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

Introduction

• Cells are the basic unit of life, enabling numerous tasks such as food digestion, nerve signaling, blood pumping, nutrient circulation, protein synthesis, and urine filtration.
• Organelles are specialized structures within cells, each responsible for specific functions.
• Cells are classified into prokaryotic (lacking an organized nucleus and membrane-bound organelles) and eukaryotic (containing a nucleus and membrane-bound organelles).
• Common components in both cell types include plasma membrane, cytoplasm, ribosomes, and DNA.
• Prokaryotic cells feature a nucleoid, numerous ribosomes, mesosomes (plasma membrane folds), and sometimes flagella for locomotion.
• Eukaryotic cells have a well-organized nucleus and membrane-bound organelles like endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, vacuoles, lysosomes, and peroxisomes.
• Advancements in microscopic techniques have been crucial for exploring detailed cell structures.

Plasma Membrane

• Forms the boundary of the cytoplasm, guarded externally by the extracellular matrix.
• Responsible for the cell’s interaction with its surroundings and is semipermeable.
• Understanding its chemical composition (mainly lipids and proteins) and electron microscopy led to the Fluid Mosaic Model, proposed by Singer and Nicolson in 1972.
• The Fluid Mosaic Model describes the plasma membrane as a lipid bilayer with embedded globular proteins, resembling a mosaic.
• Composition varies; for example, human erythrocyte membranes contain about 52% protein and 40% lipids.
• The lipid bilayer maintains a quasifluid state, allowing lateral diffusion of lipids and proteins.
• Phospholipids, the primary membrane lipid, have a hydrophilic head facing outward and a hydrophobic tail inside the bilayer.
• Two types of proteins are present: peripheral (superficially attached, involved in signaling) and integral (partially or fully embedded, including abundant transmembrane proteins).
• Prokaryotic and eukaryotic plasma membranes are structurally similar.
• In 1925, Gorter and Grendel demonstrated that lipids form a two-molecular-thick layer around cells, supporting the bilayer model using mammalian red blood cells.
• Electron micrographs show the plasma membrane as a “railroad track” with two dense lines (polar heads) and a lighter region (hydrophobic tails).
• Mesosomes, extensions of the plasma membrane in prokaryotes, form vesicles, tubules, or lamellae, increasing surface area.
• The quasifluid nature supports functions like cell division, growth, intercellular communication, secretion, and endocytosis.
• Selectively permeable, it restricts molecular movement to maintain cell composition.
• Passive transport involves molecule movement along the concentration gradient without energy, via diffusion or osmosis.
• Facilitated transport uses carrier proteins (e.g., glucose transporter) or channel proteins (e.g., aquaporins for water, ion channels for muscle/nerve cells) for molecules unable to diffuse simply.
• Active transport moves molecules against the concentration gradient using ATP, e.g., the Na⁺-K⁺ pump.
• Some active transport is ATP-independent, coupling molecule movement with another molecule’s transport along the gradient (e.g., ion, sugar, or amino acid transport using Na⁺ gradient).
• Coupled transport includes symport (same direction, e.g., glucose and Na⁺ uptake), antiport (opposite directions, e.g., Na⁺-Ca²⁺ exchanger), and uniport (single molecule, e.g., glucose via facilitated diffusion).

