Chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s contain several important membranes

Biological membrane

A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

, vital for their function. Like mitochondria, chloroplasts have a double-membrane envelope, called the chloroplast envelope. Each membrane is a phospholipid bilayer, between 6 and 8 nm

Nanometre

A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

 thick, and the two are separated by a gap of 10-20 nm, called the intermembrane space

Intermembrane space

The intermembrane space also known as IMS is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of the intermembrane space is oxidative phosphorylation....

. The outer membrane

Outer membrane

The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...

 is permeable to most ions and metabolite

Metabolite

Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...

s, but the inner membrane

Inner membrane

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

 is highly specialised with transport protein

Transport protein

A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane. Transport proteins are integral membrane proteins; that is they exist within and span the membrane across which they...

s. Carbohydrates are transported across the outer membrane by a triose phosphate translocator

Triose phosphate translocator

The triose phosphate translocator is an integral membrane protein found in the outer membrane of chloroplasts. It exports triose phosphate in exchange for inorganic phosphate and is therefore classified as an antiporter...

.

The origin of chloroplasts is now largely accepted by the botany community as occurring via endosymbiosis on an ancestral basis with the engulfment of photosynthetic bacterium within the eukaryotic 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.

Internal parts

Within the inner membrane, in the region called the stroma

Stroma (fluid)

Stroma, in botany, refers to the colourless fluid surrounding the grana within the Plastid, chloroplast.Within the stroma are grana, stacks of thylakoids, the sub-organelles, where photosynthesis is commenced before the chemical changes are completed in the stroma.Photosynthesis occurs in two...

, there is a system of interconnecting flattened membrane compartments, called the thylakoid

Thylakoid

A thylakoid is a membrane-bound compartment 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 thylakoids frequently form stacks of disks referred to as...

s. These are the sites of light absorption and ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 synthesis, and contain many proteins, including those involved in the electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

. Photosexnthetic pigments such as chlorophylls a,b and c 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, Photosytems I and II.

Functions of thylakoids

The membranes of the thylakoid contain photosystems I and II which harvest solar energy to excite electrons which travel down the electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

. 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 blue-green alga (cyanobacterium) or the stroma of a chlorolast. A steep H+ gradient is formed, which allows chemiosmosis

Chemiosmosis

Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....

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 P0-4 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 nearly thousandfold difference in concentration of H+ ions. The H+ ions pass down through the ATP-synthase catalytic gate. This chemiosmotic phenomenon occurs in mitochondria. The "osmosis" of H+ or protons not water molecules "downhill." The mean turnover rate of ATP in human cells is about 1 million ATPs-per cell-per-second!