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Cytoplasm and cell membrane

Introduction:

  • The cytoplasm of the cell is a crystallo-colloidal complex that forms the protoplasm excluding its nucleoplasm.
  • The presence of ribosomes in the cytoplasm makes the matter granular.
  • It also possesses membrane-bound cell organelles in eukaryotic cells and also the site of all biochemical pathways found in prokaryotic cells.
  • The plasma membrane surrounding the cell is a selectively permeable membrane that is made up of phospholipid bilayer and proteins of various types such as extrinsic, integral, transmembrane.

Topics covered:

  • Cytoplasm
  • Cell membrane- Salient Features
  • Lamellar/sandwich model & Unit membrane model
  • Fluid Mosaic Model

Cytoplasm
 

Introduction:

  • The protoplasm excluding the nucleus is known as the cytoplasm of the cell.
  • The cytoplasm of both prokaryotic and eukaryotic cells consists of a cytoplasmic matrix, cell organelle, and cell inclusions.

Definition and characteristics feature:

  • It lies between the nucleus and cell membrane.
  • In both prokaryotic and eukaryotic cells, a semi fluid matrix called a cytoplasm occupies the volume of the cell.
  • Cytoplasm is the main arena of cellular activities in both plant cells and animal cells.
  • Various chemical reactions occur in cytoplasm to keep the cell in living state.

    cytoplasm-and-cell-organelles
     

Detailed explanation:

  • Cytoplasm is jelly-like semi-fluid general mass of protoplasm excluding the nucleus but including all other components— cytoplasmic matrix, cell organelles and cell inclusions.
  • It includes two parts-

(1) Cytoplasmic matrix / Cytosol / Hyaloplasm / Ground plasm:

  • Liquid part of cytoplasm.
  • It can exist in sol and gel states called plasmasol and plasma gel.
  • The cytoplasmic matrix is composed of 90% water.
  • The cytoplasmic matrix consists of the crystallo-colloidal complex in water along with some chemical compounds such as minerals, sugars, amino acids, tRNAs, nucleotides, vitamins, proteins, enzymes, etc.
  • This also includes proteins as the major colloidal particles of the complex.
  • It also consists of fat in the form of globules in the cytoplasmic matrix.
  • These fat molecules are the components of various biomembranes.

(2) Trophoplasm: It involves cell organelles and cell inclusions.

(a) Cell organelles :

  • These are the subcellular structures of the cells that perform definite functions.
  • The cell contains various cell organelle such as ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, microbodies, plastids, etc.

(b) Cell inclusion:

  • The cell inclusion is the intracellular non-living substances that are not surrounded by any external membrane.
  • These structures are responsible for storing nutrients, secretory products, and pigment granules.
  • They are of three types–

(1) Reserve food:

  • It includes starch, glycogen, fat droplets and aleurone grains.

(i) Starch grains
(ii) Glycogen granules: Animal cells
(iii) Fat droplets: Animal and Plant cells
(iv) Aleurone grains

(2) Excretory or secretory products:

  • Mucus in several animal cells, essential oils, alkaloids, resins, gums, tannins, latex etc.

(3) Mineral matter:

  • Silica found in epidermal cells of grasses.
  • Calcium carbonate crystals (cystolith) found in leaf cells of Banyan (Ficus).
  • Crystals of Calcium oxalate are found in the form of star shape (druses) in Nerium ; and needle shaped (raphides) in Eichhornia.

Functions of cytoplasm :
 

(i) Raw Materials:

  • The cytoplasm is the reservoir of raw material that is used as a substrate for the functional reaction performed by cell organelles.

(ii) Products:

  • The products formed by the cell organelle are passed onto the matrix.

(iii) Exchange of material:

  • The compounds or products produced by the cell organelle within the cell are transported through the cytoplasmic matrix.

(iv) Biosynthesis:

  • The cytoplasmic matrix is responsible for the synthesis of a number of biochemicals such as fats, nucleotides, some carbohydrates, proteins, and coenzymes.

(v) Cytoplasmic streaming:

  • The motion of the cytoplasm inside the cell is known as Cytoplasmic streaming.
  • This is an autonomic process that helps in the regular distribution of various materials inside the cell.

(vi) Catabolic Activities:

  • The metabolic pathway such as anaerobic respiration, glycolysis, and pentose pathway occurs in the cytoplasm of the cell.

Cell membrane- Salient Features
 

Introduction:

  • It is the outermost covering of the cell that is elastic, living, pliable, and selectively permeable membrane.
  • It is found in both prokaryotic and eukaryotic cells.
  • It acts as a barrier separating the internal environment of the cell from the external environment.
  • The cell membrane is the surrounding membrane of the cell which was studied in detail after the advent of the electron microscope in the 1950s.
  • Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled scientists to deduce the possible structure of the plasma membrane.

Detailed explanation:

  • Cell membranes can be observed in electron microscopy. It appears as a trilaminar or tripartite layer under electron microscope.
  • Chemically a cell membrane contains proteins, Lipids, carbohydrates, and water. DNA, RNA are absent.
  • The ratio of protein and lipid in plasma membranes varies e.g. in human beings, the membrane of the erythrocyte has approximately 52 percent protein and 40 per cent lipids.
  • The thickness of the membrane is about 70 to 100 Armstrong.

    plasma-membrane
     

1. Lipids:

  • The most common lipids found in the cell membrane are phospholipids.
  • A single phospholipid molecule is made up of a polar head containing phosphate along with the two non-polar hydrocarbon tails obtained from the fatty acids.
  • The head region of the phospholipid is the hydrophilic (water-loving) part and the tail region is considered as the hydrophobic (water-repelling) part of the phospholipid bilayer.
  • Therefore, the phospholipid molecules are amphipathic or amphipathic, that is, they possess both polar hydrophilic (water-loving) and nonpolar hydrophobic (water-repelling) ends.

    lipids
     
  • The proteins present in the lipid bilayer also consist of both polar and nonpolar side chains.
  • The polar hydrophilic linkages are located towards the outer side whereas the nonpolar or hydrophobic linkages are located on the inner side.
  • Hence, the hydrophilic linkages establish connections with the hydrophobic part of the lipids.
  • Cholesterol provides rigidity and stability to the cell membrane.

2. Proteins:

  • Proteins can be fibrous or globular in nature including structural, enzymatic, carrier, permease and receptor proteins.
  • On the basis of ease of extraction, membrane proteins can be classified as integral (intrinsic) or peripheral (extrinsic) proteins.
  • This categorization is based on the degree of its association and the methods by which the protein can be solubilized.

a. Peripheral proteins:

  • Peripheral proteins are separated by the mild treatment and do not contain lipids molecules.
  • This protein is soluble in aqueous solutions.
  • Examples of the peripheral protein present in the cell membrane are spectrin of erythrocytes, cytochrome c of mitochondria.

b. Integral proteins:

  • This type of protein contains more than 70% of the membrane proteins.
  • This also requires drastic procedures for isolation.
  • Examples of integral proteins are various membrane-bound enzymes, drug and hormone receptors, histocompatibility antigens, glycophorin, etc.

3. Carbohydrates:

  • Carbohydrates of cell membranes are small unbranched or branched chains of oligosaccharides.
  • They combine with both lipids and protein molecules on the outer surface of the membrane and form glycolipids and glycoproteins respectively.

Lamellar/sandwich model & Unit membrane model
 

Introduction:

  • Several models were proposed to explain the structure of a biomembrane.
  • Danielli & Davson’s model and Robertson’s unit membrane model are the early molecular models according to which the cell membranes have a stable layered structure.

Detailed explanation:
 

A. Sandwich model/ Danielli and Davson model:

  • The first lamellar model was proposed by J. Danielli and H. Dayson.
  • The plasma membrane is made up of lipoproteins and are tripartite membranes.
  • It is formed of a denser protein monolayer on either side of the transparent phospholipid bilayer.
  • So the plasma membrane is a "protein-lipid-protein sandwich".
  • The globular protein forms the protein molecules of the plasma membrane.
  • Each phospholipid molecule is a polar molecule and has two specific ends: hydrophilic (polar or glycerol or head) end and hydrophobic (non-polar or fatty acid or tail) end.
  • Therefore, the phospholipid molecule is an amphipathic molecule.
  • The head region of the phospholipid molecules is directed towards the proteins which are held together by electrostatic forces.
  • The tails of the two lipid layers are directed towards the centre where they are held together by hydrophobic bonds and van der Waal forces.

    phospholipid-molecule
     

B. Unit-membrane model:

  • This model was proposed by J. David Robertson.
  • He modified the Lamellar model proposed by Danielli and Davson.
  • Robertson gave the concept of "Unit-membrane" which means the ultrastructure of various biomembranes of the cell is similar.

This model states the following:

  • The plasma membrane is made up of lipoproteins and is trilaminar in nature.
  • The protein molecules involved in the formation of the plasma membrane are not globular but are extended ( -protein molecules) in nature.
  • The membrane made up of a phospholipid molecule is a polar molecule containing the hydrophilic head of glycerol and hydrophobic tail of two fatty acid chains.
  • The head region of the phospholipid molecules of the bilayer is directed in opposite directions.
  • The tail region of the phospholipid molecule of bilayer is allowed to face each other.
  • The average thickness of biomembranes is around 75-100 Å.
  • The protein layer compromising the biomembrane is 20-30 Å whereas the thickness of the phospholipid bilayer is 35-40 Å.

    unit-membrane-model
     

Drawbacks of Lamellar/sandwich model & Unit membrane model :

  • These models were unable to explain the selective permeability and elasticity of plasma membranes.

Fluid Mosaic Model:
 

Introduction:

  • The most accepted model is the fluid mosaic model suggested by Singer and Nicolson, in 1972.
  • In this model, the membrane is viewed as the quasi fluid structure in which the protein molecules are embedded in the lipid bilayers.
  • This model describes both the mosaic arrangement of the protein embedded throughout the lipid bilayer as well as the fluid nature of the lipid bilayer.

Detailed explanation:
 

Fluid mosaic organization of the plasma membrane:

  • It was proposed by Singer and Nicolson (1972).
  • It is the most recognized model for plasma membranes.
  • According to this, the quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer.
  • This ability to move within the membrane is measured as its fluidity.

    fluid-mosaic-organization
     
  • They stated that plasma membranes contain lipid bilayer in which proteins are found on both outer and inner sides to form a mosaic pattern.
  • Thus they described it as protein icebergs in a sea of lipids.
  • The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
  • Eukaryotic plasma membrane contains cholesterol which provides stability to the membrane but in prokaryotes hopanoids are present instead of it.
  • Depending on ease of extraction, proteins of the membrane are of two types.
    (a) External or extrinsic proteins:
    i.   It is a peripheral protein (30% of total protein).
    ii. They lie on the surface of the membrane.
    iii. It can be easily removed e.g. Spectrin in RBC.

    (b) Integral or intrinsic proteins:
    i. It is about 70% of total protein.
    ii. They are buried partially or totally in plasma membranes.
    iii. These can not be separated easily. e.g. Cytochrome oxidase, Porin Proteins. iv. They may function as carriers, permeases, enzymes, receptors.
    v. Some large globular intrinsic proteins pass as a helix into the lipid bilayer from outside to inside to form tunnel proteins or transmembrane proteins.
    vi .The transmembrane proteins act as channels for passage of water soluble materials and water.
  • The plasma membrane is asymmetric due to oligosaccharides which form glycolipids and glycoprotein alongwith lipids and proteins respectively.
  • Both glycolipids and glycoproteins together form glycocalyx.
  • Oligosaccharide part in glycocalyx acts as a recognition centre, site for attachment and provides antigen specificity to cell membranes, blood grouping, immune response and matching of tissues in transplantation of organs.

Note:
i. Phospholipid also shows exchange of molecules from one monolayer to the monolayer of the other side; it is called flip-flop movement.
ii. Flip-flop movement is absent in protein molecules.
iii. Protein and lipid both can show rotational and lateral diffusion in the membrane.

Asymmetry of a biomembranes:

  • The two surfaces of the membrane are not symmetric.
  • The maintenance of chemical asymmetry is essential for normal membrane functions.

1. Membrane lipids are asymmetrically distributed between the two leaflets of the bilayer.The phospholipids such as phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol are mostly present in the cytosolic leaflet of the lipid bilayer, whereas sphingomyelin and phosphatidylcholine are found mainly in the non-cytosolic leaflet.
2. Extrinsic proteins present on the two sides of the membrane differ in quantity and types as they are more abundant on the inner side than on the outer side.
3. Moreover, the oligosaccharides are attached to the external surface of the lipids and proteins of the biomembrane. They are absent on the inner or cytosolic side of the membrane.

The function of the cell membrane:

  • The membrane provides protection to the cell from injury.
  • This structure is involved in the formation of various types of junctions allowing the cell to stay connected.
  • The cell membrane is responsible for causing compartmentalization in the cell.
  • This membrane separates the cells from the external environment.
  • A plasma membrane is a living and selectively permeable membrane that allows only selected substances to pass through it.
  • It forms organic connections between the adjacent cells by forming plasmodesmata and gap junctions
  • Certain biological or physiological activities such as active transport and endocytosis are performed by the plasma membrane.
  • Diffusion, osmosis, and facilitated diffusion are the physical activities (passive transport) that take place through the plasma membrane.
  • This membrane is also involved in Bulk transport which consist of two different methods that are phagocytosis and pinocytosis.
  • This membrane also contains carrier proteins for the active transport of materials such as Na+ - K+ exchange pumps.
  • Certain cell membranes such as plasma membrane of bacteria and thylakoid membrane in chloroplast organelle also persist the whole electron transport chain.
  • This structure also helps in the movement of some cells by either developing undulation ( such as fibroblast) or pseudopodia (such as Amoeba).
  • The plasma membrane also helps in the process of exocytosis as the secretary, excretory, and waste products are thrown out.
  • It also infolds to intake the bulk of material inside the cell with the help of Endocytosis.
  • The plasma membrane also shows the finger-like evaginations on the surface of the cell which are known as microvilli in cells such as intestinal cells, hepatic cells, and uriniferous tubules. They are helpful for increasing the surface area for absorption.
  • The plasma membrane also forms the sheath over cilia and flagella.

Frequently Asked Questions: FAQs
 

Q1. Describe the asymmetric nature of the plasma membrane?
Ans: Asymmetry of plasma membrane:

  • The two surfaces of the membrane are not symmetric.
  • The maintenance of chemical asymmetry is essential for normal membrane functions.
  • Membrane lipids are asymmetrically distributed between the two leaflets of the bilayer.The phospholipids such as phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol are mostly present in the cytosolic leaflet of the lipid bilayer, whereas sphingomyelin and phosphatidylcholine are found mainly in the non-cytosolic leaflet.
  • Extrinsic proteins present on the two sides of the membrane differ in quantity and types as they are more abundant on the inner side than on the outer side.
  • Moreover, the oligosaccharides are attached to the external surface of the lipids and proteins of the biomembrane. They are absent on the inner or cytosolic side of the membrane.

Q2. Explain the chemical components of the plasma membrane?
Ans:

  • Cell membranes appear as a trilaminar or tripartite layer under electron microscope.
  • Chemically a cell membrane contains proteins, Lipids, carbohydrates, and water.
  • Lipids:
  • The most common lipids found in the cell membrane are phospholipids.
  • A single phospholipid molecule is made up of a polar head containing phosphate along with the two non-polar hydrocarbon tails obtained from the fatty acids.
  • The head region of the phospholipid is the hydrophilic (water-loving) part and the tail region is considered as the hydrophobic (water-repelling) part of the phospholipid bilayer.
  • Therefore, the phospholipid molecules are amphipathic or amphipathic, that is, they possess both polar hydrophilic (water-loving) and nonpolar hydrophobic (water-repelling) ends.
  • The proteins present in the lipid bilayer also consist of both polar and nonpolar side chains.
  • The polar hydrophilic linkages are located towards the outer side whereas the nonpolar or hydrophobic linkages are located on the inner side.
  • Hence, the hydrophilic linkages establish connections with the hydrophobic part of the lipids.
  • Cholesterol provides rigidity and stability to the cell membrane.
  • Proteins:
  • Proteins can be fibrous or globular in nature including structural, enzymatic, carrier, permease and receptor proteins.
  • On the basis of ease of extraction, membrane proteins can be classified as integral (intrinsic) or peripheral (extrinsic) proteins.
  • This categorization is based on the degree of its association and the methods by which the protein can be solubilized.

a. Peripheral proteins:

  • Peripheral proteins are separated by the mild treatment and do not contain lipids molecules.
  • This protein is soluble in aqueous solutions.
  • Examples of the peripheral protein present in the cell membrane are spectrin of erythrocytes, cytochrome c of mitochondria.

b. Integral proteins:

  • This type of protein contains more than 70% of the membrane proteins.
  • This also requires drastic procedures for isolation.
  • Examples of integral proteins are various membrane-bound enzymes, drug and hormone receptors, histocompatibility antigens, glycophorin, etc.

3. Carbohydrates:

  • Carbohydrates of cell membranes are small unbranched or branched chains of oligosaccharides.
  • They combine with both lipids and protein molecules on the outer surface of the membrane and form glycolipids and glycoproteins respectively.

Q3. Describe in brief the fluid mosaic organization of the plasma membrane?
Ans:

  • It was proposed by Singer and Nicolson (1972). and is the most recognized model for plasma membrane.
  • According to this, the quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer.
  • This ability to move within the membrane is measured as its fluidity.
  • They stated that plasma membranes contain lipid bilayer in which proteins are found on both outer and inner sides to form a mosaic pattern. Thus they described it as protein icebergs in a sea of lipids.
  • The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
  • The plasma membrane is asymmetric due to oligosaccharides which form glycolipids and glycoprotein alongwith lipids and proteins respectively.

Q4. Why are phospholipid molecules called amphipathic molecules?
Ans:

  • A single phospholipid molecule is made up of a polar head containing phosphate along with the two non-polar hydrocarbon tails obtained from the fatty acids.
  • The head region of the phospholipid is the hydrophilic (water-loving) part and the tail region is considered as the hydrophobic (water-repelling) part of the phospholipid bilayer.
  • Therefore, the phospholipid molecules are amphipathic or amphipathic, that is, they possess both polar hydrophilic (water-loving) and nonpolar hydrophobic (water-repelling) ends.

Q5. What do you mean by fluidity as per the fluid mosaic model?
Ans:

  • According to the fluid mosaic model, the quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer.
  • This ability to move within the membrane is measured as its fluidity.
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