COMPONENTS OF THE CELL part(2)

(d) chloroplasts

Chloroplasts are intricate organelles found in some plant cells. Light energy can be captured by the green chlorophyll pigment of chloroplasts and used to power the process of photosynthesis where by complex food molecules are built from inorganic

raw materials.A chloroplast is a large structure (approximately

5000-10000nm in diameter) usually shaped lke a

Convex lens and surroundedby two unit

membranes. Internally it is made up of discrete

areas, the grana, Contained in a ground substance,

the stroma. The grana are composed of disc-like

membranous sacs called lamellae stacked up In

piles. amellae extend through the stroma to

provide continuity between neighbouring grana.

Layers of proteln form the lamellar membrane

Surtaces and sandwich between them layers of

chlorophyil, enzymes and phospholipids. The chloloroplasts of algae show variations from this structure which is characteristic of higher plats. Chloroplasts, like mitochondrla, possess their own supply of DNA and ribosomes. They are derived from pre-existing chloroplasts by division, developing from undifferentiated proplastids by growth and elaboration of the lamellar pigments and membranes. Chlorophyll does not develop if chloroplasts are grown in the dark but it rapidly appears on exposure to light.

(e) Ribosomes

Ribosomes are very small, dense particies

(diameter 20-25 nm) found in the cytoplasmn

where they may lie free or be closely associated

with the ER They are universaly present in cells.Ribosomes are the site of protein synthesis.

They are rich in RNA which is manufactured in thee

nucleolf. Free ribosomes are thougnt to produce the proteins required for internal cellular use whereas ribosomes bound to the ER synthesize

extracellular proteins such as digestive enzymes and hormones.

LYSOSOMES

Lysosomes are fluid-filled sacs enclosed by a single membranous envelope which are found in most types of animal cells and probabiy also in plant Cells. They are very variable in SIze and shape possibly arising by fusion of smalH vesicles budded off from the edges of the Golgi apparatus or the

ER. The high concentration of hydrolytic enzymes which they contain plays an important part in the normal intracellular digestive breakdown occurring during cell growth and repair.They are also involved in the digestion of pinocytosed and phagocytosed material entering the cell and in the cellular defenses against attack by bacteria,Viruses or toxic substances.

(g) centrioles

Most animal cells and some plant cells contain a pair of centrioles which lies in the cytoplasm near to the nuclear membrane. A centriole is made of nine sets of triplet tubules arranged in a ring to make a hollow cylinder about 400 nm long and 200nm in diameter. The members of this centriole pair lie close together, oftenorientated at right angles to each other.

Centrioles are usually self-replicating and they

have an important function during cell division

when they separate and move to form the poles of

the nuclear spindle. The basal granules (kinetosomes) of cilia and flagella are structurally

very similar to centrioles.

(h) Vacuoles

Vacuoles are fluid-filled droplets bounded by a unit

membrane, the tonoplast. They are particularly

conspicuous in plant cells.

There are many types of vacuoles:

Food vacuoles Containing food materials.

Excretory vacuoles containing waste materials.

Contractile vacuoles involved in the maintenance of internal pressure

in unicellular organisms.

Plant cell vacuoles involved in the maintenance of the turgidity of plant tissues.

A vacuole may fall into more than one of these categories. The central vacuole of a mature plant cell may fill a large portion of the cell volume and restrict the cytoplasm to a thin layer inside the

plasma membrane. The composition of the vacuole fluid is controlled by the selective permeability of

the tonoplast membrane.A Wide range of

ingredients such as ro0d matertals, salts, waste substances and pigments Occur in the fluid and may be involved in maintaining cell turgldity, often an important aspect of the support system of plant tissues.

() GRANULES

Cells may contain a variety of granules:

storage granules, e.g. Starch, 9lycogen etc.

pigment granules, secretory granules of many types.

(1) Microtubules and micro filaments

Microtubules, hollow protein tubes 1525 nm in diameter, are found in the cytoplasm in association with centrioles, cilia, flagella, sperm tails, nerve cell processes and other structures. Microfilaments, with a smaller diameter of 4–6 nm, also Occur

4 Nuclear components

If a cell is to continue its essential functlons over a long period of time it must possess a nucleus.

Some cells, such as striated muscle cells and fungal cells, are multinucleate but most contain a Single nucleus.

(K) The nuclear membrane (nuclear envelope) the nuclear membrane separates the nucleus from the surrounding cytoplasm. It is a flattened sac consisting of two layers of unit membrane which are pierced by small pores. The traffic of materials

between the nucleus and the cytoplasm is

controlled by this membrane.

(1) The nucleokus

A nucleolus is a dense spherical body of variable Size which is not enclosed in a unit membrane.

Several nucleoli may ocCur in one nucleus, each consisting largely of RNA and protein. The

synthesis of ribosomal RNA takes place in the nucleoli, which are bigger in cells producing large amounts of protein. Special constricted regions on some of the chromosomes (nucleolar organisers) are associated with the nucleoli and are believed to be the site of nucleoli formation.

(m) Chromosomal material

Chiomosomes are long, slender threads of DNA

and histone protein which are usually dispersed throughout the nucleus and difficult to identify.

They become very conspicuous, however, during cell division when the threads shorten and thicken by coiling until they become visible with the light microscope.

Generally the somatic cells of multicellular organisms posses a Constant number

chromosomes, the diploid number, which characteristic of the species. The chromosome complement is made up of two matching sets chromosomes. The partners are called homologous pairs and they are similar in size and shape. But here are important exceptions to this

generalization. The gametes contain only half the

usual chromosome number, the haploid number; algae and protozoans usually possess only a single, haploid set of chromosomes and some cells, particularly those of higher plants, contain various multiples of the haploid number such as triploid or tetraploid.

Functionally chromosomes are the carriers of the genes, those sections of the DNA strands which control RNA production and, ultimately, the protein synthesis in the cell. By this means the genes on the chromosomes are the fundamental directors of cellular structure and function.

Updated: July 25, 2020 — 12:03 am

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