CELL BIOLOGY- CELL MEMBRANE

CELL BIOLOGY

(USEFULL FOR NEET, CSIR NET, DBT-JRF, ICMR,BARC,IIT-JAM, M.SC, B.SC

WHAT IS CELL?

          Cell is the structural an functional unit of life. what actually mean by structural unit? we all know that every organism starts its life from one single cell, zygote. Zygote go through cell division to generate new cell. gradually frequent and regulated cell division result in cell growth and growth of the enzyme. Cell can be prokaryotic or eukaryotic depending upon there organelle present in the cell. Basically wheather the cell have developed nucleus or not the cell can be of eukaryotic or prokaryotic. 

Wheather its a prokaryotic or eukaryotic cell the cell membrane shows similar characteristics. Details are discussed below.

CELL MEMBRANE:

             Cell is the structural and functional unit of life. Cell membrane protects the cell i.e all internal organelle and the intracellular molecules from the external environment. Cell membrane also participates in communications and transportation of biomolecules between intracellular and extracellular environment.

Cell membrane shows a typical structure having a lipid bi-layer (thick) and embedded protein layer according to the ‘Fluid Mosaic Model’ proposed by Singer and Nicolson in 1972.

CHEMICAL COMPONENETS OF CELL MEMBRANE:

                

Lipid- forms the lipid bilayer of the cell membrane.

Protein- forms the membrane proteins which are of two types integral protein and peripheral protein.

Carbohydrate – carbohydrates are present in the cell membrane in the form of glycolipid (bound to lipids) and glycoprotein (bound to protein)

LIPID BILAYER:

  Lipid molecules constitute the bulk and thick components of cell membrane, lipid bilayer. There are about 10⁹ lipid molecules in the cell membrane of a single animal cell. All the lipid molecules in the plasma membrane are amphiphilic in nature that is they have a hydrophilic (water loving) or polar end and a hydrophobic (water fearing) or non polar end.

POLAR MOLECULES- A polar molecule is a chemical species in which the distribution of electrons between the covalently bonded atoms is not even.

Example – water, ammonia.

NON POLAR MOLECULES - A molecules make of electronegatively similar atoms, which distribute electrons equally.

Example- carbon dioxide, methane.

Three classes of lip[id molecules are present in the cell membrane- phospholipids, glycolipids and sterols.

The most abundant lipid molecule of the cell membrane is the phospholipids.

PHOSPHOLIPIDS- Phospholipids are composed of one primary alcohol, phosphate group, one secondary alcohol, two fatty acid chain. Depending upon the primary alcohol phospholipids can be of two types- glycerophospholipids (glycerol), sphingolipids (sphingosine).

                                         Figure - Phospholipids of cell membrane

The main phospholipids in most animal cell membrane are phosphoglycerides. Phosphoglycerides contain a three carbon glycerol. Two long chain fatty acids are linked through esters bond with two adjuscent carbon atom of glycerol. These structure form the hydrophobic tail part of the phospholipids as it is non-polar so can not dissolve in water. This hydrophobic parts are dissolved in organic solution. The third carbon atom of the glycerol is attached to the phosphate group(with one negative charge) which in turn attached with a another group, the secondary alcohol (serine, choline, ethanolamine) with a positive charge or neutral. These form the head part of phospholipids which is polar, dissolve in water, i.e., hydrophilic in nature.

                                   

The other important class of phospholipid are sphingolipids. It contains sphingosine instead of glycerol. In case of sphingomyeline, the most common sphingolipids, a fatty acid tail is attached to the amino group, and a phosphocholine group is attached to the terminal hydroxyl group.

At neutral pH (pH 7), phosphatidylecholine and phosphatidyleethanolamine carry no net electric charge, whereas phosphatidyleionositol and phosphatidyleserine carry a net negative electric charge.

MEMBRANE ASYMMETRY OF LIPIDS:

The lipid composition is different in the cytosolic leaflet and non-cytosolic or exoplasmic leaflet of the phospholipid bilayer. In plasma membrane of human erythrocyte cells sphingomyeline and phosphatidylecholine found more in number in the exoplasmic side of phospholipid bilayer, whereas phosphatidyleserine, phosphatidyleethanolamine and phosphatidyleionositol are preferentially located in the cytosolic side of the phospholipid bilayer. Although these phospholipids molecules are generated in the ER they oriented in preferential manner in the phospholipid bilayer with the involvement of three enzymes flippase, floppase and scramblase. After synthesis phospholipid molecules are equally distributed in two phospholipid monolayers, and this is mediated by scramblase,  a phospholipid translocator protein. Thereafter flippase, a P-type active transporter protein transport phospholipids (phosphatidyleserine, phosphatidyleethanolamine, phosphatidyleionositol) from outside to inside. On the other hand another floppase, another active transporter protein transport phospholipid molecules (phosphatidylecholine, sphingolipids) from inside to outside.  Scramblase is a ATP independent transporter protein.

FLIPPASE	FLOPPASE	SCRAMBLASE
P-type	ABC type
ATP dependent	ATP dependent	ATP independent
Outside to inside	Inside to outside	Both direction and lateral direction

However there is a significant differences is available in lipid asymmetry between apoptotic or cancer cell and normal cell. In apoptotic cell or cancer cell phosphatidylecholine found more in cytosolic end and phosphatidyleserine found more in exoplasmic cell. This is a very useful difference between cancer cell and normal cell.

