Functions of Cell Membranes

 Modifications of Cell Membrane:

What are the modifications of cell Membrane?

Cell membrane may be modified to serve functions. Common modifications include folding, intercellular junctions, and extracellular coats.

1): Folding: Cell membrane may from infolds or outfolds .Both are of many types.

(a) Infolds ( Invaginationss):Infolding of cell membrane occurs in several

 (i) Pores:Certain unfoldings may become continuous with the endoplasmic reticulum, thus forming pores in the cell membrane. 

(ii) Endocytotic Vesicles:  A pit of the cell membrane may receive a fluid droplet or a solid particle from the environment, pinch off and move in as a vacuole,  the endocytotic vesicle,to utilize the material. The process is called endocytosis.It is described ahead in detail.

(iii) Mesosomes:These are infolds of plasma membrane in bacteria to increase the surface area. A mesosome is often joined to the prochromosome and helps in prochromosome replication and cell division. 

 (b) Outfolds ( Evaginatuons): Outfolds of the cell membrane are also of many types__ 

 (i) Microvilli:The free surface of the cells in the intestinal epithelium has numerous fine, cylindrical  Outfolds called Microvilli.There may be about 2000 microvilli per cell. A microvillus is about 2 ųm long and some 800 __ 1000 A  thick, and is supported by a bundle of microfilaments. Proteins fimbrin and Villin  crosslink the microfilaments to each other, and a protein calmodulin connects them to the microvillar membrane. This makes the microfilament bundle a rigid structure. The microvilli increase the absorptive surface. Microvilli are revealed by electron microscope.Light microscope shows the microvilli_ bearing cells as having Striated or brush borders.

(ii) Interdigitations:The membranes of adjacent cells fit into each other by finger__ like folds called Interdigitations. The latter not only keep the cells adhered together but also provide a large surface contact between the cells for the exchange of materials.

 (iii) Ciliary and Flagellar Sheaths: The cell membrane  continues as Outfolds over the cilia and flagella when these are present on a cell.

 2): Intercellular Junctions There are often narrow gaps between adjacent cells in animal tissues. These are called the intercellular spaces and contain tissue fluid.There are at places specialized contact regions between the cell membranes of neighboring cells. These are of many types__ 

(i) Desmosomes:In these regions, circular patches of cell membranes are held together by interaction of proteins that extend through each membrane into the space between cells. The cell membrane has on the inner the cell. The desmosomes act as " spot welds" and keep the cells firmly together. They occur between epithelial cells subject to stress.

Desmosomes are also called maculae adherentes ( singular is macula adherens).

 (ii) Intercellular Bridges:These consist of minute projections that arise from adjacent cell membranes and make contact with one another. They allow a more rapid conduction of electrical excitation of the cells than if the electric currents were to pass through the materials between the cells.

(iii) Tight Junctions:These are regions were the membranes of adjacent animal cells are held close together by  Sealing strands.They tie the cells firmly and check the movement of materials between them.

The right Junctions are also called zonulae occludentes ( singular is simulated occludens).

(iv) Terminal Bars: These are Similar to the Desmosomes but lack tonofibrils.

(v) Gap Junctions:These are direct cytoplasm _ to_ cytoplasm  connections between many kinds of animal cells by protein "pipes" called connexons. Each connexon is a channel  by 6 identical protein subunits .They permit ins and small molecules to pass from cell to cell without leaking into the space or gap between them.Heart muscle cells have many gap Junctions that allow the electrical currents of moving ins to pass quickly between them.This coordinates contractions, making all muscle cells in certain areas of the heart to contract simultaneously. 

 (vi) Plasmodesmata: These are intercellular Junctions in plant tissues. At places, cell walls and middle lamella have tubular passages. The plasma membranes, cytoplasm and endoplasmic reticulum of adjacent cells are continuous through these passages. This arrangement Unifies most of the plant into one living continuum. Water and small solutes may pass from cell to cell through the Plasmodesmata (singular, plasmodesma). Recent experiments have shows that even certain protein and RNA molecules can also pass via Plasmodesmata.

3): Extracellular Coats:The animal cells form many kinds of extracellular Coats__ 

 (i) Chitin:It forms exoskeleton in insects. It consists of a heteroplysaccharide. It protects the cells from injury and loss of water.

(ii) Glycocalyx:It covers the epithelial lining of intestine. It consists of polysaccharides joined to the proteins on the free surface of the cell membrane. It protects the cells from digestive enzymes. Glycocalyx has been found to surround all cell surfaces, whether in contact with the surfaces of other cells or free.

(iii) Basement Membrane:It is a compact sheet under the epithelial cells. It consists of collagen fibrils associated with glycoproteins. It anchors the cells to the connective tissue beneath and acts as a selective diffusion barrier. 

(iv) Cell wall  It is a rigid coat around the plant cells. It is chiefly composed of the polysaccharide cellulose. It may have deposited of additional materials in it.It protects and supports the cells.

 Prokaryotic cells and fungal hyphae also have a protective cell wall. The cell wall consists of peptidoglycan in prokaryotes, and of chitin in fungi. A slimy polysaccharide or polypeptide capsule surrounds the cell wall in prokaryotic cells. This cell coat has a physiological importance. If it is removed, the bacterial cells are affected by osmotic pressure which would not affect them if their Coats were intact.

Functions of Cell Membranes:

The cell membrane serves many functions __

(i) It maintains the individuality and form of the cell.

(ii) It keeps the semi fluid cell contents in place and distinct from the environmental materials. 

(iii) It protects the cell from injury. 

(iv) It regulates the flow of materials into and out of the cell. A cell remains alive as long as the cell membrane is able to determine which materials should enter or leave the cell.

(V) It forms organelles within the cytoplasm. 

(vi) Its junctions keep the cells together. 

(vii) Its infolds  help in the intake of materials by endocytosis. 

(viii) Its Outfolds ( microvilli) increase the surface area for absorption of nutrients. The outfolds also from protective sheaths around cilia and flagella.

(ix) Its receptor molecules permit flow of information into the cell.
(x) Its Oligosaccharide molecules help in recognising self from nonself.

(xi) By controlling flow of materials and information into the cell, the cell membrane makes metabolism possible. 

(xii) It permit exit of secretions and wastes by exocytosis. 

 (xiii)  It controls cellular interactions necessary for tissue formation and defense against microbes.

(xiv) It help certain cells in movement by forming pseudopodia as in Amoeba and leucocytes.
(xv) Some membrane enzymes, such as respiratory enzymes in the inner mitochondrial membrane, catalyze chemical reactions as a part of the cell's metabolism. 

 (xvi) Certain membrane receptors provide irritability to the cells. For example, in nerve and muscle cells, stimulation of receptors initiates changes which lead to muscle contraction and nerve impulse conduction. 

(xvii)  By forming Plasmodesmata, the cell membrane establishes cytoplasmic continuity between adjacent cells.

(xviii) The extracellular Coats formed by ce membrane provide additional protection or anchorge to the cells. 
(xix) Cell membrane retains the materials taken up by the cell.

(xx)  The biomembranes around the organelles help th to 

 (a) maintain their identity, specific internal environment and functional individuality, and 

(b)  receive and turn out required materials. 

Q. What are fibronectins?

Ans . Fibronectins are glycoproteins that attach the animal cells to the extracellular matrix ( cell coat ) and are also connected to the integral proteins of cell membrane. 


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