Cell membrane Questions in English

(A) The cell membrane of all bacteria,whether Gram-positive or Gram-negative,is a fluid mosaic structure primarily composed of a phospholipid bilayer and embedded proteins.
This structure is consistent with the fluid mosaic model proposed by Singer and Nicolson.
While the cell wall composition differs significantly between Gram-positive and Gram-negative bacteria (peptidoglycan thickness,presence of lipopolysaccharides,etc.),the fundamental structure of the plasma membrane remains the same across both groups.

(B) The concentration of $0.8\% \ NaCl$ is isotonic to the internal environment of human $R.B.Cs$.
In an isotonic solution,the net movement of water across the cell membrane is zero.
Therefore,the $R.B.Cs$ will neither shrink nor burst and will remain the same.

(D) According to the Davson-Danielli model (also known as the sandwich model),the plasma membrane consists of a bimolecular lipid layer (phospholipid bilayer) that is sandwiched between two layers of globular proteins. This structure is often described as a protein-lipid-protein sandwich.

(D) The term "plasmalemma" was coined by the scientist $J.Q. \text{Plowe}$ in $1931$ to describe the plasma membrane of plant cells. Therefore, the correct option is $D$.

(B) The plasma membrane acts as a selective barrier. Anthocyanin is a water-soluble vacuolar pigment. Under normal conditions,the plasma membrane is impermeable to anthocyanin molecules,preventing them from leaking out of the cell into the surrounding cold water. If the membrane were damaged (e.g.,by heat),it would lose this property and the pigment would leak out.

(D) According to the Unit Membrane Model proposed by $J$.$D$. Robertson,the cell membrane is composed of a lipid bilayer sandwiched between two protein layers.
Each lipid layer is approximately $25 \ \mathring{A}$ thick,making the total lipid zone thickness approximately $50 \ \mathring{A}$.
However,the specific lipid bilayer region (the hydrophobic core) is generally cited as being between $35 \ \mathring{A}$ and $50 \ \mathring{A}$ in various structural studies based on his model.

(D) According to the fluid mosaic model,the lipid molecules in the plasma membrane are arranged in a bilayer.
In this arrangement,the hydrophilic heads of the lipid molecules are directed towards the outside (aqueous environment),while the hydrophobic tails are directed towards the inside.
This specific orientation is often described as head-to-head or parallel arrangement of the polar heads towards the aqueous surfaces.

(D) Ion carriers are specialized proteins embedded within the plasma membrane.
These proteins facilitate the active or passive transport of ions across the lipid bilayer,which is essential for maintaining cellular homeostasis and membrane potential.

(D) The plasma membrane is described by the fluid mosaic model.
It consists primarily of a lipid bilayer in which proteins are embedded.
Therefore,the plasma membrane is composed of both proteins and lipids.

(D) Desmosomes are specialized intercellular junctional complexes that provide strong adhesion between adjacent cells.
They act as 'spot welds' that anchor the intermediate filaments of the cytoskeleton to the plasma membrane,thereby maintaining the structural integrity of tissues,particularly in epithelial cells subjected to mechanical stress.
Therefore,the primary function of desmosomes is cell adherence.

(B) The 'Unit membrane model' was proposed by $J$.$D$. Robertson in $1959$.
According to this model,all cellular membranes (plasma membrane and organelle membranes) have a similar structure consisting of a lipid bilayer sandwiched between two layers of proteins.
The total thickness of this trilaminar structure is approximately $7.5 \ nm$ $(75 \ \mathring{A})$.

(B) The $Cell \text{ } membrane$ (also known as the plasma membrane) is a selectively permeable membrane that surrounds the cytoplasm of a cell.
It regulates the movement of substances, such as nutrients, ions, and waste products, into and out of the cell.
This selective permeability is essential for maintaining the internal environment of the cell, a process known as homeostasis.

(A) The plasma membrane (or cell membrane) is a universal component of all living cells.
It acts as a selective barrier that separates the internal environment of the cell from the external environment.
While structures like cell walls,chloroplasts,and food vacuoles are found in specific types of cells (e.g.,plant cells or certain protists),the cell membrane is present in all prokaryotic and eukaryotic cells.

(B) The process by which a cell engulfs or surrounds a particle and forms a vesicle around it is a form of bulk transport known as $Endocytosis$.
Specifically,when a cell engulfs solid particles,this process is called $Phagocytosis$ (often referred to as 'cell eating').
$Exocytosis$ is the reverse process,where materials are expelled from the cell.
Therefore,$Phagocytosis$ is the specific term for the engulfment of solid particles.

