2.3 Plasma Membrane
- Syllabus
- 2025
- Topic
- 2.3
- Level
- —
Learning objectives
2.3A—Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the…Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.• Phospholipids have both hydrophilic and hydrophobic regions. The polar hydrophilic phosphate regions of the phospholipids are oriented toward the aqueous external or internal environment, while the nonpolar hydrophobic fatty acid regions face each other within the interior of the membrane.• Embedded proteins can be hydrophilic (with charged and polar side groups), hydrophobic (with nonpolar side groups), or both.- i. Hydrophilic regions of the proteins are either inside the interior of the protein or exposed to the cytosol (cytoplasm).- ii. Hydrophobic regions of proteins make up the protein surface that interacts with the fatty acids in the interior membrane.2.3B—Describe the fluid mosaic model of cell membranesDescribe the fluid mosaic model of cell membranes.• Plasma membranes consist of a structural framework of phospholipid molecules embedded with proteins, steroids (such as cholesterol in vertebrate animals), glycoproteins, and glycolipids. All of these can move around the surface of the cell within the membrane, as illustrated by the fluid mosaic model.
Membrane Components Match Their Chemical Environment
A phospholipid is amphipathic: its phosphate region is polar and hydrophilic, while its fatty-acid regions are nonpolar and hydrophobic. In water, phospholipids form a bilayer with hydrophilic regions facing the aqueous cytosol and extracellular fluid and hydrophobic regions facing one another inside the membrane.
| Protein region | Compatible position | Why |
|---|---|---|
| Hydrophobic / nonpolar surface | Against fatty-acid regions inside the bilayer | It matches the membrane's nonpolar interior |
| Hydrophilic / charged or polar surface | Exposed to cytosol or another aqueous environment | It can interact with water |
| Hydrophilic region inside a folded protein | Shielded within the protein | Protein folding can place polar regions away from the lipid interior |
This chemical matching stabilizes a continuous membrane boundary while allowing proteins with both hydrophobic and hydrophilic regions to sit within it. The membrane can therefore separate the cell's internal environment from its surroundings without requiring every component to have the same chemistry.
The Membrane Is a Fluid Mosaic
The fluid mosaic model describes the plasma membrane as a phospholipid framework containing proteins, steroids such as cholesterol in vertebrate animals, glycoproteins, and glycolipids. These components form one membrane but are not fixed in a permanent pattern.
| Term | What it means in the model |
|---|---|
| Fluid | Phospholipids and embedded components can move around the cell surface within the membrane |
| Mosaic | Different kinds of molecules are interspersed through the phospholipid framework |
Fluid does not mean that the membrane dissolves or loses its boundary. Mosaic does not mean a rigid, tiled wall; it refers to the varied components distributed through a dynamic membrane.