<h5 id="51-cell-signaling">5.1 Cell Signaling</h5> <ul> <li>Cells receive and interpret environmental cues such as light, heat, sound, and touch, which influence cell fates during development through signal transduction pathways.</li> <li>A receptor is a glycoprotein located on the cell surface, cytoplasm, or nucleus, which binds to a specific ligand, a chemical messenger, to initiate a message relay system and bring about changes in the cell.</li> <li>Cells communicate over short distances through paracrine signaling, where a chemical message released in the extracellular space by sender cells is sensed by recipient cells instantly.</li> <li>Autocrine signaling occurs when a cell that secretes a ligand also possesses receptors specific for that ligand, allowing it to respond to its own signals.</li> <li>Endocrine signaling is long-distance signaling that requires the ligand to be synthesized by the cell in the extracellular space, travel to the bloodstream, and reach the recipient or target cell, as exhibited by hormones.</li> </ul> <p>======</p> <h5 id="52-metabolic-pathways">5.2 Metabolic Pathways</h5> <ul> <li>Metabolism is the process by which organisms utilize free energy to carry out life processes.</li> <li>Phototrophs convert light energy into chemical energy, while chemotrophs obtain energy by oxidizing electron donors.</li> <li>Central to energy transactions in metabolism is the energy currency ATP.</li> <li>Energy obtained is used for various cellular processes such as creating gradients, converting chemical energy into mechanical energy, and powering reactions that result in biomolecule synthesis.</li> <li>Metabolic pathways, classified into anabolic and catabolic pathways, are a series of interlinked biochemical reactions that synthesize and breakdown biomolecules.</li> </ul> <p>======</p> <h5 id="i-anabolic-pathways">(i) Anabolic pathways</h5> <ul> <li>Anabolic pathways are responsible for synthesizing larger and more complex molecules from smaller ones.</li> <li>These pathways are endergonic, meaning they require energy input.</li> <li>Anabolic reactions, also known as anabolism, include the synthesis of glucose, fats, proteins, and DNA.</li> <li>The overall reaction for anabolism can be represented as: Useful energy + Small molecules → Complex Molecules</li> <li>Anabolic pathways are essential for growth, maintenance, and reproduction in living organisms.</li> </ul> <p>======</p> <h5 id="ii-catabolic-pathways">(ii) Catabolic pathways</h5> <ul> <li>Catabolic pathways involve the breakdown of larger molecules such as carbohydrates, proteins, and fats.</li> <li>These are exergonic reactions that release energy and produce reducing equivalents and ATP.</li> <li>The useful forms of energy produced in catabolism are utilized in anabolism.</li> <li>Anabolism generates complex structures from simple ones or energy-rich states from energy-poor ones.</li> <li>The reaction for catabolism can be represented as: Fuel (carbohydrate, protein, fats) $$\downarrow$$ Catabolism $$\rightarrow{\mathrm{CO} _2+\mathrm{H} _2 \mathrm{O}}+\text{Useful energy}$$</li> </ul> <p>======</p> <h5 id="521-overview-of-carbohydrate-metabolism">5.2.1 Overview of carbohydrate metabolism</h5> <ul> <li>Glucose, the primary energy source in animals, is metabolized into pyruvate through glycolysis and further broken down through the citric acid cycle to complete oxidation into $\mathrm{CO} _{2}$ and $\mathrm{H} _{2} \mathrm{O}$.</li> <li>In anaerobic conditions, pyruvate is converted into lactic acid.</li> <li>Glycolysis intermediates contribute to various metabolic processes, including glycogen synthesis and storage, pentose phosphate pathway, fatty acid synthesis, and amino acid synthesis.</li> <li>Acetyl CoA, a glycolysis and citric acid cycle intermediate, is a precursor for fatty acids, cholesterol, and other steroids in animals.</li> <li>During starvation, non-carbohydrate precursors like lactic acid, amino acids, and glycerol can synthesize glucose through gluconeogenesis when glycogen reserves are depleted.</li> </ul> <p>======</p> <h5 id="522-overview-of-lipid-metabolism">5.2.2 Overview of lipid metabolism</h5> <ul> <li>Some tissues depend exclusively on glucose for fuel, while others can use long-chain fatty acids when glucose is limited.</li> <li>These fatty acids can be taken from the diet or synthesized from acetyl CoA derived from carbohydrate or amino acids.</li> <li>Acetyl CoA formed by the $\beta$-oxidation pathway can be oxidized to $\mathrm{CO} _{2}$ and $\mathrm{H} _{2} \mathrm{O}$ through the citric acid cycle.</li> <li>It can also be a precursor for the synthesis of other lipids such as cholesterol, which is used to synthesize all other steroids.</li> <li>Additionally, acetyl CoA can be used to synthesize ketone bodies, which serve as an alternative fuel for liver and some other tissues in prolonged fasting.</li> </ul> <p>======</p> <h5 id="523-overview-of-amino-acid-metabolism">5.2.3 Overview of amino acid metabolism</h5> <ul> <li>Amino acids are essential for protein synthesis, with 20 standard types.</li> <li>Nonessential amino acids are synthesized in the body through transamination, while essential amino acids must be obtained through diet.</li> <li>Transamination involves the transfer of amino nitrogen to a carbon skeleton to form other amino acids, with the remaining carbon skeletons playing roles in oxidization, glucose synthesis, and ketone body formation.</li> <li>Amino acids can also contribute to the synthesis of other biomolecules, such as hormones, purines, pyrimidines, and neurotransmitters.</li> <li>Important metabolic pathways include oxidization via the citric acid cycle, glucose synthesis through gluconeogenesis, and ketone body formation for energy or fatty acid synthesis.</li> </ul> <p>======</p> <h5 id="524-glycolysis">5.2.4 Glycolysis</h5> <ul> <li>Glycolysis, also known as the Embden-Meyerhof-Parnas pathway, is a universal catalytic pathway in all living cells.</li> <li>It is a major pathway of carbohydrate metabolism, starting with the phosphorylation of glucose to glucose6-phosphate by enzyme hexokinase. This reaction is irreversible and inhibited by its product, glucose-6-phosphate.</li> <li>Glucose-6-phosphate is an important intermediate in carbohydrate metabolism, leading to the formation of fructose6-phosphate by enzyme phosphoglucose isomerase. Fructose-16-bisphosphate is then formed by enzyme phosphofructokinase (PFK).</li> <li>Oxidations and phosphorylations occur, forming ATP and 3-phosphoglycerate by enzymes glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and pyruvate kinase.</li> <li>Three reactions are exergonic and irreversible, catalyzed by regulatory enzymes hexokinase, PFK, and pyruvate kinase, making them major sites of regulation of glycolysis.</li> </ul> <p>======</p> <h5 id="energy-yield-by-glycolysis">Energy yield by glycolysis</h5> <ul> <li>The net reaction of glycolysis involves the conversion of one glucose molecule into two pyruvate molecules.</li> <li>Four ATP molecules are generated from this conversion, but two ATP molecules are used in the process.</li> <li>The net ATP production during glycolysis is therefore two molecules.</li> <li>NAD+ is converted into NADH and H+, and water is also produced.</li> <li>The equations provided outline the key reactants and products in the glycolysis process.</li> </ul> <p>======</p> <h5 id="fate-of-pyruvate">Fate of pyruvate</h5> <ul> <li>The fate of pyruvate, a product of glycolytic reactions, varies in different organisms and cell types.</li> <li>Pyruvate can be converted into ethanol, a process that occurs in yeast and some bacterial species during fermentation.</li> <li>Another reaction is the conversion of pyruvate into lactic acid, which takes place in certain bacteria and in muscle cells during strenuous exercise.</li> <li>Pyruvate can also be converted into carbon dioxide, which is a part of the citric acid cycle in aerobic organisms.</li> <li>The process of pyruvate’s conversion into ethanol or lactic acid is anaerobic, while its conversion into carbon dioxide is aerobic.</li> </ul> <p>======</p> <h5 id="fermentation-anaerobic-breakdown-of-pyruvate">Fermentation (anaerobic breakdown of pyruvate)</h5> <ul> <li>Glycolysis, the initial stage of cellular respiration, requires $\mathrm{NAD}^{+}$ to continue, which is reduced to NADH.</li> <li>Under anaerobic conditions, the limited supply of $\mathrm{NAD}^{+}$ is recycled through the anaerobic breakdown of pyruvate.</li> <li>Pyruvate is reduced to $\mathrm{NAD}^{+}$ in an extension of the glycolytic pathway, leading to two processes: homolactic fermentation and alcoholic fermentation.</li> <li>Homolactic fermentation occurs in muscle, where pyruvate is converted back to $\mathrm{NAD}^{+}$ and lactate.</li> <li>Alcoholic fermentation takes place in yeast, in which pyruvate is converted to $\mathrm{NAD}^{+}$, carbon dioxide, and ethanol.</li> </ul> <p>======</p> <h5 id="homolactic-fermentation">Homolactic fermentation</h5> <ul> <li>Homolactic fermentation is a type of anaerobic fermentation where pyruvate is reduced to lactate by the enzyme lactate dehydrogenase.