Cell Biology: Cell Structure, Cell Division, and Cell Transport
Cellular Anatomy and Subcellular Structures
Nucleus: The central organelle that houses genetic material and directs overall cell activities.
Nuclear Envelope: The double-membrane structure surrounding the nucleus, containing nuclear pores and breaking into fragments during nuclear division.
Nucleolus: A dense, specialized structure within the nucleus involved in ribosome assembly.
Chromatin: Uncoiled, thread-like complexes of DNA and proteins located inside the nucleus during interphase.
Endomembrane System and Synthesis:
Rough Endoplasmic Reticulum (RER): An extensive membrane network studded with ribosomes, involved in protein synthesis and membrane modification.
Smooth Endoplasmic Reticulum (SER): A membrane network lacking ribosomes, involved in lipid synthesis, metabolic processes, and detoxification.
Ribosomes: Ribonucleoprotein complexes (either floating freely in the cytosol or attached to the rough endoplasmic reticulum) that serve as sites of protein translation.
Golgi Apparatus: A series of flattened membrane-bound sacs that modifies, sorts, packages, and routes proteins; facilitates cellular secretion being released from the cell by exocytosis.
Metabolic and Degradative Organelles:
Mitochondrion: The double-membrane organelle responsible for cellular respiration and ATP energy generation.
Lysosome: Membrane-bound organelle containing digestive hydrolytic enzymes to break down macromolecules and cellular debris.
Peroxisome: Specialized membrane-bound vesicle containing oxidative enzymes for metabolic detoxification processes.
Cytoskeleton and Centrosome Complex:
Cytosol: The fluid matrix of the cytoplasm surrounding the organelles.
Cytoskeletal Elements: Structural protein networks that maintain cell shape, anchor organelles, and facilitate movement:
Microtubule: Cylindrical cytoskeletal filaments essential for cell shape, intracellular transport, and mitotic spindle assembly.
Intermediate Filaments: Tough, insoluble protein fibers that provide mechanical strength to the cytoskeleton.
Centrosome Matrix: The specialized zone of cytoplasm surrounding the centrioles.
Centrioles: Paired cylindrical structures composed of microtubules; each centrosome contains exactly centrioles.
Plasma Membrane and Exocytosis:
Plasma Membrane: The semi-permeable phospholipid bilayer boundary that encloses the cytosol and regulates transport in and out of the cell.
Exocytosis: The process by which secretory vesicles fuse with the plasma membrane to release cellular substances into the extracellular environment.
Mechanisms of Cell Transport
Diffusion:
Definition: The movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient.
Determinants of Speed: The speed of diffusion is affected by molecule size, where smaller molecules diffuse faster than larger molecules.
Osmosis:
Definition: The net diffusion of solvent molecules (such as water, ) across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Solutions and Their Effects on Cell Morphology
Isotonic Solutions:
Solute Concentration: Contains the same solute concentration inside and outside the cell (equal solute/water concentration).
Water Movement: Water molecules move into and out of the cell at equal rates in dynamic equilibrium.
Cell Morphology: Cells retain their normal size and shape.
Specific Example: A saline solution is isotonic to human red blood cells.
Hypertonic Solutions:
Solute Concentration: Contains a higher solute concentration in the solution than inside the cell.
Water Movement: Cells lose water by osmosis as water flows out toward the higher solute environment.
Cell Morphology: Cells shrink and undergo crenation as water exits.
Specific Example: Salty water.
Hypotonic Solutions:
Solute Concentration: Contains a lower solute concentration in the solution than inside the cell.
Water Movement: Cells take on water by osmosis as water flows into the higher solute environment inside the cell.
Cell Morphology: Cells swell and take on water until they become bloated and burst (lyse).
Specific Example: Distilled water.
The Cell Cycle: Mitosis and Cytokinesis
Overview of Cell Division:
Mitosis: Process specifically referring to nuclear division.
Cytokinesis: Process specifically referring to the division of the cytoplasm.
Product: The final product of cell division is two daughter cells that are genetically identical to the parent cell.
Phases of Cell Division:
Interphase:
Non-dividing phase where cellular growth occurs.
Key structures include an intact plasma membrane, a distinct nucleolus, an intact nuclear envelope, and diffuse chromatin.
Centrosomes are positioned outside the nucleus, with each centrosome containing centrioles.
Early Prophase:
Chromatin condenses into visible, discrete chromosomes, each consisting of two identical sister chromatids attached at a centromere.
The early mitotic spindle begins to assemble between the separating centrosomes.
Radial arrays of short microtubules called asters extend from the centrosomes.
Late Prophase:
The nuclear envelope breaks down, forming fragments of nuclear envelope.
Spindle poles establish at opposite ends of the cell.
Polar microtubules extend across the cell body toward the equator.
Kinetochore microtubules attach to the specialized kinetochore structures located on the centromere of each chromosome consisting of two sister chromatids.
Metaphase:
Chromosomes align precisely along the metaphase plate (the equatorial midplane of the cell).
The mitotic spindle is fully formed, with kinetochore microtubules attached to all centromeres.
Anaphase:
Kinetochore microtubules shorten, splitting centromeres and pulling sister chromatids apart.
The separated chromatids become individual daughter chromosomes and are pulled toward opposite spindle poles.
Telophase and Cytokinesis:
Telophase (Nuclear Division): Daughter chromosomes reach opposite poles and uncoil back into chromatin. A nuclear envelope is forming around each new set of chromosomes, and a nucleolus is forming within each developing nucleus.
Cytokinesis (Cytoplasmic Division): A contractile ring pinches the plasma membrane at the cleavage furrow, dividing the cytoplasm to form two separate, identical daughter cells.