plasids

1. PLASTIDS

2. •Plastids (Schimper - 1883) are self-duplicating
membrane-bound cell organelles, concerned with the
synthesis and storage of food.
•They are present in algae, plants and some protists.
•There are three kinds of plastids, namely leucoplasts,
chromoplasts and chloroplasts.
•They can change from one type to another, and are
derived from proplastids.
•Leucoplasts are colourless, and the others coloured.Most
plastids are coloured due to the presence of pigments.
•Some important pigments and their respective colours
are given below

3. Colour Pigment
•1. Chlorophylls Green
•2. Carotenes Yellow or red
•3. Xanthophylls Yellow, orange or brown
•4. Phycoerythrin Red
•5. Phycocyanin Blue-green
•6. Haematochrome Red
•7. Fucoxanthin Brown
•8. Bacteriorhodopsin Purple

4. •Most pigments are usually seen in plastids.
•However, certain other pigments, such as anthocyanins,
are found dissolved in the cytoplasmic matrix.
•The blue, red, pink and violet colours of some flowers,
fruits, stems and leaves are due to the presence of
anthocyanins.
•Plastids are mainly concerned with the synthesis and
storage of food.
•Also, they serve asthe centres of several biosynthetic
reactions, such as the synthesis of purines, pyrimidines,
fatty acids and amino acids.

5. LEUCOPLASTS
•Leucoplasts (leukoplasts) are colourless plastids,
without colouring pigments, thylacoids and
ribosomes.
•Usually, they are found in germ cells, embryonic
cells, meristematic cells and fully differentiated
tissues, which are not normally exposed to sunlight.
•Plastids, found in cotyledons are initially colourless,
but later on they transform to chloroplasts.

7. •Leucoplasts are specialised for the synthesis
and storage of reserve food.
•Leucoplasts which synthesise and store starch
are called amyloplasts, those which synthesise
and store proteins are called proteoplasts or
aleuroplasts, and those which synthesise and
store lipids are called elaioplasts (lipidoplasts,
oleoplasts, or oleosomes).
•Amyloplasts are found in endosperm,
cotyledons and storage tubers.

8. •Often, leucoplasts get transformed to chloroplasts and
chromoplasts under certain circumstances.
•The appearance of green colour on stored tubers of
potato is due to the conversion of leucoplasts to
chloroplasts.
•Amyloplasts are found in cotyledons, endosperm and in
storage organs, such as potato tubers.
•Elaioplasts are commonly found in the tissues of
liverworts and monocotyledons.
•Aleuroplasts are found in many seeds.
•Structurally, leucoplasts are similar to chloroplasts with
double layered limiting membrane, but with fewer
lamellae.

9. CHROMOPLASTS
•Plastids, coloured other than green, are called chromoplasts.
They occur in the petals, fruits and roots of some higher
plants.
•Chromoplasts develop either from chloroplasts (as in petals),
or rarely from leucoplasts (as in carrot roots).
•During the transformation of chloroplast to chromoplast,
chlorophylls and starch gradually decrease in quantity, large
globuli are formed and arranged along the plastid membrane,
lamellar structures break down and stroma gets disorganised.
•These changes result in the empty appearance of the plastid
centre.

11. •During the transformation of leucoplasts to
chromoplasts, certain fibrils appear which later on give
rise to crystals, filling up the whole plastid.
•The crystals are in the form of sheet - like structures,
containing large quantities of carotenoids.
•Chromoplasts may be red, orange, or yellow.
•The red colour of ripening tomatoes is due to the
presence of chromoplasts which contain the carotenoid
pigment leucopene.
•Chromoplasts, containing phycocyanin and
phycoerythrin, are found in algae.
•In green plants, they contain carotenoid pigments
(carotenes and xanthophylls).

12. •They show great variety of shape, but are mainly
irregular. Granular, angular and forked types also occur.
•The irregular and sharply pointed shapes are partly due
to the presence of coloured substances in a crystalline
form (e.g. roots of carrot).
•The chromoplasts of brown algae, dinoflagellates and
diatoms, which contain fucoxanthin, are called
phaeoplasts.
•Those of red algae, which contain phycoerythrin, are
called rhodoplasts.
•The chromoplasts of blue-green algae (Cyanobacteria)
are blue-green in colour and they contain chlorophyll-a,
carotenoids and phycobilins.

13. CHLOROPLASTS
•Chloroplasts are green, chlorophyll-containing,
photosynthetically active plastids.
•They are the active energy-transducing centres
where light energy is trapped, converted to
chemical energy and conserved in organic
molecules with simultaneous synthesis of ATP.
•Chloroplasts are found in green bacteria, green
algae, green plants and green protists.

