CHOLOROPLASTS

WAY TO TRAP SOLAR ENERGY

Plant Cell Chloroplast Structure

Overview:-

Introduction of Chloroplasts

The word chloroplast is derived from the Greek words chloros, which means green, and plast, which means form or entity.

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis.

Chloroplasts are members of a class of organelles known as plastids.

PLASTIDS

Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell.

Plastids often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's color.

TYPES OF PLASTIDS

Functions of main types of plastids

Chromoplasts - color centers There are two main types of chromoplasts -- cartenoids

which store yellow and orange pigment and chlorophyll which stores green pigment.

Chloroplasts contain chlorophyll which contains green pigment and some carotenoids which hold yellow or orange pigment.

Chlorophyll traps radiant sun energy then manufactures complex organic molecules(particularly glucose) from simple raw organic materials.

Leucoplasts - energy storage Leucoplasts are organelles where starch, oil and

protein are stored.

Genome of chloroplasts

These are considered to have originated from cyanobacteria through endosymbiosis.

 The chloroplast has its own DNA, which codes for redox proteins involved in electron transport in photosynthesis. This is called as plastome.

Chloroplasts are observable as flat discs usually 2-10 μm in diameter and 1-2.5 μm thick. The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the inter membrane space.

Chloroplasts ultrastructure :

1. outer membrane2. inter membrane space3. inner membrane (1,2&3 are envelope)4. stroma (aqueous fluid)5. thylakoid lumen (inside of thylakoid)6. thylakoid membrane7. granum (stack of thylakoids)8. thylakoid (lamella)9. starch10. ribosome11. plastidial DNA12. plastoglobule (drop of lipids)

Stroma & Thylakoids The material within the chloroplast is called the stroma and contains one

or more molecules of small circular DNA. It also contains ribosomes . The stacks of sub-organelles present Within the stroma are thylakoids,

which are the site of photosynthesis. The thylakoids are arranged in stacks called grana. A thylakoid has a flattened disk shape. Inside it is an empty area

called the thylakoid space or lumen. Photosynthesis takes place on the thylakoid membrane. It

involves the coupling of cross-membrane fluxes with biosynthesis via the dissipation of a proton electrochemical gradient.

Chemical composition of Chloroplasts Chloroplast contains: Proteins – 35 – 55% Lipids- 20-30% Carbohydrates- variable Chlorophyll- 9% Carotenoids: 4.5% RNA 3 to 4% DNA 0.5% Minerals 0.2% The chloroplast contains

three types of pigments: Chlorophylls Carotenoids Phycobilins

Chlorophyll – green pigment – in higher plant, green algae

Structure similar to haem- it contains a porphyrin ring with magnesium

Chlorophyll types: Chlorophylla,b,c,d etc. Chlorophyll a , b – higher

plants, green algae Chlorophyll c –

dinoflagellates , diatoms and brown algae

Chlorophyll d –red algae

Thylakoid membranes appear as alternating light-and-dark bands. Thylakoid membranes appear as alternating light-and-dark bands, each 0.01 μm thick. Embedded in the thylakoid membrane are antenna complexes, each of which consists of the light-absorbing pigments, including chlorophyll and carotenoids, as well as proteins that bind the pigments.

This complex both increases the surface area for light capture, and allows capture of photons with a wider range of wavelengths.

The energy of the incident photons is absorbed by the pigments and funneled to the reaction centre of this complex through resonance energy transfer. Two chlorophyll molecules are then ionised, producing an excited electron, which then passes onto the photochemical reaction centre.

photosynthesis Photosynthesis  (photo means “light” and

synthesis means “putting together“ or "composition") is a process that converts carbon dioxide , water into glucose(organic compound), using the energy from sunlight

Light reaction In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron.

This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH.

This creates a proton gradient across the chloroplast membrane. Its dissipation is used by ATP synthase for the synthesis of ATP. Photolysis :- The process of breaking of water molecule into H2 &

O2.

 2 H2O(l) → O2(g) + 4 H+(aq) + 4e−

 The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

Introduction of photo systems

The two photo system units are Photo system II(p680) and Photo system I(p700), which have their own distinct reaction center chlorophylls.

These pigments are named after the wavelength of their red-peak absorption maximum. The identity, function and spectral properties are distinct and determined by each other and the protein structure surrounding them.

The function of the reaction center chlorophyll is to use the energy absorbed by and transferred to it from the other chlorophyll pigments in the photosystems to undergo a charge separation, a specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates called an electron transport chain. The charged reaction center chlorophyll (P680+) is then reduced back to its ground state by accepting an electron.

Photo system II In Photo system II, the electron that reduces

P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates.

This reaction is how photosynthetic organisms like plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photo system I works in series with Photosystem II, thus the P700+ of Photo system I is usually reduced, via many intermediates in the thylakoid membrane, by electrons ultimately from Photo system II.

Electron transfer reactions in the thylakoid membranes are complex, however; and the source of electrons used to reduce P700+ can vary.

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