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DrX Whiz Niraj
Cellular Level of Organization
Explore the microscopic world! Complete notes on Cell Structure, Membrane Transport, Cell Division, Junctions, and Cell Signaling.
The Cell is the basic structural and functional unit of all living organisms. A generalized human cell has three main parts: the Plasma Membrane (outer boundary), the Cytoplasm (fluid containing organelles), and the Nucleus (genetic control center).
| Organelle | Main Function | Key Feature |
|---|---|---|
| Plasma Membrane | Controls entry and exit of substances. | Fluid Mosaic Model (Lipid bilayer). |
| Mitochondria | Produces energy (ATP) via cellular respiration. | Powerhouse of the cell. |
| Ribosomes | Site of protein synthesis. | Protein factories. |
| Endoplasmic Reticulum (ER) | Rough ER synthesizes proteins; Smooth ER synthesizes lipids & detoxifies. | Network of membranous tubules. |
| Golgi Complex | Modifies, sorts, and packages proteins. | Post office of the cell. |
| Lysosomes | Digests worn-out organelles & foreign bodies. | Suicide bags (hydrolytic enzymes). |
| Nucleus | Controls cell division and activities. | Contains DNA (Genes). |
The plasma membrane is selectively permeable. Materials move across it via two primary mechanisms: Passive Transport (no energy required) and Active Transport (energy required).
Types of Transport:
- 1. Passive Transport: Substances move down their concentration gradient (High to Low) without using ATP.
Examples: Simple Diffusion (O2, CO2), Facilitated Diffusion (Glucose via carriers), Osmosis (Water). - 2. Active Transport: Substances move against their concentration gradient (Low to High) using cellular energy (ATP).
Examples: Sodium-Potassium (Na+/K+) Pump. - 3. Vesicular Transport: Movement of large particles using vesicles. Includes Endocytosis (into cell) and Exocytosis (out of cell).
Cell division is the process by which cells reproduce themselves. The cell cycle consists of Interphase (G1, S, G2 phases) and the Mitotic (M) phase.
Mitosis (Somatic Cells)
Used for tissue growth and repair. One parent cell divides to produce two genetically identical diploid (2n) daughter cells.
- Prophase: Chromatin condenses into visible chromosomes.
- Metaphase: Chromosomes align at the cell equator.
- Anaphase: Sister chromatids separate and move to opposite poles.
- Telophase: Nuclear envelope reforms, followed by Cytokinesis (division of cytoplasm).
Meiosis (Reproductive Cells)
Used exclusively for the production of gametes (sperm and egg). One parent cell undergoes two divisions to produce four genetically distinct haploid (n) daughter cells.
- Involves Meiosis I (reduces chromosome number) and Meiosis II (separates chromatids).
- Crossing over occurs during Prophase I, causing genetic variation.
Cell junctions are contact points between the plasma membranes of tissue cells.
- Tight Junctions: Form a fluid-tight seal between cells to prevent leakage (e.g., Stomach and Intestine lining).
- Adherens Junctions: Contain a plaque that attaches to microfilaments, helping epithelial surfaces resist separation.
- Desmosomes: Act like "spot welds" linking intermediate filaments. Prevents cells from pulling apart under tension (e.g., Skin, Heart muscle).
- Hemidesmosomes: Anchor cells to the underlying basement membrane.
- Gap Junctions: Fluid-filled tunnels (connexons) that allow rapid communication and passage of ions/small molecules between cells (e.g., Cardiac muscle synchronization).
Cells must communicate to coordinate their functions. General Principles: An extracellular signal molecule (Ligand) binds to a specific receptor on the target cell. This receptor activates an Intracellular signaling pathway (using second messengers like cAMP or Calcium ions), leading to a cellular response (e.g., gene expression, metabolic change).
Forms of Intercellular Signaling:
- a) Contact-Dependent: Cells must be in direct physical contact. Signal molecule remains bound to the signaling cell surface. (Crucial in immune responses & embryonic development).
- b) Paracrine: Signaling cell secretes local chemical mediators (local hormones) that act only on neighboring cells (e.g., Histamine release in inflammation).
- c) Synaptic: Specialized signaling performed by nerve cells (neurons). They release neurotransmitters at synapses to rapidly communicate with specific target cells (muscles/glands).
- d) Endocrine: Specialized endocrine cells (glands) secrete hormones directly into the bloodstream, which carries the signal to target cells distributed widely throughout the body.
Clinical Correlation (Applied Physiology)
Cancer & Cell Junctions (Metastasis)
Normally, cells are held tightly together by cell junctions (like Desmosomes and Adherens junctions). In malignant cancer, these junctions are often broken or mutated. This allows cancer cells to detach from the primary tumor, enter the bloodstream, and spread to other organs (a deadly process called Metastasis).