Cellular Processes Chapter Notes | Biotechnology for Class 11 - NEET PDF Download

Cell Signaling

• Cells, both prokaryotic and eukaryotic, continuously receive and interpret environmental signals such as light, heat, sound, and touch, responding in real time.
• During development, cell fates are determined by signaling pathways that respond to extracellular signals.
• Cells communicate with neighboring cells by transmitting and receiving signals, typically in the form of chemical messengers released into the extracellular milieu.
• Cells can also respond to external signals not synthesized by the body, indicating their ability to sense a wide variety of signals.
• A cell can only respond to a signal if it possesses the corresponding receptor, which is a protein located on the cell surface, in the cytoplasm, or in the nucleus.
• The chemical messenger that binds to a receptor is called a ligand, and the receptor-ligand interaction is highly specific.
• Binding of a ligand to its receptor induces conformational changes in the receptor, initiating a message relay system that triggers significant changes in cellular activities.
• Signaling is classified based on the proximity of sender and recipient cells into three main types: paracrine, autocrine, and endocrine signaling.
• Paracrine signaling involves communication over short distances, where chemical messages released by sender cells are instantly sensed by nearby recipient cells, commonly seen in neuron communication.
• Autocrine signaling occurs when a cell secretes a ligand and possesses receptors for that ligand, allowing it to respond to its own signals, as seen in cancer cells that synthesize their own growth factors for proliferation.
• Endocrine signaling, or long-distance signaling, involves ligands synthesized by a cell, released into the bloodstream, and transported to distant target cells, as exemplified by hormones.

Metabolic Pathways

• Metabolism is the process by which living organisms acquire and utilize free energy to sustain life processes.
• Organisms are classified into phototrophs, which use sunlight to convert simple molecules into energy-rich complex molecules, and chemotrophs, which obtain energy by oxidizing organic or inorganic compounds.
• Phototrophs, such as plants and some bacteria, transform light energy into chemical energy through photosynthesis.
• Heterotrophs, like animals, derive energy indirectly from plants through their food.
• Energy uptake in organisms involves coupling exergonic nutrient oxidation reactions to endergonic processes required to maintain life.
• Adenosine triphosphate (ATP) serves as the central energy currency in all energy transactions within cells.
• Metabolic pathways consist of interlinked biochemical reactions that transform a specific molecule into another in a defined sequence.
• Energy from metabolism is used for processes such as creating gradients, moving molecules across membranes, converting chemical energy into mechanical energy, and powering biomolecule synthesis.
• Metabolic pathways are divided into anabolic pathways, which synthesize complex molecules from simpler ones (endergonic), and catabolic pathways, which break down complex molecules into simpler ones (exergonic).
• Anabolic pathways consume energy to synthesize molecules like glucose, fats, proteins, or DNA, collectively referred to as anabolism.
• Catabolic pathways release energy, producing reducing equivalents and ATP, which are used in anabolic processes to create complex structures or energy-rich states.

Overview of Carbohydrate Metabolism

• In animals, glucose is the primary metabolic fuel for most tissues, metabolized into pyruvate through glycolysis.
• Under aerobic conditions, pyruvate enters the mitochondrial matrix, where it is converted into acetyl CoA and enters the citric acid cycle for complete oxidation to CO₂ and H₂O, linked to ATP formation via oxidative phosphorylation.
• In anaerobic conditions, pyruvate is converted into lactic acid.
• Glycolysis intermediates participate in other metabolic processes, including glycogen synthesis and storage in animals.
• The pentose phosphate pathway, sourced from glycolysis intermediates, provides NADPH for fatty acid synthesis and ribose for nucleotide and nucleic acid synthesis.
• Triose phosphates from glycolysis generate the glycerol moiety of triacylglycerols.
• Acetyl CoA, derived from pyruvate, serves as a precursor for fatty acid and cholesterol synthesis, with cholesterol further synthesizing all other steroids in animals.
• Pyruvate and citric acid cycle intermediates provide carbon skeletons for amino acid synthesis.
• During starvation, when glycogen reserves are depleted, non-carbohydrate precursors like lactic acid, amino acids, and glycerol synthesize glucose through gluconeogenesis.

