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Cell organelles in terms of
structure and function
nuclear
envelope
chromatin
nucleolus
nuclear
pores
rough ER
smooth ER
The Nucleus
Structure
-The nucleus, a prominent
structure in cells.
The nucleus contains chromatin.
Chromatin has a special region called
the nucleolus, where rRNA is
produced and ribosome subunits are
assembled.
The nuclear envelope contains pores
that allow substances to enter and
exit the nucleus to and from the
cytoplasm. The nuclear envelope
is attached to the endoplasmic reticulum,
which often has attached ribosomes,
where protein synthesis
occurs
Function
stores genetic information
Ribosomes
Ribosomes are organelles composed of proteins and rRNA.
Protein synthesis occurs at the ribosomes. Ribosomes are often
attached to the endoplasmic reticulum; but they also may occur
free within the cytoplasm, either singly or in groups
called polyribosomes.
Proteins synthesized at ribosomes attached to the
endoplasmic reticulum have a different destination
from that of proteins manufactured at ribosomes free in
the cytoplasm.
The endomembrane system.
The organelles in the endomembrane system work together to produce, modify,
secrete, and digest proteins and lipids.
The Endomembrane System
The endomembrane system consists of the nuclear envelope,
the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vesicles
(tiny membranous sacs) . This system divide the cell so that chemical
reactions are regulate to specific regions. The vesicles transport molecules
from one part of the system to another.
secretion
incoming vesicle
plasma
membrane
secretory vesicle
enzyme
Golgi apparatus
modifies lipids and proteins from the ER;
sorts and packages them in vesicles
lysosome
contains digestive enzymes that break
down cell parts or substances entering
by vesicles
transport vesicle
takes proteins to
Golgi apparatus
transport vesicle
takes lipids to Golgi apparatus
protein
Lipid
Smooth endoplasmic reticulum
synthesizes lipids and has various
other functions
rough endoplasmic reticulum
synthesizes proteins and
packages them in vesicles
ribosome
Nucleus
The Endoplasmic Reticulum
The endoplasmic reticulum has two portions.
1- Rough ER is
studded with ribosomes on the side of the membrane that
faces the cytoplasm. Here, proteins are synthesized and enter
the ER interior, where processing and modifi cation begin .
Some of these proteins are incorporated into membrane, and
some are for export.
2- Smooth ER, continuous with rough ER,
does not have attached ribosomes. Smooth ER synthesizes
the phospholipids that occur in membranes and has various
other functions, depending on the particular cell. In the testes,
it produces testosterone. In the liver, it helps detoxify drugs.
The Endoplasmic Reticulum
The ER forms transport vesicles in which large
molecules are transported to other parts of the
cell. Often, these vesicles are on their way to
the plasma membrane or the Golgi apparatus.
The Golgi Apparatus
The Golgi apparatus is named for Camillo Golgi,
who discovered its presence in cells in 1898.
The Golgi apparatus consists of a stack of
slightly curved saccules, whose appearance can
be compared to a stack of pancakes. Here,
proteins and lipids received from the ER are
modified. For example, a chain of sugars
The Golgi Apparatus
may be added to them. This makes them glycoproteins
and glycolipids, molecules often found in the plasma
membrane.
The vesicles that leave the Golgi apparatus move to other
parts of the cell. Some vesicles proceed to the plasma membrane,
where they discharge their contents. In all, the Golgi
apparatus is involved in processing, packaging, and
secretion
Lysosomes
Lysosomes, membranous sacs produced by the Golgi
apparatus, contain hydrolytic enzymes. Lysosomes
are found in all cells of the body but are particularly
numerous in white blood cells that swallow up
disease-causing microbes. When a lysosome fuses
with such an endocytic vesicle, its contents are
digested by lysosomal enzymes into simpler subunits
that then enter the cytoplasm. In a process called
autodigestion, parts of a cell may be broken down by
the lysosomes .
Lysosomes
Some human diseases are caused by the lack of a
particular lysosome enzyme. Tay–Sachs disease
occurs when an undigested substance collects in
nerve cells,leading to developmental problems
and death in early childhood.
Mitochondria
Outer membrane
Inner membrane
Intermembrane space
cristae
matrix
The structure of a mitochondrion.
Mitochondria
Mitochondria (sing., mitochondrion) are often called the powerhouses of the cell
the mitochondria convert the chemical energy of glucose products into the chemical
energy of ATP (adenosine triphosphate )molecules .
In the process, mitochondria use up oxygen and give off carbon dioxide. Therefore, the
process of producing ATP is called cellular respiration
The structure of mitochondria is appropriate to the task. The inner membrane is folded
to form little shelves called cristae. These project into the matrix, an inner space filled
with a gel-like fluid , The matrix of a mitochondrion contains enzymes
The number of mitochondria in cells is highly variable; liver cells contain as many as
2000 mitochondria, whereas inactive cells contain very few.
Mitochondria
for breaking down glucose products. ATP production
then occurs at the cristae. Protein complexes that
aid in the conversion of energy are located in an
assembly-line fashion on these membranous
shelves. The structure of a mitochondrion
supports the hypothesis
Mitochondria
that they were originally prokaryotes engulfed by a
cell.
Mitochondria are bounded by a double membrane,
as a prokaryote would be if taken into a cell by
endocytosis. Even more interesting is the
observation that mitochondria have their own
genes—and they reproduce themselves!
Each mitochondrion consists of four
sections
1- The outer membrane is relatively permeable
as it contains a pore-forming protein, known as
porin,which allows free passage of small
molecules. The outer membrane contains
enzymes that convert certain lipid substrates
into forms that can be metabolised within the
mitochondrion.
Each mitochondrion consists of four
sections
2- The inner membrane, which is thinner than the
outer, is thrown into complex folds and tubules
called cristae that project into the inner cavity. In
some cell types mitochondria typically have tubular
Cristae.
Each mitochondrion consists of four
sections
3- The inner cavity filled by the mitochondrial
matrix.The matrix contains a number of dense matrix
granules, thought to be binding sites for calcium,
Which is stored in mitochondria
4- The intermembranous space between the two
membranes also contains a variety of enzymes.