Q. Write an essay on the Cell wall.
CELL WALL
The cells of all plants, bacteria and fungi have a rigid, protective covering outside the plasma membrane called cell wall. The presence of cell wall in plant cells distinguishes them from animal cells. Among the vascular plants only certain cells connected with the reproductive processes, are naked, all other cells have walls. The cell wall was first observed by Hooke in the year 1865 in cork cells. Originally it was thought that the cell wall was a non-living secretion of the protoplasm, but now it is known to be metabolically active and is capable of growth and at least during its growth, contains protoplasmic material.
Structure of the cell wall:
A typical plant cell has the following three parts.
1. Middle lamella, 2. Primary wall, 3. Secondary Wall.
1.Middle lamella
It is a thin amorphous cement like layer between two adjacent cells. Middle lamella is the first layer, which is deposited at the time of cytokinesis. It is optically inactive (isotropic). It is made up of calcium and magnesium pectates. In addition to these substances proteins are also present.
2.Primary wall
It is the first formed wall of the cell which is produced inner to the middle lamella. It is thin, elastic and extensible in growing cells. It is optically active (anisotropic). It grows by addition of more wall material within the existing one. Such a growth is termed as intussusception. Some cells like the parenchymatous cells and meristematic cells have only the primary wall. The primary wall consists of a loose network of cellulose microfibrils embedded in a gel like matrix or ground substances. In most of the plants the micro fibrils are made up of cellulose. The micro fibrils are oriented variously according to shape and thickness of the wall. The matrix of the primary wall in which the micro fibrils are embedded is mainly composed of water, hemicellulose, pectin and glycoprotein. Pectin is the filling material of the matrix. Hemicellulose binds the microfibrils with the matrix and the glycoproteins control the orientation of the microfibrils.
3.Secondary Wall
A thick secondary wall is laid inner to the primary wall after the cell has reached maturity. It is laid down is succession of at least three layers often named S1, S2 and S3. It grows in thickness by accretion (apposition) i.e deposition of materials over the existing structures. The central layer (S2)is usually the thickest layer. In some cells however, the number of layers may be more than three. The formation of secondary wall is not uniform in all the cells. This results in the
differentiation of various types of cells, such as parenchyma, collenchyma, fibres and tracheids.
The micro fibrils of secondary wall are compactly arranged with different orientation in different layers embedded in a matrix of pectin and hemicellulose. Substances like lignin, suberin, minerals,waxes, tannins, resins, gums, inorganic salts such as calcium carbonate, calcium oxalate, silica etc may be deposited in the secondary wall. The secondary wall is very strongly anisotropic and layering can be observed in it. Fine structure of the cell wall particularly that of the secondary wall, has been intensively studied. This study was stimulated because of its importance to the fibre, paper and other industries. Cell wall is built of a system of microscopic threads the micro fibrils, which are grouped together in larger bundles. the layering seen in the secondary wall is often the result of the different density of the micro fibrils. The secondary wall consists of two continuous interpenetrating systems one of which is the cellulose micro fibrils and the other, the continuous system of microcapillary spaces. These spaces may the filled with lignin, cutin, suberin, hemicelluloses and other organic substances and sometimes even some mineral crystals. The cellulose molecules consist of long chains of linked glucose residues. The chain molecules are arranged in bundles which are generally termed micellae. The hypothesis of the presence of micellae was proposed by Nageli. According to Frey-wyssling and Muhlethaler the thread like cellulose molecules are arranged in bundles. Each such bundle which forms an elementary fibril consists of about 36 cellulose molecules. The elementary fibril is mostly crytalline.
Plasmodesmata
The cell wall is not totally complete around the cell. It is interrupted by narrow pores carrying fine strands of cytoplasm, which interlink the contents of the cells. They are called plasmodesmata. They form a protoplasmic continuum called symplast. It consists of a canal, lined by plasma membrane. It has a simple or branched tubule known as desmotubule. Desmotubule is an extension of endoplasmic reticulum. Plasmodesmata serves as a passage for many substances to pass through. It is also believed that they have a role in the relay of stimuli.