(B) The concept of the unit membrane was proposed by $J.D. Robertson$ in $1959$.
He suggested that all cellular membranes (plasma membrane,nuclear membrane,and organelle membranes) share a common tripartite structure consisting of a central lipid bilayer sandwiched between two protein layers.

(A) The plasma membrane is a lipid bilayer structure. Many membrane-bound enzymes require the presence of specific lipids to maintain their structural integrity and catalytic activity. $ATPase$ (specifically $Na^+/K^+$-ATPase) is a transmembrane protein that relies heavily on the surrounding lipid environment for its conformational changes and functional activity. In the absence of these essential lipids,the enzyme loses its structural stability and becomes inactivated.

(B) The plasma membrane is primarily composed of a lipid bilayer (phospholipids),proteins,and carbohydrates (oligosaccharides).
$1$. Phospholipids form the structural framework of the membrane.
$2$. Oligosaccharides are present on the outer surface,attached to proteins (glycoproteins) or lipids (glycolipids).
$3$. Spectrins are peripheral membrane proteins found on the cytoplasmic side of the plasma membrane,particularly in red blood cells,to provide structural support.
$4$. Albumins are globular proteins primarily found in blood plasma,not as structural components of the plasma membrane.

(C) The cell membrane is composed of a lipid bilayer,which is approximately $35$ $\mathring{A}$ to $45$ $\mathring{A}$ thick.
Among the given options,$40$ $\mathring{A}$ is the most accurate representation of the thickness of the bimolecular lipid layer.

(B) The plasma membrane is a selectively permeable barrier. Small,non-polar molecules and certain small polar molecules can pass through the lipid bilayer via simple diffusion. The pore size or the effective diameter of the channels/spaces through which these molecules can pass is generally considered to be in the range of $8-10 \ \mathring A$. Molecules larger than this typically require specific transport proteins or vesicular transport mechanisms to cross the membrane.

(B) The process of cell eating is known as $Phagocytosis$.
In this process,the cell engulfs large particles or solid materials by forming a vesicle called a $phagosome$.
$Pinocytosis$ is referred to as 'cell drinking',where the cell ingests extracellular fluid.
$Endocytosis$ is a general term for the uptake of materials into the cell,which includes both $phagocytosis$ and $pinocytosis$.
$Exocytosis$ is the process of moving materials out of the cell.

(A) The plasma membrane,according to the fluid mosaic model,consists of a lipid bilayer with proteins embedded in it. Under an electron microscope,the plasma membrane appears as a trilaminar structure. The outer and inner layers are composed of hydrophilic heads of phospholipids and associated proteins,which are electron-dense (stain darkly). The middle layer consists of the hydrophobic fatty acid tails of the phospholipids,which are electron-transparent (stain lightly).

(D) The cell membrane is selectively permeable,which means it allows the passage of certain molecules and ions while restricting others. One of its most critical functions is the regulation of ions (such as $Na^+$,$K^+$,$Ca^{2+}$,and $Cl^-$) across the membrane to maintain the electrochemical gradient and cellular homeostasis. This is achieved through various transport mechanisms like ion channels,pumps,and carriers.

(C) The fluid mosaic model,proposed by $Singer$ and $Nicolson$ in $1972$,describes the plasma membrane as a fluid structure.
It consists of a phospholipid bilayer in which various proteins are embedded.
These proteins are classified as extrinsic (peripheral) proteins,which are located on the surface,and intrinsic (integral) proteins,which are partially or fully embedded within the lipid bilayer.
Therefore,the plasma membrane is composed of phospholipids,extrinsic proteins,and intrinsic proteins.

(B) The plasma membrane is described as selectively permeable or semipermeable.
It allows the movement of certain molecules and ions into and out of the cell while restricting others.
This property is essential for maintaining the internal environment of the cell,known as homeostasis.

(C) The fluid mosaic model proposed by Singer and Nicholson $(1972)$ describes the plasma membrane as a mosaic of proteins embedded in a fluid phospholipid bilayer.
In contrast,Robertson's unit membrane model proposed that the membrane consists of a phospholipid bilayer sandwiched between two continuous layers of globular proteins.
Therefore,the primary difference between the two models lies in the arrangement of proteins,where Singer and Nicholson suggested that proteins are dispersed (integral and peripheral) within the lipid bilayer rather than forming continuous outer layers.

