Gen Path

The biological hierarchy of multi-cellular organisms is as follows:
- Atoms
- Molecules
- Cells
- Tissues
- Organs
- Organ Systems
- Organism
Tissue Types:
1. Nervous Tissue: Set of tissues with specific functions related to the nervous system.
2. Muscle Tissue: Composed of cells that have the ability to contract.
3. Connective Tissue: Provides support and binds other tissues together.
4. Epithelial Tissue: Covers body surfaces and lines cavities.
Multi-cellular organisms possess a vast array of capabilities but relinquish independence, becoming dependent on other cells for function.

Cell Membrane:
- Structure: A semipermeable barrier composed of a phospholipid bilayer with an asymmetric lipid composition.
- Cholesterol: Stabilizes the bilayer.
- Membrane Proteins: Approximately 50% of a cell's protein content.
- Integral Proteins: Span the membrane with functions such as receptors and transport.
- Peripheral Proteins: Exposed on one side and may have only a transient association with the membrane.

Function: Controls cellular activity.
DNA Structure:
- Exists in the form of chromosomes, which are DNA complexed with proteins.
- Euchromatin: Represents "working" DNA, accessible for transcription.
- Heterochromatin: Functions as a dynamic platform to recruit regulatory proteins for chromosomal processes like transcription and segregation.
Nucleolus: The center for rRNA synthesis, which is exported to the cytoplasm for ribosome formation.
Nuclear Envelope: A double lipid bilayer featuring nuclear pores for communication with the cytoplasm; the perinuclear space is continuous with the rough endoplasmic reticulum.

Composed of two subunits made from RNA and proteins.
Function: Converts genetic information from mRNA into proteins, which perform a variety of cellular functions.
Size: Approximately 20 nm in diameter.

Endoplasmic Reticulum (ER): Interconnecting membranous sacs.
- Smooth Endoplasmic Reticulum (SER): Involved in lipid, steroid, and carbohydrate synthesis, as well as the catabolism of substances (e.g., CYTO P450s).
- Rough Endoplasmic Reticulum (RER): Studded with ribosomes, it synthesizes proteins for internal use and external export.
Golgi Apparatus: A membranous organelle responsible for the modification and packaging of materials for export.
- Vesicles from the ER join at the cis-face, where modifications occur as they move toward the trans-face.
- Major site for new membrane synthesis.
- Secretory vesicles bud off from the Golgi and are transported to specific destinations, including specific vesicle formation for lysosomes and peroxisomes.

Major source of ATP production, supplying approximately 90% of the cell's ATP.
Ability to divide by fusion, independent of the cell cycle, in response to physiological demands.
Key role in programmed cell death (apoptosis).
Structure:
- Double membrane consisting of an outer and an inner membrane, the latter being highly folded into structures known as cristae.
- The inner membrane space contains a proton concentration gradient utilized for ATP generation.
Mitochondria contain their own unique DNA and ribosomes, coding for 13 mitochondrial proteins, most of which are encoded by nuclear DNA.

Function: Involvement in maintenance of cell structure, cell movement, and formation of the mitotic apparatus.
Components:
- Microtubules: Composed of tubulin and generally 22 nm in diameter.
- Microfilaments: Primarily made of actin and myosin with a diameter of approximately 5 nm.
- Intermediate Filaments: Diameter of about 10 nm, with some specificity to cell types (epithelial, mesenchymal, muscular, glial, neural, etc.), which can be identified using fluorescent antibodies.

Function: Acidic vesicles filled with hydrolytic enzymes essential for digestion of:
- Phagocytized materials
- Worn-out cell components
- Residual bodies containing indigestible material
Enzymatic content varies by tissue type.
Accumulation of excess uric acid crystals can disrupt lysosomal membranes, leading to cellular issues.

Small vesicles with variable enzyme content involved in both catabolic and anabolic reactions within cellular contents, especially concerning lipids.
Their key roles include:
- Catabolism of toxins.
- Oxidation of long-chain fatty acids (subsequently shuttled to mitochondria for complete oxidation).
- Synthesis of glycerolipids and isoprenoids, which are inflammatory mediators derived from cholesterol precursors.
- Catalase: An enzyme that catalyzes the decomposition of hydrogen peroxide (H2O2).

Membrane Junctions:
1. Tight Junctions: Common in epithelial cells, providing watertight connections between neighboring cells, thus maintaining a distinct internal and external environment.
2. Desmosomes: Function to anchor cells to one another and to the basement membrane.
3. Gap Junctions: Direct channels between cells, allowing the transfer of ions and very small molecules; crucial in cardiac muscle, nervous tissue, and during embryonic development.
Microvilli: Serve to increase surface area for absorption.
Cilia: Have a structure of 9+2 arrangement of microtubules; coordinated movement aids in moving materials across epithelial surfaces.