General Science Module

Biological Principles & Cell Biology

Cell biology forms the bedrock of life sciences, governed by the Cell Theory formulated by Theodor Schwann, Matthias Schleiden, and Rudolf Virchow, which asserts that all living organisms are composed of cells arising from pre-existing cells. Cells are categorized into prokaryotes, lacking membrane-bound nuclei, and eukaryotes, possessing specialized organelles like mitochondria (the cellular site of ATP synthesis through cellular respiration). Plant cells uniquely feature cellulose cell walls, large central vacuoles, and chloroplasts for photosynthesis. Genetic inheritance follows Gregor Mendel's laws of segregation and independent assortment, while Watson and Crick discovered the double-helix structure of deoxyribonucleic acid (DNA) in 1953 based on Rosalind Franklin's X-ray crystallography data.

Practice Topic MCQs All Category Topics

Key Concepts & Examination Highlights

Rudolf Virchow added the principle 'Omnis cellula e cellula' (all cells arise from pre-existing cells) to Cell Theory in 1855.
Mitochondria produce adenosine triphosphate (ATP) through oxidative phosphorylation, functioning as the powerhouses of eukaryotic cells.
James Watson and Francis Crick elucidated the double-helix model of DNA in 1953, drawing directly on X-ray diffraction images produced by Rosalind Franklin.
Robert Hooke first discovered and coined the term 'cell' in 1665 after examining thin slices of cork under a compound microscope.
Ribosomes, discovered by George Palade in 1955, are non-membrane-bound ribonucleoprotein complexes responsible for protein synthesis (translation) in both prokaryotes (70S) and eukaryotes (80S).
The fluid mosaic model of the plasma membrane, proposed by S.J. Singer and Garth L. Nicolson in 1972, describes a phospholipid bilayer with embedded and peripheral proteins.
Lysosomes, often termed cellular suicide bags, contain hydrolytic enzymes (such as proteases, lipases, and nucleases) that function optimally at an acidic pH.
Gregor Mendel established the fundamental laws of inheritance—the Law of Segregation and the Law of Independent Assortment—through hybridization experiments on garden peas (Pisum sativum) published in 1866.
Mitosis produces two genetically identical diploid daughter cells for somatic growth and repair, whereas meiosis consists of two successive divisions yielding four genetically distinct haploid gametes.
Chloroplasts contain the green photosynthetic pigment chlorophyll and carry out the light and dark (Calvin cycle) reactions to convert carbon dioxide and water into glucose.
In DNA, adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds, in accordance with Chargaff's rules.
The endoplasmic reticulum exists as rough ER (studded with ribosomes for protein synthesis and translocation) and smooth ER (responsible for lipid synthesis, carbohydrate metabolism, and drug detoxification).
The Golgi apparatus, discovered by Camillo Golgi in 1898, modifies, packages, and sorts proteins and lipids received from the endoplasmic reticulum into secretory vesicles.
Centrioles, composed of microtubule triplets in a 9+0 arrangement, form the centrosome in animal cells and organize the mitotic spindle apparatus during cell division.
The cell wall in plants consists mainly of cellulose, hemicellulose, and pectin, whereas fungal cell walls are composed of chitin (a polymer of N-acetylglucosamine).
Peroxisomes contain catalase and peroxidase enzymes that break down toxic hydrogen peroxide ( $H_2O_2$ ) into water and oxygen during fatty acid beta-oxidation.
In eukaryotic cell division, the cell cycle consists of Interphase (divided into $G_1$ , $S$ for DNA replication, and $G_2$ phases) followed by M phase (mitosis and cytokinesis).
Crossing over occurs during the Pachytene stage of Prophase I in meiosis, facilitating genetic recombination between non-sister chromatids of homologous chromosomes.
The Central Dogma of molecular biology, proposed by Francis Crick in 1958, describes the flow of genetic information from DNA to RNA via transcription, and from RNA to protein via translation.
Transfer RNA (tRNA) possesses an anticodon loop that matches complementary codons on messenger RNA (mRNA) and carries specific amino acids to ribosomes during translation.
Enzymes act as biological catalysts that accelerate chemical reactions by lowering the activation energy without altering the overall free energy change ( $\Delta G$ ).
Glycolysis, occurring in the cytoplasm of cells, is an anaerobic pathway that breaks down one molecule of glucose into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH.
The Krebs cycle (citric acid cycle) takes place in the mitochondrial matrix, producing NADH, $FADH_2$ , and GTP/ATP from acetyl-CoA oxidation.
Osmosis is the spontaneous net movement of solvent molecules through a semipermeable membrane into a region of higher solute concentration until equilibrium is reached.
Plasmids are small, circular, double-stranded extra-chromosomal DNA molecules found in bacteria that replicate independently and frequently carry antibiotic resistance genes.
The sodium-potassium pump ( $Na^+/K^+$ -ATPase) actively transports three sodium ions out of the cell and two potassium ions in per hydrolyzed ATP molecule, maintaining the resting membrane potential.
