The inner membrane is the biological membrane (phospholipid bilayer) of an organelle or Gram-negative bacteria that is within an outer membrane.

In eukaryotic cells, this inner membrane is present within the nuclear envelope, mitochondria and plastids like the chloroplast. The lumen between the inner and outer membranes is referred to as intermembrane space.

In prokaryotic cells such as many Gram-negative bacteria, the space between the inner and outer membrane is commonly referred to as the periplasmic space or periplasm. The inner membrane may also be referred to as the cytoplasmic membrane and it is similar in structure and protein content as the cytoplasmic membrane of other bacteria that contain only one membrane (such as most Gram-positive bacteria).

This structural arrangement of an inner and outer membrane is thought to be similar in Gram-negative bacteria, mitochondria and chloroplasts due to their ancestral relationship, as outlined in the endosymbiotic theory.

Inner membrane of the nuclear envelope

The inner membrane of the nuclear envelope is connected to the outer nuclear envelope membrane through nuclear pores. It contains a number of proteins involved in the structural organization of the nucleus and the attachment of chromatin to the nuclear envelope. In metazoan cells, the inner nuclear membrane contains proteins of the nuclear lamina, a protein meshwork underlying the nuclear envelope and providing structural support. Mutations in inner nuclear envelope proteins can cause nuclear envelopathies, a number of genetic disorders in humans.

Mitochondrial inner membrane

See main article at Inner mitochondrial membrane

The mitochondrial inner membrane forms internal compartments known as cristae, which allow greater space for the proteins such as cytochromes to function properly and efficiently.

Plastidial inner membrane

The plastidial inner membrane surrounds the stroma of the plastid. In chloroplasts, extensions of the inner membrane form the thylakoid membrane where the photosystems for photosynthesis are located. The inner and thylakoid membranes are very similar to the membranes found in cyanobacteria. Similar to mitochondria, plastids are thought to have evolved from bacterial endosymbionts.

See also

For eukaryotes

• Intermembrane space
• Chloroplast membrane

For bacteria

• periplasmic space
• outer membrane