Structural Organisation of a Cell Questions in English

(N/A) Substances like $CO_2$ move in and out of the cell through the process of diffusion. When $CO_2$ accumulates in high concentration inside the cell,a concentration gradient is created between the internal and external environment. $CO_2$ moves from a region of high concentration to a region of low concentration outside the cell via diffusion. Similarly,water moves in and out of the cell through a specialized process called osmosis,which is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a selectively permeable membrane.

(N/A) The plasma membrane is called a selectively permeable membrane because it selectively allows the movement of certain substances into and out of the cell. It permits the passage of specific molecules while preventing the movement of others,thereby maintaining the internal environment of the cell.

(N/A) The completed table is as follows:

Prokaryotic Cell	Eukaryotic Cell
$1.$ Size: generally small $(1-10 \,\mu m)$	$1.$ Size: generally large $(5-100 \,\mu m)$
$2.$ Nuclear region: poorly defined due to the absence of a nuclear membrane,known as nucleoid	$2.$ Nuclear region: well defined and surrounded by a nuclear membrane
$3.$ Chromosome: single	$3.$ More than one chromosome
$4.$ Membrane-bound cell organelles absent	$4.$ Membrane-bound cell organelles (e.g.,mitochondria,plastids) are present

(B) The cell is the fundamental structural and functional unit of life. Its organization is maintained by various organelles and biochemical processes. If this organization is destroyed due to physical or chemical influence,the cell loses its ability to perform essential life processes such as respiration,nutrition,and waste excretion. Consequently,the cell will not be able to perform its basic functions and will die after some time.

(N/A) Comparison of plant cell and animal cell:
$1$. Cell Wall: Plant cells possess a rigid cell wall outside the plasma membrane,whereas animal cells lack a cell wall.
$2$. Shape: Plant cells generally have a fixed,regular shape due to the cell wall,while animal cells are generally irregular or flexible in shape.
$3$. Size: Plant cells are typically larger in size compared to animal cells.
$4$. Plastids: Plant cells contain plastids (like chloroplasts) for photosynthesis,which are absent in animal cells (except in some protists like $Euglena$).
$5$. Vacuoles: Plant cells contain a single,large,permanent central vacuole,whereas animal cells have many small,temporary vacuoles.

(N/A)

Prokaryotic Cell	Eukaryotic Cell
$1$. Size: Generally small $(1-10 \,\mu m)$ where $1 \,\mu m = 10^{-6} \,m$.	$1$. Size: Generally large $(5-100 \,\mu m)$.
$2$. Nuclear region: Poorly defined due to the absence of a nuclear membrane; known as nucleoid.	$2$. Nuclear region: Well-defined and surrounded by a nuclear membrane.
$3$. Chromosome: Single.	$3$. More than one chromosome.
$4$. Membrane-bound cell organelles are absent.	$4$. Membrane-bound organelles like mitochondria and plastids are present.

(B) If the plasma membrane ruptures or breaks down:
$1$. The cell will lose its structural integrity and its ability to maintain homeostasis.
$2$. All the useful substances (like proteins,ions,and nutrients) will move out of the cell.
$3$. There will be no difference between the cell content and its external environment,leading to the death of the cell.
$4$. The cell will lose its normal shape and eventually disintegrate.

(B) Osmosis is defined as the movement of solvent molecules (typically water) from a region of their higher concentration to a region of their lower concentration through a semipermeable membrane.
During this process,water molecules are free to move across the plasma membrane in both directions,but the net movement occurs from the region of higher water concentration to the region of lower water concentration.

(A) Unlike the other options,a virus is not a cell. $A$ virus consists of genetic material ($DNA$ or $RNA$) enclosed within a protein coat called a capsid. Because of this simple structure,viruses can be crystallized and stored like non-living chemicals,a property first demonstrated by Wendell Stanley in $1935$.

