Page 4 - Headlines Histology2024_Neat

Cell membrane

• The cell membrane (plasmalemma or plasmamembrane) is the outer
membrane of the cell that acts as a barrier between its internal and external
environment.

• With light microscope (LM) it is too thin (8-10 nm) to be seen.
• The cell boundary that is often seen is mainly due to condensation of

cytoplasm on the inner aspect of the cell membrane, condensation of the
stain (such as silver or PAS) on the carbohydrate-rich coat and obliquity of
the sections.
• With electron microscope (EM) it appears as a trilaminar structure
consisting of outer and inner electron-dense layers separated by an
intermediate electron-lucent layer.
The molecular structure of the cell membrane

• The most recent and currently acceptable model for the cell membrane is
the

Fluid mosaic model
• According to this model, the cell membrane is made up of three major

components phospholipids, proteins and carbohydrates.
• The phospholipids molecules form a central bimolecular layer. Each

molecule is formed of two ends; polar or hydrophilic (has affinity with
water) end, and non-polar or hydrophobic (has no affinity with water) tail.
• The phospholipids molecules are arranged with their hydrophilic ends are
directed outward, while hydrophobic tails are directed inward toward the
center of the membrane.
• The proteins are the second major constituents of the cell membrane. The
protein molecules are arranged as globules moving freely within the lipid
layer.
• Two types of protein globules are recognized; intrinsic or integral protein
and extrinsic or peripheral proteins.

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