WELCOME TO MY PRESENTATION

Submitted ToDr. Sayda RehanaAssistant Professor Biotechnology & Genetic EngineeringKhulna UniversityKhulna

Submitted ByMd. Moin Uddin TalukderStudent Id:1307031st YEAR, 2nd TERM Biotechnology & Genetic EngineeringKhulna University,Khulna

Course Title: Cytology

Course No: BGE-1203

DATE OF SUBMISSION : June 09,2014

PLASMA MEMBRANE

All living cells and many of the tiny organelles internal to cells are bounded by thin membranes. This membrane made up of

• a two layers of phospholipids with embedded proteins.

• It separates the contents of the cell from its outside environment, and it regulates what enters and exits the cell.

• It is also known as the cell membrane or cytoplasmic membrane.

STRUCTURE OF PLASMA MEMBRANE

ORIGIN OF PLASMA MEMBRANETurnover of membranes:• Every part of the cell is moveable, there is a constant movement and there is a constant turnover.• Structure of the cell doesn’t change visibly, but with a molecule removed and replaced here, another removed and replaced

there. It is only a matter of days before a typical cell- even a stable long live cell such as hepatocyte- is completely renewed. • Thus there is constant synthesis and turnover of cell components, including the lipids and proteins of plasma membrane and

other cellular membranes.Plasma membrane synthesis:• Membranes and their constituent proteins are assembled in the ER.• This organelle contains the enzymes involved in lipid synthesis, and as lipids are manufactured in the ER, they are inserted into

the organelle's own membranes.• This happens in part because the lipids are too hydrophobic to dissolve into the cytoplasm.• Similarly, transmembrane proteins have enough hydrophobic surfaces that they are also inserted into the ER membrane while

they are still being synthesized.• Future membrane proteins make their way to the ER membrane with the help of a signal sequence in the newly translated

protein. • The signal sequence stops translation and directs the ribosomes — which are carrying the unfinished proteins — to dock with

ER proteins before finishing their work. • Translation then recommences after the signal sequence docks with the ER, and it takes place within the ER membrane. Thus,

by the time the protein achieves its final form, it is already inserted into a membrane.

CO-TRANSLATIONAL SYNTHESIS

MODELS OF PLASMA MEMBRANE1. LAMELLAR MODEL

MODELS BASED ON LIPID BILAYER STRUCTURE

2. MICELLAR MODELMODELS BASED ON GLOBULAR ARRANGEMENT

3. PROTEIN CRYSTAL MODEL

4. FLUID MOSAIC MODEL

MOST ACCEPTABLE MODEL (FLUID MOSAIC MODEL) OF PLASMA

MEMBRANE

According to the fluid mosaic model of S.J.Singer and G.L. Nicolson (1972), which replaced the earlier model of davson and danielli.

• Biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily.

• The plasma membrane consists of a combination of phospholipids and proteins.

• These proteins are not fixed in any rigid pattern, instead they float around in the membrane, this is called the fluid mosaic model of the cell membrane.

• The cell membrane is comprised of a lipid bilayer. The lipid molecules are formed in two rows, each row is a mirror image of the other row.

Two parts make up the structure of the lipid molecules – a hydrophobic and a hydrophilic section.

• The hydrophobic section avoids water and water-like molecules and faces inwards making up the center of the lipid bilayer, while the hydrophilic section attracts water or water-like molecules and faces outwards.

• TRANSPORT PROTEINS are responsible for shipping and receiving. They move materials in and out of the cell. Some of them function as pores that allow substances to diffuse through the membrane.

• RECEPTOR PROTEINS extend through the cell membrane. As the communication office of the cell, they allow the cell to interact with other cells. The part of the receptor protein on the exterior of the cell surface binds to a molecule. The specificity of receptor proteins allow the cell to respond to the outside environment in many different ways.

FUNCTION• It must compartmentalize the cell from its environment. In other words, it must separate the cell from the

environment around it and maintain all the intracellular contents within the cell. It has to keep the stuff that is inside the cell inside the cell.

• It must protect the cell. It isn't like armor or anything-- actually, it is pliable and moveable. But it has to keep stuff that is supposed to be outside the cell outside the cell. It can't just let anything in. And it also has to bend so that it isn't rigid and so that it won't break open.

• It must allow for selective permeability so that certain needed materials from the environment can enter the cell and waste materials can leave the cell. "Selective" because not ALL things should be able to cross the membrane (that would be like poking a huge hole in the cell and it would die).

• A recognition and response function. Because the outermost part of the cell is the plasma membrane, it would be the first part of the cell to come into contact with items in its environment. These items could be: toxins, food substances, hormones, other organisms, etc. You should be able to see the importance of noticing such items in your environment. For example, if a cell noticed food in its environment, it should be able to then grab hold of the food and take it inside of the cell. Or if a protist notices toxins (poisons) in its environment, it should be able to swim away from that location into a safer one. This can be considered analogous to a communication function, and it is called signal transduction.

OTHER FUNCTIONSSome of them have already come up in our conversations

and study• Cell adhesion to other cells or to the world. This is how cells stick to one another or how they stick to

other materials in their world (which can also be within a human body).

• Attachment of the cytoskeleton to the membrane. This is the problem in some forms of MD... You see, the cytoskeleton (which is inside the cell) has to attach to the membrane to allow the entire cell to move. If the cytoskeleton were to move without being attached to the membrane, the cell itself would not move anywhere or change at all from the outside. Dystrophin is one of the proteins that attaches the cytoskeleton to the membrane.

• Provide an ID marker to the world. The membrane displays an identity marker on its outside. This identity marker prevents our white blood cells (that clean out the body of unwanted things) from eating up our own cells. This ID marker is also what tells our white blood cells to chew up transplanted tissue.

• Some cells can wrap their membranes around other cells to help with an insulation function. This can happen in the nervous system to insulate the cells that transmit electrical signals.

COMPOSITIONThe plasma membrane is composed of a bilayer, this bilayer behaves very much like a fluid, that means the lipids in this bilayer are in constant lateral motions. The phospholipids can laterally exchange their places, however they can not diffuse transversely. The other important components of the plasma membrane are proteins. Proteins can be associated with the outside (peripheral) of the membrane or they extend throughout the entire membrane (transmembrane). Many of the plasma proteins are not fixed into a certain position, but can freely move within the membrane. The protein content of differs in membranes of different origin.

Amounts of Protein, Lipid and Carbohydrate in Different Biological Membranes (Approximate Percentage of Dry Weight)

Membranes- Protein Lipid CarbohydratePlasma membranes:

Red blood cells

Liver cells

Amoeba

Myelin

49

54

54

18

43

36

42

79

8

10

4

3

Nuclear envelope 66 32 2Endoplasmic reticulum 62 27 10

Golgi complex 64 26 10Mitochondrionouter membrane

inner membrane

45

78

45

22

Trace

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Chloroplast inner membrane

70 30 ---

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