Each of their nuclei contains chromosomes that have an identical chemical composition of DNA. The difference lies in that those parts of the chromosome (DNA) used to make a skin cell are activated only on the skin and nowhere else.

Figure 4.12. The nucleus is the molecular command center for directing the activities of the cell.


The nucleus has nucleoplasm in which the solutes of the nucleus are dissolved. 

Each cell reproduces according to the direction given in the DNA of its nucleus. The nucleus will direct cell division called mitosis or meiosis. Mitosis refers to a process in which the cell completely copies itself, whereas meiosis refers to a process by which the chromosomes are mingled, and a cell with a different chromosomal arrangement is produced. Meiosis is specifically found in reproductive cells.


Question 4.09

Which of the following structures in the nucleus allow chemicals to enter and leave?

A

Nucleoplasm

B

Nuclear Pores

C

Chromosomes

D

Genes


4.2.5 Endoplasmic Reticulum: Making Protein

Ships have their own manufacturing systems. In the same manner, cells have their factory which receive their direction from the nucleus. These cellular factories are the rough and smooth endoplasmic reticula.

Figure 4.13: The ribosome is the site for protein production based on DNA instructions.

The endoplasmic reticulum is a network of internal membranes that fold over each other to form compartments which are held in place by the cytoskeleton. Part of the endoplasmic reticulum is continuous with the nuclear envelope. There are two types of endoplasmic reticula: rough and smooth. The rough endoplasmic reticulum (rER) manufactures proteins, and it has a "roughened" appearance since it has ribosomes attached to it which play a major role in protein synthesis and folding. The adrenal gland which is situated on top of the kidney, has two components, the cortex, and medulla. The cortex produces a large number of hormones such as cortisol and androgens, and it is characterized by having an extensive rough endoplasmic reticulum. 

The smooth endoplasmic reticulum (sER) has no ribosomes and thus has a smooth surface. It has different functions in various cells. In muscle cells, it stores calcium ions that are used in muscle contraction. In other cells, it is involved in lipid and cholesterol synthesis as well as repair of membranes. The sER plays a major role in detoxifying products in the liver. If large amounts of alcohol or drugs enter the liver, the sER doubles in size to metabolize the alcohol or drugs. 

An interesting point about the endoplasmic reticulum is that it is not present in mature red blood cells. The red blood cells are not producing any protein but are functioning as a carrier of oxygen. As a consequence, it does not need the endoplasmic reticulum. 


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4.2.6 Golgi Complex: Packaging and Modifying Protein

Figure 4.14: The Golgi apparatus is rich with enzymes to modify and transport molecular products produced by the endoplasmic reticulum

The nucleus also directs the molecular signals for the production of proteins in the cytoplasm. The molecular signals from the nucleus are sent out through the nuclear pores and into the cytoplasm. Every protein in a cell has to be made under the direction of the molecules coming from the nucleus. The key organelle for making the protein is the rER. The Golgi complex modifies, packages, and transports proteins. 

The endoplasmic reticulum in the cell produces proteins that need to be packaged. This process of packaging takes place in the Golgi complex. The products from the endoplasmic reticulum travel to a set of flattened, sac-like membranes, which sit on top of each other and are interconnected. When a protein is produced in the ER, the protein is pinched off within part of the membrane. The protein that is being processed by the Golgi apparatus is enclosed in the second membrane and pinched off. Protein modification in the Golgi complex is accomplished by the enzymes present in the flattened sac-like structures. The resulting package is called a vesicle. What types of products are contained within the vesicles in the Golgi apparatus? They are hormones, enzymes, etc. Some chemicals that have multiple effects on your body are produced by the Golgi complexes in the neurons in the brain and then are sent into the blood. This process of sending the contents of the vesicles outside the cell and into the blood or elsewhere is called exocytosis.


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4.2.7 Lysosomes: Janitorial Service and More!

The endoplasmic reticulum, Golgi apparatus, and lysosomes are all members of a network of membranes, but they are not directly connected to each other. Lysosomes are formed by the fusion of vesicles that have budded off from the Golgi. These membrane-bound organelles have enzymes that digest proteins, nucleic acids, lipids, and complex sugars. The internal environment of the lysosome is more acidic (pH of 4.8) than the cytoplasm, the pH of which helps to activate the enzymes. 

Lysosomes are organelles that are also packaged by the Golgi apparatus. They contain powerful digesting enzymes and are responsible for the breakdown and absorption of materials taken in by the cells. In the case of Joe Smith, who is in the hospital fighting for his life, his white blood cells are chasing down the E. coli bacteria in his body. Once the bacteria are ingested, the lysosomes go to work to digest the bacteria, breaking down the E. coli's plasma membrane so that the bacteria will die. 

Another important function of lysosomes is that it degrades cellular components so that they can be recycled. Malformed proteins or those that are no longer functional are degraded by lysosomal enzymes. The result of this process called autophagy(self-eating) is the recycling of amino acids, carbohydrates and other metabolites in the cell. During periods of stress, e.g. lack of food, the lysosomes will trigger autophagy to produce nutrients for the cell. 

The extensive role of the lysosome is still being actively investigated. Its importance cannot be overstated. Diseases due to lysosomal malfunction are based on specific enzymes that are dysfunctional. Tay-Sachs disease is a disorder of the nervous system that usually affects infants in which there is an accumulation of certain kind of fats in the brain and nerve cells. Normally, these would be metabolized by normal lysosomes. In this case, the enzymes do not break down the lipids and, as a consequence, nerve cells do not function properly. Tay-Sach is characterized by listlessness, severely diminished muscle tone, blindness, deafness, and eventually paralysis. There is no cure for this lysosomal disease. 

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4.2.8 Exocytosis and Endocytosis: Cellular Communication

Similar to the ships in a fleet, cells must communicate between and amongst themselves. One of the most common techniques is for cells to release chemicals that subsequently interact with other cells or with itself. The process by which cells release chemicals is called exocytosis meaning “to be out of the cell” whereas endocytosis refers “to be in the cell”. Exocytosis is used by nerve cells to secrete chemicals to alter the activity of other cells. In a similar fashion, hormones are chemicals produced by cells and by way of exocytosis are distributed to other cells by way of the blood. 

Figure 4.15: Exocytosis and endocytosis are processes that secrete substances which are subsequently picked up. This process can occur in one cell as shown or can involve multiple cells.

The cell produces protein under the direction of DNA by a feedback process. The DNA in cells will activate the process of protein production that creates the hormone. Once the hormone is produced, its concentration is "sensed" by the comparator in the nucleus of the cell which produced the hormone. If the amount of hormone is too low relative to the set point, the additional hormone will be produced. This process is called positive feedback since additional hormone is required and entails activating the protein production sequence in the cells.

Figure 4.16: A feedback loop demonstrates how cell organelles produce and monitor levels of a hormone. This feedback loop is positive since the output is not at the level dictated by the set point.

4.2.9 The Mitochondria: Energy Production

All ships require energy to navigate. In the modern fleet, ships are powered by some form of internal energy. In cells, mitochondria produce energy for the cell. Cells produce their own energy from the nutrients (proteins, carbohydrates, and lipids) we eat and the oxygen we breathe. Cells have two chemical pathways of producing energy. They manufacture ATP (energy) in the cytoplasm, from nutrients such as glucose, through a sequence of biochemical reactions called glycolysis. The second chemical pathway manufactures energy in specialized organelles called mitochondria by process of oxidative phosphorylation. This process requires oxygen, which diffuses into the mitochondria, as well as the oxidation of food which results in electrons, hydrogen ions, and enzymes to manufacture the energy in the form of ATP. Oxidative phosphorylation also produces carbon dioxide which leaves the cells and enters the blood. (Oxidative phosphorylation will be explained in greater detail in a separate chapter.)

Figure 4.17: A mitochondrion produces an energy-rich chemical called “ATP” that powers all functions in the cell. The mitochondrion requires oxygen, carbohydrates, lipids, and protein for this function.


