Molecular & Cell Biology

S. Rahgozar,PhD

University of Isfahan

Faculty of Science

5. Bioenergetics and Metabolism

5.2. Chloroplasts,Photosynthesis and Peroxisomes

1389-90

Structure and

Function of

Chloroplasts

Grana

Chemiosmotic generation of ATP in chloroplasts and mitochondria

The genetic system of Chloroplasts

The genome consists of circular DNA molecules. The Chloroplast

genomes of chloroplasts consists of 120-160 kb and containing approx

150 genes.

( Rubisco )

Import and

Sorting of Chloroplast

Proteins

About 95% of chloroplast

proteins are encoded by

nuclear genes.

Proteins with N terminal

transit peptides are

recognized by a guidance

complex in the chloroplast

outer membrane.

Passage through the outer

membrane requires ATP

hydrolysis by Hsp70 in the

intermembrane space and

possibly the hydrolysis of

GTP by Toc34.

The transit peptide is drawn to

the stroma by Hsp100 and

removed by SPP.

(Chloroplast stromal

processing peptidase)

Import into the stroma

Import of Chloroplast Proteins into the thylakoid lumen or membrane.

1-Sec A recognizes a thylakoid signal sequence

2-Fully folded protein containing a thylakoid transfer sequence targets a proton gradient

mediated novel translocon.

3-The thylakoid membrane proteins are recognized by a stromal SRP.

Thylakoid processing protease

Twin-arginine-translocation pathway

Signal Recognition Particle

Other Plastids

Classification

Amyloplasts Leucoplasts with starch granules

Elaioplasts Leucoplasts with lipids

o Chloroplasts Chlorophyll II (green pigment)

o Chromoplasts Carotenoids (yellow, orange, red pigments)

o Leucoplasts Energy sources in nonphotosynthetic tissues

Plastids are plant organelles with the same genome as chloroplasts, but

different structure

Chromoplast contains

droplets in which

carotenoids are stored.

Amyloplast contains large

starch granules.

0.5-1μm Proplastids

o Mature plastids are able to

change from one type to another.

o Plastid development depends on

• Evironmental signals

• Intrinsic programs of

cell differentiation

Etioplasts lack chlorophyll

and contain semicrystalline

arrays of membrane tubules.

Development of a chloroplast

Pro Etioplast

Photosynthesis

Dark reactions Light reactions

Stroma

Chlorophyll II

Thylakoid membrane

Ribulose-1,5-bisphosphate

carboxylase

Each molecule of chlorophyll is

a photosynthetic pigment

(Antenna pigment molecule).

Hundreds of pigment molecules

produce a photocenter.

Photocenters are organized in the

thylakoid membrane.

(Special pair of

chlorophyll molecules)

Quinone

Electron transport and ATP synthesis during photosynthesis

PSI,II: photosystem I, II, PQ: plastoquinone, cyt bf : cytochrome bf complex,

PC: plastocyanin , Fd: Ferrodoxin

The pathway of cyclic electron flow. Light energy absorbed at photosystem I (PSI) is used for

ATP synthesis rather than NADPH synthesis. High energy electrons generated by photon absorption

are transferred to the cytochrome bf complex rather than to NADP+. At the cytochrome bf complex

electrons are transferred to a lower energy state and protons are pumped into the thylakoid lumen.

Then electrons are then returned to photosystem I by plastocyanin(PC).

Peroxisome

Dense regions: Paracrystalline arrays of the enzyme

urate oxidase

500 per human cell

Peroxins (Pex1,2,..) are

metabolic enzymes (~50),

synthesized mostly on free

ribosomes by ~80 genes of the

nuclear genome.

Can replicate by division

Function

Oxidation of , methanol, purines, amino acids, uric acids Fatty acids

o Fatty acid oxidation happens in

Peroxisome & Mitochondria in Animal cells

Peroxisome in Yeast and Plant cells

Synthesis of Lipids and Lysine amino acid

Synthesis of Cholestrol and Dolichol in Animal cells (in addition to ER)

Synthesis of Bile acids, derived from cholesterol, in Liver

Synthesis of Plasmalogen

membrane component of Heart & Brain

in Plant cells

o In seeds:

Fatty acid Carbohydrate

Through glyoxylate cycle

o In leaves:

Photorespiration rubisco

Photorespiration is not essential, however, peroxisome allows most of the

carbon in glycolate to be recovered and utilized