Cell Structure and Organisation

The basic unit of all living things is the cell. The cell theory is one of the fundamental concepts of biology; it states that:

• all organisms are made up of cells, and that

• all cells derive from other, pre-existing cells.

As we shall see in this chapter, there may exist within a cell many smaller, subcellular structures, each with its own characteristics and function, but these are not capable of independent life.

An organism may comprise just a single cell (unicellular), a collection of cells that are not morphologically or functionally differentiated (colonial), or several distinct cell types with specialised functions (multicellular). Among microorganisms, all bacteria and protozoans are unicellular; fungi may be unicellular or multicellular, while algae may exist in all three forms. There is, however, one way that organisms can be differentiated from each other that is even more fundamental than whether they are uni- or multicellular. It is a difference that is greater than that between a lion and a mushroom or between an earthworm and an oak tree, and it exists at the level of the individual cell. All organisms are made up of one or other (definitely not both!) of two very distinct cell types, which we call procaryotic and eucaryotic cells, both of which exist in the microbial world. These differ from each other in many ways, including size, structural complexity and organisation of genetic material (Table 3.1).

The most fundamental difference between procaryotic and eucaryotic cells is reflected in their names; eucaryotic cells possess a true nucleus, and several other distinct subcel-lular organelles that are bounded by a membrane. Procaryotes have no such organelles. Most of these differences only became apparent after the development of electron microscopy techniques.

As can be seen from Table 3.2, the procaryotes comprise the simpler and more primitive types of microorganisms; they are generally single celled, and arose much earlier in evolutionary history than the eucaryotes. Indeed, as discussed later in this chapter, it

The names given to the two cell types derive from Greek words: Procaryotic = 'before nucleus'

Eucaryotic = 'true nucleus'

Table 3.1 Similarities and differences between procaryotic and eucaryotic cell structure

Similarities

Cell contents bounded by a plasma membrane Genetic information encoded on DNA Ribosomes act as site of protein synthesis

Differences

Procaryotic

Size

Typically 1-5 /m Genetic material Free in cytoplasm

Single circular chromosome or nucleoid Histones absent. Internal features

Membrane-bound organelles absent

Ribosomes smaller (70S), free in cytoplasm Respiratory enzymes bound to plasma membrane Cell wall

Usually based on peptidoglycan (not Archaea) External features Cilia absent

Flagella, if present, composed of flagellin. Provide rotating motility Pili may be present Outside layer (slime layer, capsule, glycocalyx) present in some types

Eucaryotic

Typically 10-100 /m

Contained within a membrane-bound nucleus

Multiple chromosomes, generally in pairs

DNA complexed with histone proteins

Several membrane-bound organelles present, including mitochondria, Golgi body, endoplasmic reticulum and (in plants & algae) chloroplasts

Ribosomes larger (80S), free in cytoplasm or attached to membranes

Respiratory enzymes located in mitochondria

When present, based on cellulose or chitin

Cilia may be present

Flagella, if present, have complex (9 + 2) structure. Provide 'whiplash' motility

Pili absent

Pellicle or test present in some types is widely accepted that eucaryotic cells actually arose from their more primitive counterparts. Note that the viruses do not appear in Table 3.2, because they do not have a cellular structure at all, and are not therefore considered to be living organisms. (See Chapter 10 for further discussion of the viruses).

The use of DNA sequencing methods to determine phylogenetic relationships between organisms has revealed that within the procaryotes there is another fundamental division. One group of bacteria were shown to differ greatly from all the others; we now call these the Archaea, to differentiate them from the true Bacteria.

Phylogenetic: pertaining to the evolutionary relationship between organisms.

Table 3.2 Principal groups of procaryotic and eucaryotic organisms

Procaryotes

Eucaryotes

Bacteria

Blue-green 'algae'*

Fungi

Algae

Protozoa

Plants

Animals

*An old-fashioned term: this group are in fact a specialized form of bacteria, and are known more correctly as the Cyanobacteria, or simply the blue-greens. They are discussed in more detail in Chapter 7. Animals and plants fall outside the scope of this book.

These two groups, together with the eucaryotes, are thought to have evolved from a common ancestor, and represent the three domains of life (Figure 3.1). The Archaea comprise a wide range of mostly anaerobic bacteria, including many of those that inhabit extreme environments such as hot springs. In this book we shall largely confine our discussions to the Bacteria, however in Chapter 7 there is a discussion of the principal features of the Archaea and their main taxonomic groupings.

Despite their differences, Archaea and Bacteria are both procaryotes.

Taxonomy is the science of classifying living (and once-living) organisms.

Figure 3.1 The three domains of life. All life forms can be assigned to one of three domains on the basis of their ribosomal RNA sequences. The Archaea are quite distinct from the true bacteria and are thought to have diverged from a common ancestral line at a very early stage, before the evolution of eucaryotic organisms. The scheme above is the one most widely accepted by microbiologists, but alternative models have been proposed

Figure 3.1 The three domains of life. All life forms can be assigned to one of three domains on the basis of their ribosomal RNA sequences. The Archaea are quite distinct from the true bacteria and are thought to have diverged from a common ancestral line at a very early stage, before the evolution of eucaryotic organisms. The scheme above is the one most widely accepted by microbiologists, but alternative models have been proposed

Figure 3.2 Bacterial shapes. Most bacteria are (a) rod shaped, (b) spherical or (c) curved. These basic shapes may join to form (d) pairs, (e and f) chains, (g) sheets, (h) packets or (i) irregular aggregates

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