CHLOROPLAST

by Bibrita BharM. Sc. 1st year student

of School of Biotechnology

Madurai Kamaraj University

HOUSE OF PLANTPHOTOSYNTHETIC

MACHINERY

INTRODUCTION

The term Chloroplast was first described by Nehemiah Grew and Antonie Van Leeuwenhoek.“Chloro” means green while“ Plast” means living.

Chlorophyll pigments present in the chloroplast imparts the green colour to plants.

Chloroplasts are present in plants and other eukaryotic organisms that conducts photosynthesis.

INTRODUCTION

Responsible for photosynthesis, are in many respects similar to mitochondria.

Chloroplasts are larger and more complex than mitochondria, and they perform several critical tasks in addition to the generation of ATP.

Chloroplasts synthesize amino acids, fatty acids, and the lipid components of their own membranes.

The reduction of nitrite (NO2-) to ammonia (NH3), an essential step in the

incorporation of nitrogen into organic compounds, also occurs in chloroplasts.

STRUCTUREPlant chloroplasts are large organelles (5 to 10 μm long).

Chloroplast envelope: 1) the inner membrane 2)outer membranes and 3)thylakoid membrane.

The thylakoid membrane forms a network of flattened discs called thylakoids, which are frequently arranged in stacks called grana. Because of this three-membrane structure, the internal organization of chloroplasts is more complex than that of mitochondria. The three membranes divide chloroplasts into three distinct internal compartments: (1) the intermembrane space between the two membranes of the chloroplast envelope; (2) the stroma, which lies inside the envelope but outside the thylakoid membrane; and (3) the thylakoid lumen.

Chloroplasts are highly structured, membrane-rich organelles.

Outer membrane

Inner membrane

Thylakoids

Granum

Stroma

Outer membraneInner membraneThylakoids

Granum

Stroma

STRUCTURE

Distinctive composition of lipid of thylakoid membrane: only 10% phospholipids; the majority, 80%, are uncharged mono- and digalactosyl diacylglycerols, and the remaining 10% are the sulfolipids sulfoquinovosyl diacylglycerols (quinovose is 6-deoxyglucose).The acyl chains of these lipids have a high degree of unsaturation, which gives the thylakoid membrane a highly fluid character.

CHLOROPLAST GENOMELike mitochondria, chloroplasts contain their own genetic system, reflecting their evolutionary origins from photosynthetic bacteria.

The genomes of chloroplasts consist of circular DNA molecules present in multiple copies per organelle.

Chloroplast genomes are larger and more complex than those of mitochondria, ranging from 120 to 160 kb and containing approximately 120 genes..

Function Number of genes

Genes for the genetic apparatus

rRNAs (23S, 16S, 5S, 4.5S) 4tRNAs 30Ribosomal proteins 21RNA polymerase subunits 4Genes for photosynthesisPhotosystem I 5Photosystem II 12Cytochrome bf complex 4ATP synthase 6Ribulose bisphosphate carboxylase

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CHLOROPLAST GENOME

The chloroplast genome encodes approximately 30 proteins that are involved in photosynthesis, including components of photosystems I and II, of the cytochrome bf complex, and of ATP synthase.In addition, one of the subunits of ribulose bisphosphate carboxylase (rubisco) is encoded by chloroplast DNA. Rubisco is the critical enzyme that catalyzes the addition of CO2 to ribulose-1,5-bisphosphate during the Calvin cycle. Not only is it the major protein component of the chloroplast stroma, but it is also thought to be the single most abundant protein on Earth, so it is noteworthy that one of its subunits is encoded by the chloroplast genome.

CHLOROPLAST & PHOTOSYNTHESISPhotosynthesis occurs in two distinct phases:

1. The light reactions, which use light energy to generate NADPH and ATP.

The light reactions occur in the thylakoid membrane and involve processes that resemble mitochondrial electron transport and oxidative phosphorylation. In photosynthetic prokaryotes, which lack chloroplasts, the light reactions take place in the cell’s inner membrane or in highly invaginated structures derived from it called chromatophores.

2. The dark reactions, actually light-independent reactions, which use NADPH and ATP to drive the synthesis of carbohydrate from CO2 and H2O.In eukaryotes, the dark reactions occur in the stroma through a cyclic series of enzyme-catalyzed reactions.

Light reactions occur in the thylakoids (PSII) and stroma lamella (PSI).

Dark reactions occur in the stroma

Chlorophylls a and b

Ring structure in “head”(absorbs light)

Tail

When a photon strikes its energy can be transferred to an electron in the “head” region. The electron is excited, raised to a higher electron shell, with greater potential energy

PRINCIPLE PHOTORECEPTOR

LHC light harvesting complex

LHCI, PSI, and ATP synthase are all in the stroma lamella or on the edge of a grana

ORGANISATION OF FOUR MAJOR PROTIENS

Protons diffuse to the site of ATP synthase

Transfer of electrons and protons in the thylakoid membrane

CHEMIOSMOTIC GENERATION OF ATP IN CHLOROPLASTS AND MITOCHONDRIA

In terms of its role in generation of metabolic energy, the thylakoid membrane of chloroplasts is thus equivalent to the inner membrane of mitochondria. The inner membrane of the chloroplast envelope does not function in photosynthesis. Instead, the chloroplast ETC is located in the thylakoid membrane, and protons are pumped across this membrane from the stroma to the thylakoid lumen. The resulting electrochemical gradient then drives ATP synthesis as protons cross back into the stroma.

GENERATION OF ATP IN CHLOROPLASTS AND ATP SYNTHASE

Ion concentration differences and electric potential differences across membranes are a source of energy that can be utilizedAs a result of the light reactions the stroma has become more alkaline (fewer H+ ions) and the lumen more acid (more H+ ions)

Hydrophilic

HydrophobicThe internal stalk and much of the enzyme complex located in the membrane rotates during catalysis.The enzyme is actually a tiny molecular motor

PROTEIN IMPORT INTO THE CHLOROPLAST STROMA

Proteins are targeted for import into chloroplasts by a transit peptide at their amino terminus. The transit peptide directs polypeptide translocation through the Toc complex in the chloroplast outer membrane and the Tic complex in the chloroplast inner membrane. This peptide is then removed by proteolytic cleavage within the stroma. Both cytosolic and chloroplast chaperones (Hsp60 and Hsp70) are required for protein import.

PROTEIN IMPORT INTO THE CHLOROPLAST LUMEN

Proteins are imported into the thylakoid lumen in two steps. The first step is import into the chloroplast stroma. Cleavage of the transit peptide then exposes a second hydrophobic signal sequence, which directs protein translocation across the thylakoid membrane.

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