(B) If the concentration of water molecules in the surrounding medium is higher than the concentration of water molecules in the cell,the cell is placed in a hypotonic solution.
According to the process of osmosis,water molecules move from a region of higher water concentration to a region of lower water concentration through a semi-permeable membrane.
Therefore,water will move into the cell,causing it to swell up.

(C) Chromosomes are composed of $DNA$ and proteins. In eukaryotic cells,$DNA$ is tightly coiled around histone proteins to form structures called nucleosomes,which further condense to form chromosomes.

(B) Osmosis is defined as the spontaneous net movement of solvent molecules (usually water) through a selectively permeable membrane into a region of higher solute concentration (which is equivalent to a region of lower solvent concentration) in the direction that tends to equalize the solute concentrations on the two sides. Therefore,the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a semipermeable membrane is the correct definition.

(C) Plasmolysis is the process where the cytoplasm of a plant cell shrinks away from the cell wall.
This occurs when the cell is placed in a hypertonic solution (a solution with a higher solute concentration than the cell sap).
Due to the concentration gradient,water moves out of the cell through the process of exosmosis.
As water leaves the cell,the protoplast shrinks,causing the cytoplasm to pull away from the cell wall.

(A) In prokaryotic cells,the genetic material is not enclosed by a nuclear membrane. This undefined nuclear region,which contains only nucleic acids,is called the nucleoid.

(D) The term endocytosis is composed of two parts: 'endo' means towards the inside and 'cytosis' means movement related to the cell. Amoeba uses its flexible cell membrane to engulf food particles from its external environment,a process known as endocytosis (specifically phagocytosis).

(B) The prefix $\mu$ (micro) represents a factor of $10^{-6}$. Therefore,$1 \, \mu m$ (micrometer) is equal to $10^{-6} \, m$ (meters). This unit is commonly used in biology to measure the size of cells and organelles.

(A) Option $A$ is the odd one out because it is a false statement. Plant cell membranes are primarily composed of lipids and proteins,not chitin. Chitin is a structural polysaccharide found in the cell walls of fungi and the exoskeletons of arthropods,not in plant plasma membranes. Options $B$,$C$,and $D$ are scientifically accurate descriptions of membrane composition and transport properties.

(B) When you wash clothes for a long time,the soap solution acts as a hypertonic solution relative to the cells of your skin.
According to the process of osmosis,water moves from a region of higher water concentration to a region of lower water concentration through a semi-permeable membrane.
Since the soap solution is more concentrated (hypertonic) than the cytoplasm of your skin cells,water moves out of the finger cells into the soap solution.
This loss of water causes the skin cells to shrink,resulting in the wrinkled appearance of your fingers.

(N/A) Endocytosis is the process by which cells engulf materials from their surroundings by forming vesicles from the plasma membrane.
In animal cells,the plasma membrane is the outermost boundary because a rigid cell wall is absent.
This flexibility allows the plasma membrane to fold inward and engulf external substances.
In contrast,plant cells possess a rigid,protective cell wall outside the plasma membrane,which prevents the membrane from invaginating or engulfing large particles,thus making endocytosis impossible in plant cells.

(N/A) The phenomenon responsible for this situation is $Exosmosis$. When a person consumes a concentrated salt solution (a hypertonic solution),the concentration of salt outside the cells of the alimentary canal becomes much higher than inside the cells. Due to the difference in concentration,water moves out of the cells into the surrounding medium through the process of $Exosmosis$. This leads to severe dehydration of the cells lining the digestive tract,which triggers the body's protective reflex to expel the irritant,resulting in vomiting.

(B) The absorption of digested food nutrients (like glucose and amino acids) primarily occurs through diffusion (simple or facilitated) and active transport across the intestinal epithelium.
Water is absorbed primarily through the process of osmosis,which is the movement of water molecules from a region of higher water potential to a region of lower water potential across a semi-permeable membrane.

(B) When salt is added to vegetables,the concentration of the surrounding medium becomes hypertonic compared to the cytoplasm of the vegetable cells.
Due to this concentration gradient,water moves out of the vegetable cells into the surrounding medium through the cell membrane.
This process of movement of water from a region of higher water potential (inside the cell) to a region of lower water potential (outside the cell) through a semi-permeable membrane is known as $Exosmosis$.

(N/A) bacterial cell is a prokaryotic cell,whereas an onion peel cell is a eukaryotic plant cell. The differences are summarized below:

Bacterial Cell	Onion Peel Cell
$(i)$ Cell wall is made of peptidoglycan.	$(i)$ Cell wall is made of cellulose.
$(ii)$ Nucleus is absent (nucleoid present).	$(ii)$ Nucleus is present.
$(iii)$ Membrane-bound organelles are absent.	$(iii)$ Membrane-bound organelles are present.
$(iv)$ These are prokaryotes.	$(iv)$ These are eukaryotes.

(B) Carbon dioxide $(CO_2)$ moves in and out of the cell through the process of diffusion,which is the spontaneous movement of a substance from a region of higher concentration to a region of lower concentration.
Water $(H_2O)$ moves in and out of the cell through the process of osmosis,which is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

(N/A) Chromatin is the loose,thread-like form of $DNA$ and proteins found in the nucleus during interphase. When a cell prepares to divide,this chromatin condenses into thicker,distinct structures called chromatids. $A$ chromosome is the structural unit formed by one or two identical chromatids (sister chromatids) joined at the centromere,which serves to package and organize genetic material for cell division.

(N/A) When a cell has a higher water concentration than the surrounding medium,water moves out of the cell into the medium. This process is known as exosmosis,which causes the cell to shrink or become plasmolysed.
$(b)$ When a cell has a lower water concentration than the surrounding medium,water moves from the medium into the cell. This process is known as endosmosis,which causes the cell to swell or become turgid.
$(c)$ When a cell has an equal water concentration to its surrounding medium,there is no net movement of water across the cell membrane. The cell remains in an isotonic state,and its size and shape remain unchanged.

(B) The cell membrane is a living structure composed of lipids and proteins that regulates the movement of substances in and out of the cell.
The cell wall is a non-living,rigid outer covering found in plant cells,fungi,and bacteria,primarily composed of cellulose,which provides structural support and protection.

(N/A) $(i)$ $Amoeba$ is a unicellular organism that constantly changes its shape by forming pseudopodia to move and capture food.
$(ii)$ $Nerve$ $cell$ (neuron) has a fixed,typical shape with a long axon and dendrites to facilitate the transmission of electrical impulses throughout the body.

(B) The smallest known cell is the $PPLO$ (Pleuro-Pneumonia Like Organism),also known as $Mycoplasma$.
It is approximately $0.1\, \mu m$ in diameter.
These organisms lack a cell wall,which allows them to change their shape and survive without oxygen.

(B) The largest known isolated single cell is the egg of an ostrich.
Its size is approximately $170\, mm \times 135\, mm$.

(B) The nerve cell (neuron) is the longest cell in the human body. These cells can reach lengths of up to $1 \ m$ or more,extending from the spinal cord to the extremities.

(N/A) Two examples of unicellular animals are $Amoeba$ and $Paramecium$. These organisms consist of a single cell that performs all essential life functions,such as respiration,nutrition,and reproduction.

(N/A) $RNA$ stands for Ribonucleic acid.
$DNA$ stands for Deoxyribonucleic acid.

(N/A) Prokaryotic cells are cells that lack a well-defined nucleus and membrane-bound organelles. Examples of organisms with prokaryotic cells include bacteria and blue-green algae (also known as cyanobacteria).

(B) Chromosomes are thread-like structures located inside the nucleus of animal and plant cells.
Each chromosome is made of protein and a single molecule of deoxyribonucleic acid $(DNA)$.
These proteins are primarily histone proteins,which help in the packaging of $DNA$ into compact structures.

(B) Genes are the functional units of heredity and are segments of $DNA$. These genes are physically located on the chromosomes,which are thread-like structures found within the nucleus of eukaryotic cells.

