Alyssa Simon - How Cells Divide, Reproduce, and Specialize

Published in 2015 by Britannica Educational Publishing (a trademark of Encyclopdia Britannica, Inc.) in association with The Rosen Publishing Group, Inc.

29 East 21st Street, New York, NY 10010

Copyright 2015 The Rosen Publishing Group, Inc., and Encyclopdia Britannica, Inc. Encyclopaedia Britannica, Britannica, and the Thistle logo are registered trademarks of Encyclopdia Britannica, Inc. All rights reserved.

Distributed exclusively by Rosen Publishing.

To see additional Britannica Educational Publishing titles, go to rosenpublishing.com.

First Edition

Additional content and editorial input provided by Melissa Petruzzello.

Britannica Educational Publishing

J.E. Luebering: Director, Core Reference Group

Anthony L. Green: Editor, Comptons by Britannica

Rosen Publishing

Hope Lourie Killcoyne: Executive Editor

Tracey Baptiste: Editor

Nelson S: Art Director

Michael Moy: Designer

Cindy Reiman: Photography Manager

Karen Huang: Photo Researcher

Introduction and supplementary material by Alyssa Simon

Library of Congress Cataloging-in-Publication Data

Simon, Alyssa, author.

How cells divide, reproduce, and specialize/Alyssa Simon, Amy Romano.First edition.

pages cm.(The Britannica guide to cell biology)

Includes bibliographical references and index.

ISBN 978-1-6227-5803-6 (eBook)

1. CellsJuvenile literature. 2. Cell physiologyJuvenile literature. 3. CytologyJuvenile literature. I. Romano, Amy, author. II. Title.

QH582.5.S56 2015

571.6dc23

2014020660

Cover: background Fedorov Oleksiy/shutterstock.com; diagram silhouetted iLexx/iStockphoto.com

CONTENTS

A cell is the smallest unit of living matter that can exist by itself. Known as the building block of life, every living thing on the planet is made up of at least one cell. Many cells exist as microscopic, unicellular organisms, such as bacteria or yeasts, though other living creatures are composed of multiple cells. An adult human, for example, is made up of about 37.2 trillion individual cells, all of which cooperate in order to carry out vital functions. Despite their small size, cells come in a variety of shapes, ranging from cube-shaped plant cells to disk-shaped red blood cells. Each contains the components that living creatures need to sustain life.

In complex, multicellular organisms, cells combine to form tissues, which in turn make up the organs that compose body systems, such as the respiratory system. In humans, the respiratory system consists of the nasal cavity, throat, voice box, windpipe, bronchi, and lungs, all of which are composed of individual cells. Even the simplest cells are constantly busy performing the processes necessary for life throughout the day. In addition to breathing, cells have a role in digestion of food, reproduction, and growth, among other functions.

This diagram shows a typical eukaryotic cell. July Store/Shutterstock.com.

The study of cells is a branch of biology called cytology. The scientists who specialize in this field are called cytologists. Cells were first discovered by an English physicist named Robert Hooke in 1665. Hooke used a microscope of his own design to look at thinly sliced sections of cork. He saw the tiny, boxlike units that made up the cork and named them cells because of their similarity to the monastic cells used by monks. In 1673, Dutch microscopist Antonie van Leeuwenhoek discovered blood cells and the single-celled organisms, bacteria and protozoa. In 1684, he published the first accurate description of red blood cells.

By the nineteenth century, microscope technology had improved to allow more detailed investigations. In 1831, Scottish botanist Robert Brown discovered the cell nucleus and recognized it as a constant component of plant cells. German physiologist Theodor Schwann and German biologist Mathias Schleiden stated in 1839 that cells were the elementary particles of organisms in both plants and animals, an idea now known as cell theory. Additionally, they recognized that some organisms are unicellular while others are multicellular.

However, scientists were still uncertain as to how exactly cells come to be. Early investigators hypothesized that fetal cells develop out of an unformed substance and first develop a nucleus, then the cell body, and finally the cell membrane. It was many years before cytologists understood how cells divide, reproduce, and specialize.

C ells come in an astounding variety of shapes and sizes. While most cells are tiny, a few are exceptionally large. A birds egg, for example, is a single cell yet can be as large as a baseball; some nerve cells can be more than 3 feet (1 meter) long. The variation in cell size and shape is directly related to a cells function. Skin cells, for example, are flat so that they can pack tightly into layers to protect the body from bacteria, water, and the suns damaging rays. Muscle cells, on the other hand, are longer and thin and provide movement through patterns of contracting and relaxing. Even single-celled organisms come in a variety of shapes to accommodate their environments.

Eggs, skin, muscle, and nerve cells are all examples of specialized cells. They are each responsible for specific jobs and exist as part of larger organisms. Single-celled organisms, such as amoebas, are self-sufficient and are capable of performing all necessary functions to sustain life on their own. Despite the enormous variety of shapes, sizes, and functions of cells, all cells can be classified into one of two basic groups.

TWO TYPES OF CELLS

All living things can be categorized as being either prokaryotic or eukaryotic. Prokaryotic organisms are unicellular bacteria or archaea and lack a true nucleus and organelles. Instead, their DNA (deoxyribonucleic acid) floats freely in the cell and many of the cellular tasks are carried out in the membrane that encapsulates the cell. Although they are fairly simple, prokaryotic cells perform extremely complex processes and activities. In fact, prokaryotic cells have a much broader range of biochemical reactions than their relatives, the eukaryotic cells. Eukaryotic cells are generally larger than prokaryotic cells and are more organized and efficient. Eukaryotic cells are found in all multicellular organisms, such as fungi, plants, algae, and animals, and several exist as unicellular organisms, such as paramecia and amoebas. All of the components of eukaryotic cells are housed in a number of interior compartments called organelles. Chief among these is the nucleus, which serves as the cells control center and contains the chromosomes composed of the cells double-stranded DNA.

Animal cells and plant cells contain membrane-bound organelles, including a distinct nucleus. In contrast, bacterial cells do not contain organelles. Encyclopdia Britannica, Inc.

CELL STRUCTURE

Although prokaryotic cells lack the organelles of eukaryotic cells, both cell types feature an outer cell membrane, a jellylike interior known as cytoplasm, DNA, and tiny, bead-like structures called ribosomes.

The cell, or plasma membrane, is a semipermeable wall that allows specific material to leave and enter the cell. Its tiny pores, or openings, permit nutrient and waste molecules to be actively brought into and out of the cell. The cytoplasm is a nutrient-rich fluid that houses the protein-producing ribosomes (and the organelles in eukaryotic cells). DNA is present in all cells and contains the genetically coded instructions for all cell functions, including growth and reproduction.