Membranes and Membrane Transport
How the phospholipid bilayer and membrane proteins create selective permeability and support passive, active, and vesicular transport.
The plasma membrane separates the cytoplasm from the external environment while allowing controlled exchange of substances. Its structure is based on an amphipathic phospholipid bilayer with embedded or associated proteins.
Phospholipid
A molecule with a polar, hydrophilic phosphate-containing head and non-polar, hydrophobic fatty-acid tails.
In water, phospholipids form a bilayer with hydrophobic tails facing inward and hydrophilic heads facing the aqueous environments on both sides.
Small non-polar molecules such as O2 and CO2 cross the lipid bilayer relatively easily. Small uncharged polar molecules cross more slowly, while ions and most large polar molecules require membrane proteins.
Membrane proteins
Integral proteins are embedded in the bilayer; many span it completely. Peripheral proteins associate with a membrane surface or with other proteins without entering the hydrophobic core. Membrane proteins can function as channels, carriers, pumps, enzymes, receptors, anchors, or recognition markers.
Glycoproteins and glycolipids
Carbohydrate chains attached to proteins or lipids project mainly from the extracellular surface. They contribute to cell recognition, adhesion, receptor signaling, and interactions with the immune system.
Simple diffusion and osmosis
Simple diffusion is net movement down a concentration gradient without direct cellular energy input. Osmosis is net movement of water across a selectively permeable membrane down a water-potential gradient.
Facilitated diffusion
Channel and carrier proteins allow specific hydrophilic substances to move down their electrochemical or concentration gradients. Because movement is down the relevant gradient, facilitated diffusion does not directly require ATP.
Active transport
Active transport moves substances against an electrochemical gradient by coupling transport to an energy source. ATP-driven pumps often bind specific solutes, become phosphorylated or otherwise change conformation, release the solute on the other side, and return to their original state.
| Process | Protein required? | Direction relative to gradient | Direct energy input? |
|---|---|---|---|
| Simple diffusion | No | Down gradient | No |
| Facilitated diffusion | Yes | Down electrochemical/concentration gradient | No |
| Primary active transport | Yes | Can move against gradient | Usually ATP or another direct energy source |
| Osmosis | Aquaporins can greatly increase rate | Water moves down water-potential gradient | No |
Endocytosis and exocytosis
Vesicular transport moves material that cannot simply cross the membrane. During endocytosis, the membrane encloses material and pinches inward to form a vesicle. During exocytosis, an intracellular vesicle fuses with the plasma membrane and releases its contents outside the cell. Both processes require cellular energy and cytoskeletal/membrane machinery.