Mitochondria

Lec. No. 3

Mitochondria
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 in all aerobic cells of higher animals, plants, algae, protozoa and fungi. They were first observed by Kolliker in 1850 as granular structures in the striated muscles and 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 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 the stacks of thylakoids 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.



discharge
entry solid matter

digestion


lysosome


Golgi apparatus



Fig. Lysosomes - in action as suicide sac
Vesicle
Vesicles are small membrane bounded sacs in a cell, spherical in shape, they are perform a variety of functions such as transport, storing of food and 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, and liquid. In the environment and bring it into the for example, pinocytosis and phagocytosis processes.









Fig. Peroxisome



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 O2 2H2O + O2





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

















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 example cells lining the inner surface of our trachea (fig. 2).







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




The nucleus (Greek word karyon meaning a nut or kernel) is an information center of an eukaryotic cells, it is a large spherical body has double- membranes bounded called nuclear envelope, it is perforated by nuclear pores that allow the passage of certain molecules between nucleus and cytoplasm. The nuclear envelope breaks down at onset of prophase in mitosis or meiosis divisions , and it forms again during the telophase .An intermediate filaments woven together constitute a nuclear lamina lines the inner membrane of nuclear envelope. A nucleolus is a small, generally spherical body found within the nucleus of eukaryotic cells, contain ribosomal RNA (rRNA) and many copies of the genes that encode rRNA. The nucleoplasm is a fluid material within the nucleus, suspending the chromosomes and nucleoli.

Functions of nucleus:
1- The nucleus contains DNA which represents the store of genes that carry the genetic information which determine most of the characteristics of an organism.
2- The nuclear pores allow the passage of macromolecules such as mRNA and rRNA that synthesized in the nucleus to pass into cytoplasm.
3- Transcription of ribosomal RNA takes place in the nucleoli as does the assembly of ribosomal subunits.
4- Proteins of nucleoplasm may control nuclear functions such as DNA replication, transcription and RNA transport.













Fig. the nucleus structure