Cell Membranes and Transport
Cell membranes are selectively permeable lipid bilayers that define the boundaries of cells and organelles and control what crosses them.
Definition
A biological membrane is a thin bilayer of lipids with embedded proteins that separates compartments and regulates transport; membrane transport is the collection of processes that move ions and molecules across this barrier.
Scope
This area covers the structure of biological membranes as fluid lipid bilayers studded with proteins, the mechanisms by which solutes cross them through diffusion, channels, carriers, and pumps, and the electrical properties of membranes that arise from selective ion permeability.
Sub-topics
Core questions
- How is a biological membrane structured at the molecular level?
- How do cells move solutes across an otherwise impermeable bilayer?
- What distinguishes passive from active transport?
- How does selective ion permeability generate a membrane potential?
Key theories
- Fluid mosaic model
- Biological membranes are two-dimensional fluids in which a lipid bilayer hosts proteins that can diffuse laterally, giving membranes both stability and dynamic behavior.
Mechanisms
Membranes are bilayers of amphipathic lipids in which integral and peripheral proteins are embedded. Small nonpolar molecules cross by simple diffusion, while ions and polar solutes require transport proteins: channels provide selective pores, carriers bind and shuttle solutes, and pumps use energy to move solutes against gradients. Differential ion permeability and active pumping establish electrochemical gradients that store energy and underlie membrane potential.
Clinical relevance
Membrane biology underpins how cells maintain their internal environment, communicate, and convert energy, and it provides the framework for understanding transport and electrical signaling. The treatment here is descriptive and non-prescriptive.
History
Early bilayer experiments established the lipid foundation of membranes; the 1972 fluid mosaic model of Singer and Nicolson unified the picture of proteins within a fluid bilayer, and later structural studies of pumps and channels detailed how transport is achieved.
Key figures
- S. Jonathan Singer
- Garth Nicolson
- Jens Christian Skou
- Roderick MacKinnon
Related topics
Seminal works
- singer1972
- alberts2014
Frequently asked questions
- What is the fluid mosaic model?
- It describes a cell membrane as a fluid lipid bilayer within which proteins float and move laterally, like a mosaic of components in a two-dimensional liquid.
- Why can't most ions cross a membrane freely?
- The hydrophobic interior of the lipid bilayer repels charged ions, so they can only cross through specific transport proteins such as channels, carriers, and pumps.