Fundamentals of Anatomy and Physiology

Cell Structure

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The Cell Cycle. Tissues

All cells are surrounded by a cell membrane. This
membrane is often called the plasma membrane or the
plasmalemma- (Figure 3 - 3). Under the high
magnification of an electron microscope, this membrane is
composed of a double phospholipid layer with proteins
embed-ded in the phospholipid layer. The phospholipids
look like balloons with tails. The round balloon-like part is
hydrophilic (attracts water) and the double tails are
hydrophobic- (repels water). This arrangement allows for
the easy passage of water molecules through the cell
membrane via osmosis (discussed in Chapter 2). The
proteins embedded in the double phospholipid layer al-low
for the passage of molecules and ions across the cell
membrane (Figure 3-4). Some proteins make transport

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channels for small dissolved ions, others act as enzymes for

the active transport of materials into the cell against a

concentration gradient and need adenosine triphos-phate

(ATP) to function, other proteins act as receptor sites for

hormones to gain entrance into the cell, and still other

proteins act as cell identity markers. In addition, some

proteins in the phospholipid layer act as cement-ing

materials for cell adhesion on the outside of cells to hold

cells together; others act as structural supports in-side the

cell attaching to cytoskeleton structures, which hold

organelles in place in the cytoplasm. Proteins also make up

the structure of the sodium-potassium pump, a unique

feature of certain cell membranes like muscle cell

membranes and nerve cell membranes (Figure 3-5).

These molecules of proteins and phospholipids are

currently referred to in their arrangement as a fluid

mosaic model. The phospholipid molecules are like the

tiles of a mosaic but rather than being embedded in a solid

cement-like material, they are embedded in a fluid and can

move slightly to allow the passage of water molecules

across the cell membrane and thus into the cytoplasm of the

cell. This basic molecular structure of the cell membrane is

the same for all other membrane-bound organelles of the

cell.

Outside

Plasma

membrane

Inside

Transport channel Enzyme Receptor site

Cell identity marker

Attachment of^

Cell adhesion^ cytoskeleton^
Figure 3- 4 The functions of proteins embedded in the double phospholipid layer of the cell
membrane.

(^) ®
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