Cell membrane Questions in English

(B) The correct answer is $(b)$.
Chemically,a biomembrane consists of lipids $(20-70\%)$,proteins $(20-70\%)$,carbohydrates $(1-5\%)$,and water $(20\%)$.
The primary lipids found in the cell membrane include phospholipids,sterols (such as cholesterol),glycolipids,and sphingolipids (such as sphingomyelin and cerebrosides).
Proline is an amino acid,which is a building block of proteins,but it is not a structural constituent of the cell membrane itself in the same way that lipids or membrane proteins are.

(C) Porins are large $ \text{proteins} $ that form large pores in the outer membranes of the plastids, mitochondria, and some bacteria.
They allow molecules up to the size of small proteins to pass through the membrane.
Therefore, porins are classified as $ \text{proteins} $.

(C) Porins are large $ \text{proteins} $ that form large pores in the outer membranes of the plastids, mitochondria, and some bacteria.
They allow molecules up to the size of small proteins to pass through the membrane.
Therefore, porins are essentially $ \text{proteinaceous} $ channels.

(D) The bacterial cell envelope consists of three layers: the outermost glycocalyx (slime layer or capsule),the middle cell wall,and the innermost plasma membrane.
$1$. The glycocalyx (slime layer or capsule) provides protection and helps in adhesion.
$2$. The cell wall determines the shape of the cell and provides structural support.
$3$. The plasma membrane is the innermost layer and is selectively permeable (semi-permeable) in nature,controlling the entry and exit of substances in and out of the cell.

(D) Phagocytosis is the process by which a cell engulfs solid particles or large molecules from the extracellular environment using its cell membrane.
This process involves the formation of a vesicle known as a phagosome.
Pinocytosis refers to the ingestion of liquid or small dissolved particles (cell drinking).
Endosmosis is the movement of water into a cell.
Cytokinesis is the division of the cytoplasm during cell division.

(A) Tubulin is a globular protein that polymerizes to form microtubules,which are essential components of the cytoskeleton.
Microtubules are involved in various cellular processes such as intracellular transport,maintaining cell shape,and the formation of the mitotic spindle during cell division (cytokinesis).
Pinocytosis is a process of endocytosis where the cell membrane invaginates to ingest extracellular fluid; while the cytoskeleton (containing tubulin) may assist in the movement of vesicles,tubulin itself is not a structural component of the cell membrane.
Therefore,tubulin protein is not present in the cell membrane.

(C) The fluid mosaic model,proposed by $S.J. Singer$ and $G.L. Nicolson$ in $1972$,describes the structure of the plasma membrane.
According to this model,the plasma membrane is composed of a lipid bilayer (primarily phospholipids) in which proteins are embedded.
These proteins are classified into two types based on their ease of extraction: $1.$ Peripheral proteins (lie on the surface of the membrane) and $2.$ Integral proteins (partially or totally buried in the membrane).
Therefore,the correct composition includes phospholipids,peripheral proteins,and integral proteins.

(B) The fluid mosaic model,proposed by $Singer$ and $Nicolson$ in $1973$,describes the plasma membrane as a fluid structure.
The fluidity of the membrane is primarily due to the nature of the lipid bilayer.
The phospholipids are arranged in a bilayer with their hydrophobic tails facing inward and hydrophilic heads facing outward.
These lipid molecules are capable of lateral movement within the membrane,which provides the fluid nature to the plasma membrane.
Proteins are embedded within this lipid bilayer,but the overall fluidity is a characteristic property of the lipid component.

(A) According to the $NCERT$ textbook,the chemical composition of the human erythrocyte (red blood cell) membrane has been studied extensively.
It is found that the membrane consists of approximately $52\%$ protein and $40\%$ lipids.
The remaining percentage consists of carbohydrates and other minor components.
Therefore,the correct values are $52$ percent protein and $40$ percent lipids.

(D) Assertion $(A)$ is false because a cell is not a closed compartment for all substances. It is an open system that constantly exchanges materials (nutrients,waste,gases) with its environment.
Reason $(R)$ is true because the plasma membrane is selectively permeable,meaning it allows only certain molecules or ions to pass through it by means of active or passive transport.

