Premed · Premed · Biochemistry
Lecture 12: Membrane Transport
Biochemistry
Learning Objectives
By the end of this lecture, students will be able to:
- Distinguish between passive and active transport mechanisms
- Explain simple diffusion, facilitated diffusion, and osmosis
- Describe the structure and function of ion channels and aquaporins
- Explain primary active transport using the Na+/K+-ATPase as a model
- Describe secondary active transport (symport and antiport)
- Explain the principles of electrochemical gradients and membrane potential
Lecture Content
I. Overview of Membrane Transport
The lipid bilayer is selectively permeable. Small nonpolar molecules such as O2, CO2, and N2 pass freely, and small uncharged polar molecules like water, ethanol, and urea cross slowly. However, large polar molecules (glucose, amino acids), ions (Na+, K+, Cl-, Ca2+), and charged molecules cannot cross without assistance. Transport mechanisms are needed for nutrients, waste, ions, and signaling molecules and fall into two fundamental categories: passive transport, which moves solutes down their concentration or electrochemical gradient without energy input, and active transport, which moves solutes against their gradient and requires energy.
II. Thermodynamics of Transport
The free energy change for transporting an uncharged solute is given by delta-G = RT ln([S]in/[S]out). When the internal concentration is lower than the external, delta-G is negative and transport is spontaneous (passive). When the internal concentration is higher, delta-G is positive and energy is required (active). For charged solutes, the electrochemical gradient must be considered: delta-G = RT ln([S]in/[S]out) + ZF(delta-psi), where Z is the charge, F is the Faraday constant, and delta-psi is the membrane potential. The resting membrane potential of most cells is -60 to -90 mV (inside negative), which favors the entry of cations and opposes the entry of anions.
III. Passive Transport
Simple Diffusion
Simple diffusion is the direct passage of molecules through the lipid bilayer without a protein carrier. The rate is proportional to the concentration gradient and the solute's lipid solubility, following Fick's law: J = P x A x (C_out - C_in), where P is the permeability coefficient and A is the membrane area. This mechanism is important for O2, CO2, steroid hormones, ethanol, and anesthetic gases.
Facilitated Diffusion
Facilitated diffusion is transport mediated by a membrane protein, still moving down the concentration gradient and requiring no energy input. It shows saturation kinetics similar to Michaelis-Menten enzyme kinetics. Two types of proteins mediate this process. Carriers (transporters) bind the solute, undergo a conformational change, and release the solute on the other side, as exemplified by the GLUT glucose transporters. Channels form hydrophilic pores that allow rapid, selective passage, as seen with ion channels and aquaporins.
GLUT Transporters (Glucose Transporters)
The GLUT family comprises facilitated glucose transporters numbered GLUT1 through GLUT14. GLUT1 is ubiquitous, found in erythrocytes and at the blood-brain barrier, with high affinity (low Km of approximately 1 mM). GLUT2 is found in the liver, pancreatic beta-cells, and intestine, with low affinity (high Km of approximately 15-20 mM), and serves as a glucose sensor. GLUT3 is found in neurons with high affinity. GLUT4 is found in skeletal muscle and adipose tissue and is insulin-dependent -- it is stored in intracellular vesicles and translocated to the plasma membrane in response to insulin. Defective GLUT4 translocation contributes to insulin resistance in type 2 diabetes. GLUT5 is a fructose transporter in the small intestine.
IV. Ion Channels
Ion channels are integral membrane proteins that form selective, gated pores allowing rapid ion flux of up to 10^8 ions per second per channel. Their selectivity is determined by the size and charge of the channel pore; for example, K+ channels have a selectivity filter with carbonyl oxygens that mimic the hydration shell of K+ but not Na+. Gating mechanisms include voltage-gated channels that open and close in response to membrane potential changes (Na+, K+, Ca2+ channels in neurons), ligand-gated channels that open upon specific ligand binding (nicotinic acetylcholine receptor, GABA receptor), and mechanically-gated channels that open in response to physical force such as stretch or pressure.
Aquaporins are water channels that allow rapid, selective water transport. Aquaporin-1 is ubiquitous, found in red blood cells and the kidney proximal tubule. Aquaporin-2, located in the kidney collecting duct, is regulated by vasopressin (ADH), and defects cause nephrogenic diabetes insipidus.
