Cell Theory

1. Cell Theory

Close your eyes and picture a brick wall. What is the wall's basic building block? It is a single brick. Like a brick wall, cells are the building blocks that make up your body.

Your body has many kinds of cells, each specialized for a specific purpose. Just as we use a variety of materials to build a home, the human body is constructed from many cell types.

EXAMPLE

Epithelial cells protect the body's surface and cover the organs and body cavities within. Bone cells help to support and protect the body. Immune system cells fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide.

Each of these cell types plays a vital role during the body's growth, development, and day-to-day maintenance. Despite their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.

The discovery of cells involved several individuals over many years. In the 1600s, Dutch shopkeeper Antony van Leeuwenhoek used a microscope (an instrument used to view very small objects) to observe the movements of single-celled organisms, which he collectively termed “animalcules.” In the 1665 publication Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and microscopy techniques enabled other scientists to see some components inside cells.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that one or more cells comprise all living things, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later also made important contributions to this theory.

Recall that a scientific theory is a scientific explanation for something that has been thoroughly tested. The cell theory resulted from the work of several scientists who came up with these three points that they believed explain the characteristics of cells through various types of testing, experiments, and observations.

term to know

Cell Theory

A theory that states all living things are made of cells, cells are the smallest unit of life, and all cells come from preexisting cells.

2. Basic Characteristics of All Cells

All cells, regardless of complexity, share four common components:

1. A plasma membrane (also called the cell membrane), which encloses all of the cell parts. It is the membrane that surrounds the outer body of the cell and controls what can enter and exit the cell;
2. Cytoplasm, which is the jelly-like fluid that supports the contents of the cell and is found between the plasma membrane and the nucleus;
3. DNA, which is the genetic material of the cell. All cells contain some sort of genetic information that allows the cell to function and reproduce;
4. Ribosomes, which are organelles that synthesize proteins.

terms to know

Plasma Membrane

The membrane that surrounds the outer body of the cell and controls what can enter and exit the cell. Also called cell membrane.

Cytoplasm

The jelly-like fluid that supports the contents of the cell, found between the plasma membrane and the nuclear envelope.

DNA

A large molecule that contains all of an organism's genetic information.

Ribosome

A cell organelle responsible for synthesizing proteins.

3. Eukaryotic and Prokaryotic Cells

Cells fall into one of two broad categories: prokaryotic and eukaryotic. We classify only the predominantly single-celled organisms Bacteria and Archaea as prokaryotes (pro- = “before”; -kary- = “nucleus”). Animal cells, plants, fungi, and protists are all eukaryotes (eu- = “true”).

Eukaryotic cells are cells where the DNA is contained in a nucleus. Our body cells are considered eukaryotic because they all contain a nucleus.

Another characteristic of eukaryotic cells is that they, like all cells, have a plasma membrane. This is the outer layer of the cell that encloses all of the cell organelles. The organelles in a eukaryotic cell are contained within a membrane and are referred to as membrane bound.

The plasma membrane of our cells is made up of a lipid bilayer, which is composed of two layers of phospholipids. That lipid bilayer is what makes up the plasma membrane of your cells and helps to control what goes into and what can come out of the cell. All the free space within the cell is the cytoplasm.

Generally, eukaryotic cells are more complex than prokaryotic cells, and they contain more organelles (for example, mitochondria, ribosomes, endoplasmic reticulum, and the Golgi apparatus).

Prokaryotic cells do contain DNA, as all cells contain DNA. However, the difference between a prokaryotic cell and a eukaryotic cell is that in a prokaryotic cell, the DNA is not contained within a nucleus. It still has the plasma membrane, which is the outer layer of the cell, and it still has cytoplasm. You'll notice in the prokaryotic cell that there are some ribosomes as well, which help to make proteins for the cell.

Overall, prokaryotic cells are a little bit less complex than eukaryotic cells and generally smaller in size.

terms to know

Eukaryotic Cell

Type of cell that holds all of its genetic information (DNA) inside of a membrane-bound nucleus.

Organelle

A small, organized structure within a cell. A ribosome is an organelle found within both prokaryotic and eukaryotic cells; a nucleus is an organelle found within eukaryotic cells, but not prokaryotic cells.

Prokaryotic Cell

Type of cell that does not contain a nucleus.

SOURCE: THIS TUTORIAL HAS BEEN ADAPTED FROM OPENSTAX “BIOLOGY 2E”. ACCESS FOR FREE AT OPENSTAX.ORG/BOOKS/BIOLOGY-2E/PAGES/1-INTRODUCTION. LICENSE: CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL.