JULY 2018
The cell is the basic unit of structure and function of the body. Each cell is a highly organized molecular factory ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Fox”, “given” : “Stuart Ira”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “edition” : “9th”, “id” : “ITEM-1”, “issued” : { “date-parts” : “2006” }, “number-of-pages” : “75”, “publisher” : “The McGraw hill”, “title” : “Human Physiology”, “type” : “book” }, “uris” : “http://www.mendeley.com/documents/?uuid=f5cfd0be-375d-498b-ad2e-f232ca099ac5” } , “mendeley” : { “formattedCitation” : “(Fox, 2006)”, “plainTextFormattedCitation” : “(Fox, 2006)”, “previouslyFormattedCitation” : “(Fox, 2006)” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }(Fox, 2006). For the cell to maintain its shape and contents in position, it is surrounded by a structure known as the cell surface membrane.

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Fig 1.0 cell surface membrane
The cell surface membrane also called plasma membrane is made up of a double layer of phospholipids and proteins. Both the intracellular and extracellular compartments/environment of the cell are aqueous and so, the phospholipid layer helps to maintain the shape of the cell and therefore prevent the loss of enzymes, nucleotides and other cellular molecules that are water-soluble.

Phospholipids are polar (and hydrophilic) in the region that contains the phosphate group and non-polar (and hydrophobic) throughout the rest of the molecule.

In addition to lipids and proteins, the plasma membrane also contains carbohydrates and these are primarily attached to the outer surface of the membrane as glycoproteins and glycolipids.

Image fig 2.0 fluid mosaic model of cell surface membrane
The fluid-mosaic model describes the plasma membrane of animal cells. The plasma membrane that surrounds these cells has two layers (a bilayer) of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid). And the structure of the plasma membrane supports the old saying, “Oil and water don’t mix.”
Each phospholipid molecule has a head that is attracted to water (hydrophilic) and a tail that repels water (hydrophobic) Both layers of the plasma membrane have the hydrophilic heads pointing toward the outside; the hydrophobic tails form the inside of the bilayer.

Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), the plasma membrane forms a circle around each cell so that the water-loving heads are in contact with the fluid, and the water-fearing tails are protected on the inside.

The hydrophobic middle layer therefore restricts the passage of water and water-soluble molecules and ions. There are membrane proteins present in the cell surface membrane which may be integral or peripheral. Peripheral proteins are only partially embedded in one phase of the membrane whereas integral proteins span the membrane from one side to the other.ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Davidson”, “given” : “”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “edition” : “21”, “editor” : { “dropping-particle” : “”, “family” : “Colledge N.R., Walker B.R.”, “given” : “Raston S.H.”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “id” : “ITEM-1”, “issued” : { “date-parts” : “2010” }, “number-of-pages” : “386-408”, “publisher” : “Churchill Livingstone Elsevier”, “title” : “Davidson’s Principles & Practice of Medicine”, “type” : “book” }, “uris” : “http://www.mendeley.com/documents/?uuid=91f3fc98-3cab-4575-9f08-ae9c50a38d31” } , “mendeley” : { “formattedCitation” : “(Davidson, 2010)”, “plainTextFormattedCitation” : “(Davidson, 2010)” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }(Davidson, 2010)
The arrangement of the membrane is such that, it is not a solid structure and therefore, phospholipids and proteins are free to move literally, the proteins within the phospholipid “sea” are not uniformly distributed. This results in a constantly changing mosaic pattern, known as the fluid-mosaic model structure. The membrane is described as fluid because it is not rigid and molecules can move around within it and as a mosaic because it contains many kinds of molecules. Oil floating on water is an analogy for the fluid-mosaic cell membrane model.

A fluid mosaic model is presented for the gross organization and structure of the proteins and
lipids of biological membranes. The model is consistent with the restrictions imposed by
thermodynamics of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solve.

Plasma membrane lipids
There are there main lipids found in the plasma membrane
This forms the bulk of the lipids (about 75%), they are amphipathic molecules and therefore forms lipid bilayers.

It is described as having hydrophilic head consisting of a phosphate group and two hydrophobic fatty acid tails. The two components are joined by a glycerol molecule.