15. Morphology
•Chloroplasts are among the largest cytoplasmic
organelles, clearly observable under the low power of
compound microscopes.
•Their size, shape and distribution vary with different
cells and species.
•Their average size varies from 4 to 6 u in diameter and
1 to 3 μ in thickness.
•The chloroplasts of polyploid cells may be larger than
those of diploid cells.
•In general, the chloroplasts of sciophytes (shade-plants)
may be larger than those of heliophytes (sun-plants).

17. •In some algae, such as Spirogyra, Chlorella and
Chlamydomonas, only a single chloroplast is present in
each cell.
•On the other hand, a cell in the spongy tissue of a grass
leaf may have 30 to 50 chloroplasts.
•Their average number in land plants is 20 - 40 per cell.
•Their division, which occurs in the proplastid stage
(immature stage), is not correlated with cell division.
•Blue-green algae lack definite chloroplasts; instead they
possess loosely arranged cytoplasmic membranes,
which contain photosynthetic pigments.

18. •Chloroplasts are mostly spherical, ovoid, discoid,
vesicular or cup-shaped.
•In some algae, they may be reticulate (e.g.,
Oedogonium), stellate (e.g., Zygnema), band-
shaped (e.g., Ulothrix), spiral (e.g., Spirogyra), or
cup-shaped (e.g., Volvox).
•Chloroplasts are uniformly distributed within the
cytoplasm.
•Their distribution depends mainly on external
conditions, such as light intensity.

19. Chemical composition
•Chloroplasts are formed of water, proteins, lipids,
DNA, RNAs, mineral ions, chlorophylls and other
pigments, and the enzymes and biochemical factors
required for DNA replication, RNA synthesis,
protein synthesis and photosynthesis.
•Proteins serve as enzymes and intrinsic and
extrinsic membrane proteins.
•The lipid contents include phospholipids,
triglycerides, sterols and waxes.

21. •The photosynthetic pigments are of two
groups, namely principal pigments and
accessory pigments.
•The principal pigment in green plants is
chlorophyll-a and those of bacteria include
bacteriochlorophyll, chlorobium chlorophyll,
bacteriorhodopsin, etc.
•The accessory pigments of green plants
include chlorophyll-b and carotenoids
(carotene and xanthophyll).

22. •The accessory pigments of marine algae
include chlorophylls-c and d phycoerythrin
and phycocyanin.
•Accessory pigments expand the light
spectrum from which energy can be
captured.
•The energy absorbed by them is transferred
to chlorophyll-a in the early stages of
photosynthesis.
•Photochemical reaction takes place only in
chlorophyll-a.

23. Chlorophylls
•Chlorophylls are asymmetrical, green, Mg-containing
photosynthetic pigments.
•They can absorb blue-violet (435-438 nm) and red (670-680
nm) light and transmit or reflect green light (so they appear
green).
•Their photosynthetic activity will be maximal in red light.
•There are different kinds of chlorophylls, such as chlorophylls
a, b, c and d, bacteriochlorophyll and chlorobium chlorophyll.
•Of these, chlorophylls a and b are found in green plants, and
c and d are found in cyanobacteria, some protists and some
algae.

24. •A chlorophyll molecule is a magnesium-porphyrin
derivative. It consists of a pyrole head or nucleus, a
phytol tail and a side group.
•Head is hydrophilic, and the tail hydrophobic and
lipophilic.
•Head is a ring of four pyrole molecules, often called
tetrapyrole ring or porphyrin ring, with a Mg atom in the
centre.
•Tail is a long hydrocarbon chain, attached to the
porphyrin ring.

25. •Linked to the porphyrin ring is a side group,
different in different kinds of chlorophylls.
•It is the chemical nature of the side group that
determines the properties of chlorophyll.
•The side group of chlorophyll-a is a methyl group
(CH3), and that of chlorophyll-b is an aldehyde
group (-CHO).
•Chlorophyll a is bluish-green and chlorophyll b is
yellowish-green.

28. •The empirical formula of chlorophyll-a is C55
H72 O5 N4 Mg, and that of chlorophyll-b is
C55 H70 O6 N4 Mg.
•Several varieties of chlorophyll - a may be
recognized, such as chl.a 673 ,chl.a 683,
P680, P700 etc.

