Third assignment on membrane function.

1. MANHATTANVILLE COLLEGE
Writing Assignment #4
Chemical Composition of the Cell Membrane and
its effect on Cell Processes
Stephen Corvini
11/9/2011
Molecular Cell Biology
Dr. Bettica

2. Stephen Corvini
Writing Assignment 4
Dr. Bettica
November 8th, 2011
The cell membrane is one of the most important molecular components of the human body as it
serves to protect the fundamental building block of life. It is composed of the substance cholesterol which
has shown a correlation to cell proliferation (Siperstein, 1984). Recent research studies have shown that
the enzyme 3-hydroxyl-3-methylglutaryl CoA (HMG-CoA) reductase works as a primary rate limiting
enzyme for the synthesis of cholesterol (Siperstein, 1984). Organs such as the intestines have shown a
great amount of HMG-CoA and cholesterol because they require a great amount of cell proliferation
(Siperstein, 1984). It is important to note that cholesterol is important in liver cells, especially in the
process of hepatic regeneration. Cell cultures have shown that levels of cholesterol within cells will
increase approximately 24 hours before cells begin to enter the proliferation phase (Siperstein, 1984).
Cholesterol is a steroid that is also well known for its ability to maintain the fluid nature of the cell
membrane as is described by the Fluid Mosaic Model. When compactin was introduced to a cell, levels of
proliferation were significantly lower. Cholesterol was shown to reverse the effects of this compound
which further verifies the role of this substance as a key player in cell proliferation (Siperstein, 1984).
Although cell growth is a function of the cell membrane, the semi-permeable lipid based barrier contains
vital proteins which allow it to communicate with the external environment and send signals via protein
pathways.
The cell membrane contains a series of proteins embedded within the membrane that serve a
variety of functions. Integrins are one specific class of proteins that play a key role in signaling and are
well known in the field of biology. These proteins possess a heterodimer structure that is composed of
both alpha and beta subunits (Zhong and Rescorla, 2003). The function of integrins is to allow
communication with the components of the extracellular matrix (ECM) in order to perform a variety of
functions (Zhong and Rescorla, 2003). Integrins play a crucial role in cell-ECM interactions which initiate

3. cell signaling. An example of pathway that provides a specific cell function is that of the PI3K/Akt
pathway within the cell (Zhong and Rescorla, 2003). This pathway is vital as it blocks components that
would otherwise signal the cell for receptor mediated apoptosis, or cell death. Integrin proteins bind with
components of the ECM in order to convert phosphatidylinositol-4,5-biphosphate (PIP2) into
phosphatidylinositol-3,4,5-triphosphate (PIP3). This is successfully completed by the phosphorylation of
PIP2, thus converting it into PIP3 (Zhong and Rescorla, 2003). The significance of this pathway is that
the molecule PIP3 blocks the binding of cell death signaling molecules such as Bad, NFkB, the receptor
IkB, and Akt. All of these molecules are components which bind to signal a cell for apoptosis (Zhong and
Rescorla, 2003). Another form of interaction within the membrane is that of cell-cell adhesion.
Cells that exhibit adhesion with other cells are capable of promoting cell survival by the
conduction of specific signal pathways. This is seen in cells that exhibit cell-ECM pathways as previously
stated (Zhong and Rescorla, 2003). Research in colonial epithelial cells (CECs) has shown Neural Cell
Adhesion Molecules (NCAMs) are vital to the process of cell adhesion (Zhong and Rescorla, 2003). This
is important because by carrying out the process of cell-cell adhesion the activity of β-catenin, Src-related
kinases, and the function of the PI3K/Akt signal pathway are successfully maintained (Zhong and
Rescorla, 2003). This is important because this research provides evidence that if cells are not capable of
adhering to neighboring cells, the cells will be marked for apoptosis. It has also been noted that N-
cadherin is utilized in the process of binding cells with adjacent neighbor cells (Zhong and Rescorla,
2003). The cell membrane is vital to the survival of the cell in many ways and the transport of ions into
and out of the cell is essential to the cell’s homeostatic balance.
The cell membrane contains a polar and non-polar region due the amphipathic nature of
phospholipids which are the primary component of the membrane bilayer. Molecules that possess no
electrical charge and are of the right molecular size are capable of being transported via the mechanism of
passive transport. This is because they possess no charge difference to interfere with the overall
electrochemical state of the cell membrane (Szachowicz-Petelska et al, 2000). Some examples of