These are lipids with a carbohydrate attached by a glyosidic bond or covalent bond (Donald, Judith&charlotte,2013)
Forms 5% of membrane lipids. They occur on the external surface of the cell surface membrane and the carbohydrate portion of the glycolipid extends into the intercellular space and called a glycocalyx.ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Arthur”, “given” : “Vander”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “James”, “given” : “Sherman”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Dorothy”, “given” : “Luciano”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “edition” : “Seventh Ed”, “id” : “ITEM-1”, “issued” : { “date-parts” : “1998” }, “publisher” : “McGraw-Hill Companies”, “publisher-place” : “san francisco”, “title” : “Human Physiology”, “type” : “book” }, “uris” : “http://www.mendeley.com/documents/?uuid=04c4ed92-a5a6-4734-8b4e-fc080bb02150”, “http://www.mendeley.com/documents/?uuid=1e4d23bc-9507-4d13-8704-cb2546bbbdf8” } , “mendeley” : { “formattedCitation” : “(Arthur, James, & Dorothy, 1998)”, “plainTextFormattedCitation” : “(Arthur, James, & Dorothy, 1998)”, “previouslyFormattedCitation” : “(Arthur, James, & Dorothy, 1998)” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }(Arthur, James, & Dorothy, 1998)
Their role is to maintain stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow the cell to connect to one another to form tissues and provides cushioning and protection for the plasma membrane. They are found on the eukaryotic cell membrane where they extend from the phospholipid bilayer into the cellular environment (“glycolipids”. Nature,2105)
Cholesterol: cholesterol is estimated to be 20%. It reduces the permeability of the molecules to most biological molecules. It helps mobilize the outer surface of the membrane and makes it less soluble to very small water soluble molecules that could otherwise pass through more easily. This action makes the cell membrane firm and less permeable.

Intrinsic proteins span the entire membrane of the cell have the following functions;
Act as Receptors: Some proteins recognise and bind to target molecules,eg renal cells recognise and respond to ADH.
Act AsTransport Systems: They transport nutrients,neurotransmitters or hormones into and out of the cell
Act as channels. They maintain electrochemical gradient between the inside and outside of the cell which is essential for the efficient functioning of,for example the sodium/potassium pump.
Enzymes: Some membrane proteins are enzymes that breakdown chemical messengers after the massage has been received.

Substances may move across the membrane by:Diffusion,Osmosis,Facilitated diffusion and Active transport
A. Diffusion: This is the movement of molecules or ions from a region of higher concerntration to a region of a lower concerntration until the concerntrations reach equilebrium. Some substances that diffuse across the cell membrane include oxygen,carbon dioxide,steroids,fat soluble vitamins(A,D,E,K),alcohols,gylcerol and ammonia.ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “ISBN” : “9780803615465”, “author” : { “dropping-particle” : “”, “family” : “Valerie”, “given” : “Scanlon C.”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Tina”, “given” : “Sanders”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “edition” : “5th”, “editor” : { “dropping-particle” : “”, “family” : “Lisa”, “given” : “Deitch B.”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Alen”, “given” : “Sorkowitz”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Llysa”, “given” : “Richman H.”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “id” : “ITEM-1”, “issued” : { “date-parts” : “2007” }, “publisher” : “F.A. Davis”, “title” : “Essentials of Anatomy and Physiology”, “type” : “book” }, “uris” : “http://www.mendeley.com/documents/?uuid=de484a73-3ac0-4b6c-91b5-324c13d9e35a”, “http://www.mendeley.com/documents/?uuid=c6cc27e7-0196-4cef-af4c-d4f651512ee2” } , “mendeley” : { “formattedCitation” : “(Valerie & Tina, 2007)”, “plainTextFormattedCitation” : “(Valerie & Tina, 2007)”, “previouslyFormattedCitation” : “(Valerie & Tina, 2007)” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }(Valerie & Tina, 2007)
B. Osmosis: This is a special form of diffusion and defined as diffusion or movement of water molecules from a region where they are at a high concerntration to a region where they are at a low concerntration through a partially permeable membrane.The cell controls the amount of water which enters across the plasma membrane by regulating the concerntration of ions in the cytoplasm via sodium/potassium pump.


