Comprehensive Study Guide on Cell Biology: Introduction, Organelles, and Membrane Dynamics

Fundamental Definition and Importance of Cells

• Definition of a Cell: The cell is the structural and functional unit of living organisms. They are the fundamental building blocks of all living organisms capable of independent replication and functioning.

• Importance in Biology:

  • Metabolism and Growth: Cells serve as the site for essential metabolic processes that fuel growth and energy production in living organisms.

  • Reproduction and Division: Cells are fundamental to reproduction, enabling organisms to grow and multiply through cell division and differentiation.

  • Homeostasis Maintenance: Cells play a crucial role in maintaining homeostasis, helping organisms regulate internal conditions despite external changes.

  • Response to Environment: Cells respond to environmental changes through signaling pathways, allowing organisms to adapt and survive in various conditions.

• Broad Classification of Organisms:

  • Unicellular (single-celled).

  • Multicellular (multiple cells).

Discovery and Development of Cell Theory

• Robert Hooke (1665): In the 17th century, he made the groundbreaking observation of cells through thin slices of cork. He noted a multitude of structures resembling a honeycomb.

• Antonie van Leeuwenhoek (1632–1723): A Dutch draper and scientist celebrated as the "Father of Microbiology."

• Mathias Jakob Schleiden: A German botanist who stated that "All plants have cells."

• Theodor Schwann: Stated that "Animals do have cells."

• Cell Theory (1839): Proposed by Schleiden and Schwann. Its three tenets are:

  1. All living organisms are composed of one or more cells.

  2. The cell is the basic unit of structure and organization in organisms.

  3. Cells arise from pre-existing cells.

• Rudolf Virchow (1855): Stated that all cells come from pre-existing cells, completing the classical cell theory.

• Exception to Cell Theory: Viruses are considered the exception to the cell theory.

General Cell Structure and Common Components

• Universal Cell Components: Although diverse, all cells share four parts:

  • Plasma Membrane: Also called the cell membrane, it is a thin coat of phospholipids surrounding the cell. It forms the physical boundary (the "skin") between the cell and its environment.

  • Cytoplasm: Refers to all cellular material inside the plasma membrane. It consists of a watery substance called cytosol and contains other structures.

  • Ribosomes: Structures in the cytoplasm where proteins are synthesized.

  • DNA: A nucleic acid containing genetic instructions needed to make proteins.

• Generic Organelles: Lysosome, Centriole, Ribosome, Mitochondria, Endoplasmic reticulum, Nucleus, DNA, Peroxisome, Vacuole, and Golgi complex.

Classification: Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells (Greek: pro = primitive; karyon = nucleus):

  • Characterized by the absence of a nuclear membrane, nucleus, nucleolus, and most well-developed cytoplasmic organelles.

  • Considered the most primitive type of cells.

  • Nuclear material (DNA, RNA, proteins) is found freely distributed in the cytoplasm in direct contact with protoplasm.

  • Examples: Bacteria, viruses, PPLO (Pleuropneumonia-like organisms), and Blue-green algae.

• Eukaryotic Cells (Greek: eu = good/well; karyon = nucleus):

  • Possess a definitely organized nucleus with a nuclear membrane and nucleolus.

  • Contain organized cytoplasmic organelles like mitochondria, plastids, ribosomes ($80s$), Endoplasmic Reticulum (E.R.), Lysosomes, and Golgi body.

  • Nuclear materials are in contact with karyoplasms.

Detailed Comparison of Prokaryotes and Eukaryotes

• Organisms:

  • Prokaryote: Monera, eubacteria, archaebacteria, cyanobacteria (blue-green algae), purple bacteria.

  • Eukaryote: Plants, animals, protists, and fungi.

• Nucleus:

  • Prokaryote: Absent.

  • Eukaryote: Present.

• Chromosomes:

  • Prokaryote: Single.

  • Eukaryote: Multiple.

• DNA:

  • Prokaryote: Circular molecule, naked (nucleoid).

