Cell BiologyCell BiologyLec. 2 Dr: Buthaina H. Al-SabawiLec. 2 Dr: Buthaina H. Al-SabawiDate:13/12/2011Date:13/12/2011

LysosomesLysosomes(Intracellular digestion (Intracellular digestion

centers)centers)

Lysosome( intercellular didestion centers), membrane-Lysosome( intercellular didestion centers), membrane-bounded digestive vesicles, are also components of the bounded digestive vesicles, are also components of the endomembrane system that arise from the Golgi apparatus. endomembrane system that arise from the Golgi apparatus. They contain at least 60 hydrolytic enzymes, including They contain at least 60 hydrolytic enzymes, including proteases, lipases, phospholipases, glycosidases and proteases, lipases, phospholipases, glycosidases and

nucleasesnucleases . . They digest excess or worn out organelles, food particles, They digest excess or worn out organelles, food particles,

and engulfed viruses or bacteria. The membrane and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes surrounding a lysosome prevents the digestive enzymes inside from destroying the cell. Lysosomes fuse with inside from destroying the cell. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, vacuoles and dispense their enzymes into the vacuoles, digesting their contentsdigesting their contents..At pH 4.8, the interior of the lysosomes is more acidic than At pH 4.8, the interior of the lysosomes is more acidic than

the cytosol (pH 7.2the cytosol (pH 7.2)). .

The hydrolytic enzymes in lysosomes, are The hydrolytic enzymes in lysosomes, are produced in the ebndoplasmic reticulum, and produced in the ebndoplasmic reticulum, and transported and processed through the Golgi transported and processed through the Golgi apparatus. The Golgi apparatus produces apparatus. The Golgi apparatus produces lysosomes by budding.each acid hydrolase is lysosomes by budding.each acid hydrolase is then targed to a lysosome by phosphorylation.then targed to a lysosome by phosphorylation.

FunctionsFunctions:: The lysosomes are used for the digestion of The lysosomes are used for the digestion of

macromolecules from phagocytosis (ingestion of macromolecules from phagocytosis (ingestion of other dying cells or larger extracellular material), other dying cells or larger extracellular material), endocytosis, and autophagy (where old or endocytosis, and autophagy (where old or unneeded organelles or proteins, or microbes unneeded organelles or proteins, or microbes which have invaded the cytoplasm are delivered which have invaded the cytoplasm are delivered to the lysosome). to the lysosome). AutophagyAutophagy may also lead to may also lead to autophagic cell death, a form of programmed autophagic cell death, a form of programmed self-destruction, or hydrolysis, of the cell. Which self-destruction, or hydrolysis, of the cell. Which

means that the cell is digesting itselfmeans that the cell is digesting itself . .

Lysosomes also do much of the cellular digestion Lysosomes also do much of the cellular digestion required to digest tails of required to digest tails of tadpolestadpoles and to remove the and to remove the web from the fingers of 3-6 month old fetus. This web from the fingers of 3-6 month old fetus. This process of programmed cell death is called process of programmed cell death is called apoptosisapoptosisLysosomes that have not entered into digestive event Lysosomes that have not entered into digestive event are identified as primary lysosomes. lysosomes can are identified as primary lysosomes. lysosomes can digest materials taken into the cell from its digest materials taken into the cell from its environment, a process called environment, a process called heterophagyheterophagy.digestion .digestion follows, and the composite structure is now termed a follows, and the composite structure is now termed a secondary lysosomes called secondary lysosomes called phagosomesphagosomes, , oror phagolysosomesphagolysosomes..

After digestion of the contents of the secondary lysosome, After digestion of the contents of the secondary lysosome, nutrients diffuse through the lysosomal limiting membrane nutrients diffuse through the lysosomal limiting membrane and enter the cytosol. Indigestible compounds are retained and enter the cytosol. Indigestible compounds are retained within the vacuoles, which are now called within the vacuoles, which are now called residual residual bodiesbodies..

In some long-lived cells (e.g, neurons, heart, muscle, In some long-lived cells (e.g, neurons, heart, muscle, hepatocytes), large quantities of residual bodies hepatocytes), large quantities of residual bodies accumulate and are referred to as accumulate and are referred to as lipofuscinlipofuscin, or , or age age pigmentpigment..

Another function of lysosomes concerns the turnover of Another function of lysosomes concerns the turnover of cytoplasmic organelles. The resulting secondary lysosomes cytoplasmic organelles. The resulting secondary lysosomes

are known as are known as autophagosomesautophagosomes . .In some cases, primary lysosomes release their contents In some cases, primary lysosomes release their contents extracellularly (extracellularly (osteoclasts)osteoclasts)..

Lysosomal storage Lysosomal storage diseasedisease::

Lysosomal storage disease are caused by the accumulation Lysosomal storage disease are caused by the accumulation of macromolecules( proteins, polysaccharides, lipids) in of macromolecules( proteins, polysaccharides, lipids) in the lysosomes because of a genetic failure to manufacture the lysosomes because of a genetic failure to manufacture an enzymes needed for their breakdown. Neurons of the an enzymes needed for their breakdown. Neurons of the central nervous system are particularly susceptible to central nervous system are particularly susceptible to damage.damage.

Most of these diseases are caused by the inheritance of Most of these diseases are caused by the inheritance of two defective alleles of the gene encoding one of the two defective alleles of the gene encoding one of the hydrolytic enzymes. Example; Tay-Sachs disease & hydrolytic enzymes. Example; Tay-Sachs disease & Gauchers' disease-both caused by a failure to produce an Gauchers' disease-both caused by a failure to produce an enzyme needed to breakdown sphingolipids (fatty acid enzyme needed to breakdown sphingolipids (fatty acid derivatives found in all cell membrane).derivatives found in all cell membrane).

RibosomeRibosome Ribosome are small, dense-electron particles incells

that assembles proteins. Ribosomes are about 20*30 nm in size and are composed of 65% ribosomal RNA and 35% ribosomal proteins ( almost 80 different proteins) (known as a Ribonucleoprotein or RNP). It translates messenger RNA ( mRNA) to build a polypeptid chain (e.g., a protein) using amino acids delivered by Transfes RNA ( tRNA) Ribosomes are now classified as “ ribozymes. Ribosomes consist of two subunits a small subunit and a large subunit that bind together and work as one to translate the mRNA into a polypeptide chain during protein synthesis . Usually in bacterial cells, several ribosomes are working parallel on a single mRNA,

forming what we call a polyribosome or polysome .

PeroxisomesPeroxisomesPeroxisomes are ubiquitous organelles in eukaryotes that

function to rid the cell of toxic substances. They have a single lipid bilayer membrane that separates their

contents from the cytosol. Unlike lysosomes, which are formed in the secretory pathway, peroxisomes usually self-replicate by enlarging and then dividing, although there is some indication that new ones may be formed

directly

FunctionFunction::Peroxisomes contain oxidative enzymes, such as Peroxisomes contain oxidative enzymes, such as

catalase, D-amina acid and uric acid oxidase. Certain catalase, D-amina acid and uric acid oxidase. Certain enzymes within the peroxisome, by using molecular enzymes within the peroxisome, by using molecular oxygen, remove hydrogen atoms from specific oxygen, remove hydrogen atoms from specific organic substrates (labeled as R), in an oxidative organic substrates (labeled as R), in an oxidative reaction, producing hydrogen peroxide reaction, producing hydrogen peroxide ((HH22OO22, itself), itself)

RHRH22 + O + O22 R+ H R+ H22OO22

The name peroxisme refers to the hydrogen peroxide The name peroxisme refers to the hydrogen peroxide produced as a by-product of the activities of the produced as a by-product of the activities of the oxidative enzymes in the microbody. Hydrogen oxidative enzymes in the microbody. Hydrogen peroxide isperoxide is dangerous of cells because of its violent dangerous of cells because of its violent

chemical reactivitychemical reactivity . .

However peroxisomes also contain the enzyme However peroxisomes also contain the enzyme catalase, which breaks down hydrogen catalase, which breaks down hydrogen peroxide into harmless water and oxygenperoxide into harmless water and oxygen and and uses this Huses this H22OO22 to oxidize other substrates, to oxidize other substrates, including phenols, formic acids, formaldehyde including phenols, formic acids, formaldehyde and alcohol,and alcohol, by means of the by means of the peroxidationperoxidation reaction:reaction:

HH22OO22 + RH + RH22 R+ 2H R+ 2H22OOthus eliminating the poisonous hydrogen thus eliminating the poisonous hydrogen

peroxide in the process. peroxide in the process.

This reaction is important in liver and kidney This reaction is important in liver and kidney cells where the peroxisomes detoxify various cells where the peroxisomes detoxify various toxic substances that enter the blood. About toxic substances that enter the blood. About 25% of the ethanol we drink is oxidized to 25% of the ethanol we drink is oxidized to acetaldehyde in this way. In addition, when acetaldehyde in this way. In addition, when excess Hexcess H22OO22 accumulates in the cell, catalase accumulates in the cell, catalase converts it to Hconverts it to H22O through this reaction:O through this reaction:

2H2H22OO22 2H 2H22O+ OO+ O22

A major function of the peroxisome is the A major function of the peroxisome is the breakdown of fatty acid molecules, in a breakdown of fatty acid molecules, in a process called beta-oxidation. process called beta-oxidation.

VacuolesVacuolesVacuoles are membrane -bound compartments within some eukaryotic cells that can serve a variety of secretory, excretory, and storage functions. In general, vacuole functions include

1- Removing unwanted structural debris 2- Isolating materials that might be harmful or a

threat to the cell. 3-Containing waste products.

4-Maintaining internal hydrostatic pressure within the cell

5-Maintaining an acidic internal pH

6-Exporting unwanted substances from the cell . 7-Enabling the cell to change shape.

VesiclesIn cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. Vesicles store, transport , or digest cellular products and waste.Vesicles are a basic tool of the cell for organizing metabolism, transport, enzyme storage, as well as being chemical reaction chambers. Many vesicles are made in the Golgi apparatus, but also in the endoplasmic reticulum, or are made from parts of the plasma membrane.