can any one give me notes on cell biology

deapan, 8 years ago

Grade:9

1 Answers

Abishek arun

153 Points

8 years ago

Cell Envelope

Cell membrane

· The term was originally used by Nageli and Cramer (1855) for the membranous covering of the protoplast. The same was named plasmalemma by Plowe (1931).

· Plasmalemma or plasma membrane was discovered by Schwann (1838).

· Membranes also occur inside the cytoplasm of eukaryotic cells as covering of several cell organelles like nucleus, mitochondria, plastids, lysosomes, golgi bodies, peroxisomes, etc.

· They line endoplasmic reticulum, cover thylakoids in plastids or form cristae inside mitochondria. Vacuoles are separated from cytoplasm by a membrane called tonoplast.

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Lamellar Models (= Sandwich Models)

They are the early molecular models of biomembranes. According to these models, biomembranes are believed to have stable layered structure.

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Danielli and davson model

The first lamellar model was proposed by James Danielli and Hugh Davson in 1935 on the basis of their physiological studies. Phospholipids form a double layer. The phospholipid bilayer is philic polar heads of the phospholipid molecules are directed towards the proteins. The two are held together by electrostatic forces. The hydrophobic nonpolar tails of the two lipid layers are directed towards the center where they are held together by hydrophobic bonds and van der Waals forces.

Mosaic Model

Fluid-mosaic model

It is the most recent model of a biomembrane proposed by singer and Nicolson in 1972. According to this model, the membrane does not have a uniform disposition of lipids and proteins but is instead a mosaic of the two. Further, the membrane is not solid but is quasifluid. The quasifluid nature of the biomembranes is shown by their properties of quick repair, dynamic nature, ability to fuse, expand and contract, grow during cell growth and cell division, secretion, endocytosis and formation of intercellular junctions.

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Membrane transport

Passage of substances across biomembranes occurs by three methods- passive transport, active transport and bulk transport.

Passive transport

It is a mode of membrane transport where the cell does not spend any energy nor shows any special activity. The transport is according to concentration gradient. It is of two types’ passive diffusion and facilitated diffusion.

(1) Passive diffusion or transport across cell membrane. Here the cell membrane plays a passive role in the transport of substances across it. Passive diffusion can occur either through lipid matrix diffusion can occur either through lipid matrix of the membrane or with the help of channels.

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(2) Neutral solutes and lipid soluble substances.Neutral solutes and fat soluble substances can move across the plasma membrane through simple diffusion along their concentration gradient or from the side of higher concentration to the side of their lower concentration.

(3) Open channel transport. Membranes possess some open channels in the form of tunnel proteins. Water channels or aquaporins allow water and water soluble gases (CO² and O²) to pass through according to their concentration gradient. Osmosis is an example of such a transport.

(4) Facilitated diffusion. it occurs through the agency of gated ion channels and permeases. Energy is not required. The transport is along concentration gradient.

(5) Ion channels are highly specific. There is a specific channel for each ion. Ions do not pass in dissolved state through ion channels but instead only ions move through them. Most ion channels are gated. Depending upon the stimulus required for opening the gated. More than 100 ion channels have been discovered. Movement through ion channels is according to concentration gradient. The rate of passage is quite high.

Active transport

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It is uphill movement of materials across the membrane here the solute particles move against their chemical concentration or electro-chemical gradient.

· Energy is required for the process. it is obtained from ATP.

· Active transport occurs in case of both ions and nonelectrolytes, e.g., salt uptake by plant cells, glucose and phenolphthalein in case of renal tubules, sodium and potassium in case of nerve cells, etc.it is supported by various evidences

(i) absorption is reduced or stopped with the decrease in oxygen content of the surrounding environment.

(ii) metabolic inhibitirs like cyanides inhibit absorption.

(iii) active transport is also inhibited.by substances similar to solutes.

(iv) absorption of different substances is selective.

(v) cells often accumulate salts and other substances against their concentration gradient.

(vi) decrease in temperature decreases absorption. (vii) active transport is more rapid than diffusion. (viii) it shows saturation kinetics, that is, the rate of transport increases with increase in solute concentration not increase in solute concentration till a maximum is achieved.

Bulk transport

· It is transport of large quantities of micromolecules, macromolecules and food particles through the membrane.

· It is accompanied by formation of transport or carrier vesicles. The latter are endocytotic and perform bulk transport inwardly. The phenomenon is called endocytosis.

· Endocytosis is of two types, pinocytosis and phagocytosis.

· Exocytic vesicles perform bulk transport outwardly. It is called exocytosis.

· Exocytosis performs secretion, excretion and ephagy

(1) Pinocytosis: (Lewis, 1931).

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It is bulk intake of fluid, ions and molecules through development of small endocytotic vesicles of 100 - 200 nm in diameter.

· ATP, Ca²⁺ fibrillar protein clathrin and contractile protein actin are required.

· Fluid-phase pinocytosis is also called cell drinking.

· After coming in contact with specific substance, the area of plasma membrane having adsorptive sites invigilates and forms vesicle. The vesicle separates. It is called pinosome.

· Pinosome may burst in cytosol, come in contact with tonoplast and pass its contents into vacuole, form digestive vacuole with lysosome or deliver its contents to Golgi apparatus when it is called receptosome.

(2) Phagocytosis: (Metchnikoff, 1883).

· It is cell eating or ingestion of large particles by living cells, .e.g., white blood corpuscles (neutrophils, monocytes), Kupffer's cells of liver, reticular cells of spleen, histiocytes of connective tissues, macrophages, Amoeba and some other protists, feeding cells of sponges and coelenterates.

· Plasma membrane has receptors.

· As soon as the food particle comes in contact with the receptor site, the edges of the latter evaginate, form a vesicle which pinches off as phagosome.

· One or more lysosomes fuse with a phagosome, form digestive vacuole or food vacuole. Digestion occurs inside the vacuole.

· The digested substances diffuse out, while the residual vacuole passes out, comes in contact with plasma membrane for 'throwing out its contents throughexocytosis or ephagy.

Cell Wall

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A non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants.

· Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.

· Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins.

· The cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell.

· The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.

· Pits 'are formed in lignified cell wall.

· Pits-occurs in sclerenchyma, vessels and tracheids.

· Tracheids in gymnosperms have maximum number of bordered pits.

· In many secondary walls specially 'those of xylem the cell wall becomes hard and thick due to the deposition of lignin. With the increasing amount of lignin, deposition protoplasm is lost. First the lignin is deposited in middle lamella and primary wall and later on in secondary wall.

Growth of cell wall

By intussuception: As the cell wall stretches in one or more directions, new cell wall material secreted by protoplasm gets embedded within the original wall.

By apposition: In this method new cell wall material secreted by protoplasm is deposited by definite thin plates one after the other.

Cytoplasm

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· The substance occurs around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm.

· The cytoplasm is a semisolid, jelly - like material.

· It consists of an aqueous, structure less ground substance called cytoplasmic matrix or hyaloplasm or cytosol.

· It forms about half of the cell's volume and about 90% of it is water.

· It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzyme, vitamins, etc.

· The cytosol also contains storage products such as glycogenlstarch, fats and proteins in colloidal state.

· It also forms crystallo - colloidal system.

· Cytomatrix is differentiated into ectoplasm' or plasmagel (outer) and endoplasm or plasmasol (inner).

· Cytomatrix is three dimensional structure appear like a network of fine threads and these threads are called microfilaments (now called actin filaments or microtrabecular lattice)' and it is believed to be a part of cytoskeleton. It also contains microtubules and intermediate cytoplasmic filaments.

· Hyaloplasm contains metabolically inactive products or cell inclusions called deutoplast or metaplasts.

· Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria, ribosome, centrosome, flagellum or cilia etc.

· The movement of cytoplasm is termed as cyclosis (absent in plant cells).

Endomembrane System

The endomembrane system includes endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components; these are not considered as part of the endomembrane system.

Differences between SER and RER
S.No.	SER	RER
1)	Ser does not bear ribosomes over the surface of its membranes.	Red possesses ribosomes attached to its membranes.
2)	It is mainly formed of vesicles and tubules.	It is mainly formed of cisternae and a few tubules.
3)	It is engaged in the synthesis of glycogen, lipids and steroids.	The reticulum takes part in the synthesis of proteins and enzymes.
4)	SER gives rise to sphaerosomes.	It helps in the formation of lysosomes through the agency of golgi apparatus.
5)	Pores are absent so that materials synthesized by SER do not pass into its channels.	RER possesses narrow pores below its ribosomes for the passage of synthesized polypeptides into ER channels.
6)	SER is often peripheral. It may be connected with plasmalemma.	It is often internal and connected with nuclear envelope.
7)	Ribophorins are absent.	RER contains ribophorins for providing attachment to ribosomes.
8)	It may develop from RER.	It may develop from nuclear envelope.
9)	It has enzymes for detoxification.	The same are absent.
10)	Vesicles for cis-face of Golgi apparatus are provided by SER.	It provides biochemicals for Golgi apparatus.

Endomembrane System-I

The Endoplasmic Reticulum (ER):

· A network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER).

· Garnier (1897) was first to observe the ergastoplasm in a cell. The ER was first noted by Porter, Claude, and Fullman in 1945 as a network.

· It was named by Porter in 1953.

