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2.2  Architecture of Organisms, Tissues, and Cells and the Bits Between

of other cells, which has implications for the physics of a cell. So, although a cell is indeed a

useful self-​enclosed vessel for us to use biophysical technique to monitor biological process,

we must be duly cautious in how we interpret the results of these techniques in the absence

of a truly native biological context.

2.2.3  CELLS CATEGORIZED BY THE PRESENCE OF NUCLEI (OR NOT)

A cell itself is physically enclosed from its surrounding cell membrane, which is largely

impervious to water. In different cell types, the cell membrane may also be associated with

other membrane/​wall structures, all of which encapsulate the internal chemistry in each cell.

However, cells are far more complex than being just a boring bag of chemicals. Even the sim­

plest cells are comprised of intricate subcellular architectures, in which the biological process

can be compartmentalized, both in space and time, and it is clear that the greater the number

of compartments in a cell, the greater its complexity.

The next most significant tier of biological classification of cell types concerns one of these

subcellular structures, called the “nucleus.” Cells that do not contain a nucleus are called

“prokaryotes” and include both bacteria and archaea. Although such cells have no nucleus,

there is some ordered structure to the deoxyribonucleic acid (DNA) material, not only due

mainly to the presence of proteins that can condense and package the DNA but also due to

a natural entropic spring effect from the DNA, implying that highly elongated structures in

the absence of large external forces on the ends of the DNA are unlikely. This semistructured

region in prokaryotes is referred to as the nucleoid and represents an excluded volume for

many other biological molecules due to its tight mesh-​like arrangement of DNA, which

in many bacteria, for example, can take up approximately one-​third of the total volume of

the cell.

Cells that do contain a nucleus are called “eukaryotes” and include those of relatively

simple “unicellular” organisms such as yeast and trypanosomes (these are pathogen cells that

ultimately cause disease and which result in the disease sleeping sickness) as well as an array

of different cells that are part of complex multicellular organisms, such as you and I.

KEY POINT 2.5

Basic definition of the “Cell”: (1) functionally autonomous, (2) physically enclosed,

(3) structurally independent unit of “life,” either from unicellular or multicellular

organisms, (4) contains biomolecules that have the capacity to self-​replicate independ­

ently to form a new cell.

2.2.4  CELLULAR STRUCTURES

In addition to the cell membrane, there are several intricate architectural features to a cell.

The nucleus of eukaryotes is a vesicle structure bounded by a lipid bilayer (see Section 2.2.5)

of diameter 1–​10 μm depending on the cell type and species, which contains the bulk of the

genetic material of the cell encapsulated in DNA, as well as proteins that bind to the DNA,

called “histones,” to package it efficiently. The watery material inside the cell is called the

“cytoplasm” (though inside some cellular structures, this may be referred to differently, for

example, inside the nucleus this material is called the “nucleoplasm”). Within the cytoplasm

of all cell types are cellular structures called “ribosomes” used in making proteins. These are

especially numerous in a cell, for example, E. coli bacteria contain ~20,000 ribosomes per cell,

and an actively growing mammalian cell may contain ~10⁷ ribosomes.

Ribosomes are essential across all forms of life, and as such their structures are relatively

well conserved. By this, we mean that across multiple generations of organisms of the same

species, very little change occurs to their structure (and, as we will discuss later in this chapter,