The mitochondrion's well-being spells the difference between life and death for us. If the mitochondria do not produce energy (ATP), none of the functions of the cell can continue, and the cell will die. When the “life” of a critical number of cells ceases, the life of the tissue, organ, and organism may cease as well. The critical number of cells needed to maintain the life of tissue will depend on the type of tissue. For example, skin cells die and are replaced continually, but if too many cells in the heart or brain die from a heart attack or a stroke, the individual will also die since there is no sufficient ATP to maintain the tissue

RP_CH4CellularFunctions_V1-02.jpg
Figure 4.18: Two important cellular functions: energy capture by way of the mitochondria, and cell reproduction directed by the nucleus.


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4.2.10 Cell Death: Determined by DNA

Figure 4.19: Cells have a very high turnover in multi-organismic animals.

The number of cells that are constantly being recycled or eliminated is astronomical. Estimates range from 50-70 billion cells/day. Different cells have different lifespans. Red blood cells live for four months, white blood cells for one year, skin and intestinal cells for 3-4 weeks. Sperm live cells for three days. Neurons are thought to live for a lifetime. Regardless, all cells have a programmed lifespan and will die by a process called apoptosis (self-killing). Cell death involves fragmentation of the nucleus, DNA fragmentation, and cell shrinkage. This process is used to recycle cellular components as well as to regulate cell growth. It is a recycling process that is found in all living systems. Another form of cell destruction is called autophagy (self-eating) in which there is a degradation of cytoplasmic contents and damaged organelles. Autophagy is thought to involve the lysosomes, which will open up and use their enzymes to destroy the cell from the inside. Both apoptosis and autophagy are linked and are used by cells in many situations. In cases of a decrease in nutrition, liver cells will undergo autophagy to supply other cells with vital nutrients. Necrosis is another form of cell death which is not self-induced but is a result of external factors. For example, trauma, infection, and a decrease in oxygen supply will all cause cell death. Cells compose tissues which give rise to organs. Organs and tissue can have a certain amount of cell death and still function. However, once a critical number of cells die, the organism experiences different disease states. Alzheimer's disease is characterized by selective apoptosis/autophagy of neurons in the brain resulting in memory loss and dementia. The role that apoptosis and autophagy have in normal and abnormal situations is a new area of research dealing with cells. 

Question 4.14

In Puerto Rico, a hurricane has devastated the island. Water is everywhere, and so is sewage. Which of the following would be the most dangerous in terms of affecting human life?

A

Archaea

B

Bacteria

C

Eukaryota: animal cells

D

Eukaryota: plant cells


4.3 Cells

Concept 4.3: Tissue is composed of cells, and different tissues compose organs.

The cell is the basic unit of life. The story of biology includes how life has progressed from individual cells to complex organisms involving multiple cell types. These multi-cellular organisms are composed of cells that combine to form tissues which subsequently form organs. Tissues contain cells of one kind. Muscle cells make up a tissue but not an organ. An organ is a group of tissues. Your muscle has muscle cells, fat cells, blood cells, etc. Thus the heart is an organ, not a tissue since it has multiple tissues. Organ systems are combinations of organs. The organ systems are: nervous, skeletal, cardiovascular, respiratory, endocrine, immune, muscular, digestive and skin or integumentary. Each organ system, organs, and tissues are dependent ultimately on the individual cells. Cells have specialized functions depending on the organ system. The neuron is the cell for the nervous system, but it is not alone. It has specialized interaction with other cells and with the blood. No cell is an island unto itself in a multi-organ system.


Question 4.15

Arrange in order from smallest to largest.

A

Organism

B

Cells

C

Tissues

D

Organs


What is usually not presented in most discussions about cells, tissues, and organs is that they cannot survive without a system to bring them food and oxygen. Cells need to produce energy. In Homo sapiens, all of our cells exist in conjunction with the small blood vessels, called capillaries, which carry food and oxygen to the cells and remove the metabolic wastes.

