Membrane Transport Lecture Notes

Introduction

• Mechanisms of membrane transport
• Importance of understanding these processes

Simple Diffusion

• Definition: Passive process, no energy (ATP) involved
• Movement: Molecules move from high to low concentration gradients
• Examples of Molecules:
  • Respiratory gases (Oxygen and CO2)
  • Steroid hormones (e.g., testosterone, estrogen)
  • Lipid-soluble drugs
• Cell Membrane Structure:
  • Composed of phospholipid bilayer
  • Phospholipid heads are polar, fatty acid tails are nonpolar
  • Non-polar molecules can move freely across

Factors Affecting Diffusion Rate

• Surface Area: Greater surface area increases diffusion
• Concentration Gradient: Higher gradient increases diffusion
• Membrane Thickness: Thicker membrane decreases diffusion
• Molecule Weight: Heavier molecules diffuse slower

Facilitated Diffusion

• Definition: Passive process, usually no direct ATP use
• Mechanism: Requires transport proteins (channels/carriers)
• Types: for charges
  • Osmosis: Movement of water through aquaporins
  • Ion Channels:
    • Leaky Channels (e.g., potassium in neurons)
    • Voltage-Gated Channels (e.g., sodium, calcium in neurons)
    • Ligand-Gated Channels (e.g., acetylcholine at neuromuscular junction)
    • Mechanically-Gated Channels (e.g., pain receptors)
  • Carrier-Mediated: Example - GLUT4 (in adipose and muscle) transporter regulates glucose uptake via insulin

Active Transport

Primary Active Transport

• Definition: Direct use of ATP to move substances from low to high concentration (against gradient)
• Mechanism: Involves ATPases
• Key Examples:
  • Sodium-Potassium ATPase: Moves Na+ out, K+ into the cell(insulin and thyroid increases atpases (sodium and potassium)while digoxin decreases
  • Calcium Pumps: Regulate calcium in muscle relaxation
  • Proton Pumps: Found in stomach, influenced by proton pump inhibitors

Secondary Active Transport

• Definition: Indirect use of ATP
• Mechanism: Coupled transport via symport or antiport
• Examples:
  • Sodium-Glucose Co-transporter: Important in kidneys, targeted by SGLT2 inhibitors in case of diabetes
  • Sodium-Potassium-2-Chloride Symporter: Targeted by loop diuretics
  • Sodium-Calcium Exchanger and sodim hydrogion anti port: Critical in cardiac muscle (digoxin inhibitor)and urine respectively(aldosterone-increases then na and h ion also increase therefore alkalosis and acidosis is vice versa

Vesicular Transport using atp

Endocytosis

• Types:
  • Pinocytosis: Cellular "drinking", involves small solutes(intestines) uses atp when filaments (kinesins and dynein of micro tubules)are used for transporting vesicles (pinnocytic vesicle)
  • Phagocytosis: Cellular "eating", involves large particles like bacteria(neutro and macrophages ) by using actins called psudopods. (Phagosome)
  • Receptor-Mediated Endocytosis: Specific uptake (e.g., LDL uptake in liver) (clathrin molecules form pits) ( endosome) and movt of vesicles by rab proteins Disease (famicial hypercolestrolemia here ldl increases)

Exocytosis

• Function: Expelling cellular contents
• Significance: Neurotransmitter release, hormone secretion, mucin release
• Mechanism: Involves vesicle transport via cytoskeleton and SNARE proteins(v and t snares cause fusing of vesicles with cell membrane and thus releasing substances and v and t are calcium dependent )

Conclusion

• Summary of membrane transport mechanisms
• Importance of understanding for cellular function and medical relevance