Question

Describe the structure and function of the chloroplast and mitochondrion. Include in your answer: a. a description of each compartment, the membranes, and the localized activities of both organelles b. the special properties of chlorophyll and the mechanism of action of the light harvesting complex in the chloroplast c. the sequences of events in both electron transport chains from electron sources to the electron sinks and the roles of NADH and NADPH

Answer 1

CHOLOROPLASTS: These are greenish plastids that possess photosynthetic pigments, chlorophylls and carotenoids and take part in food synthesis in plants. Majority of these are found in mesophylls of leaves. These vary in size, shape and number.

Size: thickness of 2-4 nanometer and length of 5 to 10 nanometer.

Shape: may be spherical, lens shaped, oval, discoid or ribbon shaped.

Number: number varies from one per cell in chlamydomonas to 20-40 per cell in mesophyll.

These are double membrane bound organelle and have an outer and an inner membrane. Inner membrane is relatively less permeable to substances than outer membrane and hence, have more protein including carrier proteins. The space enclosed by the inner membrane of chloroplast is called as the stroma. Stoma contains a large number of organized flattened membranous sacs called thalakoids that are arranged in stalk like piles of coins called grana ( singular: granum) or the intergranal thalakoid.

The thalakoids of different grana are connected by flat membranous tubules called stroma lamelle. The membrane of thalakoid enclose a space called lumen. The stroma of chloroplast contains enzymes required for carbohydrate and protein synthesis. It also contains small double stranded DNA molecules and ribsomes. The ribosomes of chloroplasts are 70S and smaller than cytoplasmic ribosomes that are 80S.

Functions: The chloroplasts perform various functions like photosynthesis ( light reaction in thalakoid and dark reaction in the stroma) and storage of starch.

MITOCHONDIA

Mitochondria are cylindrical or sausage shaped double membranous organelles distributed in the cytoplasm abd are not easily visible under the microscope. They are stained by vital stain Janus green to make them visible for observation. Mitochondria show a great degree of variability in shape, size and number.

Size: A typical mitochondria is sausage shaped and have a diameter of 0.2 nanometer to 1.0 nanometer and length of 1.0 nanometer and 4.1 nanometer.

Number: Their number varies in different cell types depending upon the amount of work done by the cell and energy requirement.

Structure: Mitochondria are surrounded by two membranes one outer and one inner membrane. The outer membrane is smooth and forms the continuous limited boundary of the mitochondria and the inner membrane forms a number of infoldings called as cristae ( singular: crista). The outer membrane is chemically 40% lipid and 60% protein and the inner selectively permeable membrane has 80% protein and 20% lipids and is rich in cardiolipids. Due to presence of two membranes the organelle is partitioned into two distinct chambers filled with aqueous fluid.

Outer compartment or intermembrane space: It lies between the two mitochondrial membrane and is also called as peri mitochondrial space.

Inner compartment or matrix: It lies inside the inner membrane. The cristae are inflodings of the inner membrane and are formed towards the matrix that increases the surface area of enzyme action.

The matrix contains single circular double stranded DNA molecule, a few RNA molecules, 70 S ribosomes and components required for protein synthesis. The matrix also contains enzymes for tricarboxylic acid  cycle .

The two membranes have their own specific enzymes associated with the mitochondrial function. The mitochondria divides by fission. The enzyme and electron carriers for ATP formation are only in the inner membrane.

The cristae and the inner face of the inner membrane is studded with numerous spherical knob like protuberances called elementary particles or particles of Fernandez and head piece. The head piece contains enzyme ATP synthase that brings about oxidative phosphorylation coupled with ATP release.

b) CHLOROPHYLL: Pigments are substances that have an ability to absorb light at specific wavelengths.

Chlorophyll a: Brigh or blue green in chromatogram

Chlorophyll b: Yellow green

Chlorophyll has a tadpole like structure and it consists of a porphyrin head and a phytol tail.

Porphyrin head: All chlorophyll have a complex ring structure chemically related to the porphyrin like groups found in hemoglobin and cytochromes, site of elctron rearrangments when the chlorophyll is exicted and a cyclic tetra pyrrolic structure with non ionic magnesium atom.

Phytol tail: A long hydrocarbon tail is almost always attached to ring structure. Anchors the chlorophyll to hydrophobic portion of thalakoids.

Major types of chlorophyll are chlorophyll a, b, c,d, e; bacteriochlorophyll a and b etc.

Functions:

Mitochondria are main sites for aerobic respiration. They are miniature biochemical factories where food stuffs or respiratory substrates are completely oxidized to water and carbon di oxide. The energy liberated in this process is stored as ATP. These bring about the oxidation of carbohydrates, proteins and beta oxidation of fats.

The energy- ATP is produced in the mictochondria helps in the various energy requiring processes of the cell like contraction of muscles, conduction of nerve impulse, etc. and because of ATP formation function mitochondria is called as the power house of the cell.

ELECTRON TRANSPORT : The electron transport chain is a series of electron carriers over which elctron passes inn a downhill journey releasing energy at every step that is used in generating an electrochemical proton gradient which helps in synthesizing ATP.

Based on path of electron photophosphorylation can be identified as non cyclic or cyclic photophosphorylation.

NON CYCLIC PHOTOPHOSPHORYLATION

It involves Photosystem I and Photosystem II which work in series, first PSII and then PSI, the two photosystems are connected via an electron transport chain. Both ATP and NADPH+H+ are synthesized by this kind of electron flow.

First in PSII, the P680 molecules absorb 680 nm wavelength of red light causing electrons to become excited and jump into an orbit farther from atomic nucleus.

These electrons are picked up by elctron acceptor that passes them to an electron transport system of cytochromes. This movement is downhill on redox potential scale. The elctrons are then passed to pigments of PSI without being used. Simultaneous electrons in the reaction centeer of PSI are exicted when they receive light of wavelength 400 nm and these electrons are then transferred to anoter acceptor molecule with greate redox potential. These electrons are then moved downhill again to a molecule of NADP+ which reduces it to NADPH+H+.

The whole scheme from PSII to PSI to the reduction of NADP+ is called as Z scheme as it makes a Z shape in sequence of redox potential scale.

CYCLIC PHOSPORYLATION

The process of cyclic phosporylation involves only PSI and takes place in the stroma lamellae membrane. When only PSI is funtional the electron is circulated within photosystem and phosphorylation occurs due to cyclic flow of electrons. The membrane or lamellae of grana have both PSI and PSII, the stroma lamellae lack PSII as well as NADP reductaseenzyme. The exicted electrons does not pass on to NADP+ and is cycled back to PSI complex via the elctron transport chain. Cyclic phosphorylation occurs when only light of beyond 680nm are avilable for exitation.

The NADP reductase enzyme is present on the stroma side of the membrane. Protons are required for reduction of NADP+ and these protons are removed from the stroma. Along with the NADPH produced the ATP is used in the biosynthetic reaction taking place in stroma responsible for carbon di oxide fixation and sugar synthesis.