Plasma membranes separate the organelles from the cytoplasm and separate the cytoplasm from the extra-cellular environment, allowing certain metabolic events to proceed at their own pace, independent of each other and apart from events in the environment around them, increasing the efficiency of those metabolic processes. This prevents something called "futile cycles" for reversible reactions within cells.

Plasma membrane diagram. (Source: Campbell and Reece, 2002)

The plasma membrane is made up of a phospholipid bilayer, with a diverse number of proteins—each with a different function—embedded inside. It is composed of two layers of phospholipids facing each other.

• A phospholipid molecule consists of a phosphate hydrophilic head and a lipid hydrophobic tail.
• Phospholipids, and almost all transmembrane proteins, are amphipathic molecules: possessing both hydrophobic and hydrophilic bits.
• Technically, proteins, cholesterol, and other things associated with the phospholipids in a membrane are not included when we just say “phospholipid bilayer”.

The phospholipid bilayer forms the basic structure of plasma membranes. The capacity to form membranes is inherent in the structure of the phospholipid molecule. The hydrophobic tails will naturally cluster with each other between layers of hydrophilic heads. The most efficient way of keeping the hydrophobic tails away from water is for the phospholipids to form a bilayer, and arrange themselves into a sphere…the cell/organelles.

The plasma membrane surrounds the entire cytoplasm of the cell, as well as all of the organelles except for ribosomes. It constantly forms vesicles to transport, store, and break down substances inside the cell, and the various membrane-bound vesicles are constantly merging with the cell membrane in the process of endocytosis and/or excocytosis. The plasma membrane is produced by the ER, transported to the Golgi as vesicles, and then the Golgi secretes the vesicles to the cell membrane. The vesicle membrane merges with the cell membrane and thus adds to cell membrane surface area. The inside of the vesicle membrane becomes the outside of the cell membrane and vice versa.

The plasma membrane structure is reinforced by the cytoskeleton (specifically, networks of microfilaments).

In plasma membranes, the water soluble substances are channeled across by proteins.

Fluid mosaic model

The structure of a plasma membrane is very dynamic and fluid, with a mosaic of integral proteins drifting randomly around the phospholipid bilayer. Phospholipids can move laterally through the membrane very quickly (2μm/second), and occasionally can even flip to the other side of the bilayer.

Cholesterol is a temperature buffer: scattered among the phospholipids of animal plasma membranes, it maintains membrane fluidity at different temperatures, making the membrane less fluid at body temperature and more fluid at lower temperatures. Unsaturated fatty acid tails create spaces between phospholipids, increasing the membrane’s fluidity and thus lowering the temperature at which the membrane solidifies.

The plasma membrane is selectively permeable, which makes sure that the cell can uptake the necessary nutrients and expel the necessary wastes, without absorbing or excreting unnecessary materials. Water is able to diffuse through the plasma membrane, and so can lipid-soluble substances. But since the inside of the bilayer is hydrophobic, water soluble substances cannot diffuse across, but must pass through transport proteins either passively or actively. The cell can thus regulate the type of water-soluble material entering or leaving cells.

The integral and peripheral proteins attached to the membrane serve:

• As communication devices between cells, to receive signals that trigger a particular activity within the cell (in signal transduction pathways, like receptor proteins, G-proteins, and adenyl cyclases)
• To transport molecules across the membrane either passively or actively (channel proteins called aquaporins facilitate water movement across membranes)
• To act as enzymes and perform metabolic reactions like stages of photosynthesis and cellular respiration
• To act as cell-to-cell recognition proteins (usually glycoproteins)
  • Cells recognize each other (for instance, as foreign or “self”) by binding to these glycoproteins on the cell membrane)
• To join cells together (tight junctions, desmosomes)
• To attach the membrane to both the cytoskeleton within the cell and the ECM outside the cell.


Immunogold electron micrograph showing plasma membrane. (Source: Roermund et al., 1999)