  • Eukaryote: Long DNA chain bound by histones.

• Cell Division:

  • Prokaryote: Binary Fission.

  • Eukaryote: Mitosis or meiosis.

• Organelles:

  • Prokaryote: None except ribosomes. Plasmalemma may infold as mesosome or concentric infolds.

  • Eukaryote: Mitochondria, chloroplast (plants), Golgi bodies, lysosomes, ER, etc.

• Cell Wall:

  • Prokaryote: Peptidoglycans (absent in mycoplasma).

  • Eukaryote: Cellulose (plants only); absent in animals.

• Exocytosis and Endocytosis: Absent in prokaryotes; present in eukaryotes.

• Cytoskeleton: Absent in prokaryotes; present in eukaryotes (composed of protein filaments).

• Compartments: Absent in prokaryotes; present in eukaryotes due to membrane system.

• Metabolism: Prokaryotes are anaerobic or aerobic; eukaryotes are aerobic.

• Cell Size:

  • Prokaryote: $1-10\,\mu m$ linearly.

  • Eukaryote: $10-100\,\mu m$ linearly.

Differences Between Animal and Plant Cells

• Size: Animal cells are generally small; plant cells are larger.

• Cell Wall: Absent in animal cells; plant cells have a rigid cell wall made of cellulose.

• Plastids: Absent in animal cells (except for Euglena); present in plant cells.

• Vacuoles: Many and small in animal cells; mature plant cells have a large central sap vacuole.

• Golgi Apparatus: Animal cells have a single highly complex and prominent Golgi; plant cells have many simpler units called dictyosomes.

• Centrosome/Centrioles: Present in animal cells; absent in plant cells.

The Plasma Membrane (Plasmalemma)

• Terminology: The term "cell membrane" was coined by C. Nageli and C. Cramer in 1855. J. Q. Plowe introduced the term "plasmalemma" in 1931. It is also called the cytoplasmic membrane.

• Basic Composition:

  • Lipids ($28\text{ to }79\%$).

  • Proteins ($50\%$ in PM).

• Lipid Composition:

  • Phospholipids ($75\%$): Most abundant, neutral lipids (e.g., PC, PS, PE) ($80\%$).

  • Cholesterol ($20\%$): Present in mammalian cells; absent in prokaryotic cells.

  • Glycolipids ($5\%$): Stability and cell-cell interaction.

  • Sphingolipids: May be acidic (e.g., sphingomyelin) ($20\%$).

• Phospholipid Structure: These are amphipathic molecules with two hydrophobic fatty acid tails (saturated or unsaturated) and a phosphate-containing hydrophilic head group.

• Fatty Acids:

  • Saturated: Single bonds between carbon atoms.

  • Unsaturated: Double bonds between carbon atoms. These double bonds create "kinks" that prevent tight packing and increase membrane fluidity.

• Role of Cholesterol: Maintains fluidity. It prevents the compaction of tails at low temperatures and their expansion at high temperatures. It also decreases permeability to small water-soluble molecules and enhances mechanical stability.

• Lipid Rafts: Domains found on the external leaflet of the PM, composed of cholesterol, glycosphingolipids, and glycosylphosphatidylinositol.

• Membrane Fluidity: Controlled by "flip-flop" movement of lipid molecules and the presence of unsaturated fats.

Membrane Proteins and Carbohydrates

• Protein Classification (by position):

  • Integral (Intrinsic): Firmly embedded; may penetrate one leaflet or span the entire membrane (transmembrane proteins).

  • Peripheral (Extrinsic): Weakly bound by electrostatic forces to lipids; attached to polar surfaces.

  • Ectoprotein: Exposed at the outer surface.

  • Endoprotein: Exposed at the inner surface.

• Protein Function: Form channels for large molecules/ions; relay signals across segments.

• Carbohydrates: Present as oligosaccharides (short, unbranched or branched sugar chains).

  • Found as glycoproteins (on proteins) or glycolipids (on phospholipids).