29. Carotenoids
• These are yellow, orange, brown, or red pigments, found in all
photosynthetic plants.
• Together with anthocyanins, carotenoids impart autumn colours to
leaves (when chlorophyll breaks down).
• They are found in some flowers and fruits also (e.g., carrot, tomato,
pumpkin).
• Carotenoids absorb the blue and green light of the visible spectrum
and transfer the light energy to chlorophyll to be used in
photosynthesis.
• They also protect chlorophyll molecules from photo-oxidation (by
molecular oxygen in high-intesity light).
• Carotenoids are of two groups, namely carotenes (C40H56) and
xanthophylls (C40H56O₂).

30. • Carotenes are red or orange-coloured hydrocarbons (terpenes).
• Xanthophylls are brown or yellow and oxygenated hydrocarbons.
• The common carotenes include a, ß, y and d carotenes, phytotene,
lycopene, neurosporene, etc.
• During digestion in vertebrates, B-carotene gets hydrolysed to two
identical portions, which yield vitamin A.
• Xanthophylls are more abundant than carotenes.
• They differ from carotenes in having oxygen.
• The major xanthophylls of higher plants include lutein,
violaxanthin, zeaxanthin and neoxanthin.
• The commonest xanthophyll of diatoms and brown algae is
fucoxanthin.
• Lutein gives yellow colour to autumn leaves.

31. Phycobilins
• Phycobilins are red or blue accessory photosynthetic pigments, found in
cyanobacteria and red algae.
• They differ from chlorophylls and carotenoids in being water-soluble.
• They are similar to the porphyrin part of chlorophylls, except that Mg is
absent and the tetrapyrroles are linear rather than cyclic.
• There are two major groups of phycobilins, namely phycoerythrins and
phycocyanins.
• Phycoerythrins are red-coloured.
• They absorb dim and blue-green light, which can reach ocean depths.
• So, phycobilins enable algae to live in deep waters.
• Phycocyanins are bluecoloured.
• They absorb extra orange and red light.

32. Bacteriorhodopsin
• In Halobacteria, there is a light-absorbing proteinous pigment in the
plasma membrane, called bacteriorhodopsin.
• It is formed of a single polypeptide of 247 amino acids.
• It resembles the visual pigment rhodopsin, found in the retinal rods of
vertebrate eye.
• It exists in two interconvertible forms, namely "purple" and "bleached".
• The former absorbs light at a wavelength of 570 nm, and the latter at 412
nm.
• As the pigment transforms from the purple to the bleached form, it loses
H+ ion, and in the reverse transformation it picks up and H+ ion (Racker
and Stoeckenius 1974).
• Thus, bacteriorhodopsin functions as a proton pump, that is driven by
light.

33. Structure of chloroplast
• Chloroplast is a double-walled and fluid-filled bag.
• It has a gelatinous core, called matrix or stroma, covered by two
concentric membranes.
• Across these membranes, molecular exchange takes place between
stroma and cytosol.
• The outer membrane is highly permeable, and the inner one is less
permeable.
• In between these membrane is the intermembrane space or
periplastidial space.
• The membranes have lipoprotein composition, lamellar structure
and fluid-mosaic organization.
• They consist of two separate layers of 40-60 Å thickness.

35. • Stroma is a metabolic centre where CO₂ fixation and the synthesis
of nucleic acids, sugars, starch, fatty acids and some chloroplast
proteins take place.
• It contains a lamellar membrane system, ribosomes, circular DNA,
messenger and transfer RNAs, starch grains, lipid globules (called
plastoglobuli), and the enzymes for DNA duplication, genetic
transcription, genetic translation and the dark reactions of
photosynthesis.
• Stroma contains nearly 50% of the chloroplast proteins most of
which are of the soluble type.
• Most algal chloroplasts contain pyrenoids, in association with
starch grains.
• Pyrenoids contain the active enzyme ribulose biphosphate
carboxylase.
• Hence, they are believed to be involved in carbohydrate synthesis.

36. •In lower organisms, chloroplasts are primitive.
•They are bounded by a two-layered membrane and
contain pigment-coated plates in the matrix. Such
chloroplasts are called lamellate chloroplasts.
•The extensive stromal membrane system is called
thylacoid system or lamellar system.
•Chlorophyll molecules and light-absorbing accessory
pigments are embedded in it. It mainly consists of
several cylindrical discs, called grana.
•Each granum is a parallel stack or pile of 5 to 50 or
more flattened, closed and pigmented sacs, called
granal thylacoids or granal lamellae.

39. •Adjacent grana are connected together by a system of tubular
membranes, called intergranal thylacoids or fret membranes.
•In addition to these, a few unstacked thylacoids may also be
present in the stroma.
•They are called stromal thylacoids or stromal lamellae.
•They form a system of anastomosing tubules that are joined
to the grana thylacoids
•The number of thylacoids in a granum is different in different
groups.
•There is only a single thylacoid in each granum in
Rhodophyceae, two in Cryptophyceae, three in
Phaeophyceae and Bacillariophyceae, and many in others.