</li> <li>Lactate dehydrogenase is an isozyme, which means it can exist in multiple forms with different properties.</li> <li>The overall process of anaerobic glycolysis in muscle can be represented by the equation: Glucose + 2 ADP + 2 Pi → Lactate + 2 ATP + 2 H2O + 2 H+</li> <li>During this process, much of the lactate produced by glycolysis is exported from muscle cells to the liver where it is reconverted to glucose.</li> <li>This process helps to maintain a balance between the energy needs of the muscle and the liver, allowing for continued muscle function under anaerobic conditions.</li> </ul> <p>======</p> <h5 id="alcoholic-fermentation">Alcoholic fermentation</h5> <ul> <li>Alcoholic fermentation in yeast occurs under anaerobic conditions.</li> <li>The process involves the conversion of pyruvate to ethanol and $\mathrm{CO} _{2}$, regenerating $\mathrm{NAD}^{+}$.</li> <li>The reaction takes place in two steps: decarboxylation of pyruvate to acetaldehyde and $\mathrm{CO} _{2}$ by pyruvate decarboxylase (with coenzyme TPP), followed by reduction of acetaldehyde to ethanol by alcohol dehydrogenase (oxidizing NADH to $\mathrm{NAD}^{+}$).</li> <li>Fermentation results in the production of 2 ATP per glucose molecule.</li> <li>The enzyme pyruvate decarboxylase and the coenzyme TPP play crucial roles in this process.</li> </ul> <p>======</p> <h5 id="aerobic-breakdown-of-pyruvate">Aerobic breakdown of pyruvate</h5> <ul> <li>Aerobic breakdown of pyruvate involves the transfer of pyruvate from the cytosol to the mitochondria, where it is converted to acetyl CoA by the pyruvate dehydrogenase complex (PDH).</li> <li>The reaction is catalyzed by PDH and is irreversible. It requires coenzymes and prosthetic groups, including TPP, FAD, NAD+, and lipoamide.</li> <li>The equation for this reaction is: Pyruvate + CoA + NAD+ → AcetylCoA + CO2 + NADH + H+</li> <li>This reaction is the link between glycolysis and the citric acid cycle.</li> <li>The conversion of pyruvate to acetyl CoA is a key step in the aerobic respiration process, which generates energy in the form of ATP.</li> </ul> <p>======</p> <h5 id="525-citric-acid-cycle">5.2.5 Citric acid cycle</h5> <ul> <li>The citric acid cycle, also known as Kreb’s cycle or TCA cycle, is a sequence of reactions in mitochondria that oxidize the acetyl moiety of acetyl CoA and reduce NAD+, which are then reoxidized through the electron transport chain to form ATP.</li> <li>The cycle begins with a condensation reaction between acetyl CoA and oxaloacetate to form citrate, followed by a series of reactions and enzymes that oxidize citrate into isocitrate, oxalosuccinate, a-ketoglutarate, succinyl CoA, succinate, fumarate, and malate, before forming oxaloacetate again.</li> <li>The cycle is the final common pathway for the oxidation of carbohydrates, lipids, and proteins, and it oxidizes isocitrate via a NAD+ dependent enzyme, resulting in the formation of NADH+H+ and CO2.</li> <li>The cycle harvests high energy electrons from acetyl CoA and uses them to form NADH and FADH2, which are then used in oxidative phosphorylation to generate a proton gradient across the membrane and synthesize ATP from ADP and inorganic phosphate.</li> <li>The overall reaction of TCA cycle is Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O -> 2 CO2 + 3 NADH + 3 H+ + FADH2 + GTP + CoA.</li> </ul> <p>======</p> <h5 id="atp-production-through-citric-acid-cycle">ATP production through citric acid cycle</h5> <ul> <li>The citric acid cycle, also known as the Krebs cycle, oxidizes one molecule of acetyl CoA into two molecules of carbon dioxide.</li> <li>During this process, three molecules of NAD+ are reduced to NADH, and one molecule of FAD is reduced to FADH2.</li> <li>Adenosine triphosphate (ATP) is generated in the form of guanosine triphosphate (GTP) or ATP.</li> <li>The cycle contributes to cellular energy production by regenerating essential coenzymes and producing high-energy phosphate bonds.</li> <li>The overall result is the conversion of chemical energy stored in acetyl CoA into energy stored in ATP, NADH, and FADH2.</li> </ul> <p>======</p> <h5 id="526-electron-transport-chain">5.2.6 Electron transport chain</h5> <ul> <li>The electron transport chain (ETC) is a system that oxidizes reduced coenzymes (NADH+H+ and FADH2) and passes on electrons to O2, resulting in the formation of H2O.</li> <li>ETC is located in the inner mitochondrial membrane and involves four large protein complexes (I, II, III, and IV) that are transmembrane protein complexes with multiple oxidation-reduction centers.</li> <li>Electrons are carried from complex I to III by the reduced form of coenzyme Q (ubiquinone), and from complex III to IV by cytochrome-C.</li> <li>Succinate-Q-reductase (complex II) does not pump protons, unlike the other three complexes.</li> <li>The passage of electrons through ETC is associated with the loss of free energy, which is utilized to generate ATP from ADP and Pi.</li> </ul> <p>======</p> <h5 id="oxidative-phosphorylation-atp-synthesis">Oxidative phosphorylation (ATP synthesis)</h5> <ul> <li>Oxidative phosphorylation, or ATP synthesis, involves the flow of electrons from NADH or FADH2 to O2 through four protein complexes in the mitochondrial inner membrane.</li> <li>This process pumps protons out of the mitochondrial matrix, creating a proton motive force (pmf) consisting of a pH gradient and a transmembrane electrical potential.</li> <li>ATP synthesis occurs when protons flow back to the matrix side through the ATP synthase enzyme complex, which is made up of two operational units: a rotatory component and a stationary component.</li> <li>The rotation of the gamma subunit in ATP synthase induces structural changes in the beta subunit, resulting in the synthesis and release of ATP.</li> <li>The flow of electrons through NADH-Q-oxidoreductase, Q-cytochrome-c-oxidoreductase, and cytochrome-c-oxidase can develop a gradient sufficient to synthesize 1, 0.5, and 1 molecule of ATP, respectively, for a total of 2.5 molecules of ATP per molecule of NADH oxidized or 1.5 molecules of ATP per molecule of FADH2 oxidized.</li> </ul> <p>======</p> <h5 id="photosynthesis">Photosynthesis</h5> <ul> <li>Photosynthesis is the process by which biological systems convert and trap solar energy into chemical energy.</li> <li>The basic reaction of photosynthesis is represented as $6 exttext{CO} _{2}+12 exttext{H} _{2} exttext{O} extoexto (CH exttext{2} exttext{O}) _{6}+6 exttext{O} _{2}+6 exttext{H} _{2} exttext{O}$, where $(CH exttext{2} exttext{O}) _{6}$ represents carbohydrate.</li> <li>Photosynthesis takes place in chloroplasts, and the energy of light captured by various photoreceptor pigment molecules, such as chlorophyll a, xanthophylls, carotenoids, and other chlorophyll, is used to generate high energy electrons with great reducing potential.</li> <li>The light reactions of photosynthesis generate ATP and $ exttext{NADPH}+ exttext{H}^{+}$, which are used in the production of $ exttext{NADPH}+ exttext{H}^{+}$ and the reduction of $ exttext{CO} _{2}$ into 3-phosphoglycerate by a series of reactions called the Calvin cycle or dark reactions.</li> <li>The Calvin cycle or dark reactions convert $ exttext{CO} _{2}$ into 3-phosphoglycerate, which is a precursor for glucose and other carbohydrates.</li> </ul> <p>======</p> <h5 id="light-reactions">Light Reactions</h5> <ul> <li>Light reactions, also known as the photochemical phase of photosynthesis, involve capturing solar energy, splitting water and releasing oxygen, and forming higher-energy organic intermediates (ATP and NADPH+H+).</li> <li>These reactions occur in the thylakoid membranes of chloroplasts, involving light harvesting proteins, reaction centers, electron transport chains, and ATP synthase.</li> <li>The two photosystems, PSI and PSII, respond to light at slightly different wavelengths (PSI at 700 nm and PSII at 680 nm). Under normal conditions, PSII is activated first, followed by electron transfer through the Electron Transport Chain (ETC).</li> <li>Water splitting, catalyzed by the oxygen evolving complex, generates one molecule of O2 for every four electrons sent from PSII to PSI through the ETC.</li> <li>ATP synthesis by non-cyclic photophosphorylation results from the development of a proton gradient across the membrane, driving protons to reenter the stroma through the CF0 proton channel of the ATP synthase, producing ATP and NADPH+H+.</li> <li>In certain cases, cyclic photophosphorylation occurs, where only PSI works, resulting in ATP formation without NADPH+H+ and O2 production.</li> </ul> <p>======</p> <h5 id="527-carbon-assimilation---dark-reactions-or-calvin-cycle">5.2.7 Carbon Assimilation - dark reactions or Calvin Cycle</h5> <ul> <li>The dark reactions of photosynthesis, also known as the Calvin Cycle, occur in the stroma of chloroplasts.</li> <li>These reactions utilize ATP and NADPH + H+ generated by the light-dependent reactions to convert CO2 into sugar.</li> <li>CO2 is fixed in a cyclic pathway called the Calvin Cycle, which was elucidated by Melvin Calvin and his co-workers in 1950.</li> <li>The CO2 fixation process can be understood in three stages: stage I - carbon fixation, stage II - reduction, and stage III - regeneration of the starting compound.</li> <li>The Calvin Cycle results in the synthesis of trioses and hexoses, which lead to the formation of sugar.</li> </ul> <p>======</p> <h5 id="stage-i-fixation-of-mathrmco-_2-into-3-phosphoglycerate">Stage I: Fixation of $\mathrm{CO} _{2}$ into 3-phosphoglycerate</h5> <ul> <li>The first stage of the Calvin cycle is the fixation of CO2 into a stable organic intermediate.</li> <li>This process is catalyzed by the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which has both carboxylase and oxygenase activities.</li> <li>Gaseous carbon dioxide gets condensed with five-carbon acceptor ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate (PGA).</li> <li>Three molecules of CO2 are fixed to three molecules of ribulose 1,5-bisphosphate to form six molecules of 3-phosphoglycerate (PGA).</li> <li>This carboxylation reaction is the first step in the Calvin cycle, which is the pathway used by plants, algae, and some bacteria to convert carbon dioxide into glucose and other sugars.</li> </ul> <p>======</p> <h5 id="stage-ii-reduction-of-3-phosphoglycerate-to-glyceraldehyde-3-phosphate">Stage II: Reduction of 3-phosphoglycerate to glyceraldehyde 3-phosphate</h5> <ul> <li>The reduction of 3-phosphoglycerate to glyceraldehyde 3-phosphate is a part of the process that leads to the formation of sugar in the given pathway.</li> <li>This process requires two molecules of ATP (Adenosine Triphosphate) and two molecules of NADPH+H+ for the reduction of one CO2 molecule.</li> <li>The reduction of one CO2 molecule results in the fixation of one CO2 molecule.</li> <li>For the fixation of six CO2 molecules, six turns of a complete cycle are required.</li> <li>One molecule of glucose is produced from this pathway for every six turns of the complete cycle.</li> </ul> <p>======</p> <h5 id="stage-iii-regeneration-of-ribulose1-5-bisphosphate-from-triose">Stage III: Regeneration of ribulose1, 5-bisphosphate from triose</h5> <ul> <li>The objective is to regenerate ribulose1, 5-bisphosphate from triose phosphates for continuous CO2 assimilation in carbohydrate production.</li> <li>This process involves a series of reactions.</li> <li>One ATP is utilized for phosphorylation to form ribulose 1, 5-bisphosphate.</li> <li>Ribulose1, 5-bisphosphate regeneration is crucial for maintaining the flow of CO2 into carbohydrate.</li> <li>The phosphorylation process helps convert ribulose into ribulose 1, 5-bisphosphate.</li> </ul> <p>======</p> <h5 id="528-the-mathrmc-_4-pathway">5.2.8 The $\mathrm{C} _{4}$ pathway</h5> <ul> <li>The $\mathrm{C} _{4}$ pathway involves a different anatomy of kranz in some plants, with bundle sheath cells around the vascular bundles (seen in maize).</li> <li>Bundle sheath cells have thick walls, numerous chloroplasts, and no intercellular spaces.</li> <li>In $\mathrm{C} _{4}$ plants, $\mathrm{CO} _{2}$ is first fixed by PEP carboxylase in mesophyll cells, forming a 4-carbon acid, which is then transported to bundle sheath cells.</li> <li>In bundle sheath cells, the 4-carbon acid is broken down to release $\mathrm{CO} _{2}$, which is accepted by RuBisCO for the $\mathrm{C} _{3}$ pathway or Calvin cycle.</li> <li>In $\mathrm{C} _{4}$ plants, RuBisCO is present in bundle sheath cells, while PEP carboxylase is found in mesophyll cells; this is the reverse in $\mathrm{C} _{3}$ plants.</li> </ul> <p>======</p> <h5 id="53-cell-cycle">5.3 Cell Cycle</h5> <ul> <li>Cell division is a process of cell growth and reproduction, common to all living organisms.</li> <li>It results in two daughter cells with the same genetic makeup as the parent cell.</li> <li>These daughter cells can further divide, leading to the creation of new cells.</li> <li>This ability of cells to divide is crucial, as it allows for the existence of trillions of cells in the human body, starting from a single cell at birth.</li> <li>The inability of cells to divide would lead to their death.</li> </ul> <p>======</p> <h5 id="531-phases-of-cell-cycle">5.3.1 Phases of cell cycle</h5> <ul> <li>The cell cycle is a series of tasks that a cell must complete to divide, including growth, replication of genetic material, and physical splitting into two daughter cells.