  • Principal sugars include D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (sialic acid), N-acetyl-D-glucosamine, and N-acetyl-D-galactosamine.

  • Functions include mechanical structure, acting as carriers/channels, and increasing the hydrophilic nature of proteins.

Functions of the Plasma Membrane

• Diffusion: Passive transport of small molecules ($CO_2$, $O_2$, and water).

• Osmosis: Semipermeable nature sets up osmotic flow for water.

• Mediated Transport: Movement via transport proteins or permeases (highly specific).

• Endocytosis: Absorbing molecules by engulfing them via active transport (requires ATP).

• Exocytosis: Extruding contents (undigested residues, hormones, enzymes).

• Other: Cell adhesion and cell signaling.

The Nucleus (The Cellular Command Centre)

• Discovery: Discovered by Robert Brown in 1833 in Tradescantia stamen filaments.

• Occurrence: Universal in eukaryotes except in RBCs, mature sieve tubes, sclerenchyma, tracheids, vessels, and cork cells (where they degenerate).

• Morphology:

  • Shape: Related to cell shape. Isodiametrical cells have spheroid nuclei; cylindrical cells have ellipsoid nuclei; squamous cells have discoidal nuclei.

  • Size: Occupies about $10\%$ of cell volume; diameter between $3\,\mu m$ and $25\,\mu m$.

• Nuclear Variants:

  • Mononucleate: Single nucleus (most cells).

  • Binucleate: Two nuclei (e.g., Paramecium).

  • Polynucleate: 3 to 100 nuclei. In animals, called Syncytium (fusion of cells); in plants, called Coenocytes (free nuclear division).

• Ultrastructure:

  1. Nuclear Membrane (Karyotheca): A double membrane.

     • Outer Membrane ($75\,\text{\AA}$): Studded with ribosomes; continuous with ER.

     • Inner Membrane ($75\,\text{\AA}$): Inside the outer layer.

     • Perinuclear Space: $10\text{ to }50\,nm$ wide; continuous with ER lumen.

  2. Nuclear Lamina: Fibrous meshwork of intermediate filament proteins (lamins) for structural support.

  3. Nuclear Pore Complex: Diameter ~$120\,nm$. Large channels for traffic of polar molecules, proteins, and RNAs. Composed of $30\text{ to }50$ different nucleoporins.

  4. Nucleoplasm (Karyolymph): Transparent, semisolid granular ground substance containing nucleoproteins, nucleic acids, and enzymes.

  5. Chromatin Fibres: Thread-like structures of DNA and proteins (mostly histones). During division, they thicken into ribbon-like chromosomes. Protein:DNA ratio is about $1:1$.

  6. Nucleolus: Spherical structure; lacks a membrane; discovered by Felice Fontana (1774); named by Bowman.

     • Attached to the Nucleolar Organizer Region (NOR) of specific chromosomes ($13, 14, 15, 21, \text{ and } 22$).

     • Functions as a "ribosomal factory."

Mitochondria

• Discovery: Observed by Kolliker (1850). Benda (1897) gave the name "mitochondria."

• Morphology:

  • Number: Varies by function. Chaos chaos ($50,000$), sea urchin eggs ($140,000-150,000$), amphibian oocytes ($300,000$).

  • Shape: Filamentous, granular, club, racket, or ring-shaped.

  • Size: $0.5\text{ to }2.0\,\mu m$ (length up to $7\,\mu m$).

• Ultrastructure:

  • Outer Membrane ($6\,nm$): Fluid-mosaic with porins (permeable to molecules <5000 daltons).

  • Inner Membrane: Impermeable; site of the electron transport chain and ATP synthase. Folded into cristae to increase surface area.

  • Two Spaces: Matrix (internal) and Intermembrane space.

• Functions:

  • ATP production and storage.

  • Oxidation of fats.

  • Regulates metabolic activity, growth, and multiplication.

  • Detoxifying ammonia in liver cells.

