L1 - Chloroplast & Light reaction

Page 1

Overview of Photosythesis (Chloro..)

CHLOROPLAST

Chloroplast structure:

-90% protein in chloroplast encoded on DNA

Structure:

1. Outer & inner envelope

2. stroma

3. grana thylakoid

4. stromal thylakoid

Envelope -

Outer envelope (7nm) - freely permeable to ions and metabolites of Mr - 5000 bcuz of porins; isolates chloroplast and regulate exchange of metabolites with cytosol

Inner envelope (7nm) - Selectively permeable

Stromal - Contains all enzymes for photosynthetic carbon assimilation

Thylakoid - AKA internal lamellae membrane system; individual pair of parallel membrane with joined ends/flattened sacs; contains chloroplast and carotenoids pigments

2types (granal and stromal): Granal thylakoid - Individual pair closely appressed together; Stromal thylakoid - Extends from stacked granal thylakoid to become as an individual.

Chloroplast Biogenesis:

3 transporters regulates post-translational proteins destined in chloroplast

Transit peptides - Guides proteins to the correct destination; E.g. carrier & channel proteins

Chaperones - Regulates correct folding of protein; E.g. HSP70 -> keeps protein unfolded to allow passage through protein-import apparatus

Protein import complexes -

Toc proteins - For outer envelope membrane

Tic proteins - For inner envelope membrane

Page 2

Light Reactions of Photosynthesis

Overview of Light reactions

History of light reaction:

1937 Robert hill: Observed that illumination of leaf extract containing chloroplast - caused evolution of O2 & reduced of a non-biological electron acceptor (DCPIP: blue dye become colourless)

-Hill reaction:

Light

2H2O + 2DCPIP ------------------> 2DCPIP + H2 + O2

chloroplast

Hill's theory provided - first evidence that light energy caused electrons to flow - from H20 to an electron acceptor

-An example:

Light

2H2O + 2NADP+ -----------------> 2NADPH + 2H+ + O2

chloroplast

NADPH = a biological acceptor in chloroplast

Conversion of physical energy (light) to chemical energy (NADPH & ATP):

-Light energy – excites chlorophyll (Chl) molecules in thylakoid membrane

-Excitation energy – passes from Chl to Chl to a reaction centre – leading to loss of an electron = primary charge separation event

- Electron passes through electron transport chain ETC to NADP+ to form NADPH

-Energy generated by the transfer is used to create an electrochemical gradient across the thylakoid membrane.

-Gradient drives synthesis of ATP from ADP + Pi.

-NADPH and ATP provide reductant and energy for the carbon reductive pathway and net synthesis of triose phosphate and hence sucrose

Biogenesis of the thylakoid network