Cell: The Unit of Life — Practice Questions with Answers

The cell theory states: (1) All living organisms are composed of one or more cells; (2) The cell is the fundamental, basic structural and functional unit of life; (3) All cells arise from pre-existing cells (a later addition by Rudolf Virchow), rather than through spontaneous generation.

Prokaryotic cells lack a true, membrane-bound nucleus (genetic material lies in an undefined nucleoid region) and generally lack membrane-bound organelles like mitochondria, Golgi apparatus, or endoplasmic reticulum.

Eukaryotic cells possess a well-defined, membrane-bound nucleus enclosing the genetic material, along with a variety of distinct, membrane-bound organelles compartmentalising various cellular functions.

The endoplasmic reticulum (ER) is a network of interconnected membranous tubules and sacs, involved in synthesis and transport of various cellular materials.

Rough ER is studded with ribosomes on its outer surface, and is primarily involved in the synthesis of proteins destined for secretion, membrane insertion, or specific organelles. Smooth ER lacks ribosomes, and is primarily involved in lipid/steroid synthesis and detoxification of drugs and poisons.

According to the fluid mosaic model, the plasma membrane consists of a phospholipid bilayer, with various proteins embedded within or associated with this lipid layer, capable of moving somewhat freely (lateral diffusion) within the fluid lipid matrix, giving the membrane its "fluid" quality; the varied embedded proteins create the "mosaic" pattern of composition.

The plasma membrane functions as a selectively permeable barrier, regulating the movement of substances into and out of the cell, while also housing important proteins for cell signalling, transport, and recognition.

A plant cell wall is primarily composed of cellulose, a polysaccharide made of glucose units, providing structural rigidity and support.

A fungal cell wall is primarily composed of chitin, a different nitrogen-containing polysaccharide, distinct in composition from the cellulose found in plant cell walls, despite both serving a broadly similar structural, protective role.

The nucleolus is a dense, non-membrane-bound structure found within the nucleus, typically associated with specific chromosomal regions (nucleolar organiser regions).

Its primary significance is in the synthesis and assembly of ribosomal RNA (rRNA) and ribosomal subunits — the nucleolus is essentially the site where ribosome components are produced before being exported to the cytoplasm for final ribosome assembly, making it essential for the cell's protein synthesis machinery.

In plant cells, vacuoles are typically very large, often occupying up to 90% of the cell's volume, and are primarily involved in maintaining turgor pressure (cell rigidity via water storage), storage of nutrients/waste, and pigmentation.

In animal cells, vacuoles are typically small, numerous, and more specialised (e.g., food vacuoles for digestion or contractile vacuoles for water regulation in some organisms) — animal vacuoles are considerably smaller and less prominent than the characteristically large central vacuole found in plant cells.

Chromatin is the loosely packed, thread-like complex of DNA and associated histone proteins found within the nucleus during the non-dividing (interphase) stage of the cell cycle.

During cell division, this chromatin condenses and coils tightly to form distinct, visible, compact structures called chromosomes — chromatin and chromosomes are therefore essentially the same genetic material (DNA + proteins), differing primarily in their degree of condensation/packaging at different stages of the cell cycle.

Centrioles are cylindrical structures, typically found in pairs, composed of microtubule triplets, present in most animal cells (but generally absent in higher plant cells).

Their primary significance lies in organising the mitotic spindle fibres during cell division, ensuring accurate segregation of chromosomes to daughter cells, and they also serve as the basal bodies from which cilia and flagella (whip-like or hair-like cellular projections) develop.

Passive transport involves the movement of substances across the membrane along their concentration gradient (from higher to lower concentration), without requiring the cell to expend metabolic energy (e.g., simple diffusion, facilitated diffusion, osmosis).

Active transport involves the movement of substances against their concentration gradient (from lower to higher concentration), which requires the cell to expend metabolic energy (typically in the form of ATP), often facilitated by specific carrier/pump proteins in the membrane.