(B) In the fluid mosaic model of the cell membrane:
$1$. $X$ points to the globular proteins embedded in or associated with the lipid bilayer.
$2$. $Y$ points to the sugar chains (oligosaccharides) attached to the proteins,forming glycoproteins.
$3$. $Z$ points to the sugar chains (oligosaccharides) attached to the lipids,forming glycolipids.
Therefore,the correct identification is $X-$ Protein,$Y-$ Sugar,$Z-$ Sugar. However,looking at the options provided,option $B$ is the most appropriate as it correctly identifies the protein $(X)$ and the sugar component $(Z)$. Note: In the provided diagram,$Y$ and $Z$ both represent sugar chains attached to different components. Based on standard diagrams,$X$ is Protein,$Y$ is Sugar (on protein),and $Z$ is Sugar (on lipid). Thus,$X-$ Protein,$Y-$ Sugar,$Z-$ Sugar is the accurate description. Given the options,$B$ is the best fit.

(D) The fluid mosaic model,proposed by $Singer$ and $Nicolson$ in $1972$,describes the cell membrane as a mosaic of various components.
These components primarily include a phospholipid bilayer,which provides the fluid structure,and proteins that are embedded within or attached to the surface.
The proteins are classified into two types: integral (or intrinsic) proteins,which are embedded in the bilayer,and peripheral (or extrinsic) proteins,which lie on the surface of the membrane.
Therefore,the cell membrane is composed of phospholipids,extrinsic proteins,and integral proteins.

(C) The cell membrane (plasma membrane) is primarily composed of lipids and proteins. The lipid bilayer forms the structural framework,while proteins are embedded within it. Additionally,carbohydrates are present on the outer surface of the membrane,attached to proteins (glycoproteins) or lipids (glycolipids). Therefore,the cell membrane is composed of proteins,lipids,and carbohydrates.

(B) The $Fluid$ $Mosaic$ $Model$ of the cell membrane was proposed by $S.J.$ $Singer$ and $G.L.$ $Nicolson$ in $1972$.
According to this model,the cell membrane consists of a phospholipid bilayer with proteins embedded in it,which gives it a fluid-like appearance and dynamic nature.

(C) The cell membrane (plasma membrane) is primarily composed of lipids and proteins.
According to the fluid mosaic model,the membrane consists of a lipid bilayer in which protein molecules are embedded.
Lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.
Therefore,the correct answer is proteins and lipids.

(C) Desmosomes are specialized structures found in animal cells that function as intercellular junctions.
They act as 'spot welds' or anchoring junctions that provide mechanical strength to tissues by firmly attaching adjacent cells to one another.
Therefore,they are primarily associated with cell adhesion.

(C) According to the fluid mosaic model,the cell membrane is primarily composed of a phospholipid bilayer.
These phospholipid molecules are arranged such that their hydrophilic polar heads face outward,while their hydrophobic non-polar tails face inward (toward the center).
Thus,the phospholipid bilayer forms the central structural core of the membrane,within which protein molecules are embedded like a mosaic.

(B) According to the Fluid Mosaic Model,lipids and proteins are dynamic within the membrane. However,protein molecules do not exhibit 'flip-flop' movement (transverse diffusion from one leaflet to the other) within the lipid bilayer. This is because proteins are large in size and possess hydrophilic regions that would need to pass through the hydrophobic core of the lipid bilayer,which is energetically unfavorable. Therefore,statement $B$ is incorrect.

(B) According to the fluid mosaic model,the plasma membrane is a fluid structure.
Lipids can move laterally within the membrane,and they can also undergo 'flip-flop' movement (transverse diffusion) from one leaflet to the other,although this process is slow and often requires enzymes called flippases.
Proteins,due to their large size and amphipathic nature,are generally unable to undergo flip-flop movement across the lipid bilayer.
Therefore,lipids can occasionally exhibit flip-flop movement,but proteins do not.

(A) According to the $Fluid \ Mosaic \ Model$ proposed by $Singer$ and $Nicolson$ in $1972$,the cell membrane is composed of a lipid bilayer in which protein molecules are embedded. The lipid bilayer consists primarily of phospholipids,which provide the fluid nature to the membrane,while the proteins are embedded within this bilayer,either peripherally or integrally,to perform various functions such as transport and signaling.

