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

ways to bacteria though typically live in more extreme environmental conditions for combin­

ations of external acidity, salinity, and/​or temperature than most bacteria, but they also have

some biochemical and genetic features that are actually closer to eukaryotes than to bacteria.

Complex higher organisms come into the eukaryote category, including plants and animals,

all of which are composed of collections of organized living matter that is made from multiple

unitary structures called “cells,” as well as the stuff that is between cells or collections of cells,

called the “extracellular matrix” (ECM).

2.2.1  CELLS AND THEIR EXTRACELLULAR SURROUNDINGS

The ECM of higher organisms is composed of molecules that provide mechanical support

to the cells as well as permit perfusion of small molecules required for cells to survive or

molecules that are produced by living cells, such as various nutrients, gases such as oxygen

and carbon dioxide, chemicals that allow the cells to communicate with each other, and the

molecule most important to all forms of life, which is the universal biological solvent of

water. The ECM is produced by the surrounding cells comprising different protein and sugar

molecules. Single-​celled organisms also produce a form of extracellular material; even the

simplest cells called prokaryotes are covered in a form of slime capsule called a “glycocalyx,”

which consists of large sugar molecules modified with proteins—​a little like the coating of

M&M’s candy.

The traditional view is that the cell is the basic unit for all forms of life. Some lower

organisms (e.g., the archaea and bacteria and, confusingly, some eukaryotes) are classified

as being unicellular, meaning that they appear to function as single-​celled life forms. The

classical perspective is typically hierarchical in terms of length scale for more complex multi­

cellular life forms, cells, of length scale ∼10–​100 μm (1 μm or micron is one millionth of a

meter), though there are exceptions to this such as certain nerve cells that can be over a meter

in length.

Cells may be grouped in the same region of space in an organism to perform specialist

functions as tissues (length scale ∼0.1 mm to several centimeters or more in some cases),

for example, muscle tissue or nerve tissue, but then a greater level of specialization can then

occur within organs (length scales >0.1 m), which are composed of different cells/​tissues with

what appear to be a highly specific set of roles in the organisms, such as the brain, liver, and

kidneys.

This traditional stratified depiction of biological matter has been challenged recently by

a more complicated model of living matter; what seems to be more the case is that in many

multicellular organisms, there may be multiple layers of feedback between different levels of

this apparent structural hierarchy, making the concept of independent levels dubious and a

little arbitrary.

Even the concept of unicellular organisms is now far from clear. For example, the model

experimental unicellular organisms used in biological research, such as Escherichia coli

bacteria found ubiquitously in the guts of mammals, and budding yeast (also known as

“baker’s yeast”) formally called Saccharomyces cerevisiae used for baking bread and making

beer, spend by far the majority of their natural lives residing in complex 3D communities

consisting of hundreds to sometimes several thousands of individual cells, called “biofilms,”

glued together through the cells’ glycocalyx slime capsules.

(An aside note is about how biologists normally name organisms, but these generally con­

sist of a binomial nomenclature of the organism’s species name in the context of its genus,

which is the collection of closely related organisms including that particular species, which

are all still distinctly different species, such that the name will take the form “Genus species.”

Biologists will further truncate these names so that the genus is often denoted simply by its

first letter; for example, E. coli and S. cerevisiae.)

Biofilms are intriguing examples of what a physicist might describe as an emergent struc­

ture, that is, something that has different collective properties to those of the isolated building

blocks (here, individual cells) that are often difficult, if not impossible, to predict from the

single-​cell parameters alone—​cells communicate with each other through both chemical and