40. •In the bundle sheath cells of C4 plants, a specialised type of
chloroplasts are found.
•In them, lamellae are uniformly dispersed in the stroma,
without forming grana. So, they are called agranal chloroplasts.
•The leaves of C4 plants have the peculiar kranz anatomy and
hence their agranal chloroplasts are also called kranz type
of chloroplasts.
•Different views have been expressed to explain the relation
between granal and intergranal thylacoids.
•Steinmann and Sjostrand hold that stromal and intergranal
lamellae develop from granal discs. Hedge and others are of
opinion that granal discs are locally swollen thylacoids.

41. •Thylacoids have lipoprotein composition and laminar
organization.
•They are formed of thin molecular sheets or layers,
called laminae or lamellae.
•These are the store houses of photosynthetic pigments
and the enzymes of the light reactions of photosynthesis.
•The membranes also contain photochemically inactive
light-harvesting complex proteins (LHCP) and the
electron carriers cytochrome-b, cytochrome-f,
plastoquinone and plastocyanin.

42. Photosystems or pigment systems
•The light-absorbing photosynthetic pigments in the
thylacoid membrane are arranged in two complex
pigment systems, namely photosystem I(PSI) and
photosystem II (PSII).
•PSI is present mainly in unstacked membranes, and PSII
mainly in stacked membranes.
•These two systems are structurally distinct, but
functionally related. They are linked together by mobile
electron carriers.

43. •Each photosystem has many thousand
functional photosynthetic units, called
quantasomes.
•Each unit, in turn, consists of nearly 200-400
chlorophyll molecules and many accessory
pigment molecules.
•The pigment systems can absorb all
wavelengths of the visible spectrum, especially
those between 400 and 500 nm, and those
between 600 and 700 nm.

44. •Each functional unit has a photochemical reaction
centre, often called photocentre.
•It serves as an energy trap for the collection and
conversion of light energy; it brings about the conversion
of light energy to chemical energy.
•It is formed of chlorophyll-a molecules, bound to a
protein complex.
•The reaction centre of PSI is termed P700 and that of PSII
is termed P680.
•The former has an absorption maximum at 700 nm, and
the latter has it at 680 nm.

45. •The reaction centre is surrounded by a cluster of light-
absorbing pigment molecules.
•They form a light-gathering antenna system to absorb
photons.
•Hence, they are often called light-harvesting antenna
molecules.
•They include chlorophyll-a, chlorophyll-b, ß-carotenes
and other accessory pigments.

46. • Antenna molecules remain complexed with a few polypeptides,
forming the light-harvesting complex (LHC).
• This complex is more prominent in PSII than in PSI.
• It is mainly localized in stacked thylacoids. Its main function is to
capture solar energy and it has no photochemical activity at all.
• PSI and PSII differ from each other with regard to their LHC; in the
LHC of PSII, the amount of chlorophyll-a is much lesser than those of
chlorophyll-b and ß-carotene, while in the LHC of PSI it is much
higher than those of chlorophyll-b and ßcarotene.
• In the photosystems, the pigment molecules are very closely and
orderly arranged.
• This enables each molecule to absorb photon energy and pass it on
to the next molecule and finally to the reaction centre by a chain-
transfer mechanism.

47. Quantasomes
• Quantasomes are closely packed arrays of spherical or oblate granular
particles, regularly arranged on or in chloroplast thylacoids.
• They are considered to be the smallest light harvesting photosynthetic units,
capable of carrying out photochemical reactions.
• In green plants, each of them contains nearly 200-400 chlorophyll-a molecules,
some accessory pigments, electron carriers and the enzymes of light reactions.
• [Quantasomes were first noticed by Moor, Weier and Benson, and were first
isolated by Park and Biggins-1964]
• Evidences suggest that at least two distinct classes of quantasomes are present,
namely the smaller ones on the membrane surface and the larger ones inside the
membrane.
• Functionally, the smaller and larger quantasomes are believed to represent
pigment systems (photosystems) I and II respectively.

48. Semigenetic autonomy of chloroplasts
•Cytological studies have revealed the presence of
circular DNA, RNAs and ribosomes in chloroplasts.
•Nucleic acids are of the bacterial type.
•The DNA content of a chloroplast is about ten times
greater than that of a mitochondrion.
•The presence of DNA, RNAs and ribosomes gives
chloroplasts the ability to synthesise proteins.
•This makes them genetically semiautonomous.