(D) The fluid mosaic model of the cell membrane was proposed by $S.J. Singer$ and $G.L. Nicolson$ in $1972$.
$Na^+$ and $K^+$ ions require active transport (using energy) to move across the membrane against the concentration gradient.
The protein content in the cell membrane varies; for example,in human erythrocytes,it is about $52\%$ protein and $40\%$ lipids.
In the lipid bilayer,the polar (hydrophilic) heads are directed towards the outer sides,while the hydrophobic tails are directed towards the inner side.

(C) The cell membrane is primarily composed of a lipid bilayer,which consists of phospholipids,cholesterol,and various proteins.
Glycolipids are also present on the outer surface of the cell membrane.
Proline is an amino acid,which is a building block of proteins. While proteins are components of the cell membrane,proline itself is not a structural component of the membrane in the same way that lipids or cholesterol are; it is merely one of the many amino acids that make up membrane proteins.

(B) Pinocytosis,often described as 'cell drinking' or the pinching in of the cell membrane,allows cells to engulf and internalize larger molecules or extracellular fluid that cannot pass through membrane pores.
Hydrolysis is the chemical breakdown of a compound due to reaction with water.
Cyclosis (or cytoplasmic streaming) is the directed flow of cytosol and organelles around the large central vacuole of a plant cell.
Synthesis is the process of building complex molecules from simpler ones within the cell.

(C) The Assertion is correct because the lipid composition of the two leaflets of the bilayer membrane is asymmetric. For example,in the erythrocyte membrane,lecithin (phosphatidylcholine) is primarily found on the outer leaflet,while cephalin (phosphatidylethanolamine) is more abundant on the inner leaflet.
The Reason is incorrect because oligosaccharides (forming the glycocalyx) are exclusively attached to the external surface of the plasma membrane,facing the extracellular environment. They are not present on the inner cytoplasmic surface of the biomembrane.

(A) The fluid mosaic model,proposed by $Singer$ and $Nicolson$ in $1972$,describes the cell membrane as a fluid structure.
The fluidity of the membrane is due to the lateral movement of lipids and proteins within the bilayer.
The membrane is described as a mosaic because it is composed of a diverse arrangement of proteins embedded in a phospholipid bilayer.
Since the fluid nature of the membrane allows for the movement of these embedded proteins and lipids,the mosaic composition is directly related to its fluid behavior.
Therefore,both the Assertion and Reason are correct,and the Reason is the correct explanation of the Assertion.

(A) The $Na^+ - K^+$ pump is an active transport mechanism that maintains the electrochemical gradient across the cell membrane.
During one cycle of this pump,$3Na^+$ ions are transported out of the cell,and $2K^+$ ions are transported into the cell.
This process requires energy in the form of $ATP$ to move these ions against their respective concentration gradients.

(C) Active transport is a process that requires energy in the form of $ATP$ to move molecules against their concentration gradient.
Since the transport process slows down when energy production is inhibited,it indicates that the process is energy-dependent.
Therefore,the molecules are transported by active transport.

(B) Passive transport is the movement of molecules along the concentration gradient (from higher to lower concentration) without the expenditure of metabolic energy $(ATP)$.
Active transport involves the movement of molecules against the concentration gradient (from lower to higher concentration),which requires the expenditure of metabolic energy in the form of $ATP$ and specific membrane proteins (pumps).

(N/A) The plasma membrane is the outermost covering of the cell that separates it from the environment. It regulates the movement of substances into and out of the cell. It allows the entry of only some substances and prevents the movement of others.
Hence,the membrane is selectively permeable.
Movement of neutral solutes: Neutral molecules move across the plasma membrane by simple passive diffusion. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Movement of polar molecules: The cell membrane is made up of a phospholipid bilayer and proteins. The movement of polar molecules across the non-polar lipid bilayer requires carrier proteins. Carrier proteins are integral protein particles having a specific affinity for certain solutes. As a result,they facilitate the transport of these molecules across the membrane.

(N/A) The detailed structure of the plasma membrane was studied only after the advent of the electron microscope.
First,the study of the plasma membrane of human $RBC$ was conducted.
The plasma membrane is composed of lipids that are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.
This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment. The lipid component of the membrane mainly consists of phosphoglycerides.
Biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types.
In human beings,the membrane of the erythrocyte has approximately $52\%$ protein and $40\%$ lipids.
Depending on the ease of extraction,membrane proteins can be classified as integral or peripheral. Peripheral proteins lie on the surface of the membrane,while the integral proteins are partially or totally buried in the membrane.
An improved model of the structure of the cell membrane was proposed by Singer and Nicolson $(1972)$,widely accepted as the fluid mosaic model. According to this,the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.

