Introduction:

The Plant Cell Wall:

Plant cell wall comprises of a number of layers that are different in origin and chemical composition.

1) Middle Lamella:

Layer which forms the first partition between two cells as they are created during cell division is known as middle lamella or intercellular substance. It is shared by 2 adjacent cells together. Main chemical constituent of middle lamella is pectin.

2) The Primary Wall:

Next layer is primary wall. It lies between middle lamella and plasma membrane. As cell is still growing and enlarging primary wall is thin and elastic, but when cell stops growing wall may thicken. Chemically this wall is mainly composed of cellulose, different types of sugars and proteins. (Cellulose is a complex polysaccharide that is formed by consideration of several glucose molecules).

3) Secondary Wall:

Next layer of cell wall is secondary wall that forms between primary wall and plasma membrane. Secondary wall may be thin but it is generally thickened to varying degrees. When thickened it may comprises only of cellulose but generally other chemicals may become deposited on cellulose layer. Few of these other chemicals are lignin and suberin. When these chemicals are present wall is said to be lignified or suberised. Chemicals are impervious to water and air. Cells in which they take place are thus usually non-living. Thickening material is generally not evenly laid down but takes number of different patterns.

4) Pits:

Secondary wall is not generally laid over whole primary wall. It is not present altogether in some places that remain thin. Such areas which remain thin in walls of plants (whereas rest of wall is thickened) are known as pits. When pits of two adjacent cells coincide, pit pair is formed. Structure of pit may be simple, when its side walls are simple and parallel or more complex when different alterations are there.

Kinds of Plant Cells:

Some different kinds of cells found in plants are:

1) Meristematic Cells:

Meristematic cells are the least differential of all cells found in plant body. They are found at growing points of root and shoot apices where cell division results in production of new young cells. Other kinds of cell can be supposed to be derived from meristematic cells by different alterations arising as a result of growth of cell. Its main characteristic is its small size and cubical shape. It has large outstanding nucleus compared with size of cell. It contains dense cytoplasmic content.

2) Parenchymatous Cells:

These are large and thin walled. Wall is cellulose, therefore it is living cell. Cytoplasm generally has a large vacuole that may be central, limiting cytoplasm to living of wall and displacing nucleus to one side of the wall.  Parenchymatous cells form bulk of tissue of plant. They may have chloroplasts as in leaf.

3) Collenchyma Cells:

Collenchyman cells look like parenchyma cells but they are thickened at corners. Thickening is of cellulose. Consequently the thickened corners, collenchyma cells provide some mechanical strength to plant.

4) Sclerenchyma Cells:

Sclerenchyma cels have greatly thickened walls and thickening materials comprise chemicals other than cellulose. These chemicals comprise lignin and suberin that are impervious to water and air. At maturity such cells are non-living. They do not have nuclei and cytoplasm. This group of cells comprises a number of different kinds: tracheids vessels and stone cells.

(a) Tracheids: are short and unevenly shaped. They have greatly thickened walls with very narrow lumen.

(b) Fibres: are long and narrow having tapering ends. Its lumen is also narrow.

(c) Vessels: are greatly lengthened cells. They are composed of system of lengthened cells joined end to end to create continuous pipes. They have larger lumina (sing lumen) by which water is conducted. It is in walls of vessels that different methods of thickening are conspicuous.

Kinds of Animal Cells:

Some common kinds of animal cells are:

1) Muscle Cells:

The muscle is composed of different kinds of cells whose common feature is their high contractility. The different muscle cells or fibres: smooth or unstriated muscle, striped or striated muscle and cardiac muscle.

(a) Smooth (Unstriated) Muscle Cells:

Each unstriated muscle cell is lengthening, and tapers slowly at each end. It comprises of centrally placed nucleus that is enclosed by mass of cytoplasm, sacroplasm. Rest of the cell comprises of very delicate myofibillae (myofibrils) that run length of cell. These are not enclosed by well defined membrane. Smooth muscle is mostly visceral in distribution, forming contractile portion of wall of digestive tract from midpoint of oesophagus to anus, comprising ducts of glands related with digestive system.

(b) Striated Muscle Fibres:

The individual striated muscle fibre or cell is long, cylindrical and multinucleate, ends tapering to point or being fairly rounded or notched at junction of muscle and tendon. They vary from cells of unstriated muscle in that they are surrounded in definite membrane, sarcolemma.

Below sarcolemma re embedded several nuclei each in layer of sarcoplasm. Rest of the cell is made up of microfibrillae that run through length of cell. Alternating bands of light and dark stripes cross cell giving it striated look. Striated or skeletal muscle is that which layman identifies as muscle and comprises flesh or meat of animals.

(c) Cardiac Muscle Fibres:

Cardiac muscle, that is involuntary, contract rythymically and automatically. It is found only in myocardium (muscle layer of heart) and in wall of large vessels joining the heart. They contract all through the life of animal; when they stop death takes place. Cardiac muscle fibres have the structure that is intermediate between that of unstriated and striated fibres. The cardiac muscle fibre by light microsopy is muscle cell attached end to end at specialized junctional zones known as intercalated discs.

Fibres in any region run mostly in parallel fashion but cross beams are several and this provides false impressions of synetical network. Between fibres is fine connective tissue, endomysium having small blood vessels and lymphatics. Cardiac muscle fibre is enveloped by the thin sarcolemman like that of skeletal muscle, and sarcoplasm is plentiful with many mitochondria. Myofibrils are separate by mitochondria arranged in rows between them, with consequent obvious longitudinal striation.

2) Blood Cells:

Blood has two main kinds of cells or corpuscles, red blood corpuscles (RBC) or erythrocytes and white blood corpuscles (WBC) or leucocytes.

(a) Erythrocytes:

Erythrocytes are biconcave circular discs. They are surrounded by the thin elastic envelope and therefore are simply distorted by pressure. When pressure is removed, original biconcave disc shape is restored. Shape permits blood to flow through small capillaries. Cytoplasm of erythrocyte has respiratory pigment hemoglobin that provides blood is red color. There is no nucleus in mature RBC but they are present in RBCs of other vertebrates; this is characteristic feature of human RBC. As it lacks a nucleus RBC is short-lived, with halfline of 60 days and maximum life span of 120 days. Erythrocytes function in transportation of oxygen (O2) from lungs, releasing it in tissues and carrying carbon dioxide from tissues to lungs. Hemoglobin is pigment which carries gases.

(b) Leucocytes (white blood cells, WBC) leucocytes are of two kinds (i) granular (granulocytes) and (ii) non-granular of agranular. Granulocytes contain granules inside their cytoplasm and these react to definite stains. Cells whose granules stain with acid stains are known as acidophils, while those that react with basic stains are basophils. Granulocytes are basophilis, neutrophilis and eosinophils whereas agranulocytes are lyniphocytes and monocytes. Nucleus of different leucocytes are variable in shape but commonest condition is that of unevenly lobed for which is termed as polymorphous.

iii) Nerve Cells:

Nerve cell comprises of cell body that includes system of branching fibres. Cell body comprises of cytoplasm and nucleus. Cytoplasm has granules (Nissl granules) that stain readily with methylene blue. In cytoplasm is also system of neurofibrillae. Out of the fibres which arise from cell body, one is particularly long. Shorter ones are known as dendrites and through them nerve cell (neurone) receives impulses. Especially long fibre is axon or axis cylinder and by it impulses are sent out. In most cases axis cylinder is enclosed by fatty myelin sheath outside which is thin membrane known as neurolemma. Just below neurolemma is extremely thin layer of cytoplasm in which nuclei is surrounded at intervals.

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