Cytology

1. Plasma Membrane Components and Function

  • The plasma membrane is a lipid bilayer embedded with proteins and comprises five major components:

    • Phospholipids: Make up 75%75\% of the membrane. They have hydrophilic heads and hydrophobic tails, forming the bilayer that acts as a mechanical barrier and defines the internal/external boundary of the cell.

    • Cholesterol: Constitutes 20%20\% of the membrane. Its structure helps stabilize the membrane's fluidity and integrity.

    • Glycolipids: Make up 5%5\% of the membrane. They contribute to the glycocalyx and are involved in cell interactions and recognition.

    • Membrane Proteins: Can be Integral (often transmembrane, spanning the entire bilayer) or Peripheral (loosely attached to the surface).

      • Integral proteins (e.g., channels, carriers, receptors) carry out transport, signaling, and enzymatic activities.

      • Peripheral proteins provide support and involve in cell signaling and attachment.

    • Glycocalyx: Composed of carbohydrates attached to glycolipids and glycoproteins. It provides unique cellular markers for cell-cell recognition and interaction.

2. Membrane Transport

  • Active vs. Passive Transport

    • Passive Transport: Does not require cellular energy (ATP).

      • Diffusion: Movement of lipid-soluble solutes directly across the lipid bilayer along their concentration gradient.

      • Facilitated Diffusion: Uses carrier or channel proteins to transport specific solutes down their concentration gradient, without ATP.

      • Osmosis: Diffusion of water across a selectively permeable membrane. Water moves from an area of higher water concentration (lower solute) to lower water concentration (higher solute).

        • Isotonic: No net water movement; cells maintain their size.

        • Hypertonic: Water leaves the cell; cells shrink.

        • Hypotonic: Water enters the cell; cells swell or lyse.

    • Active Transport: Requires cellular energy (ATP) to move solutes, often against their concentration gradient.

      • Primary Active Transport: Uses ATP-powered pumps (e.g., Na+-K+ pump\text{Na}^+ \text{-K}^+ \text{ pump}) to establish and maintain concentration gradients.

      • Vesicular Transport: Requires ATP to form and move vesicles for bulk transport.

  • Requiring and Not Requiring a Transport Protein

    • Requiring a transport protein: Facilitated diffusion (carrier/channel proteins) and primary active transport (ATP-powered pumps).

    • Not requiring a transport protein: Passive diffusion (for lipid-soluble solutes that cross the bilayer directly).

  • Vesicular Transport into and out of the cell

    • Endocytosis (into the cell): The cell takes in substances by engulfing them in a vesicle.

      • Phagocytosis: "Cell eating"; engulfment of large particles (e.g., by macrophages).

      • Pinocytosis: "Cell drinking"; engulfment of extracellular fluid (e.g., nutrient absorption).

      • Receptor-mediated endocytosis: Specific uptake of molecules bound to receptors.

    • Exocytosis (out of the cell): Substances are released from the cell as a vesicle fuses with the plasma membrane (e.g., secretion via secretory vesicles).

3. Cell Junctions

  • Information on cell junctions (classification, functions, locations) is not detailed in the provided notes.

4. Organelles

  • Membrane-Bound Organelles

    • Mitochondria: Responsible for energy production (cellular respiration).

    • Endoplasmic Reticulum (ER):

      • Rough ER (RER): Studded with ribosomes; involved in protein synthesis, folding, modification, and processing for secretion or insertion into membranes.

      • Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, metabolism of carbohydrates, and detoxification of drugs and poisons.

    • Golgi Apparatus: Consists of cis (receiving) and trans (shipping) faces. Modifies, sorts, and packages proteins and lipids into vesicles for transport to various destinations (secretion, membrane incorporation, lysosomal delivery).

    • Lysosomes: Contain powerful hydrolytic (digestive) enzymes responsible for breaking down waste materials and cellular debris, as well as autophagy (digesting old organelles).

    • Peroxisomes: Contain enzymes that break down fatty acids and amino acids, and detoxify harmful substances by transferring hydrogen to oxygen, producing hydrogen peroxide (which is then converted to water and oxygen).

    • Transport Vesicles: Move substances between ER, Golgi, lysosomes, and the plasma membrane.

    • Secretory Vesicles: Bud off the Golgi apparatus and release their contents outside the cell via exocytosis.

