Chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s contain several important
membranes
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
, 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
thylakoids. 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 possibly ...
, such as the
euglenids and
chlorarachniophyte
The chlorarachniophytes are a small group of exclusively marine algae widely distributed in tropical and temperate waters. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally t ...
s.
The chloroplasts come via
endosymbiosis 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 that is between 6 and 8
nm thick. The lipid composition of the outer membrane has been found to be 48%
phospholipid
Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s, 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 phospholipid ...
s and 7%
sulfolipid
Sulfolipids are a class of lipids which possess a sulfur-containing functional group. An abundant sulfolipid is sulfoquinovosyl diacylglycerol, which is composed of a glycoside of sulfoquinovose and diacylglycerol. In plants, sulfoquinovosyl ...
s, while the inner membrane has been found to contain 16%
phospholipid
Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s, 79%
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 phospholipid ...
s and 5%
sulfolipid
Sulfolipids are a class of lipids which possess a sulfur-containing functional group. An abundant sulfolipid is sulfoquinovosyl diacylglycerol, which is composed of a glycoside of sulfoquinovose and diacylglycerol. In plants, sulfoquinovosyl ...
s in spinach chloroplasts.
The outer membrane is permeable to most
ions and
metabolite
In biochemistry, a metabolite is an intermediate or end product of metabolism.
The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...
s, but the inner membrane of the chloroplast is highly specialised with
transport proteins. For example, carbohydrates are transported across the inner envelope membrane by a
triose phosphate translocator The triose phosphate translocator is an integral membrane protein found in the inner membrane of chloroplasts. It exports triose phosphate (Dihydroxyacetone phosphate) in exchange for inorganic phosphate and is therefore classified as an antiporter ...
. 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
thylakoids. The thylakoid membrane is quite similar in lipid composition to the inner envelope membrane, containing 78%
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 phospholipid ...
s, 15.5%
phospholipid
Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s and 6.5%
sulfolipid
Sulfolipids are a class of lipids which possess a sulfur-containing functional group. An abundant sulfolipid is sulfoquinovosyl diacylglycerol, which is composed of a glycoside of sulfoquinovose and diacylglycerol. In plants, sulfoquinovosyl ...
s 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 th ...
.
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, blue ...
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 membra ...
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 PO
43− 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.
References
See also
*
TIC/TOC complex
{{DEFAULTSORT:Chloroplast Membrane
Membrane biology