Overview of Lipid Metabolism

• Vital tissues such as the brain, heart, and red blood cells rely exclusively on glucose, but in fasting states, less glucose-dependent tissues (e.g., muscles, liver) use long-chain fatty acids as an alternative fuel.
• Fatty acids are sourced from the diet or synthesized from acetyl CoA derived from carbohydrates or amino acids.
• Fatty acids are oxidized to acetyl CoA via the β-oxidation pathway or esterified with glycerol to form triacylglycerols, the main fuel reserve in animal adipose tissue.
• Acetyl CoA from β-oxidation has three fates: oxidation to CO₂ and H₂O through the citric acid cycle, serving as a precursor for cholesterol and other steroids (hormones and bile pigments), or synthesis of ketone bodies (acetone, acetoacetate, 3-hydroxybutyrate) as an alternative fuel for the liver and other tissues during prolonged fasting.

Overview of Amino Acid Metabolism

• Amino acids are essential for protein synthesis as their building blocks.
• Transamination transfers amino nitrogen from one amino acid to a carbon skeleton to form other amino acids.
• Deamination involves the excretion of amino nitrogen as urea, typically occurring in the liver.

Glycolysis

• Glycolysis, also known as the Embden-Meyerhof-Parnas (EMP) pathway, is a universal catabolic pathway in all living cells, occurring in the cytosol.
• It begins with the phosphorylation of glucose (6-carbon molecule) to glucose-6-phosphate, catalyzed by hexokinase, using ATP as the phosphate donor in an irreversible reaction.
• Hexokinase is allosterically inhibited by its product, glucose-6-phosphate, and can phosphorylate sugars like fructose, galactose, and mannose.
• Liver cells contain glucokinase, an isoenzyme of hexokinase, which specifically phosphorylates glucose.
• Glucose-6-phosphate is a key intermediate in glycolysis, gluconeogenesis, pentose phosphate pathway, glycogenesis, and glycogenolysis.
• Glucose-6-phosphate is converted to fructose-6-phosphate (6-carbon molecule) by phosphoglucose isomerase through an aldose-ketose isomerization reaction.
• Fructose-6-phosphate undergoes another phosphorylation by phosphofructokinase (PFK) to form fructose-1,6-bisphosphate, an irreversible reaction regulated allosterically.
• Fructose-1,6-bisphosphate is cleaved by aldolase into two triose phosphates (3-carbon molecules): glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
• Triose phosphate isomerase interconverts the two triose phosphates.
• Glyceraldehyde-3-phosphate is oxidized to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase, a NAD⁺-dependent enzyme, reducing NAD⁺ to NADH.
• Phosphate from 1,3-bisphosphoglycerate is transferred to ADP, forming ATP and 3-phosphoglycerate (3-carbon molecule) via phosphoglycerate kinase in a process called substrate-level phosphorylation.
• Since two triose phosphates are produced per glucose molecule, two ATP molecules are generated at this stage.
• 3-phosphoglycerate is isomerized to 2-phosphoglycerate (3-carbon molecule) by phosphoglycerate mutase.
• 2-phosphoglycerate is dehydrated to phosphoenolpyruvate by enolase, requiring Mg²⁺ or Mn²⁺ for activity.
• Phosphoenolpyruvate is converted to pyruvate (3-carbon molecule) by pyruvate kinase, transferring phosphate to ADP to form ATP via substrate-level phosphorylation.
• Three glycolytic reactions (catalyzed by hexokinase, PFK, and pyruvate kinase) are exergonic and irreversible, serving as major regulatory points.
• The net reaction of glycolysis is: Glucose + 2 ATP + 2 NAD⁺ + 4 ADP + 4 Pi → 2 Pyruvate + 2 ADP + 2 NADH + 2 H⁺ + 4 ATP + 2 H₂O, yielding a net of 2 ATP per glucose.
• Pyruvate has two fates: under anaerobic conditions, it is reduced to lactate (homolactic fermentation) or ethanol (alcoholic fermentation); under aerobic conditions, it is oxidized to CO₂ and H₂O via the citric acid cycle.
• In homolactic fermentation, pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD⁺ for glycolysis, with the overall reaction: Glucose + 2 ADP + 2 Pi → 2 Lactate + 2 ATP + 2 H₂O + 2 H⁺.
• Lactate is exported from muscle cells to the liver for reconversion to glucose.
• In alcoholic fermentation (in yeast), pyruvate is decarboxylated to acetaldehyde and CO₂ by pyruvate decarboxylase (using coenzyme TPP), and acetaldehyde is reduced to ethanol by alcohol dehydrogenase, regenerating NAD⁺.
• Fermentation yields 2 ATP per glucose molecule.
• Under aerobic conditions, pyruvate is transported to mitochondria, where it is converted to acetyl CoA by the pyruvate dehydrogenase complex (PDH) through oxidative decarboxylation, an irreversible reaction requiring TPP, FAD, NAD⁺, and lipoamide.
• The reaction is: Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺, linking glycolysis to the citric acid cycle.

