Anatomy Ch 3: Cell Biology: Structure, Function, and Organization

Cell Theory: All cells originate from pre-existing cells. Cells are the fundamental structural and functional units of life.
Protocell Origin: The very first cell must have emerged abiotically, evolving into a protocell, the universal common ancestor with three essential properties:
- Energy: Primarily adenosine triphosphate ( $ATP$ ) in living systems.
- Replication: The ability to pass properties to future generations (e.g., RNA as an early blueprint).
- Membrane: Borders to contain internal components and regulate interaction with the environment.
Tree of Life: Comprises three domains: Bacteria, Archaea, and Eukarya.
- Prokaryotes: Include Archaea and Bacteria; lack a nuclear membrane.
- Eukaryotes: Possess a nuclear membrane enclosing DNA; includes multicellular organisms and are evolutionarily closer to Archaea.
Horizontal Gene Transfer: Significant events in evolution:
- Acquisition of purple bacteria (capable of aerobic respiration) by primordial Archaea, leading to mitochondria in eukaryotic cells.
- Acquisition of green bacteria (capable of photosynthesis) by some algae, leading to chloroplasts in plants.

ATP Production: Humans produce and break down approximately $65$ kg of ATP daily, yet only maintain about $50$ g at any given moment. This high turnover is critical for life.
Oxidative Phosphorylation: Main process for ATP synthesis involving:
- Breakdown of carbs into protons ( $H^{+}$ ) via glycolysis and the Krebs cycle.
- Proton gradient across the mitochondrial membrane (driven by electron transport chain).
- ATP synthase (a protein machine) uses $H^{+}$ flow to produce ATP from ADP and $P_i$ .
- Oxygen ( $O<em>2$ ) combines with protons to form water ( $H</em>2O$ ).

Cell Membrane: Semi-permeable, segregates the cell from the environment, and regulates molecular transport.
- Composed of a phospholipid bilayer, with hydrophilic heads facing outwards and hydrophobic tails inwards.
- Contains proteins for selective transport and is reinforced by cholesterol (average $20$ % of membrane).
Cytoplasm: Not a dilute solution, but an incredibly crowded, complex environment.
Organelles and Molecular Machines: Perform specialized functions.
- Membrane-bound organelles: Mitochondria and chloroplasts have double membranes (due to endosymbiosis); nuclear membrane is double. Other eukaryotic organelles (Golgi, ER, lysosomes, peroxisomes, vesicles) have single membranes.
- Molecular Machines: ATP synthase, motor proteins (kinesin, dynein, myosin), proteasomes (protein degradation), and ribosomes (protein synthesis).
Ribosomes: Essential for protein synthesis; their absence in viruses necessitates host cell infection.
Protein Processing and Targeting: Proteins are synthesized (often by ribosomes on rough ER), processed (e.g., in ER, Golgi), and directed to specific organelles via signal sequences or "barcodes."
Nucleus: Contains DNA organized into chromosomes; enclosed by a double membrane continuous with the ER, featuring selective nuclear pores.

Functions: Structural support (reinforces membranes) and motility.
Types of Filaments:
- Intermediate Filaments: Provide structural integrity (e.g., keratin in skin, lamin in nuclear envelope).
- Motile Filaments: Drive cell movement:
  - Actin Filaments: Involved in cell crawling and muscle contraction (with myosin).
  - Microtubules: Act as "railroad tracks" for motor proteins (kinesin toward plus-end periphery, dynein toward minus-end interior) and essential for chromosome segregation during cell division (built from centrosomes).
Assembly: Filaments are built and broken down dynamically from non-covalently attached subunits.

Glycocalyx (Sugar Coat): A layer of carbohydrates on the cell surface used for cell-cell recognition and immune system identification.
Multicellularity: Cells recognize and adhere to each other to form tissues. Demonstrated by sponges and dissociated amphibian embryonic cells reforming original structures.
Cell Junctions: Various connections hold cells together in tissues:
- Tight Junctions: Airtight, watertight seals preventing leakage between cells.
- Desmosomes: Strong adhesive junctions, act like "Velcro" between cells.
- Hemidesmosomes: Anchor cells to the basement membrane.
- Gap Junctions: Channels allowing direct passage of small molecules and electrical signals between cells (e.g., in cardiac muscle for synchronized contraction).
Cell Differentiation: During embryonic development, a single zygote ( $1$ cell) differentiates into over $200$ different cell types, forming specialized tissues and organs.