Mitochondria are double-membrane-bound organelles — the outer membrane is smooth, while the inner membrane is highly folded into structures called cristae, which considerably increase the surface area available for the biochemical reactions of cellular respiration.

Function: Mitochondria are the primary site of aerobic cellular respiration, where glucose-derived molecules are further oxidised (via the citric acid cycle and oxidative phosphorylation) to generate large amounts of ATP, the cell's main energy currency.

Semi-autonomous nature: Mitochondria contain their own small, circular DNA molecule and their own ribosomes (distinct from the cell's cytoplasmic ribosomes), allowing them to synthesise some of their own proteins and to divide independently by a process resembling binary fission, somewhat independently of the cell's own nuclear division cycle — this unique feature, along with structural similarities to bacteria, supports the endosymbiotic theory of mitochondrial origin.

The Golgi apparatus consists of a series of flattened, stacked, membrane-bound sacs called cisternae, typically located near the nucleus, with distinct cis (receiving) and trans (dispatching) faces.

Function: Proteins and lipids synthesised in the rough/smooth endoplasmic reticulum are transported (typically via small transport vesicles) to the cis face of the Golgi apparatus. As these materials pass through the successive cisternae, they undergo further processing and modification (such as glycosylation, the addition of sugar molecules) before being packaged into vesicles at the trans face.

Role in secretory pathway: These final, mature Golgi-derived vesicles can then be directed to various destinations — fused with the plasma membrane for secretion outside the cell (exocytosis), incorporated into the plasma membrane itself, or delivered to other organelles like lysosomes — the Golgi apparatus therefore serves as the crucial central "sorting and dispatch" hub in the cell's overall secretory pathway, working in close coordination with the endoplasmic reticulum.

Cell wall: Plant cells possess a rigid cellulose cell wall external to the plasma membrane; animal cells lack a cell wall entirely.

Chloroplasts: Plant cells (in photosynthetic tissues) possess chloroplasts for photosynthesis; animal cells entirely lack chloroplasts.

Vacuole: Plant cells typically have one large, prominent central vacuole; animal cells, if present, have smaller and more numerous vacuoles.

Centrioles: Animal cells typically possess centrioles, important for spindle formation during cell division; higher plant cells generally lack centrioles, using alternative mechanisms for spindle organisation.

These structural differences reflect the fundamentally different lifestyles and functional requirements of plant versus animal cells — plants require structural rigidity (cell wall) and photosynthetic capability (chloroplasts), while animal cells prioritise flexibility and mobility.

Osmosis is the passive movement of water molecules across a selectively permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

Hypotonic solution (lower solute concentration outside the cell than inside): Water moves into the cell — in a plant cell, this causes the cell to swell, with the rigid cell wall preventing bursting, resulting in a firm, turgid state (turgor pressure).

Hypertonic solution (higher solute concentration outside the cell than inside): Water moves out of the cell — in a plant cell, this causes the cell membrane to shrink away from the cell wall, a process called plasmolysis.

Isotonic solution (equal solute concentration inside and outside): No net movement of water occurs, and the cell remains in its normal, unchanged state.

The nuclear envelope is a double-membrane structure (composed of an outer and inner membrane) enclosing the nucleus, separating its contents from the surrounding cytoplasm — the outer membrane is often continuous with the endoplasmic reticulum, and the space between the two membranes is called the perinuclear space.

Nuclear pores: The nuclear envelope is perforated by numerous nuclear pores, complex protein structures that regulate the selective, controlled passage of molecules between the nucleus and cytoplasm.

Significance: These pores allow the essential bidirectional traffic required for gene expression — messenger RNA (mRNA) and ribosomal subunits synthesised within the nucleus must exit into the cytoplasm (for translation at ribosomes), while various proteins synthesised in the cytoplasm (such as those needed for DNA replication, transcription, or histone proteins) must be selectively imported into the nucleus — nuclear pores enable this vital, carefully regulated molecular exchange essential for coordinated cell function.

Cell wall: Composed of peptidoglycan (murein), providing structural rigidity and shape.

