Eukaryotic cell structure

Dentistry college - first class
Medical biology- Lec. 2

Eukaryotic cell structure
An eukaryote cells contain true nucleus and numerous membrane – bounded organelles on contrary to prokaryotic cells.
The cell is defined as a mass of protoplasm surrounded by a thin membrane and having one or more nuclei at least during a stage of its development.
The protoplasm ( cytoplasm) : it is a living substance of the cells, colloidal in nature composed of a mixture of organic and inorganic materials, the relative amount of these constitutes vary from cell to cell and from tissue to tissue against the age , kind and degree of differentiation of cells ( show table).

Substance Percentage Form
water 85 – 90% free and bound
protein 7 – 10% albumin , globulin
fatty substance 1 -2% lipid
organics material 1 – 1.5 % carbohydrates
inorganic element 1 – 1.5 % Na , K, Ca ,Mg ,Cl , SO4

It is an outer layer of cell made up of cellulose giving the cell rigidity, strong, protection and allows the molecules such as H2O, O2, CO2 to diffuse in and out of cells. Cell wall found in plant, fungi but not in animal cells.








Fig. cell wall in plant

Plasma membrane
It is an interface between the cell and it is environment , it is so thin(80 – 100) A? that can be visualized by electron microscope , it appears to be made of bilayers of polar lipids ( phospholipids , cholesterol , glycolipid) coated on both sides by proteins. The molecular organization of plasma membrane against fluid mosaic model of Singer and Nicholson 1972. In the membrane a certain invaginations could be recognized, they serve to increase cellular surface area to facilitate transport of materials in and out of the cell called desmosomes and act as an intercellular bridges . Plasma membrane regulates the entry and exit of molecules and ions in addition to contains receptors for various external signals.













Fig. Plasma membrane structure

Transport across the plasma membrane
There are several methods of transport mechanisms across the plasma membrane as fallow:
1- Passive transport
The spread of molecular and ions through the plasma membrane from region of higher concentration to a region of lower concentration this method is called diffusion and when the transport of certain molecular, ions and amino acids occurs through the specific carriers proteins or channels ( pores ) in plasma membrane this type of method is called facilitated transport and both of methods are not require of energy .
2- Active transport
In this type, movement of ions and molecules against the concentration gradient from the lower to higher concentration, for this it requires the energy for example when the plasma membrane transport Na from the inside of the cell (low Na) to outside (high Na) and at the same time the transport of K from the cell outside (low K) to interior of cell (high K) as shown in figure.















Fig. methods of transport across the plasma membrane




Organelles are sub cellular structures with specialized functions, among the kinds of organelles are Golgi apparatus, endoplasmic reticulum, ribosome, vesicles mitochondria, chloroplast, nucleus etc.


The Golgi apparatus is named for Camillo Golgi an Italian physician, who discovered in 1898. Golgi bodies are abundant in cells that a secret chemical substance ex. pancreatic cells secreting digestive enzyme and in nerve cells that is 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.

Secretory vesicles









Layered cisternae




Rough endoplasmic reticulum



Endoplasmic reticulum (ER)
ER. (Endoplasmic means within the cytoplasm , reticulum : little net ) it is a network of tubular , flattened sacs surrounded by very thin lipoprotein membrane composed of lipid bilayer embedded with proteins , it appears to be continuous with cell membrane on one hand and nuclear membrane on the other .
There are two types of ER are known:
1- Rough or granular type (RER) : It is characterized by presence of numerous minute particles ribosome are capable of protein synthesis ex. exocrine cells of pancreas and plasma cells.
2-Smooth or a granular type (SER): It lacks the ribosome , so it appears in cells which have no active in synthesis of protein but in lipid production such as the cells of adrenal cortex , adipose cells .











Fig. Endoplasmic reticulum structure


Ribosome is a small dense, rounded and granular particle with a diameter of 150 to 250 A?. The ribosome was described by Palade in 1952 and appear either freely in the matrix of the mitochondria, chloroplast and cytoplasm or remain attached with membrane of endoplasmic reticulum and nucleus. In the cells in which active protein synthesis, the ribosomes remain attached with the membranes of the endoplasmic reticulum such as in pancreatic cells, hepatic cells, cells of the glandular stomach, thyroid cells. Ribosome is chemically composed of RNA and proteins. The ribosomal RNA (rRNA) plays a central role in the process of protein synthesis. It is consists of two ribosomal subunits that combine in presence of Mg+2 ions to form a functional ribosome of 70S in bacteria and 80S in eukaryotes.

S: Svedberg unite (sedimentation coefficient) is a measure of size determined by the rate at which a particle in solution sediments in the centrifuge.












Sub-units








Fig. Ribosome


Mitochondria (Mito: filament, chondron: granule) are filamentous or granular cytoplasm organelles, they vary in size from 0.5 ?m to 2.0 ?m. They found of all aerobic cells of higher animals and plants, algae, protozoa and fungi. They were first observed by Kolliker in 1850 as granular structures in the striated muscles. Various steps of glycolysis in mitochondria was discovered by two German biochemists Embden and Meyerhof. The number of mitochondria in a cell depends on the type and functional state of the cell. Certain cells contain large number of mitochondria e.g., eggs of sea urchin contain 140,000-150,000 mitochondria, oocytes of amphibians contain 300,000 mitochondria, liver cells of rat contain only 500-1600 mitochondria while some algal cells may contain only one mitochondrion. Mitochondria are bounded by a double membrane, the inner ones folds producing cristae, the crista are the sites of oxidative phosphorylation in cell. The matrix of mitochondria is fluid filled interior of the mitochondrion and most of enzymes of citric acid cycle are contained in the ground. Mitochondria play a key role in the oxidation of carbohydrates and fats, they are considered as the actual respiratory organs of the cells. Mitochondria produce most of ATP in eukaryotic cells.











Fig. mitochondria structure

Chloroplast
It is a green oval organelle containing chlorophyll , it has double membrane with inner membrane modified into sacs called thylakoids this stacks is called grana .The inner most substance is gel like called stroma . Chlorophyll traps the energy of sunlight which is then used by the plant to make sugar for energy and release of oxygen to keep the balance in the environment. Chloroplast is found only in plants and algae (Fig).

Lysosome
A lysosome (lyso: dissolving, soma: body) are small vesicles their size ranges from 0.2 to 5 ?m surrounded by a membrane. Lysosomes originate either from Golgi apparatus or directly from endoplasmic reticulum, recent studies reveal that lysosomes may contain up to 40 types of hydrolytic enzymes such as proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases and sulphatases. Lysosomes are involved in an intracellular digestion and are primarily meant for destroying unwanted and aged organelles inside the cells. For example, when an animal cell ingests food into a food vacuole, lysosomes fuse with the vacuole and break down the contents. Their enzymes digest carbohydrates, fat and proteins.



discharg
entry of solid matter
digestion




lysosome


Golgi apparatus


Fig. Lysosomes - in action



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. in the environment and bring it into the for example, pinocytosis and phagocytosis processes .












Peroxisomes
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 has 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 oxidation of very long chains of fatty acids .

Catalase enzyme
2H2 O 2H2O + O2



















Fig. Peroxisomes



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