Citric Acid Cycle

• The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, oxidizes the acetyl moiety of acetyl CoA in mitochondria, reducing NAD⁺ and FAD for ATP production via the electron transport chain.
• It is the final common pathway for the oxidation of carbohydrates, lipids, and proteins, as glucose, fatty acids, and most amino acids are metabolized to acetyl CoA or TCA cycle intermediates.
• In eukaryotes, TCA cycle reactions occur in the mitochondrial matrix, with enzymes either free or attached to the inner mitochondrial membrane, near electron transport chain components.
• The cycle begins with the condensation of acetyl CoA (2-carbon) and oxaloacetate (4-carbon) to form citrate (6-carbon), catalyzed by citrate synthase.
• Citrate is isomerized to isocitrate via cis-aconitate by aconitase.
• Isocitrate is dehydrogenated to oxalosuccinate by isocitrate dehydrogenase, an NAD⁺-dependent enzyme (in mitochondria), producing NADH + H⁺.
• Isocitrate dehydrogenase exists in three forms: one NAD⁺-dependent (mitochondrial) and two NADP⁺-dependent (mitochondrial and cytosolic).
• Oxalosuccinate, a transient enzyme-bound intermediate, is decarboxylated to form α-ketoglutarate (5-carbon).
• α-ketoglutarate undergoes oxidative decarboxylation to succinyl CoA (4-carbon) by the α-ketoglutarate dehydrogenase complex, reducing NAD⁺ to NADH + H⁺ and releasing CO₂ in an irreversible reaction.
• Succinyl CoA is converted to succinate by succinyl CoA synthetase, phosphorylating GDP to GTP (which converts to ATP) via substrate-level phosphorylation.
• Succinate is oxidized to fumarate by succinate dehydrogenase, a FAD-containing enzyme attached to the inner mitochondrial membrane, reducing FAD to FADH₂.
• Fumarate is converted to malate by fumarase.
• Malate is oxidized to oxaloacetate by malate dehydrogenase, reducing NAD⁺ to NADH + H⁺.
• The cycle harvests high-energy electrons from acetyl CoA, forming NADH and FADH₂ for ATP production.
• The overall reaction is: Acetyl CoA + 3 NAD⁺ + FAD + GDP + Pi + 2 H₂O → 2 CO₂ + 3 NADH + 3 H⁺ + FADH₂ + GTP + CoA.

Electron Transport Chain

• The electron transport chain (ETC) oxidizes reduced coenzymes (NADH + H⁺ and FADH₂) from the TCA cycle and other catabolic pathways, located in the inner mitochondrial membrane.
• Electrons from NADH and FADH₂ flow through membrane proteins, generating a proton gradient used for ATP synthesis via oxidative phosphorylation.
• Oxygen acts as the final electron acceptor, forming H₂O and regenerating NAD⁺ and FAD.
• The ETC consists of four large protein complexes: NADH-Q-oxidoreductase (Complex I), succinate-Q-reductase (Complex II), Q-cytochrome C oxidoreductase (Complex III), and cytochrome C oxidase (Complex IV).
• These complexes contain oxidation-reduction centers like quinones, flavins, iron-sulfur clusters, heme, and copper ions.
• Complex II does not pump protons, unlike the other three complexes.
• Electrons from Complex I are transferred to Complex III via reduced coenzyme Q (ubiquinone), a lipid-soluble quinone that diffuses within the inner mitochondrial membrane.
• Electrons from FADH₂ enter via Complex II, then transfer to Complex III, and finally to Complex IV, which reduces O₂ to H₂O.
• Electron flow through Complexes I, III, and IV pumps protons from the mitochondrial matrix to the cytosolic side, creating a proton motive force (pH gradient and electrical potential).
• Protons flow back to the matrix through ATP synthase, driving ATP synthesis from ADP and inorganic phosphate.
• Oxidation of one NADH yields approximately 2.5 ATP, while one FADH₂ yields 1.5 ATP, as FADH₂ electrons enter at Complex III.
• The TCA cycle and oxidative phosphorylation in mitochondria are the primary ATP sources in aerobic organisms.

Photosynthesis

• Photosynthesis converts solar energy into chemical energy, with the overall reaction: 6 CO₂ + 12 H₂O → C₆H₁₂O₆ + 6 O₂ + 6 H₂O, catalyzed by chlorophyll in chloroplasts of plants and some photosynthetic bacteria.
• Light energy is captured by photoreceptor pigments (chlorophyll a, xanthophylls, carotenoids) to generate high-energy electrons.
• These electrons produce ATP and NADPH + H⁺ via light reactions, which are used in the Calvin cycle (dark reactions) to reduce CO₂ to 3-phosphoglycerate, forming carbohydrates.

Light Reactions

• Light reactions, or the photochemical phase, occur in the thylakoid membranes of chloroplasts and involve capturing solar energy, splitting water to release oxygen, and forming ATP and NADPH + H⁺.
• The thylakoid membrane contains light-harvesting proteins, reaction centers, electron transport chains, and ATP synthase.
• Two photosystems, Photosystem I (PSI) and Photosystem II (PSII), operate at different wavelengths: PSI at 700 nm and PSII at 680 nm.
• PSI contains 13 polypeptide chains, ~60 chlorophyll molecules, a quinone (vitamin K), and three 4Fe-4S clusters, with a molecular mass of 800 kDa.
• PSII has ~10 polypeptide chains, ~30 chlorophyll molecules, a non-heme iron ion, and four manganese ions.
• PSII is activated first, capturing 680 nm light to excite an electron, which enters the electron transport chain (ETC) via pheophytin, plastoquinone (PQ), cytochrome b₆f complex, plastocyanin (PC), and PSI.
• PSI accepts electrons from PC when its reaction center is excited by 700 nm light, transferring electrons to a modified chlorophyll, phylloquinone, and ferredoxin (Fd) via iron-sulfur proteins, reducing NADP⁺ to NADPH + H⁺ in the stroma by ferredoxin NADP-reductase (FNR).
• PSII’s positive charge (P680⁺) oxidizes water in the thylakoid lumen, catalyzed by the oxygen-evolving complex: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂, generating one O₂ per four electrons transferred.
• Proton transfer during electron movement (e.g., via plastoquinone) creates a proton gradient across the thylakoid membrane, with higher proton concentration in the lumen.
• Protons flow back to the stroma through ATP synthase’s CF₀ channel, synthesizing ATP on the CF₁ subunit from ADP and Pi, a process called non-cyclic photophosphorylation.
• In cyclic photophosphorylation, only PSI operates, with electrons cycling from PSI to phylloquinone, cytochrome b₆f, plastocyanin, and back to PSI, producing ATP but not NADPH or O₂.

