Premed · Premed · Cell Biology
Lecture 5: Membrane Transport: Passive and Active
Cell Biology
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 carrier proteins and channels
- Explain primary and secondary active transport with specific examples
- Calculate and predict the direction of solute movement based on electrochemical gradients
Lecture Content
I. Principles of Membrane Transport
The plasma membrane is selectively permeable, meaning it allows some substances to pass while restricting others. This selectivity depends on several molecular properties. Size matters: small molecules pass more readily than large ones. Polarity is critical: nonpolar (hydrophobic) molecules cross the lipid bilayer easily, whereas polar and charged molecules cannot. Charge is the most restrictive factor, as ions cannot traverse the hydrophobic core of the membrane without assistance.
These properties create a permeability hierarchy. Gases such as O2, CO2, and N2 are the most permeable, followed by small nonpolar molecules like ethanol and urea, then small polar molecules such as water and glycerol, then large polar molecules like glucose and amino acids, and finally ions such as Na+, K+, Cl-, and Ca2+, which are essentially impermeant.
Membrane transport falls into two broad categories. Passive transport moves solutes down their concentration or electrochemical gradient and requires no energy input. Active transport moves solutes against their gradient and requires energy, either from ATP hydrolysis directly or from a pre-existing ion gradient.
II. Passive Transport
Simple diffusion is the most straightforward form of transport: molecules move from regions of high concentration to low concentration directly across the lipid bilayer, without the involvement of any protein. The rate of diffusion is proportional to both the concentration gradient and the membrane permeability of the solute, as described by Fick's law: J = -P A (delta-C). Examples of molecules that cross membranes by simple diffusion include O2, CO2, steroid hormones, and ethanol.
Osmosis is the diffusion of water across a semipermeable membrane. Water moves from regions of low solute concentration (high water potential) toward regions of high solute concentration (low water potential). The tonicity of a solution describes its effect on cell volume. In an isotonic solution, there is no net water movement and cell volume remains unchanged. In a hypotonic solution, water enters the cell, causing it to swell (and potentially lyse in animal cells). In a hypertonic solution, water exits the cell, causing it to shrink (crenation in animal cells, plasmolysis in plant cells). Osmotic pressure is the pressure that would need to be applied to prevent osmotic flow. Aquaporins, water channel proteins discovered by Peter Agre (Nobel Prize, 2003), facilitate the rapid movement of water across membranes. Aquaporin-1 in the kidney, for example, is responsible for reabsorbing approximately 180 liters of water per day.
Facilitated diffusion moves solutes down their concentration gradient through membrane proteins, requiring no energy input. Unlike simple diffusion, it is saturable, meaning the rate reaches a maximum (Vmax) when all transport proteins are occupied. Two types of proteins mediate facilitated diffusion. Channel proteins form hydrophilic pores that allow rapid transport. Ion channels are selective for specific ions and can be gated, meaning they open and close in response to voltage, ligand binding, or mechanical stimuli. Carrier proteins (also called transporters or permeases) bind the solute and undergo a conformational change to shuttle it across the membrane. Carriers are slower than channels but exhibit high substrate specificity. The GLUT family of transporters facilitates glucose transport: GLUT1 provides basal glucose uptake in most cells, while GLUT4 is regulated by insulin in muscle and adipose tissue. The kinetics of carrier-mediated transport follow a Michaelis-Menten-like pattern, with the Km reflecting the transporter's binding affinity for its substrate.
<image>Comparison of passive transport mechanisms across a cell membrane. Panel A: Simple diffusion showing small nonpolar molecules (O2, CO2) passing directly through the lipid bilayer from high to low concentration. Panel B: Osmosis showing water molecules moving through an aquaporin channel toward higher solute concentration, with diagrams of cells in hypotonic (swollen), isotonic (normal), and hypertonic (shrunken) solutions. Panel C: Facilitated diffusion via a channel protein (ion channel with selectivity filter) and a carrier protein (GLUT transporter undergoing conformational change to transport glucose). Panel D: Graph comparing transport rate vs. concentration for simple diffusion (linear) and facilitated diffusion (saturating curve reaching Vmax).</image>
III. Active Transport — Primary
Primary active transport moves solutes against their electrochemical gradient using energy derived directly from ATP hydrolysis.
The Na+/K+-ATPase (sodium-potassium pump) is the most important primary active transporter in animal cells. It pumps 3 Na+ ions out of the cell and 2 K+ ions in for every molecule of ATP hydrolyzed. Because more positive charges are moved out than in, the pump is electrogenic, contributing to the membrane potential. It maintains the characteristic ion gradients of animal cells: low intracellular Na+ (approximately 12 mM) and high intracellular K+ (approximately 140 mM). This single transporter consumes roughly 25 percent of total cellular ATP. It belongs to the P-type ATPase family, meaning it forms a phosphorylated intermediate during its transport cycle. Ouabain, a cardiac glycoside, inhibits this pump. The transport mechanism follows the Post-Albers cycle: the pump in its E1 conformation binds 3 Na+ on the cytoplasmic side; ATP phosphorylates the pump, forming E1-P; a conformational change to E2-P releases Na+ extracellularly; E2-P then binds 2 K+ on the extracellular side; dephosphorylation returns the pump to E2; and a final conformational change back to E1 releases K+ intracellularly.
