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Agriculture LibreTexts

Section 4

Plastids (E. Haeckel, 1866): these are flat, circular, cytoplasmic cell organelles, only present in plant cells and photosynthetic protests.Which are primarily involved in formation and storage of soluble and insoluble carbohydrates.Plastids are two types on the basis presence or absence of pigment:-

  1. leucoplasts(plastids without pigments): store reserve food material colourless and occur in the cells which are not exposed to sunlight and also found in embryonic cells, meristematic cells and parenchymatous cells .Leucoplasts are the centers of starch ,oils and proteins synthesis and storage, on that basis leucoplasts are of three types (a) Amyloplasts: filled with starch found in endo (b) elaioplasts: store lipids and (c) aleuroplast: store protein crystals and granules and
  2. chromoplasts and chloroplasts(coloured) Chromoplasts are plastids with pigment colours other than green, their colour is due to two pigments, carotene and xanthophyll.Yellow-to-red colored chromoplasts manufacture carotenoids, These are present in petals and fruits, imparting them different colours(red, yellow etc.). Chromoplasts may develop from chloroplasts due to replacment of chlorophyll by other pigment tomatoes and chillies turn red on ripening due to change of chlorophyll in chloroplasts by red pigment lycopin or lycopene in tomato and capsanthin in chillies.Or from leucoplasts by the development of pigments e.g., carrots.Chromoplasta are also involved in photosynthesis.The primary function in the cells of flowers is to attract agents of pollination, and in fruit to attract agents of dispersal

Chloroplasts are the most important type of plastid having green pigment(chlorophyll), and are typically about 10 micrometers in diameter. The plastids are made up of an outer limiting membrane and inner matrix. The outer membrane is made up of two layers of lipoprotein and separated from one another by a space known as perplastidial space.The outer layer of the double membrane is much more permeable than the inner layer, which features a number of embedded membrane transport proteins. Inner matrix of a chloroplast is differentiated into grana, where light reaction of photosynthesis takes place and the stroma where dark reaction(Calvin cycle) is completed. Grana consist of the lamellar system and stroma is non-membranous both are consist of interconnected sac like structure known as thylakoids.Thylakoids are of two types (1)grana thylakoid and(2) intergranal thyalkoids. The thylakoids in each granum are continuous with those in other grana through intergranal thylakoids. In the chloroplast the thylakoids are embedded, or suspended, in a matrix, the stroma, which has a somewhat granular appearance.

Figure 2.4.1: Ultrastructure of chloroplast

The stroma a semi fluid colorless colloidal complex contains DNA fibrils and ribosomes, starch grains, osmiophilic globules and occasional extensive crystal-like structures. The thylakoid membranes contains green pigment chlorophylla and b carotenoids, cytochrome, ATP-synthetase, etc.

Function of chloroplasts: Chloroplasts are specialized for photosynthesis, the biological conversion of light energy absorbed by chlorophylls, the green leaf pigments, into potential chemical energy such as carbohydrates. Photosynthesis is the process by which plants make food. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air, that is utilised by people and animals . Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar.

6 CO2 + 6 H2O + energy (from sunlight) -------->C6H12O6 + 6 O2

The sugar--together with nitrogen, sulfur, and phosphorus from the soil--helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive.

Nucleus (Robert Brown -1831):-The nucleus is the most prominent structureof cell.Nucleus is present only in cells of higher plants i.e. eukaryotes. Generally there is a single nucleus present in each cell, but there are exceptions also in some fungi e.g., Rhizopus etc. and algae e.g., Vaucheria more than one nucleus are present. Nucleus is absent in simpler one-celled plants -prokaryotes e.g., Viruses, bacteria and cynobacteria. The spherical-shaped nucleus, consists of a semi fluid matrix known as nucleoplasm in which one or more nucleoli, and chromatin threads are suspended. Nucleus controls cellular metabolism of cells and contains all genetic informations and is able to transmit it from one generation to the other. The nucleus is the most prominent of cell. The nuclei of cell consist of four components:

Nuclear membrane (or envelope)

  1. Nuclear membrane (Erclab-1845: Hertwig, 1893): The nuclear membrane is a double-layered membrane that encloses the nucleus, and separates the contents of the nucleus from the cellular cytoplasm. The space between the layers is called the perinuclear space and connected with a network of tubules and sacs, called the endoplasmic reticulum, where protein synthesis occurs, and is usually studded with ribosomes. The membrane is perforated by numerous pores called nuclear pores. These pores regulate the flow of molecules between the nucleus and cytoplasm,The nuclear membrane is semi-permeable; permitting selective molecules to pass through the membrane,into and out of the nucleus . The inner nuclear membrane has a protein lining called the nuclear lamina,(Harris and James, 1952) which binds to chromatin and other nuclear components. The nuclear membrane disintegrates during cell division or mitosis, and is reformed as the two cells complete their formation and the chromatin begins to unravel and disperse. The function of the nuclear envelope is to confine the materials necessary for DNA and RNA synthesis inside the nucleus, and controlling movement into and out of the nucleus.
  2. Nucleoplasm (Strasburger, 1882): The nucleoplasm is semi fluid, granular substance or matrix that fills the interior of the nucleus. The nucleolus and the chromatin network lie suspended in the nucleoplasm. This dense, spherical granule found in the nucleus contains RNA (ribonucleic acid) which is responsible for protein synthesis in the cytoplasm It is mostly composed of water, containing a complex assortment of materials. Nucleoplasm is distinct from cytoplasm due to the high concentration of materials like nucleotides, which are used to make DNA and RNA, and the suite of enzymes which control the DNA and RNA construction reactions .Nucleoplasm contains one or more nucleoli (singlur "nucleolus") , organelles that synthesize protein-producing macromolecular assemblies called ribosomes, and a variety of other smaller components, such as Cajal bodies, GEMS (Gemini of coiled bodies), and interchromatin granule clusters.

Figure 2.4.2: Ultrastructure of nucleus

  1. Nucleolus(Bowman, 1848):-The nucleus contains one to four densely granular region called nucleoli (singlur "nucleolus"), but within each species the number of nucleoli is fixed.There is no membrane separating the nucleolus from the rest of the nucleus. The nucleolus, which is important in the formation of ribosomes, appears as a dense mass of RNA (ribonucleic acid), ribosomal RNA, chromatin, and proteins.When a cell reproduces the nucleolus disappearsand reappear after cell formation. The nucleolus made of three morphologically distinct components:
    • Fibrillar centres (FC), where rRNA genes of several chromosomal loci (termed nucleolar organising regionsNORs) are located;the transcription of rRNA genes(RNA synthesis on DNA template) also takes place in this region.
    • Dense fibrillar component (DFC), which surrounds the fibrillar centrewhich contains actively transcribing rRNA genes and nascent rRNA transcripts; and
    • Granular component (GC),Which is the outermost region, which is the site of late processing events in the biogenesis of rRNAs.The rRNA which is a main element of ribosomes is created in the nucleolus, and the protein portion of the ribosome is synthesized in the cell's cytoplasm. These proteins enter the nucleus through the pores in the nuclear membrane and combine with the rRNA to form the small and large subunits. The completed subunits leave the nucleus and can be found in the cytoplasm or in an organelle known as the endoplasmic reticulum.

Chromatin (Flemming, 1879) and Chromosomes (Waldeyer, 1888) The nucleus contains the complex of deoxyribonucleic acid (DNA) and associated proteins, known as chromatin in the uncondensed state and as chromosomes in the condensed stateThe chromatin is embedded in a clear matrix called the nucleoplasm. Chromatin are coiled strands of DNA that are found spread throughout the nucleus, that come together and coil tightly during cell replication.Each DNA strand wraps around groups of small protein molecules called histones, forming a series of bead-like structures, called nucleosomes, connected by the DNA strand.Histones are found only in nucleus and being basic protein interact strongly with deoxyribonucleic acid(DNA) The histone proteins are rich in lysine and arginine.

Figure 2.4.3: Chromatin fibers showing nucleosomes

Heterochromatin: During interphase chromatin becomes dispersed, even then some of its portions remain tightly coiled and are called heterochromatin. Heterochromatin is found in parts of the chromosome where there are few or no genes and are generally inactive.

Euchromatin: Loosely coiled are called euchromatin Euchromatin is found in parts of the chromosome that contain many genes. The genes in euchromatin are active.This exposes the euchromatin and makes it available for the transcription process. Chromatin network:- These are very fine thread-like, coiled filaments uniformly distributed in the nucleoplasm.

Chromosomes At the time of cell division, the chromatin becomes thick and ribbon like and are known as chromosomes.

Figure 2.4.4: Structure of chromosome

The nucleus of a eukaryotic cell contains a number of chromosomes, which are composed of DNA and histone proteins.A typical chromosomes has two similar parts called chromatids. They are joined to each other by primary constriction or Centromere. Depending upon position of Centromere chromosomes are:

Figure 2.4.5: Types of chromosomes

The number, shape and arrangement of genes on chromosomes is characteristic of the species from which the nucleus came. Genes are responsible for storing and transmitting hereditary characteristics from one generation to another. A gene is the functional unit of a chromosome. Genes are arranged in single linear order along the chromosome. One gene may be responsible for a single characteristic, or a single characteristic may be transmitted by a set of genes.Each gene is a set of instructions for the construction of a specific protein.