Plastids are found in all plant cells and in euglenoides. Plastids bear some specific pigments and hence impart characteristic colours. Based on the type of pigments, plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
Chloroplast: They contain chlorophyll and carotenoid pigments. These pigments are responsible for trapping light energy which is required for photosynthesis.
Chromoplast: They contain fat soluble carotenoid pigments; like carotene, xanthophylls and others. Different plant parts get bright colours; like yellow, orange or red; because of the pigments in chromoplast.
Leucoplast: These are colourless plastids. They store various nutrients. The leucoplast which stores carbohydrates is called amyloplast. The leucoplast which stores oil and fats is called elaioplast. The leucoplast which stores proteins is called aleuroplast.
Structure of Chloroplast
Chloroplasts are lens-shaped, oval, spherical, discoid or even ribbon-like. Their number can vary from 1 per cell (as in Chlamydomonas) to 20 – 40 (as in the mesophyll of leaves).
Chloroplast is bound by two membranes. The inner membrane of chloroplast is less permeable. The space within the inner membrane of chloroplast is called the stroma. A number of organized flattened membranous sacs are present in the stroma. These flattened sacs are called thylakoids. The thylakoids are arranged in stacks called grana. There are flat membranous tubules connecting the thylakoids of the different grana. The membrane of the thylakoids encloses a space called a lumen.
The stroma contains enzymes required for the synthesis of carbohydrates and proteins. The stroma of chloroplast also contains double stranded DNA molecules and ribosomes. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).
Ribosomes were first observed under electron microscope by George Palade (1953). Ribosomes are composed of RNA and proteins. The eukoryotic ribosomes are 80S, while the prokaryotic ribosomes are 70S. In this case, ‘S’ stands for sedimentation coefficient or Svedberg’s Unit. It is an indirect measure of density and size of ribosome. Both 70S and 80S ribosomes are composed of two subunits.
The cytoskeleton is composed of an elaborate network of filamentous proteinaceous structures in cytoplasm. Cytoskeleton is involved in many functions; like mechanical support, motility, maintenance of the shape, etc.
Cilia and Flagella
Cilia and flagella are hair-like outgrowths of the cell membrane. Cilia are smaller than flagella. Cilia work like oars and facilitate movement of either the cell or the surrounding fluid. Flagella are responsible for cell movement. Flagella in prokaryotes are structurally different from those in eukaryotes.
A flagellum or a cilium is covered with plasma membrane. Their core is called the axoneme. The axoneme has a number of microtubules running parallel to the long axis. There are usually nine pairs of radially arranged peripheral microtubules and a pair of centrally located microtubules. Such an arrangement of microtubules is called the 9 + 2 array. The central tubules are connected by bridges and are also enclosed by a central sheath. The central sheath is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers. Cilium and flagellum emerge from basal bodies; which are centriole-like structures.
Centrosome and Centrioles
Centrosome usually contains two cylindrical structures which are called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other. They are made up of nine evenly spaced peripheral fibrins of tubulin protein. Each peripheral fibril is a triplet. Each is linked to the adjacent triplets. The central part of the proximal region of the centriole is also made up of protein. The central part is called the hub. The hub is connected with tubules of the peripheral triplets by radial spokes. The radial spokes are made up of protein. Centrioles form the basal bodies of cilia and flagella and spindle fibres. The centrioles give rise to spindle apparatus during cell division in animal cells.
Nucleus was first described by Robert Brown in 1831. Nucleus is enclosed by a double-membrane nuclear envelope. The space between the two membranes is called the perinuclear space. The perinuclear space forms a barrier between the nucleic materials and cytoplasmic materials. The outer membrane is usually continuous with the endoplasmic reticulum. Ribosomes are present on the outer membrane of nuclear envelope.
The nuclear membrane is interrupted by minute pores at various places. These pores provide passage to RNA and protein molecules. Usually, there is only one nucleus in a cell, but some variations can also be observed. Some mature cells even lack nucleus.
The fluid inside the nucleus is called nucleoplasm or nuclear matrix. The nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures. Nucleolus is not a membrane bound structure. Synthesis of ribosomal RNA takes place in the nucleolus.
Nucleus also contains chromatin fibres; which are distinct during some stages of cell division. The chromatin contains DNA and some basic proteins; called histones; some non-histones and also RNA.
Chromosome: A chromosome has a primary constriction called centromere. Disc-shaped structures; called kinetochores; are present on the sides of the centromere. On the basis of position of the centromere, chromosomes can of four types, viz. metacentric, sub-metacentric, acrocentric and telocentric.
When the centromere divides the chromosomes into two identical arms, it is called metacentric chromosome. When the centromere is slightly away from the middle, it is called sub-metacentric chromosome. When the centromere divides the chromosome into a smaller and another much larger arm, it is called acrocentric chromosome. When the centromere is at the tail, it is called telocentric chromosome.
Many membrane bound minute vesicles are present in both plant and animal cells. They contain various enzymes and are called microbodies.