<image>A figure showing different types of membrane transport. Panel A: Simple diffusion of a small nonpolar molecule directly through the lipid bilayer. Panel B: Facilitated diffusion through a carrier protein (showing conformational change) with a saturation kinetics graph comparing carrier-mediated transport to simple diffusion. Panel C: An ion channel (voltage-gated) showing the open and closed conformations, with the selectivity filter enlarged to show how K+ ions are selected by backbone carbonyl interactions. Panel D: An aquaporin channel showing the single-file passage of water molecules with protons excluded by the channel architecture.</image>
V. Primary Active Transport
Primary active transport uses energy directly, usually from ATP hydrolysis, to move solutes against their gradient. These proteins are also called "pumps."
Na+/K+-ATPase (Sodium-Potassium Pump)
The Na+/K+-ATPase is the most important primary active transporter in animal cells, maintaining the Na+ and K+ gradients by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed. It is electrogenic, creating a net positive charge outside and contributing to the membrane potential. The mechanism follows the Post-Albers cycle: the E1 conformation faces the cytoplasm and binds 3 Na+; ATP phosphorylates an aspartate residue on the pump to form E1-P; a conformational change to E2-P releases Na+ extracellularly; E2-P then binds 2 K+ from outside; dephosphorylation produces E2; and a final conformational change back to E1 releases K+ into the cytoplasm. This pump consumes approximately 25% of total cellular ATP (up to 70% in neurons) and is inhibited by ouabain and digoxin (cardiac glycosides used to treat heart failure). Their inhibition increases intracellular Na+, reduces Na+/Ca2+ exchange, increases intracellular Ca2+, and produces stronger cardiac contraction.
Other Primary Active Transporters
Ca2+-ATPase (SERCA) pumps Ca2+ into the sarcoplasmic reticulum in muscle cells. H+/K+-ATPase in gastric parietal cells pumps H+ into the stomach lumen and is the target of proton pump inhibitors like omeprazole. H+-ATPase on the lysosomal membrane maintains acidic pH in lysosomes. ABC transporters (ATP-binding cassette) transport diverse substrates; CFTR is a chloride channel whose mutations cause cystic fibrosis, and MDR1 (P-glycoprotein) is an efflux pump for hydrophobic drugs that contributes to multidrug resistance in cancer.
VI. Secondary Active Transport
Secondary active transport uses the energy stored in an ion gradient (established by primary active transport) to drive another solute against its gradient. There is no direct ATP hydrolysis; instead, the "downhill" movement of one solute is coupled to the "uphill" movement of another. In symport (cotransport), both solutes move in the same direction, as with SGLT1 (sodium-glucose cotransporter), which uses the Na+ gradient to drive glucose absorption in the intestine and glucose reabsorption in the kidney. In antiport (exchange), solutes move in opposite directions, as with the Na+/Ca2+ exchanger (Na+ in, Ca2+ out, important in cardiac muscle), the Na+/H+ exchanger (regulates intracellular pH), and the Cl-/HCO3- exchanger (Band 3 protein in red blood cells, mediating the chloride shift).
VII. Ionophores
Ionophores are small hydrophobic molecules that facilitate ion transport across membranes and are used as experimental tools; some have antibiotic activity. Carrier ionophores like valinomycin bind ions, diffuse across the membrane, and release the ions on the other side (valinomycin is selective for K+). Channel-forming ionophores like gramicidin A create pores in the membrane that allow passage of monovalent cations. Ionophores disrupt ion gradients, collapse membrane potential, and can uncouple oxidative phosphorylation.
<image>A diagram of the Na+/K+-ATPase pump cycle. The figure shows the six steps of the Post-Albers cycle in a circular arrangement. Step 1: E1 conformation with 3 Na+ bound on the cytoplasmic side. Step 2: ATP phosphorylates the pump, forming E1-P. Step 3: Conformational change to E2-P with Na+ released extracellularly. Step 4: 2 K+ bind from outside. Step 5: Dephosphorylation to E2. Step 6: Conformational change to E1 with K+ released into cytoplasm. The net result (3 Na+ out, 2 K+ in, 1 ATP consumed) is shown in the center. The electrogenic nature is indicated with + and - signs on either side of the membrane.</image>