· The ER is present in almost all eukaryotic cells. A few cells such as ova, embryonic cells, and mature RBCs, however, lack ER. It is also absent in prokaryotic cell. In rapidly dividing cells endoplasmic reticulum is poorly developed.

· The ER is made up' of three components. All the three structures are bound by a single unit membrane.

· Cisternae: These are flattened, unbranched structures. They lie in stacks (piles) parallel to one another. They bear ribosomes. They contain glycoproteins named ribophorin-Land ribophorin II that bind the ribosomes. Found in protein forming cells.

· Vesicles: These are oval or rounded, vacuole like elements, scattered in cytoplasm. These are also studded with ribosomes.

· Tubules: Wider, tubular, branched elements mainly present near the cell membrane. They are free from ribosomes. These are more in lipid forming cells.

· The ER often shows ribosomes attached to their outer surface. The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).

· RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.

· The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.

Golgi apparatus:

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Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus. These were later named Golgi bodies after him. They consist of many flat, disc-shaped sacs or cisternae of 0.5 mm to 1mm diameter. These are stacked parallel to each other. Varied numbers of cisternae are present in a Golgi complex.

· It is present in all eukaryotic cells. In plants, these are scattered irregularly in the cytoplasm and called as “dictyosomes".

· These are absent in bacteria and blue green algae, RBCs, spermatozoa of bryophytes and pteridophytes, and sieve tube cells of phloem of angiosperm. The number of golgi body increased during cell division.

· Average number 10-20 per cell. Golgi body surrounded by a zone of protoplasm which is devoid of cell organelles called zone of exclusion (Morre, 1977).

· Under transmission electron microscope the structure of golgi bodies was-study by Dalton and Felix (1954), golgi body is made of 4 parts.

(1) Cisternae: Golgi apparatus is made up of stack of flat sac like structure called cisternae. The margins of each cisterna are gently curved so that the entire golgi body takes on a cup like appearance. The golgi body has a definite polarity. The-cisternae at the convex end of the dictyosome comprises forming face (F. face) or cis face. While the cisternae at the concave end comprises the maturing face (M. face) or trans face. The forming face is located next to either the nucleus or endoplasmic reticulum. The maturing face is usually directed towards the plasma membranes. IUs the functional unit of golgi body.

(2) Tubules: These arise due to fenestration of cisternae and it forms a complex of network.

(3) Secretory vesicles: These are small sized components each about 40 Å in diameter presents along convex surface of edges of cisternae. These are smooth and coated type of vesicles.

(4) Golgian vacuoles:

· They are expanded part of the cisternae which have become modified to form vacuoles. The vacuoles 'develop from the concave or maturing face.

· Oolgian vacuoles contain amorphous or granular substance. Some of the golgian vacuoles function as lysosomes

· The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.

· The cis and the trans faces of the organelle are entirely different, but interconnected.

· The golgi apparatus principally performs the function of packaging materials, to be delivered either -to the intra-cellular targets or secreted outside the cell.

· Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face.

· A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the Golgi apparatus before they are released from its trans face.

· Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

Lysosomes

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Lysosomes are electron microscopic, vesicular structures of the cytoplasm, bounded by a single membrane (lipoproteinous) which are involved in intracellular digestive activities, contains hydrolytic enzymes, so called lysosomes.

· These were first discovered by a Belgian biochemist, Christian de Duve (1955) in the liver cells and were earlier named pericanalicular dense bodies.

· Terms Lysosome was given by Novikoff under the study of electron microscope.

· Matile (1964) was first to demonstrate their presence in plants, particularly in the fungus Neurospora. Polymorphism in lysosomes were described by De Robertis et al (1971).

· These are absent from the prokaryotes but are present in all eukaryotic animal cells except mammalian RBCs. They have been recorded in fungi, Euglena, cotton and pea seeds.

· These are' membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases - lipases, proteases, carbohydrases) optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

Types of lysosomes

On the basis of their contents, four types of lysosomes are recognised.

· Primary Lysosomes: A newly formed lysosome contains enzymes only. It is called the primary lysosomes. Its enzymes are probably in an inactive state.

· Secondary Lysosomes: When some material to be digested enters a primary lysosome, the latter is named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome.

· Tertiary lysosomes/Residual bodies: A secondary lysosome containing indigestible matter is known as the residual bodies or tertiary lysosome. The latter meets the 'cell by exocytosis (ephagy).

· Autophagosomes/Autolysosomes: A cell may digest its own organelles, such as mitochondria, ER. This process is called autophagy. These are formed of primary lysosomes. The acid hydrolases of lysosomes digest the organelles thus, it is called autophagosome. The lysosome are sometimes called disposal units/suicidal bags. Sometime they get burst and cause the destruction of cell or tissue.

Vacuoles

· The vacuole is the membrane-bound space found in the cytoplasm.

