Membranes, Transport, and Cellular Energetics
Membrane Structure and Dynamics
Lipid bilayers as self-organizing systems
Biological membranes are two-dimensional fluids whose composition controls curvature, permeability, signaling, and protein organization. Amphipathic lipids spontaneously form bilayers because hydrophobic tail exposure to water is energetically costly. Sterols, sphingolipids, and asymmetric leaflet composition modulate fluidity and phase behavior, while membrane proteins create highly selective conduits for transport and signal transduction.
Biophysical Insight
Membranes are not passive barriers; they are active platforms for organizing reactions, detecting mechanical forces, and coupling gradients to work.
What is the direct energetic source for ATP synthesis by ATP synthase in oxidative phosphorylation?
ATP synthase uses the proton electrochemical gradient, not electron flow directly, to drive phosphorylation.
Correct answer: The proton motive force across the inner membrane
Define the proton motive force.
It can be expressed as an energy difference that couples proton flow to work.
Correct answer: The combined electrochemical gradient of protons across a membrane, consisting of a membrane potential and a pH difference.
Transport Modes
Transport Strategies
Simple diffusion
- Moves down concentration gradient
- No transporter required
- Limited by membrane permeability
Facilitated diffusion
- Uses channels or carriers
- Still down gradient
- Can be saturable
Which transport process is most likely to be saturable?
Carrier proteins have finite turnover and binding sites, producing saturation kinetics.
Correct answer: Carrier-mediated glucose uptake
Why is membrane asymmetry biologically important?
Asymmetry is used as a regulatory and identity code in many cellular events.
Correct answer: Because different leaflet compositions support signaling, curvature, trafficking, and recognition processes.