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2.3  Chemicals that Make Cells Work

bind oxygen in the blood). There are also several essential enzymes that utilize higher atomic

number transition metal atoms in their structure, required in comparatively small quantities

in the human diet but still vital.

2.3.7  SMALL ORGANIC MOLECULES OF MISCELLANEOUS FUNCTION

Several other comparatively small chemical structures also perform important biological

functions. These include a variety of vitamins; they are essential small organic molecules

that for humans are often required to be ingested in the diet as they cannot be synthesized

by the body; however, some such vitamins can actually be synthesized by bacteria that reside

in the guts of mammals. A good example is E. coli bacteria that excrete vitamin K that is

absorbed by our guts; the living world has many such examples of two organisms benefiting

from a mutual symbiosis, E. coli in this case benefiting from a relatively stable and efficacious

external environment that includes a constant supply of nutrients.

There are also hormones; these are molecules used in signaling between different tissues

in a complex organism and are often produced by specialized tissues to trigger emergent

behavior elsewhere in the body. There are steroids and sterols (which are steroids with alcohol

chemical groups), the most important perhaps being cholesterol, which gets a bad press in

that its excess in the body lead to a well-​reported dangerous narrowing of blood vessels, but

which is actually an essential stabilizing component of the eukaryote cell membrane.

There are also the so-​called neurotransmitters such as acetylcholine that are used to

convey signals between the junctions of nerve cells known as synapses. Nucleoside molecules

are also very important in cells, since they contain highly energetic phosphate bonds that

release energy upon being chemically split by water (a process known as hydrolysis); the

most important of these molecules is adenosine triphosphate that acts as the universal cel­

lular fuel.

Worked Case Example 2.2: DNA “Information” Storage

The human haploid genome contains ca. 3000 million DNA nucleotide base pairs.

a

What are the possible stable DNA base pairs? How many raw bits of information are

there in a single DNA base pair?

b

Is one Blue-​ray disk sufficient to store the human genome information from a typical

family? (A typical Blue-​ray dual-​layer disk, of the type you might watch a movie on

at home, has a storage capacity of 50 GB. A recent U.S. census figure suggests the

average household contains 2.58 people.)

c

If a hypothetical storage device of similar volume to a USB hard drive could be made

using DNA is in its B form to store information assuming that storage units of B-​DNA

are tightly packed cylinders whose diameter and height are equal to the double-​

helical width and pitch, respectively, what % more data could it save compared to a

typical high capacity 2 TB USB hard drive? (A typical 2 TB USB hard drive has, at the

time of writing, dimensions ca. 12 cm × 8 cm × 2 cm.)

d

What is the maximum number of copies of a single complete human genome that

could fit into a cell nucleus of diameter 10 μm if it were structured in similar storage

units? Comment on the result.

Answers:

a

Each base pair (AT or TA, CG or GC) can have a total of 2 bits of information since

they result in 2², that is, 4 possible combinations.

b

1 byte (B) contains 8 bits; therefore, each base pair contains 0.25 bytes of informa­

tion. 1 kB =​ 2¹⁰ B, 1 MB =​ 2¹⁰ kB, 1 GB =​ 2¹⁰ MB =​ 2³⁰ B ≈ 1.1 × 10⁹ B.