· It contains water, sap, excretory product and other materials not useful for the cell.

· The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.

· In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole; hence their concentration is significantly higher in the vacuole than in the cytoplasm.

· In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.

Mitochondria

· Mitochondria (sing: mitochondrion), unless specifically stained, are not easily visible under the microscope.

· Mitochondria are also called chondriosoine, chondrioplast, plasmosomes, plastosomes and plastochondriane.

· These were first observed in striated muscles (Voluntary) of insects as granules by Kolliker (1880), he called them sarcosomes.

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· Michaelis (1898) demonstrated that mitochondria play a significant role in respiration.

· Bensley and Hoerr (1934) isolated mitochondria from liver cells.

· Seekevitz called them "Power house of the cell"

· Nass and Afzelius (1965) observed first DNA in mitochondria.

· Minimum number of mitochondria is one in Microasterias, Trypanosoma, Chlorella, Chlamydomonas (green alga) and Micromonas.

· Maximum numbers are found (up to 500000) in flight muscle cell, (up to 50000) in giant Amoeba called Chaos - Chaos. These are 25 in human sperm, 300 - 400 in kidney cells and 1000 - 1600 in liver cells.

· Size of mitochondria: Average size is 0.5–1.00 mm and length up to 1 – 10 mm.

(i) Smallest sized mitochondria in yeast cells 1mm³

(ii) Largest sized are found in oocytes of Rana pipiens and are 20 – 40 mm

(iii) A dye for staining mitochondria is Janus B – green.

Enzymes of Mitochondria

(1) Outer membrane: Monoamine oxidase, glycero phosphatase, acyl transferase, phospholipase A.

(2) Inner membrane: Cytochrome b.c₁.c.a, (cyt.b, cyt.c₁, cyt.c, cyt.a, cyt.a₃ NADH, dehydrogenase, succinate dehydrogenase, ubiquinone, flavoprotein, ATPase.

(3) Perimitochondrial space: Adenylate kinase, nucleoside diphosphokinase,

(4) Inner matrix: Pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, fumarase, a-Ketoglutarate dehydrogenase, malate dehydrogenase.

Plastids

· Definition: Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope which store or synthesize various types of organic compounds as ATP and NADPH + H⁺ etc. These are largest cell organelles in plant cell.

· Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).

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· A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein.

· Park and Biggins (1964) gave the concept of quantasomes.

· The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall.

· Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome.

· Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as they are large.

· They bear some specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

· The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.

· In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others' are present.

· This gives the part of the plant a yellow, orange or red colour. The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients:

· Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins.

Pigments of chloroplast

Chlorophyll a: C₅₅ H₇₂ O₅N₄Mg (with methyl group)

Chlorophyll b: C_ss H₇₀O₆N₄Mg (with aldehyde group)

Chlorophyll c: C₃₅H₃₂O₅ N₄ Mg

Chlorophyll d: C₅₄ H₇₀ O₆N₄ Mg

· Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves.

· These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10mm) and width (2-4mm). Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll.

· Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable.

· The space limited by the inner membrane of the chloroplast is called the stroma.

· A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the inter grana thylakoids.

· In addition, there are flat membranous tubules called the stroma lamellae connecting the , thylakoids of the different grana.

· The membrane of the thylakoids encloses a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.

· It also contains small, double-stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids.

· The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

Types of plastids

According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts.

Leucoplasts

They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and occur in the cells not exposed to sunlight e.g., seeds underground stems, roots, tubers, rhizomes etc. These are of three types.

(i) Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains.

(ii) Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g. castor endosperm, tube rose, etc.

(iii) Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

Murphy and Leech (1978) have reported the synthesis of fatty acids in the spinach chloroplast.
Proplastids are precursor of all type of plastids.
Capasanthin is the pigment in carotenoids found in bacteria, fungi and chilly.
Solar energy is trapped in lamella by chlorophylls but in bacteria trapping centre is B₈₉₀.
The chloroplast with nitrogen fixing genes (nif genes) constitute nitroplast.
Pyrenoids : A proteinaceous core around which starch is deposited mostly found in the chloroplast of algae and in some bryophytes.
Algal classification is based on pigmentation pattern.
Eye spot or stigma is photosensitive carotenoid pigment.

Chromoplasts

· Coloured plastids other than green are known as chromoplasts.

· These are present in petals and fruits, imparting different colours (red, yellow, orange etc) for attracting insects and animals. These also carry on photosynthesis.

· These may arise from the chloroplasts due to replacement of chlorophyll by other pigments e.g. tomato and chillies or from leucoplasts by the development of pigments.

· All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin express different colours on different pH value. These are variously coloured e.g. in flowers. They give colour to petals and help in pollination. They are water soluble. They are found in cell sap.

· Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into chromatophores.

Chloroplast

Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments.