As you complete the assignment, keep track of the resources that you used and list ALL of them as part of your assignment submission. If you used websites

5: CELLS

Suzanne Wakim & Mandeep Grewal Butte College

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CHAPTER OVERVIEW

5: CELLS This chapter outlines the discovery of cells and cell theory. It identifies ways in which all cells are alike and ways in which they vary. The chapter describes in detail important cell structures and their functions; and it explains how cells obtain energy, grow, and divide.

5.1: Case Study: The Importance of Cells 5.2: Discovery of Cells and Cell Theory 5.3: Variation in Cells 5.4: Plasma Membrane 5.5: Cytoplasm and Cytoskeleton 5.6: Cell Organelles 5.7: Cell Transport 5.8: Active Transport and Homeostasis 5.9: Cellular Respiration 5.10: Fermentation 5.11: Case Study Conclusion: Tired and Chapter Summary

This page titled 5: Cells is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform.

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5.1: CASE STUDY: THE IMPORTANCE OF CELLS

CASE STUDY: MORE THAN JUST TIRED We all get tired sometimes, especially if we have been doing a lot of physical activity. But for Jasmin, a 34-year-old former high school track star who is now a recreational runner, her tiredness was going far beyond what she thought should be normal for someone who is generally in good physical shape. She was experiencing extreme fatigue after her runs, as well as muscle cramping, spasms, and an unusual sense of heaviness in her legs. At first, she chalked it up to getting older, but her exhaustion and pain worsened to the point where this former athlete could no longer run for more than a few minutes at a time. She also began to experience other unusual symptoms, such as blurry vision and vomiting for no apparent reason.

Figure : Exhausted

Concerned, she went to her doctor. Her doctor ran many tests and consulted with several specialists. After several months, Jasmin is finally diagnosed with a mitochondrial disease. Jasmin is surprised. She has an 8- year-old niece with a mitochondrial disease, but her niece’s symptoms started when she was very young, and included seizures and learning disabilities. How can Jasmin have the same disease but different symptoms? Why did she not have problems until adulthood while her niece had symptoms at an early age? And what are mitochondria anyway?

CHAPTER OVERVIEW: CELLS As you will learn in this chapter, mitochondria are important structures within our cells. This chapter will describe cells, which are the basic unit of structure and function in all living organisms. Specifically, you will learn:

How cells were discovered, their common structures, and the principles of cell theory. The importance of size and shape in the functions of cells. The differences between eukaryotic cells (such as those in humans and other animals) and prokaryotic cells (such as bacteria). The structures and functions of parts of cells including mitochondria, the plasma membrane, cytoplasm, cytoskeleton, nucleus, ribosomes, Golgi apparatus, endoplasmic reticulum, vesicles, and vacuoles. How the processes of passive and active transport move substances into and out of cells and help maintain homeostasis. How organisms obtain the energy needed for life, including how the sugar glucose is broken down to produce ATP through the processes of aerobic cellular respiration and anaerobic respiration.

As you read this chapter, think about the following questions related to Jasmin’s disease:

1. What are mitochondria? What is their structure, function, and where did they come from during evolution?

2. Why are fatigue and “exercise intolerance,” such as Jasmin’s extreme exhaustion after running, common symptoms of mitochondrial diseases?

3. Why do you think Jasmin has symptoms that affect so many different parts of her body including her legs, eyes, and digestive system?

ATTRIBUTIONS 1. Tired by Dace Kiršpile licensed CC BY 2.0 via Flickr 2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

This page titled 5.1: Case Study: The Importance of Cells is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform.

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5.2: DISCOVERY OF CELLS AND CELL THEORY

What is this incredible object? Would it surprise you to learn that it is a human cell? The cell is actually too small to see with the unaided eye. It is visible here in such detail because it is being viewed with a very powerful microscope. Cells may be small in size, but they are extremely important for life. Like all other living things, you are made of cells. Cells are the basis of life, and without cells, life as we know it would not exist. You will learn more about these amazing building blocks of life when you read this section.