Pits
Pits are the areas on the cell wall on which the secondary wall is not laid down. The pits of adjacent cells are opposite to each other. Each pit has a pit chamber and a pit membrane. The pit membrane consists of middle lamella and primary wall. Pit membrane has many minute pores and thus they are permeable.
Pits are of two types 1.Simple pits 2.Bordered pits. In simple pits the width of the pit chamber is uniform. There is no secondary wall in the simple pit. In bordered pit the secondary wall partly overhangs the pit. Pits help in the translocation of substances between two adjacent cells. Generally each pit has a complementary pit lying exactly opposite to it in the wall of the neighbouring cell. Such pits form a morphological and functional unit called the pit pair.
Chemical Composition
The chemical composition of cell wall varies in different kingdoms. In bacteria the cell wall is composed of peptidoglycan, in Fungi it is made up of chitin. The plant cell wall is made up of cellulose. Besides cellulose certain other chemicals such as hemicellulose, pectin, lignin, cutin, suberin, silica may also be seen deposited on the wall.
Functions of cell wall:
1. It gives definite shape to the cell.
2. It protects the internal protoplasm against injury.
3. It gives rigidity to the cell
4. It prevents the bursting of plant cells due to endosmosis.
5. The walls of xylem vessels, tracheids and sieve tubes are specialized for long distance transport.
6. In many cases, the cell wall takes part in offense and defense.
Q. Write about the Structural Organization of Plasmamembrane
Ans: Plasmamembrane is a thin layer covering the cytoplasm and separates it from the cell wall. This layer is also called as cell membrane or Plasmalemma. All biological membranes covering various cell organelles are known as cell membrane. Biologicalmembranes are having similar structure and chemical composition, like that of plasmamembrane but performing different functions.
Chemical composition of Plasmamembrane:-
Chemically Plasma membrane is made up of lipids, proteins and carbohydrates. Generally Plasma membrane is 75A0 to 150A0 in thickness but in different cells it may be of different thickness. Quantitatively proteins and lipids are more than carbohydrates in the composition of Plasma membrane.
Lipids : lipids constitute the 40% of the total chemical constituents of Plasma membrane. Four different types of lipids are present in the Plasma membrane. They are Phospholipids, Glycolipids, Sphingolipids and Sterols.
Proteins: In the construction of Plasma membrane, proteins constitute 50%. The proteins are of three types. They are Structural proteins(important component of Plasma membrane-both intrinsic and extrinsic proteins), functional proteins (mostly enzymes catalyzing different chemical reactions) and Carrier proteins (meant for the transportation of different ions and other material through the membrane).
Carbohydrates :Carbohydrates may be in smaller proportion. These Carbohydrates may be branched or unbranched chains of sugars. Glucose derived Carbohydrates generally occur in an attached condition as Glycoproteins and Glycolipids in Plasma membrane.
Ultra structure of Plasma membrane:-
Several cellbiologists attempted to understand the molecular structure of the Plasmamembrane in various ways and proposed models for the same. Some of the models are explained here.
Lipid Bilayer model:
• Proposrd by E.Gorter and F.Grendel in 1925.
• This is the first proposal stating that the Plasmamembrane might contain a lipid bilayer.
• Polar groups of each molecular layer of lipid bilayer were directed towards the outside of the bilayer
Danielli-Davson Model:
• Proposed by James Danielli and Hugh Davson
• They proposed that the Plasmamembrane consists of two layers of lipid molecules. Bimolecular leaf let – formed in such a way that the polar ends of the lipid facing outward and the hydrophobic ends are associated in the central region of the leaf let. They further stated that the polar ends of the lipid molecules as being associated with a monomolecular layer of polar globular protein molecules.
• This is known as Sandwich model as the lipid bilayer is sandwiched between two continous layers of globular proteins.
• This sandwich model is first structural interpretation to account for the physical charecteristics of the membrane.
