Medical Biology
Lec -2-
Shorooq Wessam
Endoplasmic reticulum
The endoplasmic reticulum or EN is an organelle found in all eukaryotic
cells that is an interconnected network of tubules , vesivcles and cisternae that
is responsible for several specialized functions : protein translation , folding and
transport of proteins to be used in the cell membrane or to be secreted
( exocytosed) from the cell (e.g. digestive enzymes) ; production and storage of
glycogen , steroids and other macromolecules . the endoplasmic reticulum is
part of the endomembrane system . the basic structure and composition of the
EN membrane is similar to the plasma membrane .
Structure : the general structure of the endoplasmic reticulum is an extensive
membrane network of cisternae (sac – like structures) held together by the
cytoskeleton . the phospholipid membrane encloses a space , there are three
varieties of it are called :
Rough endoplasmic reticulum
The surface of the rough endoplasmic reticulum is studded with protein
manufacturing ribosomes giving it a "rough" appearance but it should be noted
that these ribosomes are not resident of the endoplasmic reticulum initially . the
ribosomes only bind to the EN once it begins to synthesize a protein destined
for sorting . the membrane of the rough endoplasmic reticulum is continuous
wirh the outer layer of the nucler envelope . although there is no continuous
membrane between the rough ER and Golgi apparatus , membrane bound
vesicales shuttle proteins between these two compartments .
Smooth endoplasmic reticulum
The smooth endoplasmic reticulum has functions in several metabolic
processes including synthesis of lipids , metabolism of carbohydrates and
calcium concentration , drug detoxification and attachment of receptors on cell
membrane proteins . It is connected to the nuclear envelope . the smooth ER
consists of tubules and vesicles that branch forming a network . the network of
smooth for the action or storage of key enzymes and the products of these
enzymes .
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Mitochondria
Is a membrane – enclosed organelle that is found in most eukaryotic cells ,
mitochondria are sometimes described as " cellular power units " because they
generate most of the cells supply of ATP used as a source of chemical energy .
the number of mitochondria in a cell varies widely by organism and tissue type
Although most of a cells DNA is contained in the cell nucleus, the
mitochondrion has own independent genome.This DNA shows similarity to
bacterial genomesandaccording to the endosymbiotic theory,mitochondria are
descended from free-living prokaryotes.
Mitochondria structure
A mitochondrion contains inner and outer membranes composed of
phospholipid bilayers and protins.The two membranes, however, have
different properties.Because of this double-membraned organization, there are 5
distinct compartments withn the mitochondrion .There is the outer
membrane,the intermembrane space (the space between the outer and inner
membranes),the inner membrane,the cristae space(formed by in folding s of the
inner membrane),and the matrix (space within the inner membrane).
Outer membrane The outer mitochondrial membrane,which encloses the
entire organelle,has a protein-to-phospholipid ratio similar to that of the
eukaryotic plasma.
Inner membrane The inner mitochondrial membrane contains proteins with
four types of functions:
. That carry out the oxidation reactions of the respiratory chain.
. ATP synthase,which makes ATP in the matrix.
. Specific transport proteins that regulate the passage of matrix metabolites.
. Protein import machinery.
The inner mitochondrial membrane is divided into numerous cristae,which
expand the surface area of the inner mitochondrial membrane,enhancing its
ability to generate ATP,for example in typical liver mitochondria.
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Golgi apparatus
The Golgi apparatus (also called the Golgi body , Golgi complex ) the golgi is
composed of membrane – bound sacs known as cisternae . between five and
eight are usually present ; however as many as sixty have been observed
vesicles from the endoplasmic reticulum (via the vesicular – tubular cluster)
fuse with the cis – golgi network and subsequently progress through the stack to
the trans-golgi network , where they are packaged and sent to the required
destination , each region contains different enzymes .
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