1.1 Some Unifying Principles

Cell Theory

To begin our discussion of cells, we first need to understand the fundamental tenets of cell theory. Cell Theory has three main components; 1) all living organisms are composed of one or more cells, 2) the cell is the basic unit of structure and organization in organisms, and 3) cells arise from pre-existing cells. Like all scientific theories, this theory is stronger than a hypothesis because it has been supported by many repeated observations from independent scientific groups. The theory originated in a time when microscopes were much simpler than the ones we use today, and most people (incorrectly) thought that organisms arrived spontaneously, a concept known as spontaneous generation.

Cell theory was developed over many years (from the mid 1600s to the mid-1800s) by several scientists that studied cells and tissues using early microscopes. Dutch shopkeeper, Anton van Leeuwenhoek was one of the first to document single cells in the 1600s. Skilled in crafting lenses, van Leeuwenhoek observed 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 staining techniques enabled other scientists to see some components inside cells. By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann used both earlier research and their own observations on tissues to propose the unified cell theory, which states the three tenets described in the first paragraph. Rudolf Virchow later made important contributions to this theory, particularly helping promote the idea that new cells arise from existing cells (omnis cellula e cellula) rather than via spontaneous generation.

Phylogenetic Tree of Life

If all organisms are composed of one or more cells, and cells arise from pre-existing cells, a natural hypothesis is that all organisms are evolved from a common ancestor – the first cell. To understand the evolution of cellular life in the chapter sections below, we must quickly review the basics of interpreting phylogenetic trees. We often depict the history of evolution using a phylogenetic tree. When an ancestral species diverges into two new species, the tree gains a new branch. We call the place from where the branches divide, a node. Each node on a tree represents a common ancestor in the lineage of the organisms which diverge from said node. Branches that share a more recent node (near the “tip” of the tree) are more closely related than branches that share a less recent node (near the “root” or bottom of the tree). For example, animals and fungi are more closely related to each other than either of these groups are related to plants because animals and fungi share a more recent common ancestor than animals and plants, or animals and fungi. Today we have three domains of life – Bacteria, Archaea, and Eukarya – all of which have undergone billions of years of evolution since the first cell evolved (Figure 1.2).

Although there are three domains of life, cells are often classified into one of two broad categories based on the presence of absence of membrane-bound organelles such as a nucleus. We classify the predominantly single-celled organisms from the domains Bacteria and Archaea as prokaryotes (pro- = “before”; -kary- = “nucleus”). Cells in multicellular animals, plants, and fungi, along with unicellular protists are all eukaryotes (eu- = “true”) in the domain Eukarya. Although Archaea are more closely related to Eukarya than to Bacteria (Figure 1.2), this prokaryote vs. eukaryote classification is useful when comparing the structure and ultrastructure (interior structure) of cells from different domains of life.

Components of All Modern Cells

In spite of the diversity described above, all cells share four common components due to their common evolutionary history (Figure 1.3). 1) The plasma membrane is an outer covering that separates the cell’s interior from its surrounding environment. 2) The cytoplasm consists of a jelly-like cytosol within the cell in which there are other cellular components. 3) The DNA is the cell’s genetic material. 4) Ribosomes synthesize proteins, based on genetic information stored in the DNA. In the next chapter sections, we will explore how these common components of cells evolved in early prokaryotes and were retained by all modern cells (including prokaryotes and eukaryotes).