Dark Reactions or Calvin Cycle

• Dark reactions, or the Calvin cycle, occur in the chloroplast stroma (or cyanobacterial cytosol), using ATP and NADPH + H⁺ from light reactions to convert CO₂ into sugars via CO₂ assimilation or fixation.
• The Calvin cycle, elucidated by Melvin Calvin, is a cyclic pathway with three stages: CO₂ fixation, reduction, and regeneration.
• Stage I: CO₂ fixation involves condensing three CO₂ molecules with three ribulose-1,5-bisphosphate (5-carbon) molecules to form six 3-phosphoglycerate (PGA, 3-carbon) molecules, catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).
• RuBisCO has both carboxylase and oxygenase activities, and since PGA is a 3-carbon molecule, the cycle is also called the C3 cycle.
• Stage II: Reduction converts six PGA molecules to six glyceraldehyde-3-phosphate molecules using six ATP (to form 1,3-bisphosphoglycerate) and six NADPH + H⁺ (to reduce 1,3-bisphosphoglycerate).
• Stage III: Regeneration diverts one glyceraldehyde-3-phosphate for glycolysis, starch, or sugar synthesis, while five glyceraldehyde-3-phosphate molecules (15 carbons) are converted to three ribulose-5-phosphate molecules, releasing two inorganic phosphates.
• Three ribulose-5-phosphate molecules are phosphorylated to ribulose-1,5-bisphosphate using three ATP, completing the cycle.

C4 Pathway

• The C4 pathway, followed by plants like maize, sorghum, and sugarcane, enhances photosynthetic efficiency by ~50% compared to C3 plants.
• C4 plants exhibit Kranz anatomy, with mesophyll cells clustered around bundle sheath cells, which have more chloroplasts.
• In mesophyll cells, phosphoenolpyruvate (PEP, 3-carbon) accepts CO₂, catalyzed by PEP carboxylase, forming oxaloacetate (OAA, 4-carbon), hence the name C4 cycle.
• OAA is converted to malate by NADP-malate dehydrogenase and transported to bundle sheath cells.
• In bundle sheath cells, malate is decarboxylated to release CO₂ and pyruvate (3-carbon), with pyruvate returning to mesophyll cells to regenerate PEP.
• Released CO₂ in bundle sheath cells is fixed by RuBisCO for the C3 pathway (Calvin cycle).
• Bundle sheath cells are rich in RuBisCO but lack PEP carboxylase, while mesophyll cells have PEP carboxylase but lack RuBisCO.
• In C3 plants, RuBisCO in mesophyll cells is the first CO₂ acceptor, whereas in C4 plants, PEP carboxylase in mesophyll cells is the first CO₂ acceptor.

Cell Cycle

• All living organisms originate from a single cell, which grows and divides to produce new cells, a fundamental process for survival, as failure to divide leads to cell death.
• Cell division produces two daughter cells with the same genetic makeup as the parent cell, enabling the formation of trillions of cells from a single cell.
• The cell cycle consists of interphase (preparation for division) and the mitotic (M) phase (division), with specific tasks accomplished in each phase.

Phases of Cell Cycle

• Interphase, sometimes called the resting phase, is when the cell prepares for replication and is divided into G1 (Gap 1), S (Synthesis), and G2 (Gap 2) phases.
• G1 Phase: The cell is metabolically active, grows larger, and accumulates RNA, organelles, and molecular building blocks, but does not replicate DNA.
• S Phase: DNA synthesis occurs in the nucleus, doubling the DNA content from 2C to 4C, while chromosome number remains unchanged (e.g., a diploid cell remains 2n).
• Centrioles, microtubule-organizing structures, are duplicated in the cytoplasm during S phase for chromosome segregation.
• G2 Phase: The cell synthesizes more proteins and organelles, further growing and preparing for the M phase.
• Some cells, like cardiac cells and neurons, exit G1 to enter a quiescent stage (G0), becoming differentiated and incapable of division.
• The M phase includes karyokinesis (nuclear division) and cytokinesis (cytoplasmic division), producing two daughter cells with equal chromosome numbers (equational division).
• Karyokinesis is divided into four stages: prophase, metaphase, anaphase, and telophase.
• Prophase: Chromatin condenses into chromosomes, centrioles move to opposite poles to form the mitotic spindle, and organelles like the Golgi apparatus, endoplasmic reticulum, nucleolus, and nuclear membrane disappear.
• Metaphase: The nuclear envelope disintegrates, chromosomes condense fully, align at the metaphase plate, and sister chromatids (held at the centromere) attach to spindle fibers via kinetochores from opposite poles.
• Anaphase: Sister chromatids are pulled apart toward opposite poles.
• Telophase: Chromosomes reach the poles, decondense, and the nuclear envelope reforms.

Cytokinesis

• Cytokinesis is the physical division of the parent cell’s cytoplasm into two daughter cells, following karyokinesis.
• In animal cells, cytokinesis begins with a plasma membrane furrow that deepens, forming a contractile ring of actin filaments, which divides the cytoplasm equally.
• In plant cells, an inextensible cell wall prevents furrow formation; instead, a cell plate forms as a precursor to a new cell wall, synthesized during cytokinesis.
• Cytokinesis ensures equal distribution of cell organelles into the daughter cells.

Meiosis

• Meiosis produces haploid gametes from diploid cells, which fuse during fertilization to form diploid offspring, and occurs during gametogenesis in plants.
• It involves one interphase followed by two nuclear divisions: Meiosis I (reductional division, halving chromosome number) and Meiosis II (equational division, maintaining chromosome number).
• Meiosis results in four haploid cells and differs from mitosis in that chromosome number is halved, and recombination shuffles genes between homologous chromosomes.
• Meiosis I and II each include prophase, metaphase, anaphase, and telophase, denoted as Prophase I, Metaphase I, etc., for Meiosis I, and Prophase II, Metaphase II, etc., for Meiosis II.
• Prophase I, the longest phase, is divided into five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• Leptotene: Chromosomes appear as thin threads, each with two sister chromatids, visible under a light microscope.
• Zygotene: Homologous chromosomes pair via the synaptonemal complex, forming bivalents or tetrads.
• Pachytene: Recombinational nodules appear, facilitating crossing over between non-sister chromatids of homologous chromosomes, leading to genetic recombination.
• Diplotene: The synaptonemal complex degenerates, and homologous chromosomes separate except at crossover sites (chiasmata), which can persist for months or years in some vertebrate oocytes.
• Diakinesis: Chiasmata terminalize, chromosomes fully condense, the meiotic spindle assembles, and the nuclear envelope and nucleolus disappear.
• Metaphase I: Bivalents align on the equatorial plate, with microtubules from opposite spindle poles attaching to the kinetochores of homologous chromosomes.
• Anaphase I: Homologous chromosomes separate, but sister chromatids remain attached at their centromeres.
• Telophase I and Cytokinesis: The nuclear envelope and nucleolus reform, followed by cytokinesis, with a short interkinesis phase (no DNA replication) between Meiosis I and II.
• Meiosis II resembles mitosis: chromosomes condense, align, separate as sister chromatids, and form four haploid cells after telophase II and cytokinesis.
• Meiosis ensures consistent chromosome numbers in sexually reproducing organisms, maintains species stability, generates genetic variation via crossing over, and produces genetically distinct daughter cells.

Programmed Cell Death (Apoptosis)

• Apoptosis, or programmed cell death, is a controlled, energy-dependent process critical during development, such as forming gaps between fingers to prevent webbed hands.
• It is highly regulated, irreversible, and essential for normal development and protection against diseases like viral infections and cancers.
• Inefficient apoptosis can lead to unregulated cell growth and division, potentially causing cancer.
• Apoptosis involves morphological and physiological changes, including DNA fragmentation, plasma membrane blebbing, nuclear envelope breakdown, and increased DNA susceptibility to deoxyribonuclease (DNAase).
• The cell splits into small membrane-enclosed fragments, which are phagocytosed by macrophages.
• Unlike apoptosis, necrosis is an unregulated, traumatic cell death caused by injury or toxic exposure, involving inflammation and release of cell contents without DNA fragmentation or membrane blebbing.

Cell Differentiation

• Multicellular organisms have trillions of cells with identical DNA, but different cell groups have distinct structural and functional roles due to differential gene expression.
• For example, neurons conduct impulses, while red blood cells (RBCs) transport oxygen, determined by unique gene expression patterns.
• Cell differentiation is the process by which unspecialized cells become specialized, acquiring specific properties during embryonic development.
• Examples of differentiated cells include epithelial cells, RBCs, white blood cells (WBCs), cardiac cells, neurons, and muscle cells.
• Not all cells differentiate; stem cells, found in embryos and adult tissues, remain unspecialized and divide to produce one stem cell and one differentiated cell.
• Embryonic stem cells, derived from early blastocysts (about a week old), are extracted from the inner cell mass and cultured to divide without differentiating unless stimulated.
• Adult stem cells, found in tissues like bone marrow, blood vessels, brain, skeletal muscles, and liver, remain quiescent until activated by tissue injury.
• Differentiation is controlled by signaling molecules, primarily growth factors, which facilitate cell communication.
• Stem cell potency, the ability to differentiate into other cell types, classifies stem cells into totipotent, pluripotent, and multipotent categories.
• Totipotent cells, like zygotes and asexual spores, can differentiate into all cell types, including embryonic and extraembryonic tissues, exhibiting the highest differentiation potential.
• Pluripotent cells, such as those from the inner cell mass, differentiate into most body tissues (endoderm, mesoderm, ectoderm) but not extraembryonic tissues like the placenta.
• Multipotent cells, like hematopoietic stem cells (HSCs), have limited differentiation potential, producing specific cell types such as RBCs, WBCs, and platelets.
• Induced pluripotent stem cells (iPSCs), developed in 2006 by Shinya Yamanaka, are adult cells reprogrammed to pluripotency using transcription factors, bypassing ethical issues with embryonic stem cells.
• iPSCs model diseases like cancer and congenital heart disease and produce patient-matched cells to avoid transplant rejection.

Cell Migration

• Cell migration is the movement of cells from one location to another, occurring in unicellular organisms (e.g., amoeba) and multicellular organisms (e.g., mammals) across various environments.
• It is critical during embryogenesis, organogenesis, regeneration, feeding, and immune responses.
• Cells migrate as single units or in groups, influenced by intrinsic factors like cytoskeleton organization, extracellular matrix, adhesion strength, and migratory signals.
• Migration involves coordinated signaling networks with four stages: polarization, protrusion and adhesion, translocation, and rear retraction.
• Polarization establishes a cell front and rear, driven by plasma membrane specialization and directional signals (chemical, electric, mechanical, or substrate-based), with actin filament branching facilitating leading-edge extension.
• Protrusion forms membrane extensions (e.g., pseudopodia) in the migration direction, involving plasma membrane spreading, a core backbone for support, and substrate contact for foothold and actin polymerization signaling.
• Protrusions adhere to the substrate, and the lagging edge detaches and retracts into the cell body.
• Adhesions are molecular communication sites between the cell and substrate, assembling at the leading edge and disassembling at the trailing edge in response to extracellular signals.
• In motile cells, adhesions in protruding areas may stabilize into mature adhesions, while highly motile cells lack well-built adhesions.
• Rear adhesion disassembly, via phosphatase phosphorylation or protease-mediated proteolysis, prevents cell tearing under tension.
• Cell migration is essential for embryogenesis (forming embryonic layers and organs), homeostasis (tissue repair, inflammation), and pathological processes like cancer metastasis.
• During gastrulation, cell migration forms embryonic layers (endoderm, mesoderm, ectoderm) and organs, with cells differentiating at their destinations.
• In homeostasis, immune cells migrate from lymph nodes to circulation, responding to injury or infection, and failure to migrate can cause autoimmune diseases or defective wound repair.
• In cancer, loss of cell-cell interactions and increased motility enable tumor cells to invade and spread from the primary site (metastasis).