(A) $\Rightarrow$ One of the most important functions of the plasma membrane is the transport of molecules across it.
$\Rightarrow$ The membrane is selectively permeable to some molecules present on either side of it.
$\Rightarrow$ Many molecules can move briefly across the membrane without any requirement of energy,and this is called passive transport.
$\Rightarrow$ Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient,i.e.,from higher concentration to lower. Water may also move across this membrane from higher to lower concentration. The movement of water by diffusion is called osmosis.
$\Rightarrow$ As polar molecules cannot pass through the nonpolar lipid bilayer,they require a carrier protein of the membrane to facilitate their transport across the membrane.
$\Rightarrow$ $A$ few ions or molecules are transported across the membrane against their concentration gradient,i.e.,from lower to higher concentration. Such transport is an energy-dependent process in which $ATP$ is utilized,and it is called active transport,e.g.,the $Na^{+}/K^{+}$ pump.

(B) Porins are specialized proteins that form large-sized pores in the outer membranes of plastids (such as chloroplasts),mitochondria,and the cell membranes of some bacteria.
They play a crucial role in diffusion by facilitating the passive transport of small-sized molecules,including certain proteins,across these membranes.

(B) The scientist $J.D. Robertson$ proposed the $Unit \text{ } Membrane \text{ } Concept$ in $1959$.
According to this concept, all cellular membranes (plasma membrane and membranes of cell organelles) have a similar structure.
He described the membrane as a trilaminar structure consisting of a central lipid bilayer sandwiched between two protein layers.
This model suggests that the protein layers are non-uniform and cover both sides of the lipid bilayer.

(B) Singer and Nicolson $(1972)$ proposed the Fluid Mosaic Model of the cell membrane.
According to this model,the quasi-fluid nature of lipids enables the lateral movement of proteins within the overall bilayer.
This model is the most widely accepted structure for the plasma membrane.

(N/A) $\Rightarrow$ Carbohydrates account for approximately $2-10 \%$ of the total mass of the plasma membrane. They are covalently linked to proteins (forming glycoproteins) or lipids (forming glycolipids).
$\Rightarrow$ Cellular identification: The specific arrangement of carbohydrate chains on the surface of cells,such as $RBCs$,determines blood group antigens,which are crucial for cell-cell recognition.
$\Rightarrow$ Biological significance: These carbohydrate moieties act as receptors for hormones,neurotransmitters,and signaling molecules. They also play a vital role in the interaction of cells with viruses,bacteria,and various drugs,ensuring the specificity of cellular responses.

(N/A) The biochemical composition of the plasma membrane includes lipids,proteins,and carbohydrates.
$1$. Lipids: The lipid component mainly consists of phosphoglycerides. These lipid molecules are arranged within the membrane in a bilayer,with the polar hydrophilic heads facing the outer sides (aqueous environment) and the non-polar hydrophobic tails facing the inner part. This arrangement protects the non-polar tails of saturated hydrocarbons from the aqueous environment.
$2$. Proteins: Depending on the ease of extraction,membrane proteins are classified as integral or peripheral. Peripheral proteins lie on the surface of the membrane,while integral proteins are partially or totally buried in the membrane.
$3$. Carbohydrates: These are also present in the membrane,often attached to proteins (glycoproteins) or lipids (glycolipids).
In human erythrocytes,the membrane composition is approximately $52\%$ protein and $40\%$ lipids. According to the fluid mosaic model proposed by Singer and Nicolson $(1972)$,the quasi-fluid nature of lipids enables the lateral movement of proteins within the overall bilayer,which is referred to as membrane fluidity.

(N/A) $\Rightarrow$ The structure and function of the plasma membrane are closely related. The membrane consists of a lipid bilayer,proteins,and carbohydrates.
$\Rightarrow$ Integral proteins act as channels or carriers for the transport of ions and molecules across the membrane. For example,in active transport,proteins function as carriers to move minerals against the concentration gradient.
$\Rightarrow$ Carbohydrates (oligosaccharides) attached to the membrane surface act as recognition sites for cell-to-cell signaling and identification,preventing the entry of unwanted external substances.
$\Rightarrow$ The amphipathic nature of phospholipids,with polar hydrophilic heads facing the aqueous environment and non-polar hydrophobic tails facing inward,creates a stable barrier that allows for the selective permeability and fluidity of the membrane.