Unitmembrane Model or Robertson’s Model:
• Proposrd by Robertson in 1957
• He found that all Biligical membranes appear to be made up of lipid layer having a thickness of 35A0 and dense band of proteins of 20A0 thickness present on either side.
• The two outer electron dense layers of proteins separated by the light middle layer of phospholipids together form a unitmembrane.
• The outer surface has mucoproteins while the inner surface proteins are non-mucoid proteins.
The Fluid Mosaic Model:
• Proposed by Singer and Nicolson in 1972.
• In this model, attention is focused on the physical state of the lipid bilayer.
• Here the lipid molecules are present in a fluid state capable of rotating and moving laterally within the membrane. The proteins occur as a mosaic of discontinuous particles that penetrate deeply into the lipid sheet.
• In a way, these protein particles resemble floating icebergs in the sea of lipids.
• Thus the Plasmamembrane is a double layer of phospholipids with their polar ends away from one another and the globular molecules of proteins and sterols scattered in between.
• Integrated proteins penetrate lipid bilayer partially or wholly and project out from both the extracellular and cytoplasmic sides of the membrane. The globular peripheral proteins are loosely attached to the membrane lipids.
Micellar Model:
• Proposed by Hilleir and Hoffman
• They suggested that the membrane have a non-lamellar pattern,consisting of globular subunits known as “micelles”.
• Accordind to this model the membrane is consisting of closely packed repeating units which are similar in structure.
• The protein components of the membrane form a monolayer on either side of the plane of lipid micelles.
Q. Describe the functions of Plasmamembrane
FUNCTIONS OF PLASMA MEMBRANE
In all cells the plasma membrane has several essential functions to perform. These include transporting nutrients into and metabolic wastes out of the cell preventing unwanted materials from entering the cell. In short, the intercellular and intra cellular transport is regulated by plasma membrane. The plasma membrane maintains the proper ionic composition pH (~7.2) and osmotic pressure of the Cytosol. To carry out all these functions, the plasma membrane contains specific transport proteins that permit the passage of certain small molecules but not others. Several of these proteins use the energy released by ATP hydrolysis to pump ions and other molecules into or out of the cell against concentration gradients. Small charged molecules such as ATP and amino acids can diffuse freely within the Cytosol but are restricted in their ability to leave or enter it across the plasma membrane. In addition to these universal functions, the plasma membrane has other important functions to perform. Enzymes bound to the plasma membrane catalyze reactions that would occur with difficulty in an aqueous environment. The plasma membranes of many types of eukaryotic cells also contain Receptor proteins that bind specific Signalling molecules like hormones, growth factors, neurotransmitters etc. leading to various cellular responses. Like the entire cell, each organelle in eukaryotic cells is bounded by a unit membrane containing a unique set of proteins essential for its proper functioning.
Membrane Transport
Based on the permeability a membrane is said to be:
1. Permeable: If a substance passes readily through the membrane
2. Impermeable: If a substance does not pass through the membrane
3. Selectively permeable: If the membrane allows some of the substances to pass through but does not allow all the substances to pass through it. The permeability of a membrane depends on 1)the size of pores in the Plasma membrane. 2)The size of the substance molecules 3)The charge on the substance molecules.
All the biological membranes are selectively permeable. Its permeability properties ensure that essential molecules such as glucose, amino acids and lipids readily enter the cell, metabolic intermediates remain in the cell and waste compounds leave the cell. In short it allows the cell to maintain a constant internal environment.
Substances are transported across the membrane either by:
1. Passive Transport or 2. Active Transport
Passive Transport
Physical processes
Passive Transport of materials across the membrane requires no energy by the cell and it is unaided by the transport proteins. The physical processes through which substances get into the cell are 1.Diffusion 2.Osmosis
Diffusion
Diffusion is the movement of molecules of any substance from a region of it’s higher to a region of it’s lower concentration (down its own concentration gradient) to spread uniformly in the dispersion medium on account of their random kinetic motion.
