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Юриспунденкция

CELL STRUCTURE

CELL:

1. cell membrane (plasma membrane);

2. protoplasm.

PROTOPLASM :

1. nucleus;

2. cytoplasm.

NUCLEUS:

1. nuclear membrane (nuclear envelope);

2. nucleoli;

3. karyoplasm (nucleoplasm).

CYTOPLASM:

1. cytosol (hyaloplasm)

2. organelles.

CELL ORGANELLES:

1. membranous organelles;

2. non-membranous organelles.

MEMBRANOUS ORGANELLES:

1. endoplasmic reticulum;

2. mitochondria;

3. Golgi complex;

4. lysosomes.

NON-MEMBRANOUS ORGANELLES;

1. ribosomes;

2. centrioles;

3. cytoskeleton.

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CELL (PLASMA) MEMBRANE (7,5 nanometers thick):

1. lipid;

2. protein;

3. carbohydrate.

It consists of two densely stained layers separated by a lighter zone, thus creating a trilaminar appearance. The trilaminar structure of the membrane is produced by the arrangement of lipid molecules (predominantly phospholipids) that constitute the basic framework of the membrane.

Each phospholipid molecule consists of an enlarged head in which the phosphate portion is located; and of two thin tails. The head end is also called the polar end while the tail end is the non-polar end. The head end is soluble in water and is said to be hydrophilic. The tail end is insoluble and is said to be hydrophobic.

In addition to molecules of lipids the cell membrane contains several proteins. The proteins are present in the form of irregularly rounded masses. Most of them are embedded within the thickness of the membrane and partly project on one of its surfaces (either outer or inner). However, some proteins occupy the entire thickness of the membrane and may project out of both its surfaces. These are called transmembrane proteins.

In addition to the phospholipids and proteins, carbohydrates are present at the surface of the membrane. They are attached either to the proteins (forming glycoproteins) or to the lipids (forming glycolipids). The carbohydrate layer is specially well developed on the external surface of the plasma membrane forming the cell boundary. This layer is referred to as the cell coat or glycocalyx.

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THE NUCLEUS

The nucleus constitutes the central, more dense part of the cell. It is usually rounded or ellipsoid. Occasionally it may be elongated, indented or lobed. It is usually 4 - 10 micrometers in diameter. In some cells the nuclei are relatively large and light staining. Such nuclei appear to be made up of a delicate network of fibres: the material making up the fibres of the network is called chromatin. At some places (in the nucleus) the chromatin is seen in the form of irregular dark masses that are called heterochromatin. At other places the network is loose and stains lightly: the chromatin of such areas is referred to as euchromatin. Nuclei which are large and in which relatively large areas of euchromatin can bee seen are referred to as open-face nuclei. Nuclei which are made up mainly of heterochromatin are referred to as closed-face nuclei.

In addition to the masses of heterochromatin (which are irregular in outline), the nucleus shows one or more rounded, dark staining bodies called nucleoli. The nucleus also contains various small granules, fibres and vesicles. The spaces between the various constituents of the nucleus are filled by a base called the nucleoplasm.

The nucleus is surrounded by a double layered nuclear membrane or nuclear envelope. The outer nuclear membrane is continuous with endoplasmic reticulum. The space between the inner and outer membranes is the perinuclear space. This is continuous with the lumen of rough endoplasmic reticulum. The inner layer of the nuclear membrane provides attachment to the ends of chromosomes.

At several points the inner and outer layers of the nuclear membrane fuse leaving gaps called nuclear pores. Nuclear pores represent sites at which substances can pass from the nucleus to the cytoplasm and vice versa.

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ENDOPLASMIC RETICULUM

The cytoplasm of most cells contains a system of membranes that constitute the endoplasmic reticulum. The membranes form the boundaries of channels that may be arranged in the form of flattened sacs (or cisternae) or of tubules.

In most places the membranes forming the endoplasmic reticulum are studded with minute particles of RNA called ribosomes. The presence of these ribosomes gives the membrane a rough appearance. Membranes of this type form the rough (or granular) endoplasmic reticulum. In contrast some membranes are devoid of ribosomes and constitute the smooth or agranular endoplasmic reticulum.

Rough endoplasmic reticulum represents the site at which proteins are synthesized. The attached ribosomes play an important role in this process. The lumen of rough endoplasmic reticulum is continuous with the perinuclear space (between the inner and outer nuclear membranes). It is also continuous with the lumen of smooth endoplasmic reticulum.

Smooth endoplasmic reticulum is responsible for further processing of proteins synthesized in rough endoplasmic reticulum. It is also responsible for synthesis of lipids, specially that of membrane phospholipids (necessary for membrane formation). Most cells have very little smooth endoplasmic reticulum. It is a prominent feature of cells producing lipids.

Products synthesized by the endoplasmic reticulum are stored in the channels within the reticulum. Ribosomes, and enzymes, are present on the “outer” surfaces of the membranes of the reticulum.

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MITOCHONDRIA

The number of mitochondria varies from cell to cell being greatest in cells with high metabolic activity (secretory cells). Mitochondria vary in size, most of them being 0,5 to 2 micrometers in length. Mitochondria are large in cells with a high oxidative metabolism.

The mitochondrion is bounded by a smooth outer membrane within which there is an inner membrane, the two being separated by an intermembranous space. The inner membrane is highly folded on itself forming incomplete partitions called cristae. The space bounded by the inner membrane is filled by a granular material called the matrix. This matrix contains numerous enzymes, it also contains RNA and DNA.

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GOLGI COMPLEX (GOLGI APPARATUS)

The Golgi complex is made up of membranes that form the walls of a number of flattened sacs. Towards their margins the sacs are continuous with small rounded vesicles. The cisternae of the Golgi complex form an independent system. Their lumen is not in communication with that of endoplasmic reticulum. Material from endoplasmic reticulum reaches the Golgi complex through vesicles.

Material synthesized in rough endoplasmic reticulum travels through the lumen into smooth endoplasmic reticulum. Vesicles budding off from smooth endoplasmic reticulum transport this material to the Golgi complex.

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LYSOSOMES

Lysosomes are small membrane-bound bodies that contain a variety of acid hydrolases. They constitute an intracellular digestive system capable of breaking down material originating both outside and within the cell.

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RIBOSOMES

Ribosomes are present in relation to rough endoplasmic reticulum. They may also lie free in the cytoplasm. They may be present singly in which case they are called monosomes; or in groups which are referred to as polysomes (polyribosomes). Each ribosome consists of proteins and RNA (ribonucleic acid) and is about fifteen nanometers in diameter. The ribosome is made up of two subunits one of which is larger than the other. Ribosomes play an essential role in protein synthesis.

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CENTRIOLES

All cells capable of division (and even some which do not divide) contain a pair of structures called centrioles. The centrioles are short cylinders that lie at right angles to each other. Each centriole consists of series of microtubules arranged in circle. There are nine groups of tubules, each group consisting of three tubules.

Centrioles play an important role in the formation of various cellular structures that are made up of microtubules. These include the mitotic spindles of dividing cells, cilia, flagella, and some projections of specialized cells (the axial filaments of spermatozoa).

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CYTOSKELETON

The cytoplasm is permeated by a number of fibrillar elements that collectively form a supporting network. This network is called the cytoskeleton. Apart from maintaining cellular architecture the cytoskeleton facilitates cell motility.

The elements that constitute the cytoskeleton consist of the following.

1. Microfilaments.

2. Microtubules.

3. Intermediate filaments.

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MITOSIS

Many cells of the body have a limited span of functional activity at the end of which they undergo division into two daughter cells. The daughter cells in turn have their own span of activity followed by another division. The period during which the cell is actively dividing is the phase of mitosis. The period between two successive divisions is called the interphase.

Mitosis is conventionally divided into a number of stages called prophase, metaphase, anaphase and telophase.

Prophase.In prophase, four main structural changes occur relatively simultaneously:

1. The chromatin threads become condensed (shortened and thickened) so that the chromosomes become visible as short, dark, rod-like structures. Each chromosome is split longitudinally in half, and the two halves are attached at some points along their length at a small region called thy centromere. Each half is a chromatid. In fact, as a result of DNA duplication which occurred prior to mitosis, each chromatid is a completely replicated chromosome, although it is not so called at this stage.

2. The pair of centrioles, usually adjacent to the nucleus of the interphase cell, start to duplicate, a daughter centriole forming adjacent to each, and the pairs of centrioles then start to move away from each other to take up positions at opposite poles or ends of the cell.

3. The nucleolus gradually disappears, its content being attached to some of the chromatids.

4. Finally, the nuclear envelope starts to disintegrate. It becomes less obvious and thinner as a result of movement of chromatin material away from its inner surface, and then it breaks down into small vesicles indistinguishable from elements of the granular endoplasmic reticulum.

Metaphase.At metaphase, all the chromosomes (pairs of chromatids) move to the centreof the cell in relation to the spindle and are arranged at the equatorial plate.

Anaphase.After complete splitting of the chromosomes at their centromeres, daughter chromosomes move to opposite poles of the cell, one diploid set (forty-six) to each end.

Telophase.At each pole of the cell, the chromosomes detach from chromosomal microtubules and the microtubules disintegrate. The nucleoli of each nucleus reappear, and the nuclear envelopes reform from cytoplasmic membranous vesicles. Cytoplasmic components are distributed equally between the two daughter cells.

Questions:

1. What are the main structures of the cell?

2. What are the main structures of the protoplasm?

3. What structural features is the nucleus characterized by?

4. Which of the cell organelles are referred to membranous organelles?

5. Which of the cell organelles are referred to non-membranous organelles?

6. What structural features is the plasma membrane characterized by?

7. What structural features is the nuclear membrane characterized by?

8. What structural features is the endoplasmic reticulum characterized by?

9. What structural features are the mitochondria characterized by?

10. What structural features is the Golgi complex characterized by?

11. What structural features are lysosomes characterized by?

12. What structural features are ribosomes characterized by?

13. What structural features are centrioles characterized by?

14. What structural features is cytoskeleton characterized by?

15. What are the main stages of mitosis?

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