Cells and Subcellular Organelles

History:

• Robert Hooke (1665): Discovered cells in cork.
• Schleiden (1838) & Schwann (1839): Proposed that all plants and animals are made of cells.
• Rudolf Virchow (1855): Proposed 'Omnis cellula e cellula' (new cells come from pre-existing cells).

Main Postulates:

1. All living organisms are composed of one or more cells.
2. The cell is the fundamental structural and functional unit of life.
3. New cells arise from pre-existing cells division.

Definition: The outermost, rigid, non-living boundary in plant cells, fungi, and bacteria. Absent in animal cells.

Structure in Plants:

• Middle Lamella: First layer formed between dividing cells, made of sticky pectins and calcium/magnesium salts.
• Primary Wall: Thin, elastic layer formed inside the middle lamella. Composed of cellulose, hemicellulose, and pectin. Capability to grow.
• Secondary Wall: Formed inside the primary wall in mature cells. Thick and rigid, made of cellulose, lignin, and inorganic salts. Provides support.

Function: Provides shape, mechanical support, and protection.

Definition: The outer boundary of animal cells and inner to the cell wall in plants. It is differentially permeable.

Fluid Mosaic Model (Singer & Nicolson, 1972): Describes the membrane as a dynamic structure.

• Phospholipid Bilayer: Forms the basic fluid matrix. Hydrophilic heads face out, hydrophobic tails face in.
• Proteins: 'Float' in the lipid sea. Types: Integral (Intrinsic) proteins (embedded) and Peripheral (Extrinsic) proteins (surface).
• Carbohydrates: Attached to lipids (Glycolipids) or proteins (Glycoproteins) on the outer surface, acting as cell markers.
• Cholesterol: Maintains fluidity and stability (in animal cells).

Functions: Transport (Active/Passive), Cell signaling, Endocytosis/Exocytosis, Protection.

Cytoplasm: The semi-viscous substance between the plasma membrane and nucleus. Contains organelles and metabolic chemicals (Cytosol). Site of Glycolysis.

Cytoskeleton: Network of protein filaments.

• Microtubules: Made of Tubulin. Hollow tubes. Form cilia, flagella, centrioles, and spindle fibers. Maintain cell shape.
• Microfilaments: Made of Actin. Thin and solid. Involved in muscle contraction and cyclosis (cytoplasmic streaming).
• Intermediate Filaments: Provide mechanical stress resistance.

Known as the control center of the cell.

• Nuclear Envelope: Double membrane with Nuclear Pores which control material exchange.
• Nucleolus: Darkly stained, non-membranous region. Site of ribosome synthesis (rRNA production).
• Chromatin/Chromosomes: Genetic material made of DNA and Histone proteins. Chromatin serves as the thread-like form during interphase; condenses into Chromosomes during division.

Network of channels extending from the nuclear membrane to the plasma membrane.

• Rough ER (RER): Surface studded with ribosomes. Involved in Protein Synthesis.
• Smooth ER (SER): Lacks ribosomes. Involved in Lipid Metabolism, Detoxification of drugs/poisons, and Calcium storage (in muscles).

Tiny granular structures involved in Protein Synthesis.

• Composition: RNA + Protein (Ribonucleoproteins).
• Types:
  • 70S: Found in Prokaryotes and Mitochondria/Chloroplasts. (Subunits: 50S + 30S).
  • 80S: Found in Eukaryotic cytoplasm. (Subunits: 60S + 40S).
• Polysome: A cluster of ribosomes attached to a single mRNA strand.

Discovered by Camillo Golgi. Consists of stacks of flattened sacs called Cisterna.

• Function: 'Post Office' of the cell. Modifies, packages, and transports proteins and lipids (Cell Secretions).
• Faces: Forming Face (Cis): Convex, receives vesicles from ER. Maturing Face (Trans): Concave, releases contents as secretory vesicles or lysosomes.

Single-membrane organelles containing powerful hydrolytic enzymes (acid hydrolases). Discovered by De Duve.

• Primary Lysosome: Newly formed, inactive.
• Secondary Lysosome (Phagosome): Formed when primary lysosome fuses with a food vacuole. Active digestion occurs here.
• Functions: Intracellular digestion, Autophagy (recycling old organelles), Autolysis (Self-destruction of cell). Known as 'Suicide Bags'.

Double-membrane bound organelle known as the 'Powerhouse of the Cell'.

• Structure: Outer membrane is smooth. Inner membrane is folded into Cristae to increase surface area. Filled with fluid called Matrix containing DNA, ribosomes (70S), and enzymes.
• Function: Site of Aerobic Respiration and ATP production (Krebs Cycle, Electron Transport Chain). It is a self-replicating organelle.

Found only in plant cells. Three types:

• Chloroplasts: Green, contain Chlorophyll. Site of Photosynthesis. Structure includes Thylakoids (stacked into Grana) and Stroma.
• Chromoplasts: Colored (red, orange, yellow). Found in flower petals and fruits. Attract insects for pollination.
• Leucoplasts: Colorless. Store food (Starch, Proteins, Oils). Found in roots and tubers.

Cylindrical structures found in animal cells and some lower plants. Absent in higher plants.

• Structure: Composed of 9 triplets of microtubules (9+0 arrangement). Usually distinct pair near the nucleus.
• Function: Form Spindle Fibers during cell division. Also form basal bodies of cilia and flagella.