  • Non-Membrane-Bound Organelles

    • Ribosomes: Composed of ribosomal RNA and proteins. Responsible for protein synthesis.

      • Free ribosomes: Synthesize proteins that function in the cytosol (e.g., enzymes, structural proteins) or in the nucleus.

      • Bound ribosomes: Attached to the RER; synthesize proteins destined for secretion, insertion into membranes, or delivery to certain organelles (e.g., lysosomes).

  • Inclusions (Non-membranous)

    • Glycogen granules: Stored form of glucose.

    • Lipid droplets: Store lipids/fats.

    • Pigments: Stored colored substances (e.g., melanin).

  • Cellular distribution of organelles: Organelles are dispersed throughout the cytoplasm, each with specific roles contributing to overall cell function, often in a functionally organized manner (e.g., ER near nucleus, Golgi near ER).

5. External Cell Structures

  • Cilia:

    • Characteristics: Short, hair-like, numerous.

    • Functions: Motility, move fluids or substances across the cell surface.

    • Locations: Found on the surface of some epithelial cells (e.g., in the respiratory tract or fallopian tubes).

  • Flagella:

    • Characteristics: Long, whip-like, typically one or few.

    • Functions: Motility, propels the cell.

    • Locations: Primarily found on sperm cells, enabling their movement.

  • Microvilli:

    • Characteristics: Small, finger-like, actin-supported extensions of the plasma membrane; immotile.

    • Functions: Increase the surface area for absorption.

    • Locations: Abundant in cells specialized for absorption (e.g., epithelial cells of the small intestine or kidney tubules).

6. Parts of the Nucleus

  • Nuclear Envelope:

    • Composition: A double membrane that encloses the nucleus, continuous with the Rough ER.

    • Functions: Separates the nucleoplasm from the cytoplasm; contains nuclear pores that regulate the transport of molecules (e.g., RNA, proteins) between the nucleus and cytoplasm; supported by the nuclear lamina.

  • Nucleolus:

    • Composition: A dense, spherical structure within the nucleus, not membrane-bound; composed of ribosomal RNA (rRNA) and proteins.

    • Functions: Primary site of ribosome production and assembly (synthesis of rRNA and ribosomal subunits).

  • Chromatin:

    • Composition: A complex of DNA, histones (proteins), and RNA components.

      • DNA is wrapped around histone proteins to form nucleosomes, giving a "beads-on-a-string" structure.

    • Functions: Packages DNA into a compact form that fits within the nucleus; involved in DNA replication, transcription (gene regulation), and cell division (when it condenses into chromosomes).

7. Mitosis and the Cell Cycle

  • The Cell Cycle:

    • Interphase: A period of cell growth and DNA replication, preparing for cell division.

      • G1 phase: Cell grows, synthesizes proteins and organelles; metabolically active.

      • S phase: DNA replication occurs, resulting in two identical sister chromatids for each chromosome.

      • G2 phase: Cell continues to grow and synthesizes enzymes and proteins necessary for mitosis; centrosomes replicate.

    • Mitotic (M) phase: The phase of active cell division.

      • Mitosis: Division of the nucleus.

      • Cytokinesis: Division of the cytoplasm.

  • Mitosis Phases and Events:

    • Prophase:

      • Chromatin condenses into visible chromosomes (each consisting of two sister chromatids joined at the centromere).

      • The nuclear envelope disintegrates.

      • Centrioles (part of the centrosome) migrate to opposite poles of the cell, forming the mitotic spindle.

    • Metaphase:

      • Chromosomes align at the equatorial plate (metaphase plate) of the cell.

      • Spindle fibers (microtubules) attach to the kinetochores (protein structures on the centromeres) of each sister chromatid.

    • Anaphase:

      • Sister chromatids separate and are pulled apart by the shortening spindle fibers towards opposite poles of the cell, becoming individual chromosomes.

      • The cell elongates.

    • Telophase:

      • Chromosomes de-condense back into chromatin at the poles.

      • A new nuclear envelope reforms around each set of chromosomes at the poles.

      • The nucleoli reappear in each forming nucleus.

    • Cytokinesis:

      • Begins during late anaphase and continues through telophase.

      • The cytoplasm divides, typically through the formation of a cleavage furrow (in animal cells).

      • Results in two genetically identical daughter cells.