49. Functions of chloroplasts
•(i) Energy fixation and the synthesis of organic molecules:
• Chloroplasts are the centres of photosynthesis.
• By photosynthesis, they manufacture organic molecules and bring about
energy fixation.
• Energy fixation is the overall process by which solar energy is trapped,
converted to chemical energy and conserved in the energy-bonds of organic
molecules.
• Energy fixation is significant in that it brings about the transduction of light
energy to chemical energy and thereby initiates the dynamics of the
ecosystem.
• Thus, the functional state of the ecosystem seems to be largely dependent
on the energy fixation by chloroplasts.

50. (ii) Synthesis of nucleic acids and proteins:
• Just as mitochondria, chloroplasts are also semiautomous organelles.
• With some autonomy they too can synthesise nucleic acids and
protein and thus bring about gene expression through the
replication, transcription and translation of the genetic code.
• Chloroplasts contain the complete molecular machinery for the
synthesis of nucleic acids and proteins.
• Chloroplast DNA can underego enzymatic replication and produce
multiple copies of it.
• Similarly, it can undergo transcription and synthesise RNAs.
• Making use of their DNA, RNAs, ribosomes and enzymes chloroplasts
can carry out genetic translation and synthesise some chloroplast
proteins also.

51. (iii) Extra-chromosomal (cytoplasmic) inheritance:
•Chloroplast DNA contains extra-chromosomal
genes, called plastogenes or plasma genes.
•They can govern non-Mendelian inheritance
and control plastid characters, generally in co-
operation with chromosomal genes.
•Plastogenes are believed to control the
inheritance of leaf colour in some plants.

52. (iv) Provide energy and carbon for
all heterotrophs:
•Chloroplasts carry out photosynthesis and
produce organic molecules, which form
the basic source of energy and carbon for
all heterotrophic organisms.
•So, most organisms directly or indirectly
depend on chloroplasts for their energetic
and nutritional requirements.

53. v) Enrich atmospheric O, and maintain the
stratospheric ozone layer:
•Oxygenic photosynthesis in chloroplasts removes CO₂
from the atmosphere and releases O₂ to the
atmosphere.
•This enriches the atmospheric oxygen store.
•Virtually, the entire atmospheric oxygen build up is the
product of oxygenic photosynthesis over the past 3300
million years.
•A portion of this oxygen treasure is converted to
stratospheric O, by the action of UV radiation.

54. •This ozone layer screens out harmful solar and
cosmic radiations and thereby protects the life on
earth.
•Thus, it becomes clear that without chloroplasts
and green plants there would be no oxygen in the
atmosphere, and life on earth would be almost
impossible.

55. (vi) Reduction of nitrite to ammonia :
•The reducing power of energised (light-
activated) electrons in chloroplasts drives
the reduction of NO₂ to NH3.
•The ammonia thus formed provides nitrogen
for the synthesis of nitrogenous compounds,
such as amino acids, nucleotides, etc.

56. Origin of plastids
• Many theories have been put forward to explain the origin and development of
plastids. Some of them are the following:
(a) Monotropic development
• This theory holds that plastids are autonomous organelles and they can change
from one form to another either reversibly or irreversibly.
• So, according to it, plastids are not formed de novo, but are always formed by
interconversion.
• It also holds that chromoplast is a degenerated form of chloroplast, formed by
the irreversible transformation of chloroplast.
• This transformation involves an irrecoverable loss of chlorophyll.

57. b) Development from proplastids
• Several workers hold that plastids develop from tiny cytoplasmic bodies, called
proplastids or plastid precursors.
• Proplastids are simple self-duplicating structures with a clear matrix, bounded by
a double-layered membrane.
• The matrix contains DNA and RNAs.Proplastids develop first in the cytoplasm of
meristematic cells.
• On exposure to light, they elongate and develop 'blebs' from the inner side of
their double-layered envelope.
• These blebs become lamellae, which in turn, multiply through linear splitting.
• Simple lamellae undergo growth and differentiation and give rise to granal,
intergranal and stromal lamellae. This results in the transformation of proplastids
to mature chloroplasts.

58. (c) Division and budding
•In lower plants, new plastids originate by the
division and budding of the existing ones.
•During division, ring-like constrictions appear,
which gradually deepen and finally divide the
plastid into fragments.
•These fragments then differentiate into mature
plastids.
•Budding of plastids also takes place under certain
special circumstances.

59. (d) Nuclear origin
•Proplastid-like structures have been observed
near the nucleus in some cases.
•This has led some workers to think that
proplastids originate from nucleus by the
evagination of nuclear envelope.
•Then, proplastids develop to mature plastids.