Cell Wall

• Present in bacteria, algae, fungi, and higher plants, but absent in animal cells.
• Structurally differs between bacteria (polysaccharide cross-linked by peptides) and eukaryotes (primarily polysaccharides like cellulose or chitin).
• Provides rigidity, shape, protection from osmotic pressure, cell-cell interaction, mechanical strength, and infection resistance.
• In Gram-positive bacteria, a thick cell wall surrounds a single plasma membrane.
• In Gram-negative bacteria, a thin cell wall is surrounded by a dual plasma membrane.
• Bacterial cell walls are made of peptidoglycan (linear chains cross-linked by tetrapeptides), which antibiotics target to inhibit growth.
• Bacterial cell walls are covered by glycocalyx (heavily glycosylated protein), acting as a barrier to pathogens, protecting against stress, and aiding cell-cell interactions.
• Glycocalyx may form a loose slime layer or a thick, tough capsule.
• In plants, the primary cell wall is thin and expandable; the secondary cell wall, formed later, is rigid and thick due to lignin deposition.
• The middle lamella (calcium pectate) holds adjacent plant cells together, connected by plasmodesmata for cytoplasmic exchange.
• Plant cell walls contain cellulose (glucose polymer); fungal cell walls contain chitin (N-acetylglucosamine polymer).

Endomembrane System

• Comprises membrane-bound organelles (endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles) with coordinated functions in protein and lipid synthesis, processing, packaging, and transport.
• These organelles are distinct in structure and function but work together.

Endoplasmic Reticulum

• An extensive network of membrane-enclosed tubules and cisternae near the nucleus and Golgi apparatus, exclusive to eukaryotic cells.
• Involved in protein synthesis, calcium storage, and lipid metabolism.
• Classified into rough ER (with ribosomes) and smooth ER (without ribosomes).
• Rough ER has ribosomes on its cytosolic surface, synthesizing proteins for secretion or organelle use.
• Proteins from free ribosomes are released into the cytoplasm for use in the nucleus, mitochondria, chloroplasts, or peroxisomes.
• Bound ribosomes transfer the ribosome-protein complex to ER receptors, inserting nascent proteins into the ER for retention or transport via the Golgi complex.
• ER facilitates trafficking of secretory proteins to the Golgi, lysosomes, or plasma membrane.
• Smooth ER, lacking ribosomes, is the primary site for lipid synthesis (e.g., phospholipids, cholesterol), occurring on its cytosolic side due to lipid hydrophobicity.
• George Palade’s 1960s research demonstrated ER’s role in protein processing and sorting, tracing the secretory pathway: Rough ER → Golgi Apparatus → Secretory Vesicles → Cell Exterior.

Golgi Apparatus

• First observed by Camillo Golgi in 1898, it is a membrane-bound organelle of flattened sacs (cisternae) near the nucleus.
• Cisternae are arranged concentrically with a cis face (near ER, forming face) and trans face (away from ER, maturing face).
• Functions as the cell’s packaging and shipping department, modifying and transporting materials.
• Transport vesicles from the ER fuse with the cis face; vesicles from the trans face deliver contents to other organelles or the plasma membrane for secretion.
• Modifies proteins (e.g., adding sugars to form glycoproteins) and lipids (e.g., forming glycolipids) in its cisternae.
• Central organelle for trafficking and post-translational modification of proteins and lipids.

Lysosomes

• Small, spherical, single-membrane vesicles (0.2-0.5 μm) in animal cells and some eukaryotes, containing hydrolytic enzymes (acid hydrolases).
• Formed from the Golgi apparatus or directly from the ER, with enzymes active at acidic pH.
• Break down macromolecules (carbohydrates, proteins, fats) from within or outside the cell, e.g., fusing with food vacuoles to digest contents.
• Perform intracellular digestion and recycle cell material via autophagy, renewing the cell.
• In Tay-Sachs disease, lipid accumulation in brain cells occurs due to defective lipid-digesting enzymes.

Vacuoles

• Membrane-bound organelles in plant, fungal, and some animal cells, covered by a tonoplast membrane.
• Function in storage, structural support, and recycling; appear transparent due to lack of cytoplasmic material.
• In young plant cells, multiple small vacuoles merge into a large central vacuole (up to 90% of cell volume) as the cell matures.
• The central vacuole contains water, cell sap, metabolites, and solid inclusions, maintaining turgor and protecting against biotic stress.
• Types include lytic vacuoles, protein storage vacuoles, and other storage vacuoles.
• Fungal vacuoles aid in degradation, osmoregulation, and pH maintenance.
• In protists, food vacuoles engulf food particles, and contractile vacuoles (e.g., in amoeba) handle excretion and osmoregulation.

Mitochondria

• Present in nearly all eukaryotic cells, varying in number (single large or thousands) and location based on cell function.
• Typically sausage-shaped, 3.0-10.0 μm long and 0.5-1.5 μm wide, appearing rod-shaped or cylindrical in electron micrographs.
• Double-membrane-bound, with a smooth outer membrane and an inner membrane with cristae (infoldings) for increased surface area.
• Divided into the peri-mitochondrial space (between membranes) and the mitochondrial matrix (innermost compartment).
• The matrix contains enzymes for the TCA cycle, cellular respiration, and ATP synthesis, plus mitochondrial DNA, 70S ribosomes, and RNA.
• Some proteins are synthesized by mitochondrial DNA, supporting DNA replication, transcription, and protein synthesis in the matrix.
• Contain over 1000 proteins, varying by species and organism needs.

Plastids

• Large, pigment-containing organelles in plant cell cytoplasm, visible under a microscope.
• Impart specific colors to plants based on pigment type.
• Types include:
  • Chromoplasts: Colored (yellow, red, pink, violet) due to fat-soluble pigments like carotene and xanthophylls, found in flowers, fruits, and leaves.
  • Leucoplasts: Colorless, store reserve materials (amyloplasts for starch, aleuroplasts for protein, elaioplasts for oil).
  • Chloroplasts: Green, containing chlorophyll (a, b, carotenoids, xanthophylls), essential for photosynthesis.
• Chloroplasts are lens-shaped (2-4 μm wide, 5-10 μm long), predominantly in leaf mesophyll cells.
• Bound by a double membrane with a narrow intermediate space, containing thylakoids (flattened sacs) arranged in grana.
• Stroma (fluid outside thylakoids) contains chloroplast DNA, 70S ribosomes, and enzymes.
• Stroma lamellae connect thylakoids between grana; thylakoid membranes contain light-harvesting proteins, reaction centers, electron-transport chains, and ATP synthase for photosynthesis.

Ribosomes

• Membraneless organelles, protein synthesis factories, found in both prokaryotic and eukaryotic cytoplasm.
• First observed by George Palade in 1955 via electron microscopy.
• Number varies; a growing mammalian cell may have ~10 million ribosomes.
• Classified by sedimentation rate: 70S (prokaryotic, also in mitochondria/chloroplasts) and 80S (eukaryotic).
• Composed of rRNA and proteins, with a small and large subunit (70S: 30S and 50S; 80S: 40S and 60S).
• Subunits dissociate in the cytoplasm when not synthesizing proteins.
• rRNAs form secondary structures via base pairing, associating with ribosomal proteins for a 3D structure.
• Some rRNAs in the large subunit are catalytic (ribozymes).
• Svedberg unit (S) measures sedimentation rate, named after Theodor Svedberg, who developed the ultracentrifuge.

Microbodies

• Small, single-membrane-bound organelles in eukaryotic cells, near the ER.
• Classified as peroxisomes and glyoxysomes based on function.

Peroxisomes

• Small, membrane-bound organelles derived from ER, replicating by fission, lacking their own genome.
• Involved in energy metabolism, containing oxidases for peroxide production and catalase to neutralize harmful oxidation products.
• In animal cells, they handle oxidation and lipid biosynthesis; in liver cells, they detoxify alcohol.
• In plant cells, they convert fatty acids to carbohydrates in seeds and aid photorespiration in leaves.

Glyoxysomes

• Specialized peroxisomes in fungi and higher plants, especially in fat storage tissues of germinating seeds.
• Increase in number and activity during seed germination.
• Contain enzymes for fatty acid oxidation, glyoxylate cycle, and gluconeogenesis, converting lipids to sugars for seedling growth until photosynthesis begins.
• Coordinate with mitochondria and plastids for sugar synthesis.