(C) Desmosomes are specialized intercellular junctions that provide strong adhesion between cells.
They are formed by the interaction of transmembrane proteins (like cadherins) with the cytoskeleton (specifically intermediate filaments or tonofilaments) within the cytoplasm.
These structures are essential for maintaining the structural integrity of tissues subjected to mechanical stress,such as the skin and cardiac muscle.
Therefore,they are associated with the plasma membrane and the cytoplasmic cytoskeleton.

(A) The $Fluid \, mosaic \, model$ is the most widely accepted model for the structure of the plasma membrane.
It was proposed by $S.J. \, Singer$ and $G.L. \, Nicolson$ in $1972$.
According to this model, the plasma membrane is composed of a phospholipid bilayer with proteins embedded within it, which can move laterally, giving it a fluid nature.

(C) Kavanau's lipid pillar model proposed that the cell membrane consists of lipid pillars with protein layers. This model was primarily static and failed to account for the dynamic nature of the plasma membrane. Specifically,it could not explain the process of $Phagocytosis$ (cell eating),which involves significant membrane remodeling,invagination,and vesicle formation. While it attempted to describe membrane structure,it lacked the fluidity required to explain complex transport mechanisms like $Phagocytosis$.

(B) Carbohydrates on the cell membrane are typically found as glycoproteins or glycolipids. These molecules extend from the outer surface of the plasma membrane and act as molecular markers. They play a crucial role in cell-to-cell recognition,allowing cells to identify each other,which is essential for immune responses and tissue formation.

(C) The concept of the 'unit membrane' was proposed by $J$.$D$. Robertson.
According to this model,all cellular membranes (plasma membrane,nuclear membrane,etc.) share a common structure consisting of a lipid bilayer sandwiched between two layers of proteins.
Therefore,the unit membrane is composed of lipids and proteins,which are collectively referred to as lipoproteins.

(A) The cell membrane consists of a phospholipid bilayer where the fatty acid tails are hydrophobic (water-fearing) and the phosphate heads are hydrophilic (water-loving).
Integral membrane proteins are embedded within this bilayer.
The hydrophobic amino acid side chains of these proteins interact with the hydrophobic fatty acid tails of the phospholipids through hydrophobic interactions.
These hydrophobic interactions are the primary force that stabilizes and anchors the proteins within the lipid bilayer.

(C) The plasma membrane is a biological membrane that separates the interior of all cells from the outside environment.
Based on electron microscopic studies,the thickness of the plasma membrane is generally observed to be approximately $75\ \mathring{A}$ $(7.5\ nm)$.
Therefore,the range of $50\ \mathring{A}$ to $70\ \mathring{A}$ is the most accurate representation among the given options for the typical thickness of a cell membrane.

(B) The $Fluid$ $Mosaic$ $Model$ of the plasma membrane was proposed by $S. J. Singer$ and $G. L. Nicolson$ in $1972$.
According to this model,the membrane is a quasi-fluid structure where proteins are embedded in a phospholipid bilayer,allowing for lateral movement of molecules.

(B) The biological membrane,specifically the plasma membrane,is primarily composed of lipids and proteins.
According to the fluid mosaic model,the membrane consists of a phospholipid bilayer in which protein molecules are embedded.
Lipids,specifically phospholipids,provide the structural framework,while proteins perform various functions such as transport,enzymatic activity,and cell signaling.

(B) The chemical composition of biological membranes varies depending on the cell type.
In human erythrocytes (red blood cells),the membrane is composed of approximately $52\%$ proteins and $40\%$ lipids.
Among the given options,the ratio of $50\%$ proteins and $50\%$ lipids is the closest approximation to the general composition of biological membranes as described in the $NCERT$ textbook.

(A) The Danielli and Davson model (also known as the sandwich model) describes the plasma membrane as a phospholipid bilayer sandwiched between two layers of globular proteins. In this model,the phospholipid molecule consists of a hydrophilic (polar) head and a hydrophobic (non-polar) tail. The polar head is typically represented as a circular structure,while the non-polar tail is represented as a rod or thread-like structure.

(B) Pinocytosis (cell drinking) and phagocytosis (cell eating) are types of endocytosis.
These processes involve the ingestion of extracellular fluid or solid particles by the cell membrane.
Since the cell membrane is the outer boundary of the protoplasm and these processes involve the movement of materials into the cytoplasm,they are fundamentally concerned with the activity of the protoplasm.

(D) The plasma membrane (plasmalemma) consists of a lipid bilayer,proteins,and carbohydrates.
Carbohydrates in the plasma membrane are not present as free polysaccharides like starch or cellulose.
Instead,they are covalently linked to lipids to form glycolipids or to proteins to form glycoproteins.
These molecules are primarily located on the outer surface of the plasma membrane and play a crucial role in cell recognition and adhesion.

