Chloroplasts are surrounded by two membranes. This double-membrane structure, known as the chloroplast envelope, consists of an outer and an inner membrane that encase the stroma and thylakoids inside.

Membrane Structure

The outer membrane is smooth and permeable, allowing small molecules to pass through easily. The inner membrane is selective, regulating transport into the stroma where photosynthesis occurs. Inside, thylakoid membranes form stacks called grana, but these do not count toward the outer envelope.

Why Two Membranes?

Chloroplasts evolved from endosymbiotic cyanobacteria, inheriting their double membrane (inner from plasma membrane, outer from cell wall). This setup protects photosynthetic machinery while enabling metabolite exchange. In primary chloroplasts (plants, green algae), exactly two surround the organelle; secondary ones in some protists may have more.

Key Facts

  • Outer membrane : Freely permeable via porins.
  • Inner membrane : Contains transporters for ions, sugars.
  • Thylakoids : Internal, not part of envelope; host chlorophyll.

TL;DR: Two membranes surround a chloroplast.

Information gathered from public forums or data available on the internet and portrayed here.