The rate of diffusion is directly proportional to
1. The concentration of the substance
2. Temperature of the medium
3. Area of the diffusion pathway
The diffusion is inversely proportional to
1. The size of the substance molecules
2. The molecular weight of the substance molecule
3. The distance over which the molecules have to diffuse
Diffusion through Biomembranes
Gases and small hydrophobic molecules diffuse directly across the phospholipid bilayer at a rate proportional to their ability to dissolve in a liquid hydro carbon. Transport of molecules takes place along the concentration gradient and no metabolic energy is expended in this process. This can be described as ‘Downhill transport’. Diffusion through the bio membrane takes place in two ways.
1. Diffusion of fat-soluble substances through plasma membrane simply by dissolving in the lipid bilayer.
2. Diffusion of water soluble substances and ions: This takes place through pores in the membranes.
Diffusion of charged particles water soluble substances and ions such as K + Cl - and HCO3- diffuse through the pores in the membranes. An ion diffuses from the side richer in like charges to the side with an excess of opposite charges. The difference of electrical charges between the two sides of a membrance is called electro chemical gradient. The integral proteins of the membrane act as protein channels extending through the membrane. The movement of gas molecules occurs down its pressuregradient.
Osmosis
It is the special type of diffusion where the water or solvent diffuses through a selectively permeable membrane from a region of high solvent concentration to a region of low solvent concentration.
Uniporter Catalyzed Transport
The plasma membrane of most cells (animal or plant) contains several uniporters that enable amino acids, nucleosides, sugars and other small molecules to enter and leave cells down their concentration gradients. Similar to enzymes, uniporters accelerate a reaction that is thermodynamically favoured. This type of movement sometimes is referred to as facilitated transport or facilitated diffusion.
Three main features distinguish uniport transport from passive diffusion. 1. the rate of transport is far higher than predicted 2.transport is specific 3.transport occurs via a limited number of transporter proteins rather than throughout the phospholipids bilayer.
Active transport
It is vital process. It is the movement of molecules or ions against the concentration gradient. i.e the molecules or ions move from the region of lower concentration towards the region of higher concentration. The movement of molecules can be compared with the uphill movement of water. Energy is required to counteract the force of diffusion and the energy comes from ATP produced by oxidative phosphorylation or by concentration gradient of ions. Thus active transport is defined as athe energy dependent transport of molecules or ions across a semi permeable membrane against the concentration gradient. Active transport takes place with the help of carrier proteins that are present in the plasma membrane. In the plasma membrane there are a number of carrier molecules called permeases or translocases present. For each type of solute molecule there is a specific carrier molecule. It has got two binding sites; one for the transportant and other for ATP molecule. The carrier proteins bind the transportant molecule on the outer side of the plasma membrane. This results in the formation of carrier-transportant-complex. As the ATP molecule binds itself to the other binding site of the carrier protein it is hydrolysed to form ADP and energy is released. This energy brings conformational change in the carrier transportant-complex and the transportant is carried through the channel on the other side of the membrane. The carrier molecule regains its original form and repeats the precess.
There are two forces which govern the movement of ions across selectively permeable membranes, the membrane electric potential and the ion concentration gradient. ATP driven ion pumps generate and maintain ionic gradients across the plasma membrane.
Endocytosis and exocytosis
Endocytosis and exocytosis are active processes involving bulk transport of materials through membranes, either into cells (endocytosis) or out of cells (exocytosis). Endocytosis occurs by an in folding or extension of the plasma membrane to form a vesicle or vacuole or vauole. It is of two types.
1. Phagocytosis:(cell eating)-Substances are taken up in solid form. Cells involving in this process are called phagocytes and said to be phagocytic. (eg.) some white blood cells. A phagocytic vacuole is formed during the uptake.
2. Pinocytosis (cell drinking)-Substances are taken up in liquid form. Vesicles which are very small are formed during intake. Pinocytosis is often associated with amoebiod protozozns, and in certain kidney cells involved in fluid exchange. It can also occur in plant cells. Exocytosis is the reverse of endocytosis by which materials are removed from cells such as undigested remains from food vacuoles.