Figure : Healthy human T-cell

If you look at a living matter with a microscope — even a simple light microscope — you will see that it consists of cells. Cells are the basic units of the structure and function of living things. They are the smallest units that can carry out the processes of life. All organisms are made up of one or more cells, and all cells have many of the same structures and carry out the same basic life processes. Knowing the structure of cells and the processes they carry out is necessary to understanding life itself.

DISCOVERY OF CELLS The first time the word cell was used to refer to these tiny units of life was in 1665 by a British scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under a microscope. The microscopes of his day were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see what looked like a honeycomb. Hooke made the drawing in the figure below to show what he saw. As you can see, the cork was made up of many tiny units, which Hooke called cells.

Soon after Robert Hooke discovered cells in cork, Anton van Leeuwenhoek in Holland made other important discoveries using a microscope. Leeuwenhoek made his own microscope lenses, and he was so good at it that his microscope was more powerful than other microscopes of his day. In fact, Leeuwenhoek’s microscope was almost as strong as modern light microscopes. Using his microscope, Leeuwenhoek was the first person to observe human cells and bacteria.

Figure : Robert Hooke sketched these cork cells as they appeared under a simple light microscope.

CELL THEORY By the early 1800s, scientists had observed the cells of many different organisms. These observations led two German scientists, named Theodor Schwann and Matthias Jakob Schleiden, to propose that cells are the basic building blocks of all living things. Around 1850, a German doctor named Rudolf Virchow was studying cells under a microscope when he happened to see them dividing and forming new cells. He realized that living cells produce new cells through division. Based on this realization, Virchow proposed that living cells arise only from other living cells.

The ideas of all three scientists — Schwann, Schleiden, and Virchow — led to cell theory, which is one of the fundamental theories unifying all of biology. Cell theory states that:

All organisms are made of one or more cells. All the life functions of organisms occur within cells. All cells come from already existing cells.

SEEING INSIDE CELLS Starting with Robert Hooke in the 1600s, the microscope opened up an amazing new world — the world of life at the level of the cell. As microscopes continued to improve, more discoveries were made about the cells of living things. However, by the late 1800s, light microscopes had reached their limit. Objects much smaller than cells, including the structures inside cells, were too small to be seen with even the strongest light microscope.

Then, in the 1950s, a new type of the microscope was invented. Called the electron microscope, it used a beam of electrons instead of light to observe extremely small objects. With an electron microscope, scientists could finally see the tiny structures inside cells. In fact, they could even see individual molecules and atoms. The electron microscope had a huge impact on biology. It allowed scientists to study organisms at the level of their molecules and led to the emergence of the field of cell biology. With the electron microscope, many more cell discoveries were made. Figure

shows how the cell structures called organelles appear when scanned by an electron microscope.

 A BIG BLUE CELL

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Figure : An electron microscope produced this image of the structures inside a cell.

STRUCTURES SHARED BY ALL CELLS Although cells are diverse, all cells have certain parts in common. These parts include a plasma membrane, cytoplasm, ribosomes, and DNA.

1. The plasma membrane (also called the cell membrane) is a thin coat of phospholipids that surrounds a cell. It forms the physical boundary between the cell and its environment, so you can think of it as the “skin” of the cell.

2. Cytoplasm refers to all of the cellular material inside the plasma membrane. The Cytoplasm is made up of a watery substance called cytosol and contains other cell structures such as ribosomes.

3. Ribosomes are structures in the cytoplasm where proteins are made. 4. DNA is a nucleic acid found in cells. It contains the genetic

instructions that cells need to make proteins.

These parts are common to all cells, from organisms as different as bacteria and human beings. How did all known organisms come to have such similar cells? The similarities show that all life on Earth has a common evolutionary history.

REVIEW 1. Describe cells. 2. Explain how cells were discovered. 3. Outline how cell theory developed. 4. Identify structures shared by all cells. 5. True or False. Cork is not a living organism. 6. True or False. Some organisms are made of only one cell. 7. True or False. Ribosomes are found outside of the cytoplasm of a cell.

