Cell: The Unit of Life

• Postulates of Cell Theory:
1. All living organisms are composed of one or more cells and their products.
2. The cell is the fundamental structural and functional unit of all life.
3. All new cells arise solely from pre-existing living cells by cell division ("Omnis cellula e cellula").
• Exception: Viruses are non-cellular (acellular) particles lacking cytoplasm, cell membranes, and independent metabolic machinery. They only reproduce inside a living host cell, thus defying the cell theory.

1. Cell Wall: Plant cells are bounded by a rigid, protective, non-living cellulose cell wall outside the plasma membrane; animal cells completely lack a cell wall.
   2. Plastids: Plant cells contain plastids (such as green chloroplasts for photosynthesis); animal cells have no plastids.
   3. Vacuoles: Plant cells possess a single, permanent large central vacuole that maintains turgidity; animal cells have only small, non-permanent, or absent vacuoles.
   (Bonus: Animal cells have centrosomes with centrioles; higher plant cells lack centrosomes).

• Mitochondria are the sites of aerobic cellular respiration in eukaryotic cells.
• They oxidize respiratory substrates (pyruvate/glucose) through the Krebs cycle and electron transport system on their folded inner cristae membranes.
• The energy released is trapped and stored in the form of high-energy chemical bonds in ATP (Adenosine Triphosphate) molecules.
• Because ATP powers all mechanical, chemical, and transport work of the cell, mitochondria are called the "Powerhouses of the Cell".
• Energy Currency: ATP (Adenosine Triphosphate).

• Lysosomes are membrane-bound spherical vesicles filled with powerful hydrolytic (digestive) enzymes capable of breaking down proteins, nucleic acids, lipids, and carbohydrates.
• When a cell becomes damaged, diseased, or reaches the end of its life cycle, its lysosomal membranes rupture.
• The released hydrolytic enzymes digest and destroy their own host cell from within (a process called autolysis).
• Hence, lysosomes are famously nicknamed "Suicide Bags".

• Rough Endoplasmic Reticulum (RER):
- Structure: Membranes are densely studded with ribosomes on their outer cytoplasmic surface, giving a rough granular appearance.
- Function: Actively involved in the synthesis, folding, and transport of proteins.
• Smooth Endoplasmic Reticulum (SER):
- Structure: Membranes are tubular and completely free of ribosomes, having a smooth surface.
- Function: Synthesizes lipids, phospholipids, and steroid hormones (e.g., testosterone, estrogen) and detoxifies drugs in liver cells.

1. Chloroplasts: Green plastids containing the pigment chlorophyll; primary function is Photosynthesis (synthesizing glucose using sunlight).
   2. Chromoplasts: Colored plastids containing red, yellow, and orange carotenoid pigments; impart vibrant colors to petals and fruits to attract pollinating insects and birds.
   3. Leucoplasts: Colorless plastids; primary function is the storage of reserve food materials (amyloplasts store starch, elaioplasts store oils, aleuroplasts store proteins).

• Function: The centrosome (containing two mutually perpendicular centrioles) serves as the primary Microtubule Organizing Center (MTOC) of the animal cell.
• During Cell Division: At the onset of prophase in mitosis, the centrosome duplicates and the two centrioles migrate to opposite poles of the cell, organizing and radiating spindle fibers (the mitotic spindle) that pull chromosomes apart.

• Chromatin fibers are an organized macromolecular complex consisting of:
1. DNA (Deoxyribonucleic Acid): Approximately $40\%$ of the mass, carrying the hereditary genetic instructions (genes).
2. Histone Proteins: Approximately $60\%$ of the mass, acting as structural spools around which DNA strands wind tightly into nucleosomes.
• During cell division, chromatin threads condense and coil into visible, rod-like Chromosomes.