 MEMBRANE FLUIDITY: Cell membrane has a quasi-fluid structure. Cell membrane maintains a critical fluidity so as to maintain its semi-permeability. Fluidity is essential to transport gasses, nutrients, signalling molecules, and also for movement of phospholipids molecule along the membrane plane. For example human live in different climatic region. But there a lot of differences between cell membrane compositions among people live in colder region and temperate region. The cell membrane fluidity is controlled by various factors,

1)      Temperature: As the temperature increases the fluidity of the membrane increases. Increasing in temperature increase the kinetic energy of phospholipids molecules and also decrease the interaction between lipid molecules thus increase the fluidity. Decrease in temperature in turn decrease the membrane fluidity and make the cell membrane more rigid in nature. The temperature at which membrane behaves like 50% fluid and 50% gel like structure is known as transition melting point (T_m).

2)      Fatty Acid Chain Length: Long saturated fatty acid chains have more tendency to aggregate and packed tightly by van der walls interaction and hydrophobic interactions, thus make the membrane more gel like state or increase the rigidity and decrease the membrane fluidity. Meanwhile presence of short fatty acid chains increase the membrane fluidity as there is less surface area available for van der walls interaction and hydrophobic interaction.

3)      Degree of Saturation: Phospholipids with unsaturated fatty acid chains that is fatty acid chains having double bond or triple bond structure, increase the membrane fluidity. Unsaturation that is presence of double bond structure in the fatty acid chain form kink structure and increase the distance between adjacent fatty acid chains of neighbouring phospholipid molecules. This in turn reduces the tendency to interact with other phospholipid molecules. So more the unsaturation more will be the membrane fluidity.

Sterol: There is another molecule sterol, is also responsible for maintaining the cell membrane fluidity. Sterols are lipid soluble molecules and these are amphipathic in nature. These are mainly responsible for maintaining of membrane rigidity. In case of animal cell the sterol present is known as cholesterol, in case of plant the sterol present is known as phytosterol. Most common phytosterol are brassicasterol, campesterol, sitosterol, stigmasterol, avenasterol. In case of fungi the sterol present is known as ergosterol. Bacteria contain hopanoids instead of sterol. In animal cells Cholesterols  mainly interact with phospholipids  present in the cell membrane. Cholesterol present in between lipid molecules and interacts with phospholipids by the means of hydrophilic interaction and hydrophobic interaction as cholesterol shows amphipathic character. Basically in cold temperature cholesterol concentration increases so as to prevent possibilities to be more rigid as it prevent phospholipid molecules to came more closer. Whereas in high temperature cholesterol also prevent lipid molecules from being separated out from each other by means of hydrophobic interaction with fatty acid chains. So the main role of cholesterol in the plasma membrane to maintain membrane fluidity.   

MEMBRANE PROTEINS:

As we know cell membrane is composed mainly of lipids and proteins. Protein molecules are embedded in the phospholipid bilayer. Membrane proteins play a major role in maintaining the functions of a specific cell. Membrane proteins are amphipathic in nature. Membrane proteins can be classified as

                     1) Integral protein

                     2) Peripheral protein

                     3) Lipid anchor protein

Arrangement of various membrane protein

1) Integral Protein: Integral proteins also called trans membrane protein. These type of protein shows three part cytosolic domain, exoplasmic domain and transmembrane domain. The cytosolic and exoplasmic domain have hydrophilic surface which interact with the aqueous of the cytosolic and exoplasmic environment. The transmembrane domain or the membrane spanning segment of the protein is hydrophobic in nature and it interacts with the fatty acid chain of the phospholipid molecules. In all transmembrane proteins examined till date, the membrane spanning domains consists of one or more α-helix or of multiple β-strands. Glycophorin, BAND-3, GPCR, Aquaporin, Ion channels, ATP Binding Cassette, Human Leukocyte Antigen (HLA), are some example of transmembrane protein.

Depending upon the number of time a transmembrane protein pass the lipid bilayer TM protein may be single pass (monotopic) or multi-pass (polytopic). Glycophorin, the major protein the human erythrocyte cell membrane, is a representative single pass transmembrane protein, which contains only one single membrane spanning α-helix. It is composed of 131 amino acid residues. BAND-3 is a multi-pass TM protein which contains 929 amino acid residues and 14 transmembrane segment. The aquaporins are a large family of highly conserved which transport water, glycerol and other hydrophilic molecule across the cell membrane. Aquaporins have 6 membrane spanning α-helix. Porin a class of transmembrane protein have β-barrel strand in transmembrane segment. Porins are present in outer membrane of gram-negative bacteria like E.coli.

2.Peripheral Protein:  Peripheral proteins do not directly contact the hydrophobic core of the phospholipid bilayer. Instead they are bound to the membrane either indirectly by interactions with integral or lipid anchored membrane proteins  can be bound to either the cytosolic or the exoplasmic face of the plasma membrane. Peripheral proteins covalently attached to the cell membrane. Spectrin, anchyrin, BAND-4.1 are well known example of peripheral proteins.

3.Lipid Anchor Protein:  Lipid anchor proteins are covalently bounded to one or more lipid molecules. The hydrophobic segment of the attached lipid is embedded in one leaflet of the membrane and anchor the proteins to the membrane. The polypeptide chain itself does not enter the phospholipid bilayer. GPI- anchor protein is a type of lipid anchor protein.

FOR MORE DETAIL FOLLOW THIS BOOKS

1. MOLECULAR BIOLOGY OF THE CELL- 

       BUY NOW

buy now

2. LODISH CELL BIOLOGY:

VARIOUS STUDY MATERIALS FOR NEET, NET, DBT etc.