The Ca2+-ATPase (SERCA) pumps calcium from the cytosol into the ER or sarcoplasmic reticulum lumen, maintaining the very low cytosolic Ca2+ concentration of approximately 100 nM against the much higher ER concentration of approximately 0.5 mM. This P-type ATPase is essential for muscle relaxation and for the precise regulation of Ca2+ signaling. The H+/K+-ATPase, found in gastric parietal cells, pumps protons into the stomach lumen to create the extremely acidic environment (pH 1-2) required for digestion. It is the target of proton pump inhibitor drugs such as omeprazole.
V-type ATPases are vacuolar proton pumps that acidify lysosomes, endosomes, and vacuoles. Unlike P-type ATPases, they do not form a phosphorylated intermediate. ABC transporters (ATP-Binding Cassette) constitute a large family of ATP-driven transporters. MDR1 (P-glycoprotein) pumps drugs out of cells and is a major cause of multidrug resistance in cancer. CFTR, mutated in cystic fibrosis, is an unusual ABC family member that functions as a chloride channel regulated by ATP binding and hydrolysis.
<image>Mechanism of the Na+/K+-ATPase. A six-step cycle diagram showing: Step 1 — E1 conformation with 3 Na+ bound on the cytoplasmic side. Step 2 — ATP phosphorylation of the pump forming E1-P. Step 3 — Conformational change to E2-P with Na+ released extracellularly. Step 4 — 2 K+ bind on extracellular side. Step 5 — Dephosphorylation returning to E2. Step 6 — Conformational change back to E1 with K+ released intracellularly. Net result: 3 Na+ out, 2 K+ in, 1 ATP consumed per cycle.</image>
IV. Active Transport — Secondary (Coupled Transport)
Secondary active transport harnesses the energy stored in an existing ion gradient, established by primary active transport, to drive the movement of another solute against its gradient. No direct ATP hydrolysis occurs; the energy coupling is indirect.
In symport (cotransport), two solutes move in the same direction. The Na+-glucose symporter (SGLT1) in the intestinal epithelium is a classic example: Na+ moves down its concentration gradient into the cell, and this favorable movement drives the simultaneous uptake of glucose against its gradient. The Na+ gradient that powers this transporter is itself maintained by the Na+/K+-ATPase. Other symporters include Na+-amino acid cotransporters in the intestine and kidney and the Na+/K+/2Cl- cotransporter (NKCC) in the thick ascending limb of the kidney.
In antiport (exchange or countertransport), two solutes move in opposite directions. The Na+/H+ exchanger (NHE) brings Na+ into the cell while expelling H+, thereby helping to regulate intracellular pH. The Na+/Ca2+ exchanger (NCX) exchanges 3 Na+ (in) for 1 Ca2+ (out), assisting in the maintenance of low cytosolic calcium. The Cl-/HCO3- exchanger (Band 3 protein) in red blood cells facilitates CO2 transport by exchanging chloride and bicarbonate across the membrane.
The Na+ gradient serves as the primary driving force for most secondary active transport in animal cells. In bacteria and plants, H+ gradients typically play this role instead.
V. The Electrochemical Gradient and Membrane Potential
The membrane potential (Vm) is the voltage difference across the plasma membrane. In typical animal cells, the resting membrane potential ranges from -40 to -80 mV, with the inside of the cell negative relative to the outside. This potential arises from three factors: ion concentration gradients (especially for K+), the selective permeability of the membrane to K+ through K+ leak channels at rest, and the small contribution of electrogenic pumps (the Na+/K+-ATPase contributes approximately -10 mV).
The Nernst equation calculates the equilibrium potential for a single ion species: E_ion = (RT/zF) ln([ion]outside / [ion]inside). At 37 degrees Celsius, this simplifies to E_ion = (61.5/z) log([ion]outside / [ion]inside) mV. The equilibrium potentials for the major ions are approximately: E_K = -90 mV, E_Na = +60 mV, E_Cl = -70 mV, and E_Ca = +120 mV.
The Goldman equation extends this analysis to consider multiple ion species and their relative permeabilities simultaneously. Because the membrane at rest is far more permeable to K+ than to Na+ (P_K >> P_Na), the resting membrane potential lies close to E_K.
The electrochemical gradient is the sum of the concentration gradient and the electrical gradient, and it determines the direction and magnitude of ion movement across the membrane. The free energy change for moving an ion across the membrane is given by delta-G = RT ln([ion]in / [ion]out) + zF Vm.
<image>Diagram of electrochemical gradients across the plasma membrane of a typical animal cell. Panel A: Table showing intracellular and extracellular concentrations of major ions — Na+ (12 mM in, 145 mM out), K+ (140 mM in, 5 mM out), Ca2+ (0.0001 mM in, 1.8 mM out), Cl- (4 mM in, 116 mM out). Panel B: Diagram of the membrane showing the Na+/K+-ATPase creating gradients, K+ leak channels establishing resting potential, and arrows indicating the direction of electrochemical driving forces for each ion. Membrane potential shown as -70 mV (inside negative).</image>