CELL WALL
The cells of all plants, bacteria and fungi have a rigid, protective covering outside the plasma membrane called cell wall. The presence of cell wall in plant cells distinguishes them from animal cells. Among the vascular plants only certain cells connected with the reproductive processes, are naked, all other cells have walls. The cell wall was first observed by Hooke in the year 1865 in cork cells. Originally it was thought that the cell wall was a non-living secretion of the protoplasm, but now it is known to be metabolically active and is capable of growth and at least during its growth, contains protoplasmic material.
Structure of the cell wall:
A typical plant cell has the following three parts.
1. Middle lamella, 2. Primary wall, 3. Secondary Wall.
1.Middle lamella
It is a thin amorphous cement like layer between two adjacent cells. Middle lamella is the first layer, which is deposited at the time of cytokinesis. It is optically inactive (isotropic). It is made up of calcium and magnesium pectates. In addition to these substances proteins are also present.
2.Primary wall
It is the first formed wall of the cell which is produced inner to the middle lamella. It is thin, elastic and extensible in growing cells. It is optically active (anisotropic). It grows by addition of more wall material within the existing one. Such a growth is termed as intussusception. Some cells like the parenchymatous cells and meristematic cells have only the primary wall. The primary wall consists of a loose network of cellulose microfibrils embedded in a gel like matrix or ground substances. In most of the plants the micro fibrils are made up of cellulose. The micro fibrils are oriented variously according to shape and thickness of the wall. The matrix of the primary wall in which the micro fibrils are embedded is mainly composed of water, hemicellulose, pectin and glycoprotein. Pectin is the filling material of the matrix. Hemicellulose binds the microfibrils with the matrix and the glycoproteins control the orientation of the microfibrils.
3.Secondary Wall
A thick secondary wall is laid inner to the primary wall after the cell has reached maturity. It is laid down is succession of at least three layers often named S1, S2 and S3. It grows in thickness by accretion (apposition) i.e deposition of materials over the existing structures. The central layer (S2)is usually the thickest layer. In some cells however, the number of layers may be more than three. The formation of secondary wall is not uniform in all the cells. This results in the
differentiation of various types of cells, such as parenchyma, collenchyma, fibres and tracheids.
The micro fibrils of secondary wall are compactly arranged with different orientation in different layers embedded in a matrix of pectin and hemicellulose. Substances like lignin, suberin, minerals,waxes, tannins, resins, gums, inorganic salts such as calcium carbonate, calcium oxalate, silica etc may be deposited in the secondary wall. The secondary wall is very strongly anisotropic and layering can be observed in it. Fine structure of the cell wall particularly that of the secondary wall, has been intensively studied. This study was stimulated because of its importance to the fibre, paper and other industries. Cell wall is built of a system of microscopic threads the micro fibrils, which are grouped together in larger bundles. the layering seen in the secondary wall is often the result of the different density of the micro fibrils. The secondary wall consists of two continuous interpenetrating systems one of which is the cellulose micro fibrils and the other, the continuous system of microcapillary spaces. These spaces may the filled with lignin, cutin, suberin, hemicelluloses and other organic substances and sometimes even some mineral crystals. The cellulose molecules consist of long chains of linked glucose residues. The chain molecules are arranged in bundles which are generally termed micellae. The hypothesis of the presence of micellae was proposed by Nageli. According to Frey-wyssling and Muhlethaler the thread like cellulose molecules are arranged in bundles. Each such bundle which forms an elementary fibril consists of about 36 cellulose molecules. The elementary fibril is mostly crytalline.
Plasmodesmata
The cell wall is not totally complete around the cell. It is interrupted by narrow pores carrying fine strands of cytoplasm, which interlink the contents of the cells. They are called plasmodesmata. They form a protoplasmic continuum called symplast. It consists of a canal, lined by plasma membrane. It has a simple or branched tubule known as desmotubule. Desmotubule is an extension of endoplasmic reticulum. Plasmodesmata serves as a passage for many substances to pass through. It is also believed that they have a role in the relay of stimuli.
