Cell membrane Questions in English

(B) The lipid component of the cell membrane mainly consists of phosphoglycerides.
Phosphoglycerides are glycerol-based phospholipids and are the primary structural components of biological membranes.
Each glycerophospholipid molecule consists of a small polar head group and two long hydrophobic fatty acid chains.

(A) $RBC$ (Red Blood Cell) of humans is the best material for the study of the structure of the cell membrane.
Chemical studies on the $RBC$ cell membrane enabled scientists to infer the possible structure of the plasma membrane.
The plasma membrane of the $RBC$ consists of a complex,well-ordered group of lipids and proteins stretched over the outer surface of the cell in the form of a lipid bilayer,punctuated by penetrating or attached proteins.
This membrane has numerous properties that arise in part from specialized interactions between specific membrane proteins or lipids,or both.

(A) In $1972$,$S$.$J$. Singer and Garth $L$. Nicolson proposed the fluid mosaic model to describe the structure of the plasma membrane. 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) Based on the fluid mosaic model of the plasma membrane:
$A$ represents the Sugar (carbohydrate) chain attached to a protein or lipid.
$B$ represents a Protein molecule.
$C$ represents an Integral protein (transmembrane protein) that spans the entire membrane.
$D$ represents the Lipid bilayer,which forms the structural framework of the membrane.
Therefore,the correct identification is: $A$: Sugar,$B$: Protein,$C$: Integral protein,$D$: Lipid bilayer.

(B) The plasma membrane is selectively permeable,meaning it allows only certain substances to pass through it.
It is elastic in nature,which allows for cell movement and shape changes.
It is asymmetric because the composition of proteins and lipids differs between the inner and outer leaflets of the lipid bilayer.

(C) The fluid mosaic model,proposed by Singer and Nicolson,describes the plasma membrane as a quasi-fluid structure.
It consists of a phospholipid bilayer where the hydrophilic heads face outward and the hydrophobic tails face inward.
This bimolecular layer of phospholipids forms the core or middle part of the membrane structure.
Proteins are embedded within this bilayer,either peripherally or integrally,rather than forming a continuous middle layer.

(B) The $RBC$ membrane is highly flexible due to the fluid mosaic nature of the plasma membrane,which allows for the movement of proteins and lipids.
Peripheral (extrinsic) proteins are indeed more abundant on the cytoplasmic face of the plasma membrane compared to the extracellular face.
However,the flexibility of the $RBC$ membrane is primarily attributed to the fluid nature of the lipid bilayer and the underlying cytoskeletal network (like spectrin),rather than the specific distribution of extrinsic proteins.
Therefore,while both statements are factually correct,the Reason is not the direct explanation for the Assertion.

(A) The cell membrane is composed of a lipid bilayer where the lipids are arranged with their hydrophobic tails facing inward and hydrophilic heads facing outward.
This arrangement is crucial because the hydrophobic tails consist of non-polar saturated hydrocarbons,which are insoluble in water.
By pointing inward,these tails are shielded from the aqueous environment (water) present both inside and outside the cell.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the arrangement described in the Assertion.

(B) To facilitate the transport of hydrophilic substances across the cell membrane,the membrane contains specialized proteins known as aquaporins or water channels.
These water channels are responsible for the passive transport of water-soluble substances.
Structurally,these channels are composed of eight different types of aquaporins.

(C) Membrane channels are specialized integral membrane proteins that facilitate the transport of specific molecules or ions across the cell membrane.
$1$. They are indeed proteins.
$2$. They are usually gated, meaning they can switch between open and closed states to regulate transport.
$3$. The statement "All ions pass through the same type of channel" is incorrect because channels are highly selective; for example, potassium channels only allow potassium ions to pass, and sodium channels only allow sodium ions to pass.
$4$. Porins are specific types of channel proteins that form large pores in the outer membranes of plastids, mitochondria, and some bacteria, allowing the passage of molecules up to the size of small proteins.

(C) Porins are proteins that form large pores in the outer membranes of plastids,mitochondria,and some bacteria. Specifically,they are found in the outer membrane of Gram-negative bacteria,not Gram-positive bacteria. Therefore,option $C$ is the odd one out.

(A) Pumps are membrane proteins that use energy (usually in the form of $ATP$) to carry substances across the cell membrane against the concentration gradient.
Like enzymes,these proteins are highly specific in the substances they transport.
Since both statements are scientifically accurate and the specificity of pumps is a characteristic property similar to enzymes,the Reason supports the nature of the pumps described in the Assertion.

(A) The permeability of the plasma membrane is influenced by the charge and size of ions.
Monovalent ions (like $Na^+$ and $K^+$) tend to decrease the permeability of the membrane.
Conversely,divalent ions (like $Ca^{2+}$) act to increase the permeability of the plasma membrane.
Therefore,the correct sequence is $A$ = Decrease and $B$ = Increase.

(D) $\Rightarrow$ The transport of substances is primarily regulated by the cell membrane because it is a selectively permeable membrane. It consists of a lipid bilayer embedded with proteins and carbohydrates.
$\Rightarrow$ Lipid-soluble substances diffuse through the membrane easily,while specific transport proteins (like porins) facilitate the movement of other small molecules.
$\Rightarrow$ In contrast,the cell wall is a non-living,rigid structure that provides mechanical support and protection to the cell,but it does not regulate the selective transport of molecules into or out of the cytoplasm.

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

(D) The chemical composition of the human erythrocyte (red blood cell) membrane has been studied extensively.
It consists of approximately $52 \%$ protein and $40 \%$ lipid.
The remaining percentage is composed of carbohydrates and other minor components.
Therefore,$P = 52 \%$ and $Q = 40 \%$.

