Cell Biology: Chromatin and Filaments

Chromatin

• DNA and histone proteins form the nucleosome, a fundamental unit of chromatin structure.
• Euchromatin is a loosely packed form of chromatin, associated with active gene expression, where DNA is transcribed into mRNA.
• Heterochromatin is a tightly packed form of chromatin, typically associated with inactive or silenced genes.
• Chromatin is thread-like, while a chromatid is a condensed form of chromatin.
• DNA is transcribed into mRNA, which is then translated into proteins.

Intermediate Filaments

• These filaments regulate cell adhesion to the extracellular membrane and matrix.
• Involved in cell-to-cell (intracellular) and cell-to-organelle interactions.
• They help maintain cell shape and integrity.

Microtubules

• Composed of α-tubulin and β-tubulin subunits.
• Function as a "railroad system" for intracellular transport, facilitated by motor proteins like dynein and kinesin.
• Play a crucial role in cell division, forming spindle fibers that separate chromosomes.

Rough Endoplasmic Reticulum (RER)

• Studded with ribosomes, making it the site of protein synthesis.
• Proteins synthesized on the RER are destined for:
  • Lysosomes (digestive enzymes)
  • Plasma membrane (membrane proteins)
  • Excretion out of the cell
• RER also participates in protein folding and N-type glycosylation, adding sugar residues to proteins.

Smooth Endoplasmic Reticulum (SER)

• Lacks ribosomes.
• Involved in lipid metabolism, breaking down lipids into fatty acids, phospholipids, and cholesterol, including steroid hormones like estrogen and testosterone.
• Plays a role in biotransformation, detoxifying xenobiotics through enzymes like CYP450 (important for liver function).

Golgi Apparatus

• Composed of flattened membrane-bound sacs called cisternae.
• Has a trans face (exit side) and a cis face (entry side).
• Functions:
  • Receives materials from SER and RER.
  • Modifies proteins, including O-type glycosylation (adding sugar residues to serine and threonine).
  • Packages large molecules for transport to:
    • Plasma membrane
    • Lysosomes
    • Excretion out of the cell

Ribosomes

• Sites of protein synthesis.
• Composed of ribosomal RNA (rRNA) and proteins.
• Found free in the cytoplasm or bound to the RER.
• Function: Translate mRNA into amino acid chains to produce proteins.

Lysosomes

• Discovered by Christian de Duve in 1955.
• Known as "suicide bags" or "recycling bodies."
• Contain hydrolytic enzymes (hydrolases) for breaking down large molecules, including:
  • Nucleases
  • Lipases
  • Proteases
  • Glucosidases
• Involved in:
  • Autophagy: Breakdown of non-functional organelles.
  • Autolysis: Self-destruction of damaged cells, releasing enzymes for reuse.

Peroxisomes

• Protect cells from the harmful effects of hydrogen peroxide production.
• Contain an oxidation enzyme, catalase, which breaks down hydrogen peroxide into oxygen and water.
• Involved in:
  • Neutralizing free radicals.
  • Oxidative reactions, where oxygen is added.

Cytoplasm

• The jelly-like substance inside the cell membrane, containing the nucleus, organelles, and other cellular components.
• Supports and suspends organelles, facilitates the movement of materials within the cell.
• Two components:
  • Cytosol: The liquid portion surrounding the organelles.
  • Organelles: Membrane-bound structures with specific functions.

Nucleus

• Contains the cell's genetic material (DNA) in the form of chromatin.
• Surrounded by a nuclear envelope with pores for the passage of molecules.
• Contains nucleoplasm/karyoplasm, a fluid similar to cytoplasm, within the nucleus.

Vacuoles

• Large, membrane-bound sacs filled with fluid, primarily water.
• More prominent in plant cells.
• Functions:
  • Storage of nutrients and waste products.
  • Maintenance of turgor pressure in plant cells.

Chloroplasts (Plants)

• Sites of photosynthesis.
• Contain chlorophyll, the green pigment that captures light energy.
• Have a system of membranes, including thylakoids (flattened sacs containing chlorophyll) and grana (stacks of thylakoids).

Transport of Materials Across Membrane

Passive Transport

• Does not require energy, moving molecules down their concentration gradient (from high to low concentration).
• Types:
  • Simple diffusion: Movement of non-polar molecules across the membrane.
  • Facilitated diffusion: Movement of polar molecules with the help of transport proteins.

Active Transport

• Requires energy (ATP) to move molecules against their concentration gradient (from low to high concentration).
• Examples:
  • Sodium-potassium pump.

Bulk Transport

• Movement of large molecules or particles across the membrane.
• Types:
  • Exocytosis: Movement of materials out of the cell.
  • Endocytosis: Movement of materials into the cell, enclosed in a membrane vesicle.
    • Phagocytosis: "Cell eating" of large particles.
    • Pinocytosis: "Cell drinking" of fluids and small molecules.

Diffusion

• The spontaneous movement of particles from a region of higher concentration to a region of lower concentration.

Osmosis

• The movement of water or solvents across a semi-permeable membrane from a region of lower concentration to a region of higher concentration, in response to the concentration gradient of solutes.
• Types:
  • Endosmosis: Movement of water into the cell, causing it to swell or become turgid.
  • Exosmosis: Movement of water out of the cell, causing it to shrink or become flaccid.
• Osmotic solutions:
  • Isotonic: Same solute concentration inside and outside the cell.
  • Hypertonic: Higher solute concentration outside the cell, causing water to move out of the cell.
  • Hypotonic: Higher solute concentration inside the cell, causing water to move into the cell.

Osmotic Pressure

• The minimum pressure required to stop the movement of water across a semi-permeable membrane.
• In a hypertonic solution, the osmotic pressure is higher than the pressure inside the cell, forcing water out. In hypotonic, the pressure is lower, drawing water in.
• Examples:
  • Feeling thirsty after salty food (hypertonic solution).
  • Swelling of resins and seeds when soaked in water (hypotonic solution).
  • Dialysis of the kidney (regulation of water and solute levels in the blood).
  • Water and mineral movement in plants.

This quiz covers essential concepts in cell biology, focusing on chromatin structure, intermediate filaments, and microtubules. You'll learn about the roles of euchromatin and heterochromatin, the significance of intermediate filaments in cell adhesion, and the function of microtubules in intracellular transport and cell division.