Chloroplasts contain several important membranes, vital for their function. Like mitochondria, chloroplasts have a double-membrane envelope, called the chloroplast envelope, but unlike mitochondria, chloroplasts also have internal membrane structures called

thylakoid Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...

s. Furthermore, one or two additional membranes may enclose chloroplasts in organisms that underwent

secondary endosymbiosis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possi ...

, such as the

euglenid Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typical o ...

s and chlorarachniophytes. The chloroplasts come via

endosymbiosis An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

by engulfment of a photosynthetic cyanobacterium by the eukaryotic, already mitochondriate cell. Over millions of years the endosymbiotic cyanobacterium evolved structurally and functionally, retaining its own DNA and the ability to divide by binary fission (not mitotically) but giving up its autonomy by the transfer of some of its genes to the nuclear genome.

Envelope membranes

Each of the envelope membranes is a

lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...

that is between 6 and 8 nm thick. The lipid composition of the outer membrane has been found to be 48% phospholipids, 46%

galactolipid Galactolipids are a type of glycolipid whose sugar group is galactose. They differ from glycosphingolipids in that they do not have nitrogen in their composition. They are the main part of plant membrane lipids where they substitute phospholipids ...

s and 7% sulfolipids, while the inner membrane has been found to contain 16% phospholipids, 79%

s and 5% sulfolipids in spinach chloroplasts. The outer membrane is permeable to most

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

and metabolites, but the inner membrane of the chloroplast is highly specialised with

transport protein A transport protein (variously referred to as a transmembrane pump, transporter, escort protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that serves the function of moving other materials within ...

s. For example, carbohydrates are transported across the inner envelope membrane by a triose phosphate translocator. The two envelope membranes are separated by a gap of 10–20 nm, called the intermembrane space.

Thylakoid membrane

Within the envelope membranes, in the region called the stroma, there is a system of interconnecting flattened membrane compartments, called the

s. The thylakoid membrane is quite similar in lipid composition to the inner envelope membrane, containing 78%

s, 15.5% phospholipids and 6.5% sulfolipids in spinach chloroplasts. The thylakoid membrane encloses a single, continuous aqueous compartment called the

thylakoid lumen Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...

. These are the sites of light absorption and ATP synthesis, and contain many proteins, including those involved in the electron transport chain. Photosynthetic pigments such as chlorophylls a,b,c and some others, e.g., xanthophylls, carotenoids, phycobilins are also embedded within the granum membrane. With exception of chlorophyll a, all the other associated pigments are "accessory" and transfer energy to the reaction centers of Photosytems I and II. The membranes of the thylakoid contain photosystems I and II which harvest solar energy to excite electrons which travel down the electron transport chain. This exergonic fall in potential energy along the way is used to draw (not pump!) H⁺ ions from the lumen of the thylakoid into the cytosol of a

cyanobacterium Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

or the stroma of a chloroplast. A steep H⁺ gradient is formed, which allows

chemiosmosis Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a memb ...

to occur, where the thylakoid, transmenbrane ATP-synthase serves a dual function as a "gate" or channel for H⁺ ions and a catalytic site for the formation of ATP from ADP + a PO₄³⁻ ion. Experiments have shown that the pH within the stroma is about 7.8, while that of the lumen of the thylakoid is 5. This corresponds to a six-hundredfold difference in concentration of H⁺ ions. The H⁺ ions pass down through the ATP-synthase catalytic gate. This chemiosmotic phenomenon also occurs in mitochondria.

Envelope membranes

Thylakoid membrane

References

See also