Chapter 2 – Orientation for the Bio-Curious  15

detection precision is not required to infer molecular-​level behavior of a biological system.

One such can be found unnaturally in x-​ray crystallography of biological molecules and

another more naturally in muscles.

In x-​ray crystallography, the process of crystallization forces all of the molecules, barring

crystal defects, to adopt a single favored state; otherwise, the unit cells of the crystals would

not tessellate to form macroscopic length scale crystals. Since they are all in the same state,

the effective signal-​to-​noise detection ratio for the scattered x-​ray signal from these molecules

can be relatively high. A similar argument applies to other structural biology techniques, such

as nuclear magnetic resonance, (see Chapter 5) though here single energetic states in a large

population of many molecules are imposed via a resonance effect due to the interaction of a

large external magnetic field with electron molecular orbitals.

In muscle, there are molecular machines that act, in effect, as motors, made from a protein

called “myosin.” These motor proteins operate by undergoing a power stroke–​type molecular

conformational change, allowing them to impose force against a filamentous track composed

of another protein called “actin,” and in doing so cause the muscle to contract, which allows

one to lift a cup of tea from a table to our lips, and so forth. However, in a normal muscle

tissue, the activity of many such myosin motors is synchronized in time by a chemical trigger

consisting of a pulse of calcium ions. This means that many such myosin molecular motors

are in effect in phase with each other in terms of whether they are at the start, middle, or end

of their respective molecular power stroke cycles. This again can be manifested in a relatively

high signal-​to-​noise detection ratio for some bulk ensemble biophysical tools that can probe

the power stroke mechanism, and so again this permits molecular-​level biological inference

without having to resort to molecular-​level sensitivity of detection. This goes a long way to

explaining why, historically, so many of the initial pioneering advances in biophysics were

made through either structural biology or muscle biology research or both.

KEY POINT 2.3

Exceptional examples of biological systems exhibiting molecular synchronicity, for

example, in muscle tissue, can allow single-​molecule interferences from ensemble

average data.

To understand the nature of a biological material, we must ideally not only explore the soft

condensed matter properties but also focus on the fine structural details of living things,

through their makeup of constituent cells and extracellular material and the architecture of

subcellular features down to the length scale of single constituent molecules.

But life, as well as being highly complex, is also short. So, the remainder of this chapter is

an ashamedly whistle-​stop tour of everything the physicist wanted to know about biology but

was afraid to ask. For readers seeking further insight into molecular-​ and cell-​level biology,

an ideal starting point is the textbook by Alberts et al. (2008). One word of warning, however,

but the teachings of biology can be rife with classification and categorization, much essential,

some less so. Either way, the categorization can often lead to confusion and demotivation in

the uninitiated physics scholar since one system of classification can sometimes contradict

another for scientific and/​or historical reasons. This can make it challenging for the physi­

cist trying to get to grip with the language of biological research; however, this exercise is

genuinely more than one in semantics, since once one has grasped the core features of the

language at least, then intellectual ideas can start to be exchanged between the physicist and

the biologist.

2.2  ARCHITECTURE OF ORGANISMS, TISSUES,

AND CELLS AND THE BITS BETWEEN

Most biologists subdivide living organisms into three broad categories called “domains” of

life, which are denoted as Bacteria, Eukaryotes, and Archaea. Archaea are similar in many