(B) $\Rightarrow$ Gap junctions facilitate the movement of ions,small molecules,and chemical signals between adjacent cells through intercellular channels.
$\Rightarrow$ These are hydrophilic pathways between two animal cells,composed of two protein cylinders called connexons.
$\Rightarrow$ Each connexon consists of six protein subunits that surround a central hydrophilic pore.
$\Rightarrow$ The opening and closing of these pathways are regulated by the $pH$ level and the concentration of $Ca^{+2}$ ions.

(B) The intracellular concentration of $K^+$ is maintained higher than the extracellular concentration primarily through the action of the $Na^+/K^+$-ATPase pump.
This pump actively transports $3$ $Na^+$ ions out of the cell for every $2$ $K^+$ ions it brings into the cell,utilizing energy from $ATP$ hydrolysis.
Because more positive charges $(Na^+)$ are pumped out than are brought in $(K^+)$,the interior of the cell becomes negatively charged relative to the exterior.
This electrochemical gradient,combined with the continuous activity of the pump,ensures that $K^+$ ions accumulate inside the cell at higher concentrations than in the extracellular fluid.

(N/A) The cell membrane of beetroot cells acts as a semi-permeable barrier that prevents the leakage of pigments (betalains) at room temperature.
When placed in cold water,the membrane remains intact and the pigments stay within the cells.
However,when placed in hot water,the high temperature causes the denaturation of membrane proteins and increases the fluidity of the lipid bilayer.
This leads to the disruption of the cell membrane,making it permeable,which allows the red pigments to diffuse out of the cells into the surrounding water.

(B) The plasmalemma (cell membrane) is a selectively permeable membrane.
It allows the passage of certain molecules and ions while restricting others based on the cell's requirements.
Therefore,the correct classification for the plasmalemma is Selectively Permeable $(SL)$.

(B) $\Rightarrow$ The correct answer is $B$. Proteins.
Substances that have a hydrophilic moiety find it difficult to pass through the lipid bilayer of the cell membrane.
Their movement across the membrane is facilitated by specific membrane proteins,which act as transporters or channels.

(A) Membrane fluidity refers to the ability of lipids and proteins to move laterally within the bilayer.
In cold weather,phospholipids tend to pack more tightly,which decreases fluidity.
Plants adapt to cold temperatures by increasing the proportion of unsaturated fatty acids in their membrane phospholipids.
The 'kinks' in the unsaturated hydrocarbon tails prevent the phospholipids from packing too tightly,thereby maintaining membrane fluidity.
Cholesterol,which acts as a temperature buffer,is primarily found in animal cell membranes,not plant cell membranes.

(A) The given diagram shows the fluid mosaic model of the cell membrane.
In the diagram,the components marked as $A, B, C, D$ and $E$ are respectively:
$A$ - Sugar (oligosaccharide chain),
$B$ - Protein (peripheral protein),
$C$ - Lipid bilayer,
$D$ - Integral protein,
$E$ - Cytoplasm.
The fluid mosaic model was proposed by $SJ$ Singer and $GL$ Nicolson in $1972$. It describes the cell membrane as a two-dimensional liquid in which phospholipid and protein molecules can diffuse easily.

(A) The cell membrane (marked as $X$) helps control the entry and exit of substances into the cell.
Nucleus passes the genetic information from the parent cell to the new cell; it is also called the control center of the cell.
$A$ cell wall is the tough,rigid layer present outside the plant cell,which provides structural support and protection.
Chloroplasts occur in plant cells and contain chlorophyll,which helps the cell in synthesizing food through photosynthesis.

(C) Integral proteins (also known as intrinsic proteins) are embedded within the lipid bilayer of the cell membrane.
They can be partially or totally buried in the phospholipid matrix.
Some of these proteins,known as transmembrane proteins,span the entire thickness of the membrane.

(B) Active transport is an energy-dependent process in which $ATP$ is utilized to move molecules against their concentration gradient.
For example,the $Na^+/K^+$ pump uses energy to maintain ion gradients across the cell membrane.

(B) The ratio of proteins and lipids varies considerably in different cell types.
In the human erythrocyte membrane, the composition is approximately $52 \%$ proteins and $40 \%$ lipids.
The remaining percentage consists of carbohydrates and other components.