Pits
Pits are the areas on the cell wall on which the secondary wall is not laid down. The pits of adjacent cells are opposite to each other. Each pit has a pit chamber and a pit membrane. The pit membrane consists of middle lamella and primary wall. Pit membrane has many minute pores and thus they are permeable.
Pits are of two types 1.Simple pits 2.Bordered pits. In simple pits the width of the pit chamber is uniform. There is no secondary wall in the simple pit. In bordered pit the secondary wall partly overhangs the pit. Pits help in the translocation of substances between two adjacent cells. Generally each pit has a complementary pit lying exactly opposite to it in the wall of the neighbouring cell. Such pits form a morphological and functional unit called the pit pair.
Chemical Composition
The chemical composition of cell wall varies in different kingdoms. In bacteria the cell wall is composed of peptidoglycan, in Fungi it is made up of chitin. The plant cell wall is made up of cellulose. Besides cellulose certain other chemicals such as hemicellulose, pectin, lignin, cutin, suberin, silica may also be seen deposited on the wall.
Functions of cell wall:
1. It gives definite shape to the cell.
2. It protects the internal protoplasm against injury.
3. It gives rigidity to the cell
4. It prevents the bursting of plant cells due to endosmosis.
5. The walls of xylem vessels, tracheids and sieve tubes are specialized for long distance transport.
6. In many cases, the cell wall takes part in offense and defense.
Q. Write about the Structural Organization of Plasmamembrane
Ans: Plasmamembrane is a thin layer covering the cytoplasm and separates it from the cell wall. This layer is also called as cell membrane or Plasmalemma. All biological membranes covering various cell organelles are known as cell membrane. Biologicalmembranes are having similar structure and chemical composition, like that of plasmamembrane but performing different functions.
Chemical composition of Plasmamembrane:-
Chemically Plasma membrane is made up of lipids, proteins and carbohydrates. Generally Plasma membrane is 75A0 to 150A0 in thickness but in different cells it may be of different thickness. Quantitatively proteins and lipids are more than carbohydrates in the composition of Plasma membrane.
Lipids : lipids constitute the 40% of the total chemical constituents of Plasma membrane. Four different types of lipids are present in the Plasma membrane. They are Phospholipids, Glycolipids, Sphingolipids and Sterols.
Proteins: In the construction of Plasma membrane, proteins constitute 50%. The proteins are of three types. They are Structural proteins(important component of Plasma membrane-both intrinsic and extrinsic proteins), functional proteins (mostly enzymes catalyzing different chemical reactions) and Carrier proteins (meant for the transportation of different ions and other material through the membrane).
Carbohydrates :Carbohydrates may be in smaller proportion. These Carbohydrates may be branched or unbranched chains of sugars. Glucose derived Carbohydrates generally occur in an attached condition as Glycoproteins and Glycolipids in Plasma membrane.
Ultra structure of Plasma membrane:-
Several cellbiologists attempted to understand the molecular structure of the Plasmamembrane in various ways and proposed models for the same. Some of the models are explained here.
Lipid Bilayer model:
• Proposrd by E.Gorter and F.Grendel in 1925.
• This is the first proposal stating that the Plasmamembrane might contain a lipid bilayer.
• Polar groups of each molecular layer of lipid bilayer were directed towards the outside of the bilayer
Danielli-Davson Model:
• Proposed by James Danielli and Hugh Davson
• They proposed that the Plasmamembrane consists of two layers of lipid molecules. Bimolecular leaf let – formed in such a way that the polar ends of the lipid facing outward and the hydrophobic ends are associated in the central region of the leaf let. They further stated that the polar ends of the lipid molecules as being associated with a monomolecular layer of polar globular protein molecules.
• This is known as Sandwich model as the lipid bilayer is sandwiched between two continous layers of globular proteins.
• This sandwich model is first structural interpretation to account for the physical charecteristics of the membrane.