(B) In prokaryotic cells,the genetic material is not enclosed by a nuclear membrane. This undefined nuclear region containing only nucleic acids is known as the nucleoid.

(A) Endocytosis is the cellular process by which cells ingest external materials,such as nutrients or fluids,by engulfing them within the plasma membrane.
This process involves the plasma membrane folding inward to form a vesicle that encloses the substance,which is then transported into the cytoplasm.

(B) Osmosis is a special case of diffusion.
It is defined as the net movement of water molecules from a region of higher water concentration (or higher water potential) to a region of lower water concentration (or lower water potential) through a selectively permeable membrane.
This process continues until equilibrium is reached.

(N/A) The movement of $CO_{2}$ into and out of the cell occurs through the process of diffusion,which is the spontaneous movement of a substance from a region of higher concentration to a region of lower concentration.
The movement of $H_{2}O$ into and out of the cell occurs through the process of osmosis,which is the movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.

(B) virus lacks its own metabolic machinery,such as ribosomes,enzymes,and energy-generating systems,which are necessary for protein synthesis and replication. Because it lacks these essential components,it must hijack the metabolic machinery of a living host cell to multiply.

(A) When a living plant cell is placed in a hypertonic solution,water moves out of the cell due to osmosis.
This loss of water causes the protoplast to shrink and pull away from the cell wall.
This specific phenomenon is known as Plasmolysis.

(B) The functional unit of $DNA$ that carries genetic information is known as a $Gene$. Genes are specific segments of $DNA$ sequences that code for functional molecules,typically proteins,which determine the traits and characteristics of an organism.

(B) The process involved is $Osmosis$.
When raisins are placed in a liquid medium like porridge,the concentration of water inside the raisins is lower than the concentration of water in the surrounding medium.
Due to this concentration gradient,water molecules move from the surrounding medium into the raisins through their semi-permeable cell membranes.
This movement of water from a region of higher water concentration to a region of lower water concentration through a semi-permeable membrane is known as $Osmosis$.

(A) In a dividing cell,$DNA$ is organized into compact,thread-like structures called chromosomes.
$(b)$ In a non-dividing cell,$DNA$ is present as a loose,tangled network of thread-like structures known as chromatin material.

(N/A) Cells exhibit a wide variety of shapes and sizes,which are often related to their specific functions in an organism.
For example:
$1$. Fat cells are generally spherical or oval in shape,which helps them in storing fat droplets efficiently.
$2$. Bone cells (osteocytes) have a branched or star-like shape,which allows them to connect with other cells and form a structural network in the bone tissue.
These differences in shape demonstrate that cell morphology is adapted to perform specialized tasks within the body.

(N/A)

Nucleus	Nucleoid
$1.$ Nuclear membrane is present.	$1.$ Nuclear membrane is absent.
$2.$ Chromatin network and more than one chromosome are present.	$2.$ Only one circular chromosome is present.

(N/A) Fungi can withstand very dilute (hypotonic) external media without bursting due to the presence of rigid cell walls. When placed in a hypotonic solution,water enters the fungal cells through osmosis,causing them to swell. As the cell expands,it exerts turgor pressure against the cell wall. The cell wall,being rigid and strong,exerts an equal and opposite wall pressure,which prevents the cell from bursting.

(N/A) $DNA$ stands for Deoxyribonucleic acid. It is a complex molecule that contains the genetic instructions necessary for the development, functioning, growth, and reproduction of all known organisms. In a non-dividing cell, $DNA$ is found within the nucleus as a component of chromatin material. When the cell prepares to divide, this chromatin material condenses and organizes into structures called chromosomes.
$(b)$ The functional segments of $DNA$ are known as genes. Genes are the basic units of heredity and are responsible for the transmission of traits from parents to offspring.

(N/A) Genes are the segments of $DNA$ occupying specific positions on the chromosomes.
These are the fundamental units of heredity that are transmitted from one generation to another via chromosomes.
They carry the genetic information required for the synthesis of proteins,which in turn control various cellular activities,determine specific cell functions,and regulate the expression of traits.