Prokarvotes and Eukarvotes

Prokaryotes and Eukaryotes

All cellular organisms fall into two major groups, the

Prokaryotes and Eukaryotes

Prokaryotes (pro.:before;karyon:nucleus) comprise bacteria and blue-green algae cells of prokaryotes(prokaryotic cells) lack true nuclei. Their genetic material (DNA) is not enclosed by nuclear membranes and lies free in the Cytoplasm

Eukaryotes (eu:true)

- include green plants, fungi, slime moulds and animals

The table below shows the major difference between prokaryotes and eukaryotes

　

Structure of cells

All living organisms are made of CELLS. A typical cell is about 10 um(l um = micro-meter = 10-6m) in diameter. it is.made ,of living material called Protoplasm.

Protoplasm consists of a central nucleus and the surrounding colloidal mass called cytoplasm, which is bound by a thin layer of cell Membrane. The cytoplasm contains many sub-cellular organelles.

Important: Electron micrographs should be studied.

I. Cell Membrane (plasma lemma; plasmamembrane)

Important: The trilaminar structure of membranes and a brief treatment of the fluid mosaic model.

(A) Occurrence

Membrane occurs not only at the surface of cell but also around nucleus and the sub-cellular organelles such as endoplasmic reticulum(ER), mitochondria, chloroplast and lysosome.

(B) Structure

Under light microscope(LM) the membrane appears as a single line.

1.Trilaminar structure (Davson-Danielli model 1935) Under the high magnification of the electron microscope(EM) the membrane appears as a three layered (Trilaminar) structure of about 7.5 nm(l nm = 10-9 m) thick: a bimolecular layer of lipid (phospho-lipid) of about 3.5 nm thick which is sandwiched between two layers of protein, each of the protein layer is about 2.0 nm thick Such structure forms the unit membrane.

2. Fluid mosaic model

Recent research find that the protein does not form a continuous layer covering both sides of the membrane, as envisaged by Danielli and Davson. In 1972, J. J. Singer and G. L. Nicholson suggested a modified structure for the cell membrane. the bimolecular phospholipid layer with its inwardly directed hydrophobic tails remains unchanged. However, it is suggested that the protein molecules vary in size and have a much less regular arrangement. Some proteins occur on the surface of the phospholipid layer, while others extend into it. Some even extend completely across. Viewed from the surface the proteins are dotted throughout the phospholipid layer in a mosaic(鑲嵌) arrangement.Other research suggests that the lipid molecules have considerable sideway movement i.e. is fluid. It was these facts which give rise to its name, the fluid-mosaic model.

The proteins in the membrane giving structural supports and they are very specific. it is this specificity which allows cell to' be recognized by other agents in the body e.g. enzymes, hormones and antibodies. The proteins also assist the active transport across the membrane.

(C) Functions

1. To restrict the exit and entry of substances into and out of the cell

2. To provide a structure on which protein molecules can be assembled. eg. in the

mitochondria the respiratory enzymes are arranged on the inner membrane in a

specific order.

(D) Special features

The plasma membrane is often thrown into numerous folds to increase area in order to facilitate exchange with environment.

eg. micro-villi of intestinal epithelial cell membranous folds and vesicles associated with pinocytosis(cell drinking)

　

II. Cytoplasm

The cytoplasm is a viscous substance composed of many molecules in forms of soluble proteins, carbohydrates, oil droplets, glycogen granules and vacuoles for secretion. It is not static but capable of mass flow, which is called cytoplasmic streaming.

Suspended in the cytoplasm are numerous membranous structures called the endoplasmic reticulum and many organelles specialized for performing particular functions inside the cell.

Apart from acting as a store of vital chemicals , the cytoplasm is the site of certain metabolic pathways, an important example being glycolysis. Synthesis of fatty acids, nucleotide and some amino acids also takes place in it.

　

　

III. Endoplasmic reticulum(ER)

(A) Structure

It is a system of flattened, membrane-bounded sacs called cisternae, forming tubes and sheets. It is continuous with the outer membrane of the nuclear envelope. The membranes of ER may be lined with ribosomes and these ER are called rough endoplasmic reticulum. The rough ER is concerned with protein synthesis and is consequently most abundant in those cells which are rapidly growing or secrete enzymes. The other type of ER lack ribosomes on their membranes and they are called smooth endoplasmic reticulum. The smooth ER is concern with lipid is

consequently most abundant in those cells producing lipid-related secretions, eg. the sebaceous glands of mammalian skin and cells secreting steroids.

(B) Functions

  1. The ER is connected with the cell membrane and the nuclear membrane so

    2. that it provides a kind of intra-cellular transport system facilitating movements

    of material from one part of cell to another.2. It transport proteins which have been synthesized be ribosome. Some of these

    proteins are secreted out as enzymes or hormones.

    3. It produce and transport lipids and steroids.

    4. It provides large area for enzymes attachment in specific order so as to

    facilitate the occurrence of biochemical reactions in a regular pattern inside

    the cell.5. It provides a supplementary mechanical support for the colloidal nature of

    cytoplasm.IV. Golgi body( Golgi apparatus, dictyosome in plant cell)(A) Structure

    It consists of a stack of flattened, membrane-bound sacs called cisternae. At the edges the cisternae pinch off numerous vesicles called G'Oicgi vesicles.These vesicles contain secretory granules and move to the other parts of the cell, particularly the cell membrane and the cell wall.

    (B) FormationIt is formed by the fusion of vesicles which are pinched off from the cavities of ER.

    (C) Functions 1. They act as sites to concentrate and pack the materials in other parts of the

    cell. eg. Protein newly synthesized in the rough ER are transported in the

    vesicles to the Golgi body where carbohydrates are added to form

    glycoproteins. These glycoproteins are paced in vesicles which are pinched

    off the Golgi body and moves to the cell membrane where the glycoprotein

    is discharged ie. secretion occurs. They are particularly numerous in newly

    secreting cells. eg. liver cells

    　

  2. They may play a part in primary wall formation in plant cell because they are found to aggregate in large number in the region of cell plate (the region between two newly formed daughter nuclei where the new cell wall is laid down after nuclear division) formation during cell division.

    3. V) Ribosomes

    (A) Structure

    1. It is a RNA-containing particle of about 2Onm in length.

    2. It is composed of two sub~units of unequal size. with the sedimentation

    coefficient of 30S and 50S. (S is a measure of the rate of sedimentation of

    molecules in a centrifugal field. The sedimentation rate is proportional to the

    weight and shape of a particle.)

    A ribosome particle is composed of RNA and proteins.

    3. They may be free in cytoplasm or attached to ER. They are found in the

    mitochondria and chloroplast.

  3. A number of ribosomes may be linked together by a piece of messenger-RNA

to form polysome.

(B) Function

They provide the sites for protein synthesis.

Functional relationship between nucleus, ER. ribosomes and Golgi Apparatus

Functionally these organelles work as a team in protein synthesis. Nucleus containing DNA molecules which reside on chromosomes and contain the genetic messages which determine the structure of a protein. mRNA is transcripted from DNA, which carries the genetic code for one polypeptide chain. mRNA molecules then migrate through nuclear pores into the cytoplasm where they associate with ribosomes to start the process of protein synthesis. Efficient working of ribosome takes place on the surface of rough ER. The polypeptide chains formed on ribosomes are passed into cisternae of endoplasmic reticulum. Besides containing protein are pinched off from the rough ER and carry the protein to the Golgi apparatus carbohydrate is added to protein before it is secreted out of the cell.

　

VI) Lysosomes

(A) Structure1. It is a dark spherical body which is bounded by a single membrane.2. It contains many enzymes that can break down the macro-molecular structures

of the cell.

(B) Functions

1.They digest material which the cell consumes from the environment. In the

case of white blood cells, this may be bacteria or other harmful material.In

Protozoan, it is the food which has been consumed by phagocytosis(cell

eating). The lysosomes may move towards the vacuole or vesicle which engulf

the nutrient particles. They fuse with the vacuole or vesicle and discharge

their enzymes to achieve INTRA-CELLULAR DIGESTION.2.They digest parts of the cell, such as worn-out organelles, in a similar way to

that describe in 1.After the death of the cell they are responsible for its

complete breakdown, a process known as autolysis.

3.They release their enzymes outside cell in order to break down other cells, eg.

in the reabsorption of tadpole tails during metamorphosis.

VII) Mitochondria

  1. Structure

1.It is an oval or rod-shape structure with a diameter of 1.0um It is a hollow

structure bounded by two membranes. 2.The outer membrane is smooth and regular. It controls the entry and exit of

chemicals.

3.The inner membrane is folded inwards to form many tubular processes called

cristae, into the central fluid matrix.These cristae greatly increase the area

for attachment of more respiratory enzymes (electron transport system and

cytochrome system) in a small space.The surface of these cristae has stalked

granules along its length.4.The central cavity is filled with a matrix which is semi-rigid material

containing protein, lipids and trace of DNA. It also contains enzymes for

Kreb's cycle and beta-oxidation of fatty acids

  1. Functions

The enzymes (electron transport system )in the mitochondrial cristae and the enzymes of the Kreb's cycle in the matrix are responsible for the oxidativeThus mitochondria are present in great number in cells that require a lot of energy.

VIII) micro-tubules

Microtubules are elongated, proteinous non-membranous cylinders and are widely distributed in the cytoplasm. They are very fine tubes, with walls made up of helically arranged globular subunits of protein called tubulin. There are three different types of micro-tubules that differ from one another in size, structure and functions:

  1. Cytoplasmic micro-tubules they are most abundant near the cell membrane

and are usually parallel with cellulose fibres in cell wall. They play a role in

cytoplasmic streaming.

2. Spindle micro-tubules

They are abundant in the nuclear region of the dividing cells and play several

roles in cell division.

3. Flagella and cilia

Flagella movement results from the sliding of one micro-tubule along another.