Unitmembrane Model or Robertson’s Model:
• Proposrd by Robertson in 1957
• He found that all Biligical membranes appear to be made up of lipid layer having a thickness of 35A0 and dense band of proteins of 20A0 thickness present on either side.
• The two outer electron dense layers of proteins separated by the light middle layer of phospholipids together form a unitmembrane.
• The outer surface has mucoproteins while the inner surface proteins are non-mucoid proteins.
The Fluid Mosaic Model:
• Proposed by Singer and Nicolson in 1972.
• In this model, attention is focused on the physical state of the lipid bilayer.
• Here the lipid molecules are present in a fluid state capable of rotating and moving laterally within the membrane. The proteins occur as a mosaic of discontinuous particles that penetrate deeply into the lipid sheet.
• In a way, these protein particles resemble floating icebergs in the sea of lipids.
• Thus the Plasmamembrane is a double layer of phospholipids with their polar ends away from one another and the globular molecules of proteins and sterols scattered in between.
• Integrated proteins penetrate lipid bilayer partially or wholly and project out from both the extracellular and cytoplasmic sides of the membrane. The globular peripheral proteins are loosely attached to the membrane lipids.
Micellar Model:
• Proposed by Hilleir and Hoffman
• They suggested that the membrane have a non-lamellar pattern,consisting of globular subunits known as “micelles”.
• Accordind to this model the membrane is consisting of closely packed repeating units which are similar in structure.
• The protein components of the membrane form a monolayer on either side of the plane of lipid micelles.
Q. Describe the functions of Plasmamembrane
FUNCTIONS OF PLASMA MEMBRANE
In all cells the plasma membrane has several essential functions to perform. These include transporting nutrients into and metabolic wastes out of the cell preventing unwanted materials from entering the cell. In short, the intercellular and intra cellular transport is regulated by plasma membrane. The plasma membrane maintains the proper ionic composition pH (~7.2) and osmotic pressure of the Cytosol. To carry out all these functions, the plasma membrane contains specific transport proteins that permit the passage of certain small molecules but not others. Several of these proteins use the energy released by ATP hydrolysis to pump ions and other molecules into or out of the cell against concentration gradients. Small charged molecules such as ATP and amino acids can diffuse freely within the Cytosol but are restricted in their ability to leave or enter it across the plasma membrane. In addition to these universal functions, the plasma membrane has other important functions to perform. Enzymes bound to the plasma membrane catalyze reactions that would occur with difficulty in an aqueous environment. The plasma membranes of many types of eukaryotic cells also contain Receptor proteins that bind specific Signalling molecules like hormones, growth factors, neurotransmitters etc. leading to various cellular responses. Like the entire cell, each organelle in eukaryotic cells is bounded by a unit membrane containing a unique set of proteins essential for its proper functioning.
Membrane Transport
Based on the permeability a membrane is said to be:
1. Permeable: If a substance passes readily through the membrane
2. Impermeable: If a substance does not pass through the membrane
3. Selectively permeable: If the membrane allows some of the substances to pass through but does not allow all the substances to pass through it. The permeability of a membrane depends on 1)the size of pores in the Plasma membrane. 2)The size of the substance molecules 3)The charge on the substance molecules.
All the biological membranes are selectively permeable. Its permeability properties ensure that essential molecules such as glucose, amino acids and lipids readily enter the cell, metabolic intermediates remain in the cell and waste compounds leave the cell. In short it allows the cell to maintain a constant internal environment.
Substances are transported across the membrane either by:
1. Passive Transport or 2. Active Transport
Passive Transport
Physical processes
Passive Transport of materials across the membrane requires no energy by the cell and it is unaided by the transport proteins. The physical processes through which substances get into the cell are 1.Diffusion 2.Osmosis
Diffusion
Diffusion is the movement of molecules of any substance from a region of it’s higher to a region of it’s lower concentration (down its own concentration gradient) to spread uniformly in the dispersion medium on account of their random kinetic motion.
