Cell membrane

 TOPIC 2         CELL MEMBRANE 

CELL MEMBRANE 

INTRODUCTION

                              It is the protective and selectively permeable membrane which sorrounds the cell.

It is the outer most layer in animals because they lack cell wall.

In Plants,prokaryotes(except mycoplasma) and fungi it lies inner to cell wall.

SIZE 

       Its size is about 7 nanometer.

BASIC DIFFERENCE BETWEEN CELL WALL AND CELL MEMBRANE

        The basic difference between cell wall and cell membrane is that of permeability.

The cell wall is completly permeable means allow every substance to enter to cell while cell membrane is selectively permeable which allow only those substance to enter into cell which cell needs and also determines its polarity.

COMPOSITION OF CELL MEMBRANE 

                                                             Till to this date four models are proposed about the composition of cell membrane which we will explain in detail:

1)GORTER AND GRINDLE MODEL

                                                             This model was proposed by Gorter and Grindle in 1925 they said that the cell membrane is just a lipid bilayer.

2)DANIEL AND DAVON MODEL

                                                        This model was proposed by Daniel and Davon in 1935 thdey said that the cell membrane is made up of lipid bilayer which are covered by protein layer or pores.

3)UNIT MEMBRANE MODEL

                                                This model proposed by Robertson in 1959 he said that cell membrane sandwiched between two layers of proteins.

4)FLUID MOSAIC MODEL

                                             This model was proposed by Sanger and Nicholson in 1972.

It is the most acceptable model of cell membrane worldwide.According to this model "The cell membrane is a sea of lipids in which proteins are floating".

we will explain this model in detail according to this model cell membrane is made up of following molecules:

1-LIPOPROTEIN(lipids 20-40%  + proteins 60-80%)

2-INTEGRAL PROTEINS (extrinsic and intrinsic )

3-CARBOHYDRATES(2-3%)

4-CHOLESTROL(less than 1%)

5-GLYCOLIPIDS AND GLYCOPROTEINS(carbohydrates +lipids and proteins)

Now we will explain the function of these molecules in detail:

1)LIPOPROTEIN

                            It is the basic and functional part of all living membranes.it is a conjugated molecule formed from lipids and proteins.

In cell membrane LIPIDS make cell membrane differentially permeable means allow only non polar molecules to enter into cell and prevent the entery  of polar and ionic substances.

In cell membrane LIPIDS present in form of PHOSPHOLIPIDS(phosphate + lipids) because it provide strength and enhance and regulate the permeability of cell membrane because it have two parts:Head part (water loving) and tail part (water hating part) these two parts have important role in dissolving process of molecules so in this way the contribute in regulation of molecules.  

In cell membrane PROTEIN make it selectively permeable and allow it to enter those substances only which cell needs.

2)INTEGRAL PROTEINS

                      These proteins are of two types:

INTRINSIC protein they float throughout the cell membrane.It help in diffusion ,active transport and osmosis.

EXTRINSIC protein they present in fixed form and inner to cell surface.

It also recieve the stimulus from outside enviornment.

These proteins also act as Permeases.

3) CARBOHYDRATES

                                    They are responsible for reception.

Responsible for hormone receptor site

Nerve impulse recognition 

Recognition of antigen

Recognition of Food

Regulate endocytosis and exocytosis

4)CHOLESTROL

                                  It have role in fluidity.

5)GLYCOLIPIDS AND GLYCOPROTEINS:

                                    They are surface cell makers.

OVERALL FUNCTION OF CELL MEMBRANE  

1-Involve in secretion and excretion

2-obtain nutrients

3.maintain ionic gradients(responsible for nervous and muscular activity)

4-Prevent water escaping

5-Also help in maintainance of PH

IMPORTANT MCQS IN THIS TOPIC

1-The basic component of cell membrane is:

Answer: LIPOPROTEIN

2-The surface area provided to cell membrane by:

Answer: GOLGI BODIES

3-The cell membrane is selectively permeable because of:

Answer: PROTEINS

4-The most acceptable model of cell membrane is:

Answer: FLUID MOSAIC

5-The lipid portion of cell membrane allows non polar molecules:

Answer: LIPIDS

6-Permeases is the another name of:

Answer: INTEGRAL PROTEINS

7-The percentage of lipids in cell membrane is:

Answer: 20-40%

8-The fluidity of cell membrane is regulated by:

Answer: CHOLESTROL

9-The lipids in cell membrane is present in form of:

Answer: PHOSPHOLIPIDS

10-Little damage to which organelle cause cell death:

Answer: CELL MEMBRANE

11-The eating process of cell is called as :

Answer: PHAGOCYTOSIS

12- The drinking process of cell is called as:

Answer: PINOCYTOSIS

13-Which prokaryotes have outermost layer cell membrane:

Answer: MYCOPLASMA

14-The cell surface makers are :

Answer: GLYCOLIPIDS AND GLYCOPROTEINS

15-The size of cell membrane is: 

Answer: 7 NANOMETER