Cytoskeleton

• A multi-component system of fibrous proteins maintaining cell organization and shape.
• Provides mechanical support, aiding cell division, movement, and intracellular transport.
• Composed of three filament types:
  • Microtubules (25 nm, tubulin protein): Hollow rods of 10-15 protofilaments, involved in cilia/flagella movement.
  • Actin filaments (6 nm, actin protein): Found near plasma membrane, aid muscle contraction, cytokinesis, and cell movement.
  • Intermediate filaments (10 nm, various proteins): Rope-like, provide mechanical strength.

Cilia and Flagella

• Hair-like, microscopic structures involved in cell motility, arising from a basal body (centriole-like).
• Cilia are smaller (5-10 μm), numerous, and move rhythmically (e.g., in Paramecium, mammalian respiratory tract).
• Flagella are longer (up to 150 μm), fewer (1-2), and move independently (e.g., in spermatozoa, Chlamydomonas).
• Both are fibrillar, bounded by a 90Å membrane continuous with the plasma membrane, with a 9+2 microtubule axoneme (nine peripheral, two central microtubules).
• Prokaryotic cilia/flagella have a 9+0 array, differing structurally.
• Examples: Cilia in protozoans (Ciliata), ciliated epithelium; flagella in protozoans (Flagellata), spermatozoa, and algae.

Centrosome and Centrioles

• Centrosome, found in animal cell cytoplasm near the nucleus, contains two perpendicular centrioles in pericentriolar material.
• Duplicates in S phase and separates during mitosis M-phase.
• Centrioles are cylindrical, with nine microtubule triplets arranged around a central cavity, acting as mitotic spindle assembly centers.
• Form the basal body for cilia and flagella.

Nucleus

• A well-defined organelle in eukaryotes, controlling cell activities and storing genomic information.
• Regulates gene expression via mechanisms unique to eukaryotes.

The Nuclear Envelope

• A double lipid bilayer surrounding the nucleus, similar to the plasma membrane, acting as a barrier.
• Contains nuclear pores where inner and outer membranes are continuous, fitted with nuclear pore complexes (nucleoporins in a ring around a central channel).
• Nuclear pores allow transport of RNA, proteins, and small polar/charged molecules.
• The outer membrane is continuous with the ER; the inner membrane has a nuclear lamina (lamin proteins) for structural support.

Nucleoplasm

• A clear fluid (karyolymph) enclosed by the nuclear envelope, containing a nuclear matrix (protein fibrils) to maintain nuclear shape.
• Houses enzymes for DNA replication and transcription.
• Contains suspended nucleolus and chromatin.

Nucleolus

• A distinct, membrane-less nuclear body involved in rRNA and ribosome synthesis.
• Located in the nucleolar organizing region, rich in rRNA-synthesizing genes.
• Other nuclear bodies handle transcriptional regulation, gene silencing, DNA repair, and rRNA processing.

Chromosome

• Thread-like structures of coiled DNA packaged with proteins, containing all genetic material.
• Types: autosomes (body chromosomes) and allosomes (sex chromosomes).
• Humans have 46 chromosomes (22 autosome pairs, 1 sex chromosome pair).
• Chromosome numbers vary: Arabidopsis (10), maize (20), wheat (42), fruit fly (8), human (46), dog (78).
• Prokaryotes have a single circular chromosome (nucleoid); some (e.g., Vibrio cholerae) have multiple.
• In eukaryotes, chromosomes are in the nucleus; during interphase, they form chromatin fibers (nucleosomes of DNA wrapped around histones).
• Unwound human DNA stretches 6 feet; during division, chromatin condenses into visible chromosomes.
• Chromosomes replicate and divide during cell division, with errors potentially causing serious issues.
• In 1923, Theophilus Painter incorrectly reported 48 human chromosomes; Joe Hin Tjio corrected this to 46 in 1956.