eukaryotic cell structure - Golgi apparatus , vesicles , cytoskeleton and others structures

The Golgi apparatus is named for Camillo Golgi an Italian physician , who discovered in 1898 .Golgi bodies are abundant in cells that secret chemical substances ex. pancreatic cells secreting digestive enzyme and in nerve cells that secreting neurotransmitters . The structure of Golgi apparatus consist of elongated flattened sacs or cisternae ( cisterna :collecting vessels closed spaces serving as fluid reservoirs) running parallel to each other a group of large vacuoles lying near the margin of the flattened sacs and clusters of small vesicles , it has a convex outer face or cis face which associated with the endoplasmic reticulum and a concave inner face or trans face away from the reticulum . Materials were processed through the apparatus moved from the cis region to the trans region then to plasma membrane .

Functions of Golgi complex:
1-Formation of secretory vesicles , protein initially synthesized in ER. passes to Golgi complex forming face transition (concave) vesicles and there found enzymes called glycosyl transferasas helps in adding sugar groups to form glycoproteins and latter leave the Golgi complex as membrane – bounded secetory granules (secretary vesicles ).
2- Golgi complex is involved in linking sugar molecules to form polysaccharides
3- Golgi forms certain intra – cellular structures such as certain yolk substances and cortical granules of oocyst and the complex also shares in the formation of lysosomes .
















Fig. Golgi apparatus structure




Vesicle
Vesicles are small membrane bounded sacs in a cell, spherical in shape, they are perform a variety of transport functions store of food , water, metabolic and excrete toxic waste through two processes :
a- Exocytosis is the process by which eukaryotes excrete most molecules from their cells , vesicles containing the molecules to be excreted moving to the cell surface, and the vesicle membrane fuses with the plasma membrane, releasing the vesicle contents into the environment. In exocytosis , membrane from the interior of the cell becomes part of the plasma membrane.
b- Endocytosis : localized regions of plasma membrane can surround materials macromolecules, particles, liquid, etc. in the environment and bring it into the interior of the cell as a membrane-bounded vesicle , this process is called endocytosis Ex. pinocytosis and phagocytosis .







Lysosome
A lysosome (lyso : dissolving , soma : body ) is an organelles bounded by a single membrane and containing hydrolytic enzymes. The internal pH of the lysosome is acidic, around 4-5 , lysosomes collaborate with vesicles formed by endocytosis or phagocytosis to digest material imported from the environment.

Peroxisomes or microbodies
Peroxisomes are small particles about 0.5 Mm in diameter derived from endoplasmic reticulum , found in cytoplasm of all animal and plant cells . The Peroxisomes like lysosome small in size and spherical in shape and bounded by single membrane of lipid and protein molecules , so the distintiction of lysosome is difficult . The mature one contain small crystal and the matrix contain more than 50 enzyme involved in diverse activities such as peroxidase , catalase ,D. amino acid oxidsae , ? – hydroxyl acid oxidase which play role in protect the cell from the toxic effect of hydrogen peroxide (H2O2) by breaking down to oxygen and water in addition to oxidatio of very long chains of fatty acids .











Cytoskeleton
The cytoplasm of all eukaryotic cells is connected by a network of protein fibers that support the shape of the cell and anchors organelles to fixed locations, this network is called the cytoskeleton (fig. ) . In eukaryotic cells there are three major classes of protein fibers making up the cytoskeleton they are :
a- Microtubules
b-Intermediate filaments
c- Microfilament

Cytoplasm
The cytoplasm is a jelly – like material surrounding the nucleus of a cell which contains most of the cells structures , it is support and protect cell organelles . The liquid (non-particulate) part of the cytoplasm is called the cytosol , the organelles and cytoskeleton lie in cytosol.











Cilia and flagella
Cilia ( cilium : eyelashes ) and flagella ( flagellum : small whips) are elongated appendages on the surface of some cells . In most prokaryote and eukaryote , the cilia and flagella are used for locomotion ( fig. 1) however, many animals have ciliated cells for which the cilia serve simply to move materials far from the cell for ex. cells lining the inner surface of our trachea (fig. 2) .







Fig.2 ciliated trachea cell Fig.1 flagella and cilia



The nucleus is the information center of an eukaryotic cells , a double- membranes bound the nucleus and linear chromosomes it is called a nuclear envelope and is perforated by nuclear pores that allow the passage of certain molecules between nucleus and cytoplasm. The nuclear envelope breaks down at onset of prometaphase in mitosis or meiosis , and it forms again during the telophase . In Eukaryotic cell a nuclear lamina lines the inside of the inner membrane of the nuclear envelope , intermediate filaments woven together constitute a nuclear lamina. The nucleoplasm is the fluid material within the nucleus, suspending the chromosomes and nucleoli. A nucleolus is a small, generally spherical body found within the nucleus of eukaryotic cells. Transcription of ribosomal RNA (rRNA) takes place in the nucleoli, as does the assembly of ribosomal subunits. The nucleoli contain ribosomal RNA (rRNA) and many copies of the genes that encode rRNA.















Fig. the nucleus structure

Chromosome
Chromosome is a thread – like structure composed of chromatin that carry the genes which determine all the individual characters . A single Chromosome contains a large number of genes, each with its own locus (place) on the chromosome. A chromosome consists of a large molecule of DNA and a number of protein molecules called histone . Chromosome change dramatically in structure during the cell cycle.
Chromatin : is the DNA-protein complex found in eukaryotic chromosomes .It is replicated during the S phase of the cell cycle.

The histone : a group of water soluble proteins found in association with the DNA chromosomes and deficient in bacteria , they believed to be involved in the condensation and coiling of chromosome during cell division .

The centriol
It is usually lies adjacent to the outer side of the nuclear membrane towards the larger area of the cytoplasm . The cytoplasm immediately surrounding it is called the centrosome. The centriol appears as a small hollow cylinder with nine sets of microtubules on the wall , it plays an important role in cell division during mitotic prophase , the centriole divided into two and followed by the division of the centrosome and each migrates towards the opposite pole with the radiating astral rays .