Carbonizing and Singeing

Cellulosic impurities such as burrs and vegetable fibers that were not removed during carding must be removed from wool cloth by chemical treatment before it can be further processed. In the carbonizing process, the wool cloth is treated with dilute sulfuric acid, dried, and then heated until the impurities are converted to carbon. Next, the cloth is fed through corrugated rollers, which crush the charred particles and shake them free. Finally, the cloth is washed and neutralized to remove residual acid. Sometimes it is necessary to carbonize wool fibers or yarn before they are made into cloth. The strength of the wool is not appreciably affected by carbonizing if the process conditions are carefully controlled.

When cotton cloth comes from the loom, the fiber ends or fuzz must be removed, because a smooth surface is necessary, particularly if the cloth is to be printed. This is especially important for fabrics known for their smooth surface. Singeing is the name given to the finishing process used to remove these fiber ends or short hairs by burning without causing damage to the cloth. Since burning entails oxidation, this process may be thought of as a chemical process.

Before singeing, the cloth is thoroughly dried and brushed so the short fiber ends will be brought to the surface. There are two singeing techniques. In the plate technique, the cloth is passed rapidly over a curved copper plate heated to a bright red. In the gas technique, two gas burners are placed so that both the front and back of the cloth is singed in a single pass through the equipment. The hot cloth is then passed through water or steam to extinguish any sparks that may be present.

Singeing is traditionally the first finishing process for woven cotton cloth. But sometimes starches and waxes may be more firmly fixed during singeing, making desizing more difficult. So, even though it is more economical to singe first, in some cases it is advantageous to modify or reverse the sequence of these finishing operations.

The use of polyester staple in blends with cotton, with the resultant pilling problem during wear, has increased the importance of singeing for these cloths. It is also important to singe knitted cloth to obtain a smooth surface. Improved machines have been introduced which insure clean singeing across the entire width of the cloth with less tension in the lengthwise direction.

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CYTOSKELETON AND NUCLEUS

Cytoskeleton

The cytoskeleton is a mesh of proteinaceous, filamentous elements termed microtubules, microfilaments, and intermediate filaments. It provides a supporting network within the cytoplasm and is important in cell movement and in the rearrangement of cytoplasmic compo-nents. Microtubules also form discrete cytoplasmic structures such as centrioles.

Nucleus

At least one nucleus is present in all eukaryotic cells, although erythrocytes and blood platelets lack nuclei. Nuclei vary greatly in shape, but usually are spherical or ovoid. They generally have a nuclear envelope, chromatin, nucleoplasm, and one to several nucleoli.

The nuclear envelope is a double membrane with a narrow intermembranous space termed the perinuclear cisterna, or perinuclear space. The envelope is perforated by many nuclear pores, each of which is covered by a proteinaceous diaphragm. Nuclear chromatin is an intensely basophilic substance consisting of DNA and associated histone and non-histone proteins.

Active DNA does not stain with the nuclear stain haematoxylin. However, inactive DNA is readily stained with haematoxylin, toluidine blue and other similar basic dyes. The stainable DNA may appear in clumps or in a reticulated pattern. The functional DNA is termed euchromatin, whereas the non-functional, or inactive DNA, is termed heterochromatin.

During cell division, the stainable, inactive DNA appears in the form of threads or rods called chromosomes. The nucleolus is composed, most importantly, of RNA, and is the source of cytoplasmic RNA. The nucleoplasm is the matrix in which the other intranuclear components are embedded.

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Cytoplasm

Cytoplasm is enclosed by the plasrrfalemma, and surrounds the nucleus. It contains structures which can be divided into three groups, organelles cytoplasmic inclusions and cytoskeleton.

Organelles

The organelles include mitochondria the Crolgi apparatus, ribosomes, the endoplasmic reticulum lysosomes and centrioles. They are membrane-bound permanent subceliuiar compartments.

Mitochondria

Mitochondria are the largest of the cytoplasmic organelles, usually 2-6 /xrr, in length, and 0. 2 fim in diameter and having thread-like, ovoid or spherical shapes. They all have a double limiting membrane. The outer mitochondrial membrane has a smooth contour and forms a continuous but relatively porous covering. The inner mitochondrial membrane infolds into the centre of the organelle as a series of plate-like projections, the cristae mitochondriaies. Mitochondria synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP). They are therefore the energy-providers (“powerhouses”)of the cell.

Golgi apparatus

The Golgi apparatus (Golgi complex) is composed of a stack of membrane-bound cisternae, or saccules, with associated vesicles and vacuoles. The stack of membranes is curved, with convex (immature) and coMave (mature or secreting) faces. This organelle is typically near the nucleus and is often found near the centrioles. It plays a major role in the transport and release of secretory materials from the cell. The Golgi apparatus is best developed in neurons and glandular cells, which are specialized for secretion.

Ribosomes

Ribosomes are protein-synthesizing organelles. Each ribosome has two unequal ribosomal subunits9 named after their sedimentation rates during ultracentrifugation (but often called simply “large” and “small”). Cytoplasmic ribosomes are composed of ribosomal RNA (rRNA) and occur in two forms, free ribosomes and polyribosomes. Free ribosomes are individual ribosomes dispersed in the cytoplasm. Polyribosomes are groups of ribosomes evenly distributed along a single strand of messenger RNA (mRNA). Polyribosomes are found free in the cytoplasm and attached to membranes of the endoplasmic reticulum. They play a critical role in assembling amino acids into specific proteins.

Lysosomes

Lysosomes are small, membrane-limited vesicles of varying size and shape. They contain a variety of hydrolytic enzymes and function as the cellular digestive system. The enzyme most widely exploited for their identification is acid phosphatase because it occurs almost exclusively in lysosomes. Generally, lysosomes are of two types, primary lysosomes and secondary lysosomes. Primary lysosomes are the storage form of lysosomes. They contain newly -synthesized enzymes which are yet to be active. Secondary lysosomes are formed by fusion of a primary lysosome with some other body arising within or outside the cell. In the secondary lysosomes, enzymatic digestion is occurring or has occurred. The results of enzymatic digestion in a lysosome may leave in the cell a “residual body” which contains undigestible materials such as pigments, crystals, and certain lipids.

Endoplasmic reticulum

The endoplasmic reticulum is a freely anastomosing network of membranes which form vesicles, or cisternae. These may be elongated, flattened, rounded or tubular. In fully-differentiated cellsthe endoplasmic reticulum has two forms: rough and smooth. The rough endoplasmic reticulum (RER), also termed granular endoplasmic reticulum, is studded with ribosomes. The RER is particularly abundant in cells specialized for protein secretion. The smooth endoplasmic reticulum (SER) lacks ribosomes and thus appears as a smooth-surfaced membrane in electron micrographs. It is most well-developed in cells producing a class of hormones termed steroids.

Cytoplasmic inclusion

Cytoplasmic inclusions are structures, membrane-bound or not, that are transient than organelles and less actively involved in cell metabolism. They als such as stored foods (e. g. lipid droplets, glycogen granules), pigments materials.

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Cell Membrane

Cells are the structural and functional units of life. There are two basic cell types: prokaryotic cells and eukaryotic cells. Prokaryotic cells are small, single-celled organisms, e. g. bacteria, which lack a nuclear envelope, histones, and membranous organelles. Eukaryotic cells exist primarily as components of multicellular organisms. This article covers only the basic structural and functional features of eukaryotic cells.

A eukaryotic cell has three major components: a cell membrane, cytoplasm and a nucleus. All such cells have certain characteristics in common, but they also differ strikingly in their size, shape, organelle content and function.

Cell membrane

The cell membrane, or plasmalemma, is an outer limiting membrane which separates a cell from its environment. Chemically, the cell membrane consists of lipids9 protein and carbohydrates. The widely accepted fluid mosaic mode describes the plasmalemma as “protein icebergs in a sea of lipid arranged in a bilayer”. The cell membrane is about 10 nm in thickness. It can only be seen under a light microscope when associated material on its external surface increases the apparent thickness. In electron micrographs of osmium-stained tissue, the cell membrane has a trilaminar appearance. This is due to the deposition of a heavy metal stain on the polar (hydrophilic) heads of the lipid molecules which make up the bimolecular lipid leaflet. Each of the dense lines is separated by an unstained central region corresponding to the hydrophobic region of the lipid leaflet. Most intracellular membranes have the same basic organization as the cell membrane. The cell membrane is essential to maintaining the functional steady state, or homeostasis, required for cell survival.

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