</li> <li>The cell cycle is controlled by genetics and typically takes about 24 hours, although this can vary between organisms.</li> <li>It is divided into two major phases: Interphase and Mitotic (M) Phase.</li> <li>Interphase is characterized by cell growth and replication of DNA content. It is the phase between two successive M phases.</li> <li>During the Mitotic (M) Phase, the cell separates its DNA content (replicated during interphase) and cytoplasm, ensuring both daughter cells receive the same amount of DNA at the end of the cell cycle.</li> </ul> <p>======</p> <h5 id="interphase">Interphase</h5> <ul> <li>Interphase is the initial stage of the cell cycle, divided into G1, S, and G2 phases.</li> <li>During <strong>G1 phase</strong>, the cell is metabolically active, growing in size without replicating genetic content.</li> <li>In the <strong>S phase</strong>, DNA synthesis occurs, doubling the DNA content from 2C to 4C while keeping the chromosome number constant. Centrioles are also duplicated in the cytoplasm.</li> <li><strong>G2 phase</strong> involves the synthesis of additional proteins and organelles to prepare the cell for M phase.</li> <li>Some cell types, like differentiated cells (e.g., cardiac cells, neurons), exit the cell cycle and enter a quiescent stage (G0) where they cannot undergo division.</li> </ul> <p>======</p> <h5 id="mitotic-m-phase">Mitotic (M) Phase</h5> <ul> <li>Mitosis is a phase in the cell cycle where chromosomes are condensed and segregated equally into two daughter cells, referred to as equational division.</li> <li>The four stages of mitosis are:</li> </ul> <ol> <li>Prophase: Chromatin condenses into chromosomes, centrioles move towards opposite poles, and cell organelles begin to disappear.</li> <li>Metaphase: Nuclear envelope disintegrates, chromosomes align at the equator, and sister chromatids are held together by centromeres with kinetochores serving as sites of attachment to spindle fibers.</li> <li>Anaphase: Chromosomes are pulled apart towards opposite poles by spindle fibers radiating from centrosomes.</li> <li>Telophase: Nuclear envelope, endoplasmic reticulum, and Golgi apparatus reform, and chromosomes decondense and reach their respective poles.</li> </ol> <p>======</p> <h5 id="532-cytokinesis">5.3.2 Cytokinesis</h5> <ul> <li>Cytokinesis is the physical process of dividing the cytoplasm of a parent cell into two daughter cells.</li> <li>It occurs after karyokinesis (nuclear division) and is marked by the formation of a furrow in the plasma membrane.</li> <li>The furrow deepens and ultimately forms a contractile ring, which is composed of actin filaments.</li> <li>In plant cells, cytokinesis involves the synthesis of a new cell wall, initiated by the formation of a cell plate, due to the inextensible cell wall.</li> <li>The process is characterized by the equal distribution of cell organelles into the two daughter cells.</li> </ul> <p>======</p> <h5 id="533-meiosis">5.3.3 Meiosis</h5> <ul> <li>Meiosis is a process of producing haploid gametes from diploid cells.</li> <li>It consists of a single interphase, followed by Meiosis I and Meiosis II.</li> <li>Meiosis I is the reductional division, reducing chromosome number to half.</li> <li>Meiosis II is equational, similar to mitosis, with no change in chromosome number.</li> <li>Prophase I, the longest phase, has five substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.</li> <li>Crossing over, a genetic recombination, occurs in Pachytene between non-sister chromatids of homologous chromosomes.</li> </ul> <p>======</p> <h5 id="_metaphase-i_"><em><strong>Metaphase I</strong></em></h5> <ul> <li>Metaphase I is a stage in Meiosis where bivalents align on the equatorial plate.</li> <li>This alignment is facilitated by microtubules from opposite poles of the spindle attached to the kinetochore of homologous chromosomes.</li> <li>The attachment of microtubules creates tension that straightens and aligns the chromosomes on the metaphase plate.</li> <li>Metaphase I is followed by Anaphase I, where homologous chromosomes are separated.</li> <li>The process ensures proper segregation of genetic information, maintaining diploidy in sexually reproducing organisms.</li> </ul> <p>======</p> <h5 id="_anaphase-i_"><em><strong>Anaphase I</strong></em></h5> <ul> <li>Anaphase I is a phase of meiosis I.</li> <li>It is characterized by the separation of homologous chromosomes.</li> <li>Sister chromatids do not separate at this stage; they remain attached at their centromeres.</li> <li>The separated homologous chromosomes move towards opposite poles of the cell.</li> <li>This stage sets the basis for genetic diversity as each future gamete will have a unique combination of chromosomes.</li> </ul> <p>======</p> <h5 id="_telophase-i-and-cytokinesis_"><em><strong>Telophase I and cytokinesis</strong></em></h5> <ul> <li>Telophase I is the final stage of Meiosis I, characterized by the reappearance of the nuclear envelope and nucleolus.</li> <li>This stage is followed by cytokinesis, the process of dividing the cytoplasm and cell body.</li> <li>Interkinesis, a short gap between the two meiotic divisions, occurs next. It does not involve another round of DNA replication.</li> <li>Meiosis II then begins, which is similar to Mitosis with the exception of having only one round of DNA replication.</li> <li>The stages of Meiosis I and Meiosis II are illustrated in Fig. 5.18.</li> </ul> <p>======</p> <h5 id="_meiosis-ii_"><em><strong>Meiosis II</strong></em></h5> <ul> <li>In Prophase II, the nuclear envelope breaks down and the nucleolus disappears, followed by another compaction of chromosomes.</li> <li>During Metaphase II, chromosomes get attached at the centromeres containing kinetochores and align at the equator.</li> <li>At Anaphase II, detachment of centromeres of each chromosome occurs, leading to the movement of detached chromosomes towards opposite poles.</li> <li>Telophase II and cytokinesis mark the end of the meiotic cycle, with two sets of chromosomes enclosed by a newly formed nuclear membrane and then undergoing cytokinesis.</li> <li>The final result of Meiosis II is the formation of four haploid cells.</li> </ul> <p>======</p> <h5 id="534-significance-of-meiosis">5.3.4 Significance of meiosis</h5> <ul> <li>Mitosis and meiosis are processes of cell division, with mitosis occurring in both sexual and asexual organisms, and meiosis only in sexual organisms.</li> <li>Mitosis occurs in somatic cells, with one nuclear division and DNA replication at interphase I, while meiosis occurs in germ cells, with two nuclear divisions and DNA replication only at interphase I.</li> <li>Prophase in mitosis is simple, while in meiosis it is divided into further subphases. Synapsis and crossing over of homologous chromosomes occur in prophase of meiosis.</li> <li>In mitosis, the number of chromosomes in daughter cells is equal to that of the mother cell, while in meiosis, daughter cells receive half the number of chromosomes as the mother cell.</li> <li>Mitosis results in the formation of two daughter cells, while meiosis results in the formation of four daughter cells.</li> </ul> <p>======</p> <h5 id="54-programmed-cell-death-apoptosis">5.4 Programmed Cell Death (Apoptosis)</h5> <ul> <li> <p>Apoptosis, or Programmed Cell Death, is a property of cells that enables them to die during development.</p> </li> <li> <p>It is a controlled, energy-dependent process that occurs in a specific order, including DNA fragmentation, blebbing of the plasma membrane, breakdown of the nuclear envelope, and increased susceptibility to deoxyribonuclease (DNAase).</p> </li> <li> <p>Apoptosis results in the cell splitting up into small membrane-enclosed fragments, which are then phagocytosed by nearby macrophages.</p> </li> <li> <p>Inefficient apoptosis can lead to unregulated growth and division of cells, potentially resulting in cancer.</p> </li> <li> <p>Necrosis is another mechanism of cell death, characterized by unregulated digestion of cell fragments and inflammation of the surrounding area, which occurs as a result of injuries or exposure to toxic substances.</p> </li> <li> <p>Unlike apoptosis, necrosis does not involve DNA fragmentation or blebbing of the plasma membrane, and the cell’s contents are released into the surrounding area.</p> </li> </ul> <p>======</p> <h5 id="55-cell-differentiation">5.5 Cell Differentiation</h5> <ul> <li>Cell differentiation is the process by which unspecialised cells become specialised, acquiring specific properties and functions over time.</li> <li>Examples of differentiated cells include epithelial cells, RBCs, WBCs, cardiac cells, neurons, and muscle cells.</li> <li>Stem cells are undifferentiated cells that can divide to produce two daughter cells, one remaining a stem cell and the other becoming differentiated.</li> <li>Sources of stem cells are embryos and adult tissues, with embryonic stem cells derived from early-stage embryos and adult stem cells found in various tissues throughout the body.