Cellular Transport

How does membrane structure control transport functions?

Transport Mechanisms

1. Passive transport
   1. Does not require cellular energy expenditure
   2. Rate dependent on temperature, gradient difference, molecular size & charge
   3. Diffusion - net movement of molecules from area greater concentration to area of lesser concentration
   4. Osmosis - diffusion of water through a semipermeable membrane
2. Carrier mediated - requires protein transporter, limited by carrier saturation & specificity
   1. Facilitated diffusion - along conc gradient, not require energy
   2. Active transport - can work against conc gradient, requires cellular ATP expenditure
      • Secondary active transport - driven by energy stored in ion gradients
3. Bulk or vesicular transport
   1. Movement of particles & macromolecules through membrane
   2. Involves formation of membranous vesicles

Membrane Permeability

Membrane permeability - ability of substance to pass through membrane

1. Lipid soluble molecules (small nonpolar) & small uncharged polar molecules
   1. Diffuse through lipid bilayer
   2. Diffusion rate dependent on size & solubility
   3. ex. H₂O, O₂, CO₂, fatty acids, steroids, urea, ethanol
2. Polar molecules & small ions
   1. Diffuse through protein channels along electrochemical gradients
   2. ex. H₂O, Ma⁺, K⁺, Ca⁺², Cl^-, HCO₃^-
   3. Leakage channels - open continuously
   4. Gated channels - not open continuously
      • ex. Voltage-gated & ligand-gated (signal molecule at specific receptor site)
3. Glucose
   1. Facilitated diffusion into cells by specific transporter proteins along concentration gradient. Protein undergoes conformational change.
   2. Gradient maintained by phosphorylating intracellular glucose
   3. Limited by carrier saturation
4. Active transport of substances against concentration gradient
   1. Limited by carrier saturation & available ATP
   2. Driven by ATP hydrolysis
      1. ex. Na⁺/K⁺ pump, 3 Na out & 2 K in, antiport
5. Secondary transport
   1. Na gradient drives glucose-Na symport inintestinal cells, a.a.-Na symport
6. Vesicular transport
   1. Exocytosis - fusion of intracellular vesicle with membrane, secretory
   2. Endocytosis - membrane encloses & invaginates to produce intracellular vesicle
      1. Receptor mediated - selective concentrating mechanism, only specific ligand enters cell, ex. FE
      2. Pinocytosis - fluid or soluble matter
      3. Phagocytosis - large particles in vacuoles, specialized cells