The rate of diffusion is directly proportional to
1. The concentration of the substance
2. Temperature of the medium
3. Area of the diffusion pathway
The diffusion is inversely proportional to
1. The size of the substance molecules
2. The molecular weight of the substance molecule
3. The distance over which the molecules have to diffuse
Diffusion through Biomembranes
Gases and small hydrophobic molecules diffuse directly across the phospholipid bilayer at a rate proportional to their ability to dissolve in a liquid hydro carbon. Transport of molecules takes place along the concentration gradient and no metabolic energy is expended in this process. This can be described as ‘Downhill transport’. Diffusion through the bio membrane takes place in two ways.
1. Diffusion of fat-soluble substances through plasma membrane simply by dissolving in the lipid bilayer.
2. Diffusion of water soluble substances and ions: This takes place through pores in the membranes.
Diffusion of charged particles water soluble substances and ions such as K + Cl - and HCO3- diffuse through the pores in the membranes. An ion diffuses from the side richer in like charges to the side with an excess of opposite charges. The difference of electrical charges between the two sides of a membrance is called electro chemical gradient. The integral proteins of the membrane act as protein channels extending through the membrane. The movement of gas molecules occurs down its pressuregradient.
Osmosis
It is the special type of diffusion where the water or solvent diffuses through a selectively permeable membrane from a region of high solvent concentration to a region of low solvent concentration.
Uniporter Catalyzed Transport
The plasma membrane of most cells (animal or plant) contains several uniporters that enable amino acids, nucleosides, sugars and other small molecules to enter and leave cells down their concentration gradients. Similar to enzymes, uniporters accelerate a reaction that is thermodynamically favoured. This type of movement sometimes is referred to as facilitated transport or facilitated diffusion.
Three main features distinguish uniport transport from passive diffusion. 1. the rate of transport is far higher than predicted 2.transport is specific 3.transport occurs via a limited number of transporter proteins rather than throughout the phospholipids bilayer.
Active transport
It is vital process. It is the movement of molecules or ions against the concentration gradient. i.e the molecules or ions move from the region of lower concentration towards the region of higher concentration. The movement of molecules can be compared with the uphill movement of water. Energy is required to counteract the force of diffusion and the energy comes from ATP produced by oxidative phosphorylation or by concentration gradient of ions. Thus active transport is defined as athe energy dependent transport of molecules or ions across a semi permeable membrane against the concentration gradient. Active transport takes place with the help of carrier proteins that are present in the plasma membrane. In the plasma membrane there are a number of carrier molecules called permeases or translocases present. For each type of solute molecule there is a specific carrier molecule. It has got two binding sites; one for the transportant and other for ATP molecule. The carrier proteins bind the transportant molecule on the outer side of the plasma membrane. This results in the formation of carrier-transportant-complex. As the ATP molecule binds itself to the other binding site of the carrier protein it is hydrolysed to form ADP and energy is released. This energy brings conformational change in the carrier transportant-complex and the transportant is carried through the channel on the other side of the membrane. The carrier molecule regains its original form and repeats the precess.
There are two forces which govern the movement of ions across selectively permeable membranes, the membrane electric potential and the ion concentration gradient. ATP driven ion pumps generate and maintain ionic gradients across the plasma membrane.
Endocytosis and exocytosis
Endocytosis and exocytosis are active processes involving bulk transport of materials through membranes, either into cells (endocytosis) or out of cells (exocytosis). Endocytosis occurs by an in folding or extension of the plasma membrane to form a vesicle or vacuole or vauole. It is of two types.
1. Phagocytosis:(cell eating)-Substances are taken up in solid form. Cells involving in this process are called phagocytes and said to be phagocytic. (eg.) some white blood cells. A phagocytic vacuole is formed during the uptake.
2. Pinocytosis (cell drinking)-Substances are taken up in liquid form. Vesicles which are very small are formed during intake. Pinocytosis is often associated with amoebiod protozozns, and in certain kidney cells involved in fluid exchange. It can also occur in plant cells. Exocytosis is the reverse of endocytosis by which materials are removed from cells such as undigested remains from food vacuoles.
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