</li> <li>Cell potency, the ability of stem cells to differentiate into other cell types, is controlled by signalling molecules called growth factors, with stem cells being the most potent type.</li> </ul> <p>======</p> <h5 id="1-_totipotent-cells_">1. <em><strong>Totipotent Cells</strong></em></h5> <ul> <li>Totipotent cells have the ability to differentiate into all possible cell types in an organism.</li> <li>These cells exhibit the highest differentiation potential.</li> <li>Totipotent cells can give rise to both embryonic and extraembryonic cells.</li> <li>Examples of totipotent cells include a zygote formed after fertilization and asexual spores.</li> <li>A zygote can differentiate into various specialized cells in an organism, as illustrated in Fig. 5.20.</li> </ul> <p>======</p> <h5 id="2-_pluripotent-cells_">2. <em><strong>Pluripotent Cells</strong></em></h5> <ul> <li>Pluripotent cells have the ability to differentiate into most tissues of the body, but not all.</li> <li>Their differentiation potential is less than that of totipotent cells.</li> <li>An example of pluripotent stem cells is the cells derived from the inner cell mass.</li> <li>They give rise to three germ layers: endoderm, mesoderm, and ectoderm.</li> <li>These germ layers can further differentiate into most, but not all, body tissues and organs.</li> </ul> <p>======</p> <h5 id="3-_multipotent-cells_">3. <em><strong>Multipotent Cells</strong></em></h5> <ul> <li>Multipotent cells have a limited differentiation range, such as haematopoetic stem cells (HSCs) that differentiate into various blood cells.</li> <li>Pluripotent cells, like embryonic stem cells (ESCs), can differentiate into any cell type, but ethical constraints have led to the development of Induced Pluripotent Stem Cell (IPSC) technology.</li> <li>IPSC technology, developed in 2006, uses adult tissue cells and specific transcription factors to induce pluripotency, bypassing the need for embryos.</li> <li>IPSCs can create patient-matched pluripotent cells, reducing transplant rejection risk.</li> <li>IPSCs are important for modelling human diseases, such as cancer and congenital heart disease.</li> </ul> <p>======</p> <h5 id="56-cell-migration">5.6 Cell Migration</h5> <ul> <li> <p>Cell migration is the movement of cells from one place to another, occurring in various organisms and conditions.</p> </li> <li> <p>It involves several stages: polarisation (specialisation of the plasma membrane/cell cortex), protrusion (formation of leading edge through actin polymerisation), adhesion (attachment and detachment to the substratum), and retraction (movement of the cell body).</p> </li> <li> <p>The leading edge is formed through the creation of protrusions like pseudopodia, which are membrane extensions in the direction of cell migration. The lagging edge then separates from the substratum and is pulled back into the cell body.</p> </li> <li> <p>Adhesions are sites of molecular communication between the cell and the substrate, assembling and disassembling in response to extracellular signals. The adhesive contacts at the rear end are downregulated or proteolysed for cell movement.</p> </li> <li> <p>The organisation of the cytoskeleton, extracellular matrix, adhesion strength, and migratory signals are intrinsic factors affecting cell migration.</p> </li> <li> <p>Cells can move as single units or in groups, with the mode of migration depending on the type of cell and conditions.</p> </li> </ul> <p>======</p> <h5 id="role-of-cell-migration">Role of Cell Migration</h5> <ul> <li> <p>Cell migration is crucial for gastrulation, the process of forming embryonic layers (endoderm, mesoderm, and ectoderm) by the migration of large numbers of cells. It also plays a role in organogenesis, leading to the formation of organs and tissues.</p> </li> <li> <p>Cell migration is essential for homeostasis, the ability to maintain a steady internal environment in response to external changes. It is vital for tissue regeneration/repair and inflammation during injury or infection. Failure of cell migration can lead to severe defects such as autoimmune diseases and defective wound repair.</p> </li> <li> <p>In the context of metastasis, cell migration plays a significant role in the spread of cancer from one place to another. Cancer cells develop invasive properties due to lost cell-cell interactions and increased cell motility, enabling them to migrate and spread from the primary tumor site.</p> </li> </ul> <p>(Note: There are no specific equations or formulae related to cell migration in this text.)</p> <p>======</p>