Cell Structure and Organelles (Membranous and Nonmembranous)

Membrane-bound organelles

• Nucleus: stores genetic information; DNA primarily as chromatin most of the time; chromosomes visible during cell division (mitosis). Mitosis not the focus here.
• Nucleolus: smallest observable structure under light microscopy; ribosome production site.
• Mitochondria: powerhouses of the cell; inner membrane folds (cristae) with matrix between; site of ATP production from sugars via respiration; O extrm{2} consumed, CO extsubscript{2} produced. ATP is the cellular energy currency.
  • Simplified energy equation: \text{Glucose} + 6\,\text{O}2 \rightarrow 6\,\text{CO}2 + 6\,\text{H}_2\text{O} + \text{ATP}
• Endoplasmic reticulum (ER): network of membranes forming fluid-filled chambers for intracellular transport.
  • Rough ER: studded with ribosomes; synthesizes proteins.
  • Smooth ER: lacks ribosomes; synthesizes lipids (phospholipids, cholesterol) and other lipids; site for lipid-based membrane maintenance and steroid hormone synthesis (notably in testes/ovaries).
• Golgi apparatus: the cell’s post office; receives vesicles from ER, sorts and packages them; packages into vesicles with three fates:
  1) vesicle becomes a lysosome (inside the cell),
  2) vesicle fuses with the cell membrane to donate phospholipids (recycling/replacement),
  3) vesicle becomes a secretory vesicle and releases contents via exocytosis (e.g., hormones).
• Lysosomes: membranous sacs with digestive enzymes; digest old/damaged organelles and pathogens; autolysis (programmed cell death) can occur during development.
• Peroxisomes: contain oxidase enzymes; detoxify harmful substances (e.g., alcohol, formaldehyde, free radicals); break down hydrogen peroxide to water and oxygen.

Non-membrane bound organelles

• Cytoskeleton: network of protein filaments that provide support and enable movement; composed of microfilaments, intermediate filaments, and microtubules (smallest to largest).
  • Microfilaments: just under the plasma membrane; maintain cell shape and surface integrity; important in cells like erythrocytes.
  • Intermediate filaments: high tensile strength; resist pulling forces.
  • Microtubules: hollow tubes; anchor and position organelles; track for movement; form the mitotic spindle during cell division.
• Centrosomes and centrioles: Centrosome = central body near the cell’s center; contains two centrioles; microtubule organizing center; essential for mitotic spindle formation.

Surface area to volume ratio concepts

• SA:V ratio limits cell size; larger organisms adapt shapes to optimize heat exchange and nutrient/waste exchange; examples include limb/ear size differences for thermoregulation.
• Why SA:V matters for cells: large cells have diffusion challenges; a high SA promotes absorption and exchange.
• Microvilli: finger-like extensions of the plasma membrane that dramatically increase surface area to enhance absorption (e.g., intestinal cells).
• Not to be confused with cilia: cilia are motile extensions that move substances along the cell surface (e.g., mucus in airways, movement of eggs in fallopian tubes); built from microtubules.
• Flagella: long extensions used for cellular locomotion (e.g., sperm tail).

Quick reference concepts

• Distinguish membranous vs non-membranous organelles.
• Key membranous organelles: nucleus/nucleolus, mitochondria, ER (RER & SER), Golgi, lysosomes, peroxisomes.
• Key non-membranous components: cytoskeleton (microfilaments, intermediate filaments, microtubules) and centrosomes/centrioles.
• Energy/life cycle: ATP production in mitochondria; respiration consumes O extrm{2} and releases CO extsubscript{2}.
• Nucleolus as the clue to locating the nucleus under light microscopy.