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

in terms of localized structural features caused by the heterogeneous makeup of lipids in the

cell membrane, resulting in dynamic phase transition behavior that can be utilized by cells in

forming nanoscopic molecular confinement zones (i.e., yet another biological mechanism to

achieve compartmentalization of biological matter).

The cell membrane is a highly dynamic and heterogeneous structure. Although structured

from a phospholipid bilayer, native membranes include multiple proteins between the

phospholipid groups, resulting in a typical crowding density of 30%–​40% of the total mem­

brane surface area. Most biomolecules within the membrane can diffuse laterally and rota­

tionally, as well as phospholipid molecules undergoing significant vibration and transient

flipping conformational changes (unfavorable transitions in which the polar head group

rotates toward the hydrophobic center of the membrane). In addition, in eukaryotic cells,

microscale patches of the cell membrane can dynamically invaginate either to export

chemicals to the outside world, a process known as exocytosis, which creates phospholipid

vesicle buds containing the chemicals for export, or to import materials from the outside

by forming similar vesicles from the cell membrane but inside the cell, a process known as

endocytosis, which encapsulates the extracellular material. The cell membrane is thus better

regarded as a complex and dynamic fluid.

The most basic model for accounting for most of the structural features of the cell mem­

brane is called the “Singer–​Nicholson model” or “fluid mosaic model,” which proposes that

the cell membrane is a fluid environment allowing phospholipid molecules to diffuse laterally

in the bilayer, but with stability imparted to the structure through the presence of transmem­

brane proteins, some of which may themselves be mobile in the membrane.

Improvements to this model include the Saffman–​Delbrück model, also known as the

2D continuum fluid model, which describes the membrane as a thick layer of viscous fluid

surrounded by a bulk liquid of much lower viscosity and can account for microscopic

dynamic properties of membranes. More recent models incorporate components of a pro­

tein skeleton (parts of the cytoskeleton) to the membrane itself that potentially generates

semistructured compartments with the membrane, referred to as the membrane fence model,

with modifications to the fences manifested as “protein pickets” (called the “transmembrane

protein picket model”). Essentially though, these separately named models all come down to

the same basic phenomenon of a self-​assembled phospholipid bilayer that also incorporates

interactions with proteins resulting in a 2D partitioned fluid structure.

Beyond the cell membrane, heading in the direction from the center of the cell toward

the outside world, additional boundary structures can exist, depending on the type of cell.

For example, some types of bacteria described as Gram-​negative (an historical description

relating to their inability to bind to a particular type of chemical dye called “crystal violet”

followed by a counterstrain called “safranin” used in early microscopy studies in the nine­

teenth century by the Danish bacteriologist Hans Christian Gram, which differentiated them

from cells that did bind to the dye combination, called “Gram-​positive” bacteria) possess a

second outer cell membrane.

Also, these and many other unicellular organisms, and plant cells in general, possess an

outer structure called the “cell wall” consisting of tightly bound proteins and sugars, which

functions primarily to withstand high osmotic pressures present inside the cells. Cells con­

tain a high density of molecules dissolved in water that can, depending on the extracellular

environment, result in nonequilibrium concentrations on either side of the cell boundary

that is manifested as a higher internal water pressure inside the cell due to pores at various

points in the cell membrane permitting the diffusion of water but not of many of the larger

solute molecules inside the cell (it is an example of osmosis through a semipermeable

membrane).

Cells from animals are generally in an isotonic environment, meaning that the extracellular

osmotic pressure is regulated to match that of the inside of the cells, and small fluctuations

around this can be compensated for by small changes to the volume of each cell, which the

cell can in general survive due to the stabilizing scaffold effect of its cytoskeleton. However,

many types of nonanimal cells do not experience an isotonic environment but rather are

bathed in a much lower hypotonic environment and so require a strong structure on the out­

side of each cell to avoid bursting. For example, Staphylococcus aureus bacteria, a modified