Endocytosis encompasses phagocytosis (cellular eating of large particles) and pinocytosis (cellular drinking of extracellular fluid via invaginating vesicles).
Gap junctions in animal cells and plasmodesmata in plant cells provide direct cytoplasmic channels between adjacent cells for intercellular communication and transport.
Cilia and flagella in eukaryotic cells possess a characteristic '9+2' axonemal arrangement of microtubule doublets powered by the motor protein dynein.
Apoptosis is programmed cell death characterized by cellular shrinkage, chromatin condensation, membrane blebbing, and orderly phagocytic clearance without causing inflammation.
Necrosis is unprogrammed cell death resulting from acute physical or chemical trauma, characterized by cellular swelling, plasma membrane rupture, and surrounding tissue inflammation.
Epigenetics refers to heritable modifications in gene expression (such as DNA methylation and histone acetylation) that occur without altering the underlying nucleotide sequence.
Telomeres are repetitive nucleotide sequences ( $TTAGGG$ in vertebrates) located at the ends of linear chromosomes that protect chromosomes from degradation and shorten with successive cell divisions.
Telomerase is a ribonucleoprotein reverse transcriptase enzyme that elongates telomeres in germ cells, stem cells, and cancer cells, conferring cellular immortality.
The lac operon in Escherichia coli, described by François Jacob and Jacques Monod in 1961, is a model of inducible gene regulation controlling lactose catabolism.
Splicing in eukaryotic pre-mRNA involves the removal of non-coding introns and the joining of coding exons by a dynamic ribonucleoprotein complex called the spliceosome.
Alternative splicing allows a single gene to encode multiple distinct protein isoforms, significantly expanding the functional diversity of the proteome.
The genetic code is degenerate (multiple codons can code for the same amino acid), universal across almost all taxa, and non-overlapping, consisting of 64 triplet codons.
The start codon $AUG$ codes for methionine in eukaryotes (and N-formylmethionine in prokaryotes), while $UAA$ , $UAG$ , and $UGA$ serve as universal stop codons.
Chaperone proteins (chaperonins like Hsp70 and GroEL-GroES) assist in the non-covalent folding and conformational assembly of newly synthesized polypeptides.
The cytoskeleton of eukaryotic cells consists of three primary filamentous components: microfilaments (actin), intermediate filaments (keratin, lamin), and microtubules (tubulin).
Active transport moves molecules against their electrochemical concentration gradient across a biological membrane, requiring metabolic energy expenditure.
Facilitated diffusion is a form of passive transport where transmembrane channel or carrier proteins assist solute movement along a concentration gradient without consuming ATP.
Autophagy is a catabolic cellular recycling process where cytoplasmic components and damaged organelles are sequestered in double-membrane autophagosomes and degraded by lysosomes.
The endosymbiotic theory, championed by Lynn Margulis in 1967, posits that mitochondria and chloroplasts originated from ancient endosymbiotic prokaryotes engulfed by ancestral eukaryotic cells.
Primary active transport directly uses ATP hydrolysis to pump ions across membranes, while secondary active transport (cotransport: symport and antiport) harnesses the electrochemical gradient created by primary pumps.
Prions are misfolded infectious proteins that induce normal cellular proteins ( $PrP^C$ ) to adopt the abnormal scrapie conformation ( $PrP^{Sc}$ ), causing fatal transmissible spongiform encephalopathies.
Cytochrome c is a small, highly conserved heme protein associated with the inner mitochondrial membrane that transfers electrons between Complex III and Complex IV and acts as a trigger for apoptosis.
Plasmolysis occurs in plant cells when placed in a hypertonic solution, causing water to leave the cell and the protoplast to shrink away from the rigid cell wall.
Hardy-Weinberg equilibrium states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences ( $p^2 + 2pq + q^2 = 1$ ).

Curriculum & Reference Sources: NCERT Biology Class XI/XII, National Institutes of Health (NIH), Nature Education

Sample Solved Questions & Concept Explanations

8 Verified Concept Questions

Q1.EASY

What organelle is known as the 'Powerhouse of the Cell' for generating ATP?

Q2.EASY

Which pigment in plant leaves absorbs sunlight for photosynthesis, giving leaves their green color?

Q3.EASY

Which vitamin is synthesized in human skin when exposed to ultraviolet B (UVB) sunlight?

Q4.EASY

What is the normal body temperature of a healthy human being on the Celsius scale?

Q5.MEDIUM

Which enzyme present in human saliva initiates the chemical digestion of dietary starch into maltose sugars?

Q6.HARD

The enzyme 'Ribulose-1,5-bisphosphate carboxylase-oxygenase' (RuBisCO) is biologically renowned for what critical function?

Q7.EASY

Which cellular organelle is universally known as the "Powerhouse of the Cell" because it synthesizes ATP energy?

Q8.EASY

Which green pigment found inside chloroplasts absorbs sunlight to drive the process of photosynthesis in plants?