Figure 4.20: Organization in Multicellular Animals: Different Levels of Biological Organization


Question 4.16

Which of the following organs do not have tissues?

A

Circulatory

B

Respiratory

C

Neural

D

Muscular

E

None of the above answers are correct. All organs are composed of different tissues


4.3.1 Connections Between Cells

In multicellular systems, cells are connected to each other by various kind of anatomical/chemical links such as tight junctions, adherens junctions, gap junctions, and desmosomes.

4.3.2 Tight Junctions

Cells have an orientation with the apical, or top part, being exposed to the lumen (opening), and the basal part is the bottom part that interacts with the extracellular fluid. These cells are connected at their apical ends, and this connection is called a tight junction since it limits the passage of molecules and ions. If molecules or ions are to go from the lumen to the extracellular fluid, they will have to go through the cell. They also block the movement of molecules and ions from the other direction so that molecules cannot mix with each other. Tight junctions are found in the lungs. Smoking will cause an increase in permeability of lung cells, which means that the tight junctions are not that effective. This increase in permeability will allow the molecules from the basolateral surface to interact with those on the apical site triggering an increase in cell growth. 

Figure 4.21: A tight junction connects certain parts of adjoining cells.

4.3.3 Adherens Junctions

Cardiac cells continuously beat, and they need to have a strong connection between each other. The adherens junctions hold cardiac cells tightly together as they contract and relax. They are composed of a central element called cadherins, which are attached to an anchoring structure called catenin. Catenins, in turn, are attached to contracting filaments composed of actin. Thus, as the cells contract, the entire adherens junction moves in concert with the contraction. 

4.3.4 Gap Junctions

Some cells require a much easier flow of ions and molecules amongst themselves. The gap junction fulfills this role as it permits the free passage of ions and small molecules between cells. The movement of ions supports the transmission of electrical signals from one cell to another. This connection allows for the very fast transmission of electrical signals since there is no need for any chemical transmitters to be released. 

4.3.5 Desmosomes

Some cells have only part of their membrane attached to another by the desmosomes instead of the Adherens Junctions. The desmosome is found connecting skin cells in which they are attached to intermediate filaments in the cytoplasm. 

Figure 4.22: Cells are connected by way of different junctions: adherens junctions, tight junction, desmosomes, and gap junctions.

4.3.6 Plasmodesmata

Animals cells are not the only cells that have communication with each other. Plant cells are also linked by plasmodesmata, which is the continuation of the cytoplasm from one cell into the adjoining cell. These are the specialized junctions between plant cells that allow for the movement of ions, molecules—such as sugar and amino acids—and even larger molecules such as proteins between cells. The plasma membrane is continuous between cells raising the question of whether the two cells are really separate. 

Figure 4.23: Plant cells are connected by way of plasmodesmata.


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4.4 Transport Systems

Concept 4.4: Cells transfer chemicals internally and externally

4.4.1 Intracellular, Extracellular, and Interstitial Spaces

Modern ships have to move in an aqueous environment, and all their food and material is transferred by way of water.

In a similar manner, the cell is a saline-filled, carbon-based balloon connected to other carbon-based balloons floating in a sea of saline, or in close proximity to each other. The term intracellular space (inside the cell) refers to the contents inside the cell, whereas the term: extracellular space refers to the space outside the cell. A subcategory of extracellular space is interstitial space, which is the space between cells. The chemical composition of these compartments may differ greatly. 

Some ships are stationary while others move. Within Homo sapiens, the white blood cell is like a military "Navy Seals Unit" that travels in the blood plasma to seek out "enemies", in this case invading bacteria or viruses. Navy Seals move from the seas to the land. In a similar fashion, white blood cells move from the blood to the interstitial space to kill microbes.

Figure 4.24: The white blood cell (colored blue) waits to neutralize an invading bacteria or virus

The red blood cell is like a quiet "submarine". It floats in the blood, delivering oxygen to all cells and picking up carbon dioxide. The actual process of oxygen delivery and accepting carbon dioxide are chemical reactions. In this case, the oxygen goes from the blood (extracellular space) into the cell (intracellular space).

Cardiac cells in the heart do not move from place to place, but they do the contract. These cells require a large amount of energy and therefore need a continuous, blood supply. Every cardiac cell must work together with other cardiac cells, which means that they are all connected physically. Cardiac cells have "gap junctions", which enable the cells to share their cytoplasm. Imagine two castles that share a common wall. There are holes (pores) in the wall to facilitate the transport of materials and information. In this case, the communication is between two cells intracellularly. 

4.4.2 Extracellular to Intracellular Movement or Intracellular to Extracellular Movement

Remember that movement of water goes both ways: 1. From the outside of the cell to the inside (endocytosis) or 2. From the inside of the cell to the outside (exocytosis). The methods of movement of water need to address the fact that the water must cross the cell membrane. The cell membrane is not a simple blob. It has electrical properties as well as physical and chemical properties that control what moves in and out of the cell. If the molecule has no electrical charge and is small enough to move through the pores of the membrane, it does not meet any resistance. Water, carbon dioxide and urea are examples of small, uncharged molecules.

Since the membrane has a large number of lipids, any molecule that is lipid soluble can enter the cell through the membrane itself. Oxygen, nitrogen and certain kinds of anesthetic gases are examples of lipophilic (lipid lover) substances. But how do the chemical enter and leave the cells?

4.4.3 Water, Electrolytes, Molecules Enter Cells either Passively or Actively

Passive transport is best exemplified by diffusion. Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules, the concentration difference, size of the molecules as well as the temperature of the solution and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are equally randomly distributed on either side of the membrane system. For oxygen to reach cells, it diffuses from the blood to the cells. As long as the cells use the oxygen, there will be an oxygen gradient favoring the movement of oxygen into the cells. The opposite occurs for carbon dioxide. It is a gas produced by the cells, and it diffuses from the cell to the blood for eventual release into the environment. Passive transport does not allow the cell to play a major role in what enters of leaves since it is purely on the basis of concentration differences. The cell has other strategies that it uses so it can be more discriminate about enters or leaves the cell. 


Question 4.18

For passive transport to function, which of the following is required?

A

Energy source such as ATP

B

Same concentration of ions on either side of the membrane

C

Same number of proteins on either side of the membrane

D

Concentration gradient between either side of the membrane.


4.4.4 Facilitated Passive Transport

This method of transport relies on the activation of membrane-embedded protein and large polar molecules or ions chemical being transported. The protein plays an active role in promoting the movement of the chemical from the extracellular to intracellular space. Facilitated passive transport can be accomplished by either carrier proteins or channel proteins. Carrier proteins are selective to transport specific molecules whereas a channel protein constitutes a channel. The difference between facilitated passive transport and active transport process is not clear since in some cases of facilitated passive transport ATP as an energy source is required. 

Figure 4.25: Facilitated diffusion can occur by two methods: channel or protein transfer

4.4.5 Active Transport: The Sodium/Potassium Pump

Active transport refers to the transportation of molecules against their concentration gradient which requires energy usually in the form of ATP. Active transport works against diffusion. If there is a higher intracellular concentration of potassium than the interstitial fluid, based on diffusion, the ions will diffuse through the membrane and out. However, to reverse this trend, energy in the form of ATP is used at the cell membrane to “pump" the potassium ion inside the cell. The ATP works with specialized proteins that play a major role in capturing the molecule in question and transporting it against a concentration gradient. It is similar to the castle having thousands of people inside the courtyard wanting to leave. As they push to leave, big, burly guards grab them and throw them back into the courtyard. The guards are the protein structures that are activated by ATP.

Active transport requires energy in the form of ATP which is formed by the mitochondria inside the cell. The cells have many different kinds of pumps. In one case, ATP is used to pump sodium outside the cell and bring potassium inside. The transfer of ions are linked, but the transfer is not 1 sodium ion for 1 potassium ion but three sodium to 2 potassium ions indicating selectivity of the activated protein structure. Without either of these important components (ATP or proteins), the cells will lose their ability to control their chemical and electrical composition and will die. 

