Plasma Membrane & Transport Lecture

Importance of Plasma Membrane

• Lipid bilayer consists of phospholipids
• Key components: lipid rafts, cholesterol, glycoproteins, transport proteins
• Fluid mosaic model: describes the dynamic nature of the plasma membrane
• Functions in the immune system and cancer evasion

High Yield for MCAT

• Plasma membrane: structure and function
• Active and Passive transport mechanisms

Mid Yield Topics

• Membrane-bound organelles and oxidative phosphorylation
• Basic understanding of other organelles:
  • Ribosome: protein synthesis
  • Rough ER: protein synthesis (ribosomes attached)
  • Smooth ER: lipid synthesis and detoxification
  • Lysosomes: breakdown of macromolecules
  • Mitochondria: ATP production, has its own genetic code

Structure and Composition of Plasma Membrane

• Main Components:
  • Phospholipids: form the bilayer
  • Proteins: ion channels, transport proteins
  • Cholesterol: maintains fluidity
  • Carbohydrates: involved in cell signaling and identification

Functions of Membrane Components

• Phospholipids: hydrophilic heads and hydrophobic tails, create a barrier for selective permeability
• Cholesterol: maintains membrane fluidity
  • At low temperatures: prevents membrane from freezing
  • At high temperatures: prevents membrane from becoming too fluid
• Proteins: channels for molecules, signaling (e.g., CTLA-4 for immune evasion)
• Carbohydrates: cell signaling and identification, not heavily tested on MCAT

Fluid Mosaic Model

• Describes the dynamic and flexible nature of the membrane
• Components are constantly moving and rotating (flipping)

Active vs Passive Transport

• Active Transport: requires energy (ATP)
  • Primary: direct use of ATP
  • Secondary: uses ion gradients created by primary transport (symporters and antiporters)
• Passive Transport: does not require energy
  • Simple Diffusion: molecules move directly through the membrane (e.g., hydrophobic molecules)
  • Facilitated Diffusion: requires transport proteins (e.g., water through aquaporins)

Prokaryotic vs Eukaryotic Membranes

• Eukaryotes: sterols, glycolipids
• Prokaryotes: peptidoglycan cell wall

Helpful Analogies

• Simplifying concepts with relatable analogies (e.g., floaties in a pool for fluid mosaic model)

Application on MCAT Questions

• Understand the context of questions on transport and membrane structure

Concepts and Processes to Remember

• Significance of cholesterol in membrane fluidity
• Transport protein specificity
• Difference between hydrophobic and hydrophilic molecules and their transport mechanisms
• Endocytosis and exocytosis mechanisms

Endocytosis and Exocytosis

• Endocytosis: cell ingests materials by engulfing them
• Exocytosis: cell expels materials by vesicles fusing with the membrane
• Maintains homeostasis in cell size

Summary of Key Points for MCAT

• Fluid mosaic model
• Hydrophobic vs Hydrophilic transport mechanisms
• Major membrane components: lipids, proteins, carbohydrates
• Differences between prokaryotic and eukaryotic membranes
• Transport mechanisms: active vs passive, facilitated diffusion

Practice Questions

1. What other cellular components are likely to be located near the lacY transcript in the cell membrane (prokaryotic)?

   • Proteins and glycolipids: Incorrect (also in eukaryotes)
   • Glycolipids and sterols: Incorrect (only in eukaryotes)
   • Sterols and phospholipids: Incorrect (sterols in eukaryotes)
   • Phospholipids and proteins: Correct (common in prokaryotes)

2. Which type of molecule is least likely to be found in a eukaryotic cell membrane?

   • Phospholipids: Incorrect (commonly found)
   • Cholesterol: Incorrect (commonly found)
   • Glycoproteins: Incorrect (commonly found)
   • Peptidoglycan: Correct (found in prokaryotes)

3. Mechanism for STN entry into the cell considering its structure:

   • Active transport: Incorrect (no concentration gradient context)
   • Receptor-mediated endocytosis: Incorrect (usually for larger molecules)
   • Diffusion directly through the membrane: Correct (hydrophobic structure)
   • Passage through ion channel: Incorrect (STN is not an ion)