Plasma membrane: A selectively permeable membrane enclosing the cytoplasm, beneath the cell wall.

Nucleoid: An irregularly shaped region containing the bacterial cell's single, circular DNA molecule, not enclosed by a nuclear membrane.

Ribosomes: Numerous smaller (70S) ribosomes scattered throughout the cytoplasm, responsible for protein synthesis.

Plasmids: Small, circular, extra-chromosomal DNA molecules, often carrying genes conferring specific advantages like antibiotic resistance, present independently of the main bacterial chromosome.

Additional structures may include flagella (for motility), pili/fimbriae (for attachment or genetic exchange), and a protective capsule/slime layer external to the cell wall in some bacterial species.

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotic organisms (bacteria) that were engulfed by a larger host cell, eventually establishing a mutually beneficial (symbiotic) relationship, and over evolutionary time became permanently integrated as organelles within the host cell.

Supporting evidence: Both mitochondria and chloroplasts possess their own small, circular DNA (similar in structure to bacterial DNA, unlike the linear DNA found in the eukaryotic nucleus); they contain their own, distinctly bacterial-type (70S) ribosomes, different from the eukaryotic (80S) cytoplasmic ribosomes; both organelles are bound by a double membrane (consistent with an original engulfment process, where the inner membrane represents the original prokaryotic membrane, and the outer membrane derives from the host cell's engulfing membrane); and both can replicate independently via a process resembling bacterial binary fission, rather than through the same mechanisms used for other organelles.

Cell signalling allows cells to communicate with each other and respond appropriately to changes in their external environment, often mediated by specific receptor proteins embedded in the plasma membrane.

Step 1 (Reception): A signalling molecule (ligand, such as a hormone or neurotransmitter) binds specifically to a complementary receptor protein located on the target cell's plasma membrane.

Step 2 (Transduction): This binding event triggers a change in the receptor's shape or activity, initiating a cascade of intracellular biochemical events (often involving secondary messenger molecules or a series of protein activations) that relay and amplify the original signal within the cell.

Step 3 (Response): This signal transduction cascade ultimately leads to a specific cellular response — this could include changes in gene expression, alterations in enzyme activity, or changes in cell metabolism/behaviour, allowing the cell to appropriately react to the original external signal.

As a cell increases in size (radius), its volume increases proportionally to the cube of the radius ($V \propto r^3$), while its surface area increases only proportionally to the square of the radius ($A \propto r^2$) — this means that as a cell grows larger, its surface-area-to-volume ratio steadily decreases.

The plasma membrane's surface area is critical for essential processes like nutrient uptake, waste removal, and gas exchange (all occurring across this membrane) — as the cell's volume (and hence its metabolic demand for these exchanges) grows faster than its available surface area for exchange, a point is eventually reached where the cell's surface area becomes insufficient to adequately support the metabolic needs of its increasingly large volume.

This fundamental surface-area-to-volume constraint is precisely why cells generally remain microscopically small, rather than growing indefinitely large — instead, multicellular organisms achieve larger overall body sizes by increasing their total number of small, individually efficient cells, rather than by increasing the size of any single cell beyond this fundamental physical constraint.

The cytoskeleton is a dynamic, interconnected network of protein filaments extending throughout the cytoplasm of eukaryotic cells, providing structural support and enabling various cellular movements.

Microfilaments (actin filaments): The thinnest cytoskeletal filaments, composed of actin protein, important for cell shape, movement (like muscle contraction), and cytoplasmic streaming.

Intermediate filaments: Provide mechanical strength and structural stability to cells, and help maintain the position of organelles like the nucleus within the cell.

Microtubules: The thickest cytoskeletal filaments, composed of tubulin protein, important for maintaining cell shape, intracellular transport (acting as tracks for motor proteins moving organelles/vesicles), and forming the mitotic spindle during cell division, as well as forming the core structure of cilia and flagella.

Overall significance: Together, these three filament types provide the cell with its overall structural framework (maintaining shape and mechanical integrity), while also enabling essential dynamic processes including cell division, intracellular transport, and cell motility.