4.5 Chapter 4 Summary

1. Cell Types & Compartments

Key distinctions between prokaryotic & eukaryotic cells

• Eukaryotes possess a true nucleus (membrane-bound) and multiple organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, chloroplasts in plants)

• Cytoplasm: the space between the plasma membrane and nuclear envelope, filled with cytosol, cytoskeleton, dissolved molecules, and where many metabolic reactions occur

• Organelle compartmentalization: Organelles create specialized internal environments (e.g., acidic lumens in lysosomes) to optimize biochemical functions

• Plasma membranes are phospholipid bilayers embedded with proteins and cholesterol that regulate molecular traffic and maintain cellular integrity

2. Plasma Membrane Structure & Components

• Explained by the fluid-mosaic model: a dynamic bilayer of phospholipids where proteins, cholesterol, and carbohydrates are interspersed and laterally mobile.

• Selectivity: amphipathic nature allows small nonpolar molecules (O₂, CO₂, fat-soluble vitamins) to diffuse directly, whereas ions and polar molecules require transport proteins

• Membrane asymmetry: inner and outer leaflets differ in lipid and protein composition; glycoproteins and glycolipids on the exterior aid cell recognition and adhesion

3. Passive Membrane Transport

Passive transport involves movement down a concentration gradient without using ATP

Main types:

1. Simple diffusion: nonpolar molecules pass directly through the bilayer (e.g., O₂, CO₂)

2. Facilitated diffusion:

   • Channel proteins form hydrophilic pores (e.g., aquaporins for water, ion channels for Na⁺, K⁺, Cl⁻)

   • Carrier proteins (e.g., GLUT transporters) bind specific substrates and undergo conformational changes to shuttle molecules.

Osmosis & tonicity

• Osmosis: water flows through a semipermeable membrane toward higher solute concentration — often via aquaporins.

• Tonicity: describes solution effects on cell volume:

  • Hypotonic: water enters → cells swell/lyse

  • Isotonic: balanced, no net change

  • Hypertonic: water exits → cells shrink/crenate.

• Biological responses: e.g., plant cell turgor, animal osmoregulation via vacuoles and kidneys

4. Active Membrane Transport

Active transport moves substances against gradients and requires energy (typically ATP) See Wikipedia’s page on Active transport for more information [New Tab].

Types:

1. Primary active transport

   • Directly uses ATP.

   • Example: Na⁺/K⁺‑ATPase pumps Na⁺ out and K⁺ in, critical for membrane potential and secondary transport mechanics.

2. Secondary active transport

   • Uses energy from existing gradients.

   • Symporters (e.g., Na⁺/glucose co-transport) move two substances simultaneously in the same direction.

   • Antiporters (e.g., Na⁺/H⁺ exchanger) move in opposite directions.

Additional mechanisms

• ATP-binding cassette (ABC) transporters, P-type pumps, V-type pumps transport ions, toxins, drugs.

• Endocytosis & exocytosis: bulk transport processes (e.g., phagocytosis, pinocytosis, receptor-mediated endocytosis, vesicle fusion for secretion)

🔍Summary Table

Summary was initially generated by ChatGPT then modified by the author.