8. Proteins are made on _____________ . 9. What are the differences between a light microscope and an electron

microscope? 10. The first microscopes were made around

A. 1965 B. 1665 C. 1950 D. 1776

11. Which of these scientists made each of the following discoveries? (Anton van Leeuwenhoek; Robert Hooke; Rudolf Virchow) A. Observed some of the first cells and first used the term “cell” B. Observed the first human cells C. Observed cells dividing

12. Robert Hooke sketched what looked like honeycombs, or repeated circular or square units when he observed plant cells under a microscope.

A. What is each unit? B. Of the shared parts of all cells, what makes up the outer surface of

each unit? C. Of the shared parts of all cells, what makes up the inside of each

unit?

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ATTRIBUTIONS 1. Healthy human T-cell by NIAID Flickr's photostream, public domain

via Wikimedia Commons 2. Cork Micrograph by Robert Hook, public domain via Wikimedia

Commons 3. Chlamydomonas by Dartmouth Electron Microscope Facility,

Dartmouth College, released into the public domain via Wikimedia Commons

4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

This page titled 5.2: Discovery of Cells and Cell Theory is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform.

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5.3: VARIATION IN CELLS

Figure shows a bacterial cell (colored green) attacking human red blood cells. The bacterium causes a disease called relapsing fever. The bacterial and human cells look very different in size and shape. Although all living cells have certain things in common — such as a plasma membrane and cytoplasm — different types of cells, even within the same organism, may have their own unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary not only in shape but also in size, as this example shows. In most organisms, however, even the largest cells are no bigger than the period at the end of this sentence. Why are cells so small?

Figure : Bacterial cell attacking a human red blood cell

EXPLAINING CELL SIZE

Figure : Surface Area to Volume Comparison. A larger cube has a smaller surface area (SA) to volume (V) ratio than a smaller cube. This also holds true for cells and limits how large they can be.

Table : Characteristics of small and large cubes Characteristic Small Cube Large Cube

sides (S) Surface Area (SA)

Volume (V) SA:V

Most organisms, even very large ones, have microscopic cells. Why don't cells get bigger instead of remaining tiny and multiplying? What limits cell size?

The answers to these questions are clear once you know how a cell functions. To carry out life processes, a cell must be able to quickly pass substances into and out of the cell. For example, it must be able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. It is this need to pass substances across the surface that limits how large a cell can be.

Look at the two cubes in Figure . As this figure and table show, a larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells; a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen and produces more wastes. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the harder it will be for the cell to get rid of its wastes and take in necessary substances. This is what limits the size of the cell.

CELL FORM AND FUNCTION

Figure : Human nerve cell Figure : Human sperm cells

Figure : Human white blood cell

Cells with different functions often have different shapes. The cells in Figure – Figure are just a few examples of the many different shapes that human cells may have. Each type of cell in the figure has characteristics that help it do its job. For example, the job of the nerve cell is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail of each tiny sperm cell? Its tail helps a sperm cell "swim" through fluids in the female reproductive tract in order to reach an egg cell. The white blood cell has the job of destroying bacteria and other pathogens. Figure shows the large white blood cell (in yellow) engulfing and destroying bacteria (in orange).

CELLS WITH AND WITHOUT A NUCLEUS There is a basic cell structure that is present in many but not all living cells: the nucleus. The nucleus of a cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear membrane) and contains DNA. Based on whether or not they have a nucleus, there are two basic types of cells: prokaryotic cells and eukaryotic cells.

PROKARYOTIC CELLS Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm rather than enclosed within a nuclear membrane. Prokaryotic cells are found in single-celled organisms, such as the bacterium represented by the model below. Organisms with prokaryotic

 BACTERIA ATTACK!

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1cm 3cm

6 = 6 × = 6cS2 12 m2 6 = 6 × = 54cS2 32

m2

= = 1cS3 13 m3 = = 27cS3 33

m3

SA/V = 6/1 = 6 SA/V = 54/27 = 2

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Figure : Prokaryotic Cell. This diagram shows the s