4.4.6 Active Transport: The Calcium Pump

Biological systems have many different kinds of pumps. For example, calcium pumps are found in the membrane of the sarcoplasmic reticulum. Do not think that all pumps are located just on the plasma membrane. Calcium is important for muscle contraction, triggering a fertilized egg to develop, secretion of hormones, and for the release of neurotransmitters. Calcium ions are pumped out of muscle cells so that the intracellular concentration is 10000xs less than the extracellular concentration. It works in conjunction with movement of sodium ions so that it is coupled with the movement of other ions. 

The science of membrane transport is interesting and complex. From an evolutionary point of view, the primitive cells that eventually developed into prokaryotes had to control their extracellular environment to survive. All organ systems utilize ionic pumps and active transport and will be discussed in their respective chapters. 


Question 4.19

For active transport to work, which of the following is required?

A

Energy in the form of ATP

B

Concentration gradient

C

Proteins in the membrane that transport chemicals

D

Energy (ATP) interacting with carrier proteins to move chemicals against a concentration gradient.



Question 4.20

The transition from single cells to multiorgan systems required how much time?

A

Hundred years

B

Million years

C

Billion years

D

Multi-billion years


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Question 4.22

Multi-organ systems require which of the following to coordinate their activity?

A

Chemical communication

B

Physical communication such as tight junctions

C

Electrical communication

D

All of the above


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Figure 4.26: Chapter overview of the cell.

4.5 Vocabulary Questions

Vocabulary Question 4.01

The smallest structural unit of an organism that is capable of independent functioning, consisting of one or more nuclei, cytoplasm, and various organelles, all surrounded by a semipermeable cell membrane.


Vocabulary Question 4.02

A thin membrane (a double layer of lipids surrounding a protein layer) enclosing the cytoplasm of a cell; proteins in the membrane control passage of ions (like sodium or potassium or calcium) in and out of the cell.


Vocabulary Question 4.03

Any of various blood cells that have a nucleus and cytoplasm that separate into a thin white layer when whole blood is centrifuged, and help protect the body from infection and disease. White blood cells include neutrophils, eosinophils, basophils, lymphocytes, and monocytes.


Vocabulary Question 4.04

Cells that transport oxygen and carbon dioxide between lungs and tissues.


Vocabulary Question 4.05

Reword the question: Any of the cylindrical hollow structures that are distributed throughout the cytoplasm of eukaryotic cells that provide structural support as well as playing a role in cellular locomotion such as spermatozoa movement.


Vocabulary Question 4.06

A specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genic characters; organizing center of the cell.


Vocabulary Question 4.07

_____ Endoplasmic Reticulum. Parts of endoplasmic reticulum without ribosomes with varying functions from cell to cell and include steroid hormone synthesis, and storage of ions.


Vocabulary Question 4.08

_____ Endoplasmic Reticulum. Parts of endoplasmic reticulum without ribosomes with varying functions from cell to cell and include steroid hormone synthesis, and storage of ions.


Vocabulary Question 4.09

Set of flattened sac-like membranes that function in the packaging and processing of newly synthesized proteins from the endoplasmic reticulum.


Vocabulary Question 4.10

The energy (ATP) producing organelle of cells.


Vocabulary Question 4.11

Liquid contained inside the cell membranes (usually containing dissolved solutes).


Vocabulary Question 4.12

Liquid found between the cells of the body that provides much of the liquid environment of the body.


Vocabulary Question 4.13

A liquid containing proteins and electrolytes including the liquid in blood plasma and interstitial fluid.

4.6 Image Credits

[1] Image courtesy of tpsdave in the Public Domain.
[2] Image courtesy of National Institutes of Health in the Public Domain.
[3] Image courtesy of geralt in the Public Domain.
[4] Image courtesy of the National Cancer Institute in the Public Domain.