(A) Desmosomes are specialized structures found on the lateral surfaces of epithelial cells. They act as intercellular junctions that provide mechanical strength by anchoring cells together. Structurally,they are formed by the modification and specialization of the plasma membrane and associated cytoskeletal proteins.

(C) The plasma membrane is a selectively permeable membrane that regulates the movement of substances into and out of the cell.
Osmoregulation is the process by which the cell maintains a constant osmotic pressure in its fluids by controlling the water and salt concentration.
Since the plasma membrane controls the passage of water and solutes (ions/molecules) across the cell boundary,it plays a critical role in osmoregulation.
Protein synthesis occurs in ribosomes,and nucleic acid synthesis occurs in the nucleus or cytoplasm,not primarily in the plasma membrane.

(A) The $Cell \text{ } membrane$ (also known as the plasma membrane) is a selectively permeable membrane.
It regulates the movement of substances into and out of the cell, thereby controlling the permeability of the cell.
While the cell wall is permeable, it does not act as the primary regulator of selective permeability in living cells.

(B) The correct answer is $(b)$.
According to the fluid mosaic model, while lipids and proteins exhibit lateral diffusion (moving within the plane of the membrane), proteins do not undergo flip-flop movements (transverse diffusion) across the lipid bilayer due to their large size and polar nature.
Option $(a)$ is correct because proteins can move laterally within the lipid bilayer.
Option $(c)$ is correct because certain proteins are restricted to specific domains (e.g., lipid rafts or specific cell surfaces) to perform localized functions.
Option $(d)$ is correct because integral proteins can be completely embedded within the lipid bilayer (non-transmembrane proteins).

(D) Membrane proteins perform a variety of essential functions for the cell.
$1$. Receptor function: They act as receptors for signaling molecules (like hormones or neurotransmitters) to initiate cellular responses.
$2$. Transport function: They act as channels or carriers to facilitate the transport of ions and molecules across the cell membrane.
$3$. Cell recognition: Glycoproteins and other membrane proteins act as markers that allow cells to recognize each other,which is crucial for immune responses and tissue formation.
Therefore,all the listed functions are performed by membrane proteins.

(A) $RNA$ (Ribonucleic acid) is a nucleic acid present in the cytoplasm,ribosomes,and chromosomes (as part of chromatin).
Plasmalemma (plasma membrane) is primarily composed of lipids and proteins.
It does not contain $RNA$ as a structural component or functional unit.
Therefore,$RNA$ is absent in the plasmalemma.

(C) Permeases are specialized proteins found in the plasma membrane of the cell.
They function as carrier proteins that facilitate the transport of specific molecules,such as ions,sugars,and amino acids,across the membrane.
Therefore,they are classified as carrier membrane proteins.

(A) The cell wall is composed of cellulose,hemicellulose,and pectin,which makes it freely permeable to water and solutes.
In contrast,the plasmalemma (cell membrane) is a selectively or differentially permeable membrane that controls the movement of specific molecules into and out of the cell.

(C) The cell membrane is described as selectively permeable because it allows the passage of certain solutes along with the solvent while restricting others.
The entry of water into root hairs occurs via osmosis,which is a passive process involving the solvent.
The entry of mineral salts into root hairs occurs via active transport or facilitated diffusion,which is a selective process involving specific solutes.
Since the cell membrane regulates the movement of both water (solvent) and mineral salts (solutes) independently based on the cell's requirements,the combination of these two processes best demonstrates the selective permeability of the membrane.

(A) The $Plasmalemma$ (or plasma membrane) is a selectively permeable membrane that regulates the movement of specific solutes and solvents into and out of the cell.
While other membranes like mitochondrial and chloroplast membranes are also selectively permeable,the $Plasmalemma$ is the primary example of a biological membrane that exhibits selective permeability to maintain cellular homeostasis.

(B) The plasma membrane acts as a selectively permeable membrane.
It regulates the movement of water and specific solutes into and out of the cell to maintain cellular homeostasis.

(C) Betanin is a pigment stored within the vacuoles of beet root cells. The plasma membrane and the tonoplast (vacuolar membrane) act as semi-permeable barriers that retain the pigment inside the cell. High temperatures cause the denaturation of membrane proteins and increase the fluidity of the lipid bilayer,leading to the disruption of membrane integrity. This damage allows the betanin pigment to leak out of the cells into the surrounding medium. Therefore,a slice of fresh beet root will release more betanin pigment under high temperature conditions.