CHLOROPLAST:STRUCTURE AND FUNCTION

Photoautotrophs Organisms that utilize the radiant energy of the sun to

convert CO2into organic compounds

Photosynthesis A process in which the energy from sunlight is

transformed into chemical energy that is stored in carbohydrates and other organic molecules

Low-energy electrons are removed from a donor compound and converted into high-energy electrons using the energy absorbed from light

Chloroplasts Generally lens-shaped, approximately 2-4 µm wide

and 5-10 µm long, and typically numbering 20-40 per cell

Located predominantly in the mesophyll cells of leaves Arise by fission from preexisting chloroplasts or from

their non-pigmented precursors called proplastids Structure:

1. Envelope> outer covering> has 2 membranes:a. outer envelope membrane

> contains porins

b. inner envelope membrane> highly impermeable; substances may pass

through it with the aid of a variety of transporters

2. Thylakoids> flattened membranous sacs formed by the chloroplast internal membrane> arranged in orderly stacks called “grana”

> contain the energy transducing machinery for photosynthesis

3. Lumen – space inside a thylakoid sac4. Stroma

> space outside the thylakoid sac and within the chloroplast envelope

2 series of reactions during photosynthesis:1. light-dependent reactions energy from sunlight is absorbed and stored as

chemical energy in 2 key biological molecules: ATP and NADPH

2. light-independent reactions/ dark reactions carbohydrates are synthesized from CO2 using the

energy stored in the ATP and NADPH molecules produced in the light-dependent reaction

Photosynthesis begins with the absorption of photons of light by photosynthetic pigments, an event that pushes electrons to its outer orbital from which they can be transferred to an electron acceptor.

Chlorophyll the most important light-absorbing photosynthetic

pigments consists of 2 parts:

a. Mg2+ containing porphyrin ring that functions in light absorption

b. A hydrophobic phytol chain that keeps the chlorophyll embedded in the photosynthetic membrane

Photosynthetic unit A group of several hundred chlorophyll molecules

acting together to trap photons and transfer energy to the pigment molecule at the reaction center

Components:a. Reaction-center chlorophyll

> only 1 per unit> transfers electrons to an electron acceptor

b. Antenna> light harvesting chlorophyll molecules that absorbs photons of varying wavelength and transfers the energy (called excitation energy) very rapidly to the pigment molecule at the reaction center

Photosystems Large pigment CHON complexes where light-absorbing

reactions of photosynthesis 2 types:

1. Photosystem II (PS II)> boosts electrons from an energy level below that of water to a midway point> reaction center is P680> complex of more than 20 different polypeptide> has a separate outer antenna pigment which resides within a separate pigment-protein complex, called LHCII (light-harvesting complex II)> uses absorbed light energy for 2 activities:a.) removing electrons from water (photolysis)b.) generating a proton gradient

2. Photosystem I (PSI)> raises electrons from a midway point to an energy level well above that NADP+> reaction center is P700> consists of a reaction-center core made up of 12-14 polypeptide subunits and a peripheral complex of protein bound pigments called LHCI

The Flow of electrons during photosynthesis:1. From H2O to PSII The formation of one molecule of oxygen during

photolysis is thought to require the simultaneous loss of 4 electrons from 2 molecules of H2O

2. From PSII to plastoquinol

a.) Excitation energy → outer antenna pigments (LHCII) → inner-antenna chlorophyll → reaction center (P680)

P680 will become positively charged (P680+) upon transfer of electrons to the 1st electron acceptor, Pheophytin. Pheophytin becomes negatively charged or reduced (Pheo-) in the process.

b.) P680+ → Pheo- → plastoquinone A (PQA-) →

plastoquinone B (PQB-)

c.) PQB- + PQB

- → PQB2-

d.) PQB2- + 2 H+ from the stroma → plastoquinol (PQH2)

→ diffuses into the bilayer

3. From PQH2 to PSI

PQH2 → cytochrome b6f → plastocyanin → P700

4. From PSI to production of NADPH

Excitation energy → LHCI → P700+ → Ao- (a monomeric chlorophyll a molecule) → phylloquinone → Iron-sulfur centers (FX, FB and FA) → out of PSI → Ferrodoxin → Ferrodoxin-NADP+ reductase (contains an FAD prosthetic group capable of accepting and transferring 2 electrons) → NADPH

The Machinery for ATP synthesis:ATP synthase

Consist of a head called CF1, which contains the catalytic site and a base called CF0, which spans the membrane

Transport of electrons is coupled with proton translocation, thus creating a proton motive force that will drive ATP synthesis.

Noncyclic photophosphorylation Formation of ATP during the process of oxygenic

photosynthesis, since electrons move in a linear path from H2O to NADP+

Cyclic photophosphorylation Formation of ATP that is carried out by PSI

independent of PSII

The conversion of one mole of carbohydrate requires the input of 3 moles of ATP and 2 moles of NADPH

CO2 Fixation and the Synthesis of Carbohydrate:1. C3 pathway/ Calvin cycle/ Calvin-Benson cycle Occurs in all eukaryotic photosynthetic cells and

cyanobacteria CO2 condenses with ribulose 1,5-bisphosphate (RuBP)

as catalyzed by RuBP carboxylase (Rubbisco) to form an unstable 6-C intermediate, which splits into 2 molecules of 3-phosphoglyceric acid (PGA). NADPH and ATP are used to convert PGA molecules to

glyceraldehydes phosphate (GAP). For every 6 molecules of CO2 fixed, 2 molecules of GAP can be directed toward the formation of sucrose or starch, while the remaining molecules of GAP can be used to regenerate RuBP for additional rounds of CO2 fixation.

Photorespiration A process wherein Rubisco catalyze a reaction in which

O2 rather than CO2 is covalently joined to RuBP, leading to the loss of CO2

2. C4 pathway Utilized by C4 plants, e.g. tropical grasses, sugarcane,

corn, sorghum CO2 is fixed to Phosphoenol pyruvate (PEP) in the

mesophyll as catalyzed by PEP carboxylase to form a 4-C acid, which is transported to the bundle sheath where it is decarboxylated leading to the accumulation of CO2, thus favoring fixation to RuBP

Enables synthesis of carbohydrates at lower CO2

levels.

3. CAM pathway Utilized by CAM plants (plants that survive in very hot,

dry habitats e.g. cactus) Also utilizes PEP carboxylase but carry out light-

dependent reactions and CO2 fixation at different times of the day, rather than in different cells of the leaf

As CO2 is fixed during the night, malic acid is formed and transported into the cell’s vacuole and eventually released in the cytoplasm during daylight hours to provide a source of CO2 which can be fixed by Rubisco under conditions of low O2.