(C) The cell membrane is composed of a lipid bilayer.
Each lipid molecule has a polar hydrophilic head and a non-polar hydrophobic tail.
The polar head is oriented towards the outer surface,while the non-polar tail,which consists of saturated hydrocarbons,is protected from the aqueous environment by being oriented towards the inner side.
Option $C$ is incorrect because the tail is non-polar,not polar,and it is composed of saturated hydrocarbons,not unsaturated ones in the standard model description.

(D) In the structure of the cell membrane,carbohydrate molecules can be attached to both lipids and proteins.
When carbohydrates are attached to lipids,they are called glycolipids.
When carbohydrates are attached to proteins,they are called glycoproteins.
Therefore,the correct answer is $D$.

(D) According to the fluid mosaic model,the quasi-fluid nature of lipids enables lateral movement of proteins within the bilayer.
Lipid molecules are amphipathic,meaning they have both hydrophilic (polar head) and hydrophobic (non-polar tail) regions.
Facilitated transport requires specific membrane proteins (carrier proteins or channels) to move molecules across the membrane.
Simple diffusion is the movement of molecules directly across the lipid bilayer without the assistance of membrane proteins.
Therefore,the statement that proteins are useful in simple diffusion is incorrect.

(A) In the fluid mosaic model of the cell membrane:
$P$ points to the sugar chains (carbohydrates) attached to the surface of the membrane.
$Q$ points to a peripheral protein,which is loosely associated with the surface of the lipid bilayer.

(C) The $Na^{+}-K^{+}$ pump is a type of active transport mechanism found in the cell membrane.
It functions as a transmembrane protein,specifically an enzyme known as $Na^{+}-K^{+}$ ATPase.
This protein uses energy derived from the hydrolysis of $ATP$ to transport $Na^{+}$ ions out of the cell and $K^{+}$ ions into the cell against their respective concentration gradients.
Therefore,the 'pump' itself is a protein molecule.

(C) Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.
In gases,the intermolecular forces of attraction are very weak and the kinetic energy of particles is very high,allowing them to move freely and rapidly.
Therefore,the rate of diffusion is highest in gases,followed by liquids,and is slowest in solids.

(D) Active transport uses energy to pump molecules against a concentration gradient. This process is carried out by specific membrane proteins known as pumps. These proteins use energy,typically derived from $ATP$,to move substances across the cell membrane.

(B) Pumps are specialized membrane proteins that use energy (usually from $ATP$) to transport molecules or ions against their concentration gradient across the cell membrane. These are essential for active transport. Therefore,pumps are made of proteins.

(B) The cell membrane is primarily composed of a lipid bilayer,which has a hydrophobic interior.
Substances that are hydrophobic or lipid-soluble can easily pass through the lipid bilayer via simple diffusion.
However,substances with a hydrophilic (water-loving) moiety are polar or charged and cannot easily pass through the non-polar,hydrophobic core of the lipid bilayer.
Therefore,hydrophilic substances require specialized transport proteins (like channels or carriers) to cross the cell membrane.

(B) Based on the structure of the plasma membrane:
$A$ points to a carbohydrate chain attached to a protein,which is a Glycoprotein,responsible for cell-to-cell recognition.
$B$ points to an integral protein forming a channel,which is a Tunnel protein,facilitating the transport of polar molecules.
$C$ represents the Lipid Bilayer,which forms the structural framework of the membrane.
$D$ points to Cholesterol molecules embedded within the bilayer,which provide stability and regulate membrane fluidity.
Therefore,the correct matching is $A \rightarrow$ Glycoprotein,$B \rightarrow$ Tunnel protein,$C \rightarrow$ Lipid Bilayer,$D \rightarrow$ Cholesterol.

(D) The plasma membrane of a living cell,including root cells,is described as selectively permeable.
This means it allows the passage of certain molecules and ions while restricting others based on the cell's requirements.
While 'semipermeable' is sometimes used interchangeably,'selectively permeable' is the more accurate biological term as it actively regulates the transport of specific solutes.

(B) The cell wall is the outermost layer of a plant cell,including root hair cells. It is composed of cellulose and is freely permeable to water and dissolved substances,allowing them to pass through easily.
The plasma membrane (cell membrane) is the layer located inside the cell wall. It is selectively permeable (or semi-permeable),meaning it regulates the entry and exit of specific molecules based on the cell's requirements.
Therefore,both Statement-$I$ and Statement-$II$ are correct.

(C) Statement $A$ is correct: The membrane of human $RBCs$ consists of approximately $52\%$ protein and $40\%$ lipids.
Statement $B$ is correct: The phospholipids are arranged in a bilayer with the polar heads towards the outer sides and hydrophobic tails towards the inner part.
Statement $C$ is incorrect: Mesosomes are specialized membranous structures formed by the extensions of the plasma membrane into the cell in prokaryotes,not eukaryotes.
Statement $D$ is correct: The hydrophobic tails of the lipid molecules are directed towards the inner side,ensuring they are protected from the aqueous environment.
Statement $E$ is correct: Glycocalyx is a sugar coat present on the outer surface of the plasma membrane.
Therefore,statements $A, B, D,$ and $E$ are correct. Since the provided options do not contain this combination,and based on standard examination patterns where $C$ is explicitly false,the intended answer is $A, B, D$ and $E$. Given the options,$(3)$ is the most appropriate choice as it contains the correct statements $A, B,$ and $D$.