Page 5 - Headlines Histology2024_Neat

? The intrinsic proteins are firmly attached to the lipid bilayer. Some of them
extend throughout the entire thickness of the membrane and constitute
transmembrane channels for the passage of water-soluble ions and
molecules

• The extrinsic or peripheral proteins are only partially embedded to either
aspects of the membrane.

• The carbohydrate fractions are conjugated with the protein (glycoprotein)
and lipid (glycolipid) molecules of the cell membrane. These glycoproteins
and glycolipids project from the outer surface of the cell membrane as cell
coat or glycocalyx.

Functions of the cell membrane
• The cell membrane is the part of the cell that regulate the exchange of

molecules and ions between its internal and external environment. This
occurs by several ways:
• Passive Diffusion: this involves the entrance of small molecules into the
cytoplasm. It depends on the presence of a concentration gradient across the
plasmalemma (e.g., diffusion of lipid soluble substances, oxygen, CO2,
water and small ions).
• Facilitated Diffusion: this type of diffusion is also concentration-
dependent and involves the transport of large water-soluble molecules such
as glucose and amino acids. It requires the presence of carriers to which the
molecules have to bind in order to pass through the plasmalemma.
• Active Transport: this process requires the utilization of energy provided
as ATP. (e.g., sodium-potassium pump).
• Selective transport: it depends on the presence of specific cell surface
receptors to pick up specific molecules into the cytoplasm (e.g., hormones).
• Endocytosis and Exocytosis
• Endocytosis involves either the engulfment of solid particles (phagocytosis)
or minute droplet of fluid (pinocytosis).

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