C.Active transport:This is a carrier- mediated transport of a solute through a unit membrane up its concerntration gradient,with this expenditure of adenosine triphosphate(ATP).Ions such as Na+,K+,CL+,H+ and others such as amino acids and glucose may be transported across the cell surface membrane by the use of energy(ATP). Example SODIUM-POTASSIUM PUMP( Na+-K+ pump),which binds three sodium ions from the intracelluler fluid and ejects them from the cell,then binds two potassium ions from the extracelluler fluid and releases these into the cell.The Na+-k+ pump controls cell volume,generates body heat.maintaining electrical excitability of nerves,muscles,and heart.it also provides energy for other transport pumps to draw upon in moving such solutes as glucose through the plasma membrane.calories are needed operate the pump.
D .Facilitated diffusion:This is considerably faster than normal diffusion and is used to transport molecules such as glucose,fructose,no fat-soluble vitamins,urea and others ions the cell membrane
The plasma membrane of a cell plays the following roles:
1. Protection
A. it protects the integrity of the interior of the cell by allowing only selected substances into the cell thereby preventing certain toxic substances from entering the cell.

B. It also protects the cell from loss of useful biological macromolecules held within the cell by its plasma membrane.

2. It acts as a Barrier
It regulates exchange of materials with its surroundings. It separates contents of the cell from its outside environment and it regulates what enters and exits the cell. The lipid bilayer is semi-permeable, which allows only selected molecules to diffuse across the membrane.

3. It provides Shape, support and Flexibility
A. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane supports the cell and helps in maintaining the shape of the cell.

B. The cell membrane is mostly made up of proteins and lipids. While lipids help to give membranes their flexibility and proteins monitor and maintain the cell’s chemical climate and assist in the transfer of molecules across the membrane.

4. Active Transport
A. Cell or plasma membrane can allow active transport of specific molecules across the membrane in either direction, by either moving against a concentration gradient (e.g.) from low concentration outside the cell to high concentration inside the cell or vice-versa using energy
Endocytosis: process by which secretory vesicles are moved into the cell via the cell membrane.
Exocytosis: cells move the contents of secretory vesicles out of cell via cell membrane).
Metabolic Activity: Cell/Plasma membrane includes as part of their structure certain proteins and enzymes that are involved in some of the metabolic processes of the cell.

Markers or Signals for Communication: Surface proteins embedded in the cell membrane help
communicate and pick signals from nearby cells. ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “ISBN” : “978-81-8052-239-0”, “author” : { “dropping-particle” : “”, “family” : “Veena”, “given” : “Ahuja Mehta”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “edition” : “2nd”, “id” : “ITEM-1”, “issued” : { “date-parts” : “2011” }, “publisher” : “Ane Books Pvt.ltd.”, “publisher-place” : “india”, “title” : “Textbook of medical physiology”, “type” : “book” }, “uris” : “http://www.mendeley.com/documents/?uuid=930b1004-45c5-43fe-87ce-317ebfed5d63”, “http://www.mendeley.com/documents/?uuid=b34b8d2d-c1d5-4f1c-a687-61e4c9d45d59” } , “mendeley” : { “formattedCitation” : “(Veena, 2011)”, “plainTextFormattedCitation” : “(Veena, 2011)”, “previouslyFormattedCitation” : “(Veena, 2011)” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }(Veena, 2011)
ADDIN Mendeley Bibliography CSL_BIBLIOGRAPHY Arthur, V., James, S., ; Dorothy, L. (1998). Human Physiology (Seventh Ed). san francisco: McGraw-Hill Companies.

Donald V.,Judith V.,Pratt, C., (2013)Fundamentals of biochemistry life at the molecular level(4th ed.)Hoboken,NJ:John Wiley ;Sons,inc
Davidson. (2010). Davidson’s Principles & Practice of Medicine. (R. S. H. Colledge N.R., Walker B.R., Ed.) (21st ed.). Churchill Livingstone Elsevier.

“Glycolipids”.Nature. Nature Publishing Group. 2015-11-01.

Fox, S. I. (2006). Human Physiology (9th ed.). The McGraw hill.

Valerie, S. C., & Tina, S. (2007). Essentials of Anatomy and Physiology. (D. B. Lisa, S. Alen, & R. H. Llysa, Eds.) (5th ed.). F.A. Davis.

Veena, A. M. (2011). Textbook of medical physiology (2nd ed.). india: Ane Books Pvt.ltd.