4. molecules that can pass through the membrane are ethanol, carbon dioxide and oxygen; all of which pass
through the membrane by means of simple diffusion (Szachowicz-Petelska et al, 2000). Although
passive diffusion allows molecules to move through there are channel proteins that are involved in the
process which include ionotropic, metabotropic, and catalytic integral channel proteins (Szachowicz-
Petelska et al, 2000). Within the membrane ionotropic proteins facilitates ions, metabotropic involve cell
signals, and catalytic are involved with enzyme facilitated reactions (Szachowicz-Petelska et al, 2000).
Larger substances that possess an electrical charge such as sodium must pass through a pump that is
facilitated by the process of ATP-phosphorylation which results in a conformational change of the
channel protein in order to allow the charged molecule to pass through the membrane (Szachowicz-
Petelska et al, 2000).
There are several key carrier proteins that are involved in the facilitation of active diffusion which
include coupled carrier proteins, ATP pumps, and light driven pumps (Szachowicz-Petelska et al, 2000).
Light driven pumps are only seen in bacterial organism cells (Szachowicz-Petelska et al, 2000). Channel
proteins that undergo physical changes are necessary in order to allow the passage of certain synthetic
molecules into the cell as well. Recent research regarding drug transport mechanisms across the cell
membrane has shown evidence to suggest the employment of transport proteins aiding in the facilitation
of drug molecules into the cell cytosol (Szachowicz-Petelska et al, 2000). Other research has shown that
abnormal changes in lipid structure can result in the occurrence of an abnormal amount of ion transfer
within the cell membrane (Vokurkova et al., 2005). Phagocytosis is an example of a key process that can
come into play as a result of substances that are transported into and out of the cell.
The role of phagocytosis is the consumption or engulfment of cell matter that is targeted for
digestion within the body. An important instant of when phagocytosis becomes important is during the
functioning of the immune system within the body. Macrophages are the most common cell types to
perform the process of phagocytosis (Lennartz, 1999). Recent research has shown that phospholipases
may be involved in the regulation of molecules that play a key role in the process of phagocytosis.

5. Among the molecules that are known to take part in phagocytosis are various kinases, GTP-binding
proteins, and specificially protein kinase C (Lennartz, 1999). Polymerization of actin filaments within the
cell membrane combine with membrane movement have also been shown to be important to the
engulfment of particulate matter by components of the cell membrane (Lennartz, 1999). It has been seen
that phospholipases act as generators for secondary messengers. Important phospholipases that contribute
to this process are phospholipase C, phospholipase D, and phosphoinositide 3-OH kinase (PI(3)K)
(Lennartz, 1999). These phospholipases help with Immunoglobin binding in cell as well (Lennartz, 1999).
There are many important cell components that are vital to this process within the membrane.
The cell membrane is perhaps one of the most simply constructed components of the cell, but it
exhibits a high level of intricate functionality. All fields of biology acknowledge the importance of this
crucial component of the cell’s foundation. The many proteins and molecules which are embedded within
the membrane assist this structure in its function. The cell membrane takes part in processes such as cell
growth, transport, signaling, preservation, and phagocytosis.

6. Works Cited
Michelle R. L. 1999. Phospholipases and phagocytosis: The role of phospholipid-derived second
messengers in phagocytosis. Int J Biochem Cell Biol 31(3-4):415-30.
Siperstein, Marvin. "Role of Cholesterogenesis and Isoprenoid Synthesis in DNA Replication." Journal of
Lipid Research 25 (1984): 1462-468. 5 Nov. 2005. Web. 3 Nov. 2011.
<http://www.jlr.org/content/25/13/1462.full.pdf+html>.
Szachowicz-Petelska B, Figaszewski Z, Lewandowski W. 2001. Mechanisms of transport across cell
membranes of complexes contained in antitumour drugs. Int J Pharm 222(2):169-82.
Vokurková M, Nováková O, Dobes˘ová Z, Kunes˘ J, Zicha J. 2005. Relationships between membrane
lipids and ion transport in red blood cells of dahl rats. Life Sci 77(13):1452-64.
Zhong X and Rescorla FJ. Cell surface adhesion molecules and adhesion-initiated signaling:
Understanding of anoikis resistance mechanisms and therapeutic opportunities. Cell Signal (0).