  • Apoptosis (programmed cell death).

  • Biosynthesis of certain parts of blood and hormones (testosterone, estrogen).

  • Calcium ion concentration regulation.

Golgi Apparatus

• History: Discovered by Camillo Golgi; originally called "internal reticular apparatus." Nicknamed the "post office of the cell."

• Occurrence: Absent in prokaryotes, some fungi, sperm cells of bryophytes/pteridophytes, and mature RBCs. Liver cells may have $~50$; algal rhizoids can have over $25,000$.

• Morphology: Contains interconnecting tubules, vesicles, and cisternae.

  • Dictyosome: A stack of cisternae. Animal cells: $5\text{ to }6$ cisternae; Plant cells: $20+$ cisternae.

  • Cisternae: Flattened sacs ($~1\,\mu m$ diameter). Space between cisternae is $20\text{ to }30\,nm$.

  • Polarity: Convex side is cis-face (forming face, near nucleus/ER); Concave side is trans-face (maturing face, near PM).

  • Vesicles:

    • Transitional: Blebs from RER that move to cis-face.

    • Secretory: Play roles in trafficking.

• Functions: Modifying/sorting/packaging macromolecules; synthesis of proteoglycans/carbohydrates; formation of lysosomes, acrosomes, and cell walls (lignin, cellulose, etc.).

Endoplasmic Reticulum (ER)

• Discovery: Named in 1953 by Porter.

• Structure: Network of closed, flattened sacs (cisternae).

• Types:

  • Rough ER (RER): Studded with ribosomes. Involved in protein synthesis for secretion and membrane maintenance. Abundant in pancreatic acinar cells and plasma cells.

  • Smooth ER (SER): No ribosomes. Involved in lipid synthesis (fatty acids, phospholipids), steroid synthesis, and detoxification of chemicals. Abundant in hepatocytes and adipose tissue.

• Enzymes involved: Stearases (fatty acid metabolism), Glucose-6-phosphatase (glucose metabolism), Mg++ activated ATPase, and NADH-cytochrome C reductase.

• Functions: Skeletal framework; exchange of molecules; intracellular transport; transmitting intracellular impulses ($sarcoplasmic reticulum$ releases calcium for muscle contraction).

Peroxisomes

• General: Simple membrane-bound circular organelles ($0.2\text{ to }1.5\,\mu m$). Present in all animal cells (except erythrocytes) and many plant cells.

• Growth: Self-replicating by fission but lack their own genome.

• Biochemical Functions:

  • Hydrogen Peroxide Metabolism: Use oxidases (D-amino acid oxidase, urate oxidase) to produce $H_2O_2$ and catalase to break it down:     $2H_2O_2 \rightarrow 2H_2O + O_2$

  • Detoxification: Oxidizes alcohol in liver/kidney cells.

  • Fatty Acid Oxidation ($\beta$-oxidation): In mammalian cells, occurs in both mitochondria and peroxisomes; in plants/yeast, exclusively in peroxisomes. Does not generate ATP; releases energy as heat.

  • Photorespiration: Oxidizes glycolic acid into glyoxylate.

  • Biosynthesis: First reactions in plasmalogen formation (abundant in myelin).

Lysosomes

• Discovery: Described by Christian de Duve in 1949; renamed "lysosome" in 1955. Also called "Pericanalicular dense bodies."

• Polymorphism (Four states):

  1. Primary Lysosome: Storage granules; inactive enzymes synthesized by ribosomes.

  2. Secondary Lysosome (Digestive Vacuole): Formed by fusion of primary lysosome and pino/phagosome. Active state during digestion.

  3. Residual Body: Indigestible material remaining after digestion of food matter.

  4. Autophagic Vacuole: (Involved in cellular digestion).

• Functions:

  • Digestion of external particles and intercellular substances.

  • Autolysis: Rupture of the lysosomal membrane causing self-digestion of the cell.

  • Fertilization: Enzymes dissolve cells surrounding the egg to allow sperm passage.