1.1 Introduction to Cell Biology and Cell Communication

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glycoproteins, solutes, and water) which together form a cell-support. The mechanical

strength of the tissue depends on both the strength of the cytoskeleton for each cell and

the strength of the ECM. The ordering of cells into tissues is also regulated by the ECM.

Cells are not static structures but grow, develop and mature, and multiply. They also

react to stimuli, outside forces and conditions, and might even commit suicide (apopto-

sis) if they are irreversibly damaged or are infected with a dangerous virus or bacterium.

The centriole starts the process of cell division (mitosis) by making the spindle fibers that

draw the two strands of the chromosomes (DNA pieces) apart. The cells can then form

two nuclei, one for each cell, by copying the one strand, reforming the complete, double-

stranded DNA chromosome and then forming a nuclear membrane around it; after that,

the complete cell divides by forming a membrane between those nuclei and separating

the identical cells from each other.

To modify a cell, specifically to modify or remove a specific protein in a cell, the

method of genetic engineering was developed. It has been most widely used in bacteria

and plants. The genetic information for a specific protein, its gene or piece of DNA se-

quence, is excised and modified. To be active in the cell, that piece of DNA needs to be

attached to a promoter sequence that regulates when and how that information is used,

then the DNA is reinserted into the cell. This method works most effectively if one func-

tion is based on only one gene, which is common in simple organisms such as bacteria

but rare for higher organisms.

With this information on cell structure in mind, let us return to cell communica-

tion. The body has two major communication systems: the endocrine system that uses

hormones as its signal and the neuronal system that uses neurotransmitters instead (Fig-

ure 1.2). Hormones are secreted into the blood stream and are widely distributed. They

act on any tissue that happens to have receptors for that specific hormone. Hormones

are generally signals for development, i. e., slower, more long-term processes such as the

signal for a stem cell to develop into a new neuron in the brain, or signals that activate

several organs, i. e. insulin, which activates several organs so that food can be broken

down and converted into energy.

Neurotransmitters will be released into a very small space between two nerve cells

and thus will act only on that subsequent neuron. Release and uptake are fast, and

the neurotransmitter will be destroyed immediately. So these signals are used to react

quickly to the environment or analyze and transmit information in the brain stemming

from the continuous input of our senses.

In either communication system, though, the signal arrives outside of the cell that

is supposed to use that information. So how does the signal on the outside of the cell

effect a change inside of that cell, given that the cell membrane poses such a formidable

barrier? Here, the specific channels or other transmembrane proteins come into play.

The structures and mechanisms of these channels will be discussed in more detail in the

next chapter (see Section 1.2). The signal binds to a channel or a transmembrane protein

on the outside of the cell. That binding event changes the three-dimensional structure

of that protein so that an enzyme at the inside of the cells gets activated to catalyze a