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الكلية كلية العلوم للبنات
القسم قسم علوم الحياة
المرحلة 3
أستاذ المادة قاسم محمد علي العامري
4/25/2011 3:39:55 AM
Parts of the Cell
Cytoplasm
The gel-like material within the cell membrane is referred to as the cytoplasm. It is a fluid
matrix, the cytosol, which consists of 80% to 90% water, salts, organic molecules and
many enzymes that catalyze reactions, along with dissolved substances such as proteins
and nutrients. The cytoplasm plays an important role in a cell, serving as a "molecular
soup" in which organelles are suspended and held together by a fatty membrane.
Within the plasma membrane of a cell, the cytoplasm surrounds the nuclear envelope and
the cytoplasmic organelles. It plays a mechanical role by moving around inside the
membrane and pushing against the cell membrane helping to maintain the shape and
consistency of the cell and again, to provide suspension to the organelles. It is also a
storage space for chemical substances indispensable to life, which are involved in vital
metabolic reactions, such as anaerobic glycolysis and protein synthesis.
The cell membrane keeps the cytoplasm from leaking out. It contains many different
organelles which are considered the insoluble constituents of the cytoplasm, such as the
mitochondria, the chloroplast, lysosomes, peroxysomes, ribosomes, several vacuoles and
cytoskeletons, as well as complex cell membrane structures such as the endoