Introduction to Cells - OpenStax CNX · Introduction to Cells Robert Bear David Rintoul Based on Studying Cells y by OpenStax College This work is produced by OpenStax-CNX and licensed

OpenStax-CNX module: m47170 1

Introduction to Cells∗

Robert Bear

David Rintoul

Based on Studying Cells† by

OpenStax College

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 4.0‡

Introduction

The history of the knowledge of the phenomena of life and of the organized world can be divided

into two main periods. For a long time anatomy, and particularly the anatomy of the human

body, was the alpha and omega of scienti�c knowledge. Further progress only became possible

with the discovery of the microscope. A long time had yet to pass until through Schwann the

cell was established as the �nal biological unit. It would mean bringing coals to Newcastle

were I to describe here the immeasurable progress which biology, in all its branches, owes to the

introduction of this concept of the cell. For this concept is the axis around which the whole of

modern science revolves.

Paul Ehrlich, "Partial Cell Functions", Nobel Lecture, December 11, 1908Ehrlich's enthusiasm for the cell is understandable. A single cell is the basic unit of life, and the starting

point for each and every human and other organism on the planet. A cell is the smallest unit of a livingthing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called anorganism. Thus, cells are the basic building blocks of all organisms, and the study of cells is at the veryheart of the research enterprise that we call biological science.

Several cells of one kind that interconnect with each other and perform a shared function form tissues,several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up anorgan system (such as the digestive system, circulatory system, or nervous system). Several systems thatfunction together form an organism (like a human being). Here, we will examine the structure and functionof cells.

There are many types of cells, all grouped into one of two broad categories: prokaryotic and eukaryotic.For example, both animal and plant cells are classi�ed as eukaryotic cells, whereas bacterial cells are classi�edas prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or eukaryotic, let's�rst examine how biologists study cells.

∗Version 1.5: Jul 3, 2014 7:22 pm +0000†http://legacy.cnx.org/content/m44405/1.7/‡http://creativecommons.org/licenses/by/4.0/

http://legacy.cnx.org/content/m47170/1.5/


1 Microscopy

Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists usemicroscopes (micro- = �small�; -scope = �to look at�) to study them. A microscope is an instrument thatmagni�es an object. Most photographs of cells are taken with a microscope, and these images can also becalled micrographs.

The optics of a microscope's lenses change the orientation of the image that the user sees. A specimenthat is right-side up and facing right on the microscope slide will appear upside-down and facing left whenviewed through a microscope, and vice versa. Similarly, if the slide is moved left while looking throughthe microscope, it will appear to move right, and if moved down, it will seem to move up. This occursbecause microscopes use two sets of lenses to magnify the image. Because of the manner by which lighttravels through the lenses, this system of two lenses produces an inverted image (binocular, or dissectingmicroscopes, work in a similar manner, but include an additional magni�cation system that makes the �nalimage appear to be upright).

1.1 Light Microscopes

To give you a sense of cell size, a typical human red blood cell is about eight millionths of a meter or eightmicrometers (abbreviated as eight µm, or eight µ) in diameter; the head of a pin of is about two thousandthsof a meter (two mm) in diameter. That means about 250 red blood cells could �t on the head of a pin.

Most student microscopes are classi�ed as light microscopes (Figure 1a). Visible light passes and isbent through the lens system to enable the user to see the specimen. Light microscopes are advantageousfor viewing living organisms, but since individual cells are generally transparent, their components are notdistinguishable unless they are colored with special stains. Staining, however, usually kills the cells.

Light microscopes commonly used in the undergraduate college laboratory magnify up to approximately400 times. Two parameters that are important in microscopy are magni�cation and resolving power. Mag-ni�cation is the process of enlarging an object in appearance. Resolving power is the ability of a microscopeto distinguish two adjacent structures as separate: the higher the resolution, the better the clarity and detailof the image. When oil immersion lenses are used for the study of small objects, magni�cation is usuallyincreased to 1,000 times. In order to gain a better understanding of cellular structure and function, scientiststypically use electron microscopes.



Figure 1: (a) Most light microscopes used in a college biology lab can magnify cells up to approximately400 times and have a resolution of about 200 nanometers. (b) Electron microscopes provide a much highermagni�cation, 100,000x, and a have a resolution of 50 picometers. (credit a: modi�cation of work by"GcG"/Wikimedia Commons; credit b: modi�cation of work by Evan Bench)

1.2 Electron Microscopes

In contrast to light microscopes, electron microscopes (Figure 1b) use a beam of electrons instead of abeam of light. Not only does this allow for higher magni�cation and, thus, more detail (Figure 2), it alsoprovides higher resolving power. The method used to prepare the specimen for viewing with an electronmicroscope kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in avacuum, so living cells cannot be viewed with an electron microscope.

In a scanning electron microscope, a beam of electrons moves back and forth across a cell's surface,creating details of cell surface characteristics. In a transmission electron microscope, the electron beampenetrates the cell and provides details of a cell's internal structures. As you might imagine, electronmicroscopes are signi�cantly more bulky and expensive than light microscopes.



(a) (b)

Figure 2: (a) These Salmonella bacteria appear as tiny purple dots when viewed with a light microscope.(b) This scanning electron microscope micrograph shows Salmonella bacteria (in red) invading humancells (yellow). Even though sub�gure (b) shows a di�erent Salmonella specimen than sub�gure (a), youcan still observe the comparative increase in magni�cation and detail. (credit a: modi�cation of work byCDC/Armed Forces Institute of Pathology, Charles N. Farmer, Rocky Mountain Laboratories; credit b:modi�cation of work by NIAID, NIH; scale-bar data from Matt Russell)

2 Cell Theory

The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek,a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses,van Leeuwenhoek observed the movements of protista (a type of single-celled organism) and sperm, whichhe collectively termed �animalcules.�

In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term �cell� forthe box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoekdiscovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniquesenabled other scientists to see some components inside cells.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues andproposed the uni�ed cell theory, which states that all living things are composed of one or more cells, thecell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made importantcontributions to this theory.

: Cytotechnologist

Have you ever heard of a medical test called a Pap smear (Figure 3)? In this test, a doctor takesa small sample of cells from the uterine cervix of a patient and sends it to a medical lab wherea cytotechnologist stains the cells and examines them for any changes that could indicate cervicalcancer or a microbial infection.

Cytotechnologists (cyto- = �cell�) are professionals who study cells via microscopic examinationsand other laboratory tests. They are trained to determine which cellular changes are within normallimits and which are abnormal. Their focus is not limited to cervical cells; they study cellularspecimens that come from all organs. When they notice abnormalities, they consult a pathologist,who is a medical doctor who can make a clinical diagnosis.



Cytotechnologists play a vital role in saving people's lives. When abnormalities are discovered early,a patient's treatment can begin sooner, which usually increases the chances of a successful outcome.

Figure 3: These uterine cervix cells, viewed through a light microscope, were obtained from a Papsmear. Normal cells are on the left. The cells on the right are infected with human papillomavirus(HPV). Notice that the infected cells are larger; also, two of these cells each have two nuclei instead ofone, the normal number. (credit: modi�cation of work by Ed Uthman, MD; scale-bar data from MattRussell)

3 The Phylogenetic Relationship of Life

All of life can be grouped into three Domains Archaea, Bacteria and Eukarya (Figure 4). Even thoughArchaea and Bacteria are prokaryotes, there are enough di�erences between Archaea and Bacteria thatwarrant them being in di�erent Domains. Within the Domain Eukarya, there are at least four KingdomsProtists (multiple kingdoms), Fungi, Plantae and Animalia. Figure 4 shows the current phylogentic tree ofall these various groups that we will be exploring.



Figure 4: The phylogenetic tree of the Domains Bacteria, Archeae and the four Kingdoms of Eukarya.Work by EVa Horne

4 Our approach to Cell Biology

In this textbook, we explore cells and the chemistry of life in the reverse order of many traditional textbooks.First, we explore the diversity of life at the cellular level and then we investigate the chemistry of life orBiochemistry. The reason why we take this approach is based on the fact that many students have hadsome exposure to cells and this exposure allows them to connect with the material. In addition, we feel amacro to micro approach to investigating this material allows the learner the ability to better connect howthe biochemistry relates to the functioning cell.


Documents

Introduction to Cells - OpenStax CNX · Introduction to Cells Robert Bear David Rintoul Based on Studying Cells y by OpenStax College This work is produced by OpenStax-CNX and licensed