Chloroplast , structure and functions in full details

What is Chloroplast

Chloroplast is a cell organelle and the site of photosynthesis in eukaryotic cells. Hence they are present in plant cells and algae, but not in animal cells or bacteria cells.

The word 'Chloroplast' comes from a greek words;

"chloros" which means "green"

"plastes" wich means  "the one who forms"

Chloroplasts are present in all green coloured eukaryotic cells. Chloroplasts are the site where the photosynthesis function is performed. In short Photosynthesis occurs in eukaryotic cell structures called chloroplasts.

Structure & Function of Chloroplast Components

Chloroplast is made up of following components:

1) Envelope

Consists of inner and outer phospholipid bilayers, each 6-8 nm thick, with a 10-20 nm inter-membrane space in between.

Function: Being semi-permeable, the envelope regulates the transportation of various molecules like glucose, ions like Fe2+ and Mg2+, oxygen and carbon dioxide, in and out of the envelope. It is also the site for synthesis of certain lipids and light-harvesting pigments.

2) Stroma (Chloroplast Matrix)

The aqueous matrix encapsulated by the envelope and containing all internal components of the chloroplast.

Function: Dark or light-independent reactions occur at stroma. "The enzymes in the stroma utilize carbon dioxide from the atmosphere, as well as the ATP and NADPH2 molecules released from grana, to synthesize sugar molecules and starch. This process is also known as carbon dioxide fixation, and occurs through a series of reactions collectively called Calvin cycle." [2] Light-dependent oxygen fixation also occurs in stroma. 

3) Thylakoids (each thylakoid has a Lumen)

A flattened membrane sac inside the chloroplast, used to convert light energy into chemical energy. recganization of the protein complexes of the thylakoid membrane. Photosystem II is located predominantly in the  stacked regions of the thylakoid membrane; photosystem I and

ATP synthase are found in the unstacked regions protruding into the stroma. Cytochrome b6 f complexes are evenly distributed. This lateral separation of the two photosystems requires that electrons and protons produced by photosystem II be transported a considerable distance before they can be acted on by photosystem I and

 Almost all the chemical processes that make up the light reactions of photosynthesis are carried out by four major protein complexes: photosystem II, the cytochrome b6 f complex, photosystem I, and the ATP synthase. These four integral membrane complexes are vectorially oriented in the thylakoid membrane to function .

• Photosystem II oxidizes water to O2 in the thylakoid lumen and in the process releases protons into the lumen.

• Cytochrome b6 f receives electrons from PSII and delivers them to PSI. It also transports additional protons into the lumen from the stroma.

• Photosystem I reduces NADP+ to NADPH in the stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP reductase (FNR).

• ATP synthase produces ATP as protons diffuse back through it from the lumen into the stroma.

4) Grana (singular: Granum)

Stack of thylakoids in a chloroplast.

Function: Light-dependent reactions occur at grana and thylakoids. It is the place where photosynthetic pigments like chlorophyll a, chlorophyll b, carotenoids, etc. absorb light energy, which in turn breaks down the water molecules, and finally gives rise to ATP, NADPH2 and oxygen.

5) Lamellae  (singular: Lamella)

The tissue that connects the grana to each other.

Function: "Lamellae act as a 'skeleton' of the chloroplast, maintaining efficient distances between the grana, thereby maximizing the overall efficiency of the chloroplast." [1]

6) Circular DNA

7) Starch Granules

Starch exists in chloroplast in the form of tiny little 'grains' or 'granules'.

Function: They are the (insoluble) storage carbohydrate product of photosynthesis.

8) Lipid Globules

9) Ribosomes

Chloroplasts contain the smaller type of ribosomes (70S ribosomes), similar to the ones found in the cytoplasm of prokaryotic cells.

Function: Their main function is to serve as the site of mRNA translation.

10) Chlorophyll

Green pigment in plants that absorbs light energy used to carry out photosynthesis.

11) Peripheral Reticulum

In certain plants, an additional set of membranous tubules budding off from the inner membrane of the envelope.

Function: Fast transport of metabolites and proteins from the inter-membrane space into the chloroplasts.

Functions of Chloroplast

1.      Absorption of light energy and converting it into biological energy

2.      Production of NAPDH2 and evolution of oxygen through the process of photosys of water.

3.   Production of ATP by photophosphorylation. NADPH2 and ATP are the assimilatory powers of photosynthesis. Transfer of CO2 obtained from the air to 5 carbon sugar in the stream during dark reaction.

4.      Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid by the utilization of assimilatory powers.

5.      Conversion of PGA into different sugars and store as stratch. The chloroplast is very important as it is the cooking place for all the green plants. All heterotrophs also depend on plasts for this food.