Chloroplast

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Chloroplast:

Chloroplast can be defined as one of three types of plastids, characterized by its high concentration of chlorophyll, the other two types, the leucoplast and the chromoplast, contain little chlorophyll and do not carry out photosynthesis.

Plastid definition:Plastids are double membrane bound semi-autonomous organelles present in all living plant cells and photosynthetic protists.

Chloroplast characteristics:

  • It has a double membrane which encloses a fluid-filled region called the stroma.
  • Embedded in the stroma is a complex network of stacked sacs.
  • Each stack is called a granum and each of the flattened sacs which make up the granum is called a thylakoid.
  • The thylakoid membrane, that encloses a fluid-filled thylakoid interior space, contains photosynthetic pigments.
  • There are many grana in each chloroplast which are interconnected by unstacked stromal lamellae.
  • The lipids of the thylakoid membrane have a distinctive composition.
  •  About 80% lipids are uncharged mono- and digalactosyl diacylglycerol and only about 10% are phospholipids.

Functions of Chloroplast:

Photosynthesis:

Photosynthesis is a photochemical process by which the green plants convert the light energy into chemical energy in the form of reducing power and ATP, and use these chemicals to drive carbon dioxide fixation. Light is harvested by a specialized pigment called chlorophyll which is located in choloroplast.

Chlorophyll:

Chlorophyll, a light-absorbing pigment, contains a polycyclic, planar tetrapyrrole ring structure. Chlorophyll is a lipid soluble pigment. It has the following important features:
  • The central metal ion in chlorophyll in Mg2+
  • Chlorophyll has a cyclopentane ring fused to pyrrole ring III
  • The propionyl group on a ring IV of chlorophyll is esterified to a long-chain tetraisoprenoid alcohol. In chlorophyll a and b, it is phytol..
Oxygenic photosynthetic organisms contain different types of chlorophyll molecules like Chl a, Chl b, Chl c, and Chl d. There chlorophyll molecules differ by having different substituent groups on the tetrapyrrole ring. An-oxygenic photosynthetic organisms contain bacteriochlorphyll molecules. They are related to chlorophyll molecules.

Accessory pigments:

Besides the major light-absorbing chlorophyll molecules, there are two groups of accessory pigments which absorb light in the wavelength region, where chlorophylls do not absorb energy. The two types of accessor pigments are carotenoids and phycobillins.

Location of photosynthetic pigments:

In oxygenic photosynthetic organisms, photosynthetic pigments are found in thylakoid membranes. In anoxygenic photosynthetic organisms such as green Sulphur bacteria, photosynthetic pigments are found in plasma membrane and vesicles called chlorosomes attached to the plasma membrane. In the purple Sulphur bacteria, the bacteriochlorophylls are located in invaginations of the plasma membrane called chromatophores.

Ultra-structure of chloroplast:

The chloroplasts are covered by two plasma membranes.  Each membrane of the chloroplast is tri-laminar and lipo-proteinous. The contents of chloroplast are heterogeneous. They consist of: 
  • The matrix or stroma: The internal space of the chloroplasts is occupied with a liquefied and transparent material called as the matrix or the stroma. It is the site where grana and inter-grana stay embedded.
  • The grana: The chloroplasts comprises of many granular and chlorophyll containing structures, known as the grana.  The size of the grana may vary from 0.3 - 1.7 microns.  The chloroplasts may sometimes consist of 40 to 60 grana in the matrix.  Each granum is further made of 10 to 100 dice-like, overlaid, membranous sections called as thylakoids.  Each thylakoid is disconnected from the stroma or the matrix of the chloroplast by its membranes.

Concept of photosynthetic unit:

  • In 1932, Robert Emerson, and William Arnold provided the first evidence for the cooperation of many chlorophyll molecules in energy conversion during photosynthesis.
  • They suggested that not all the chlorophyll molecules in a chloroplast were directly involved in the conversion of light energy into chemical energy.
  • Using suspensions of the alga Chlorella and flashing lights of very short duration a saturating intensity, they determined the minimum amount of light needed to produce maximal oxygen production during photosynthesis.
  • Based on the number of chlorophyll molecules present in the preparation, they calculated that one molecule of oxygen was being released during a brief flash of light for every 2500 molecules of chlorophyll present.
  • It was later shown that a minimum of 8 photons must be absorbed to produce one molecule of oxygen and for this about 300 or so chlorophyll molecules act together as one photosynthetic unit in which only one member of the group- the reaction center chlorophyll- actually transfers electrons to an electron acceptor.
  • The majority of the chlorophyll molecules serve as an antenna complex, collecting light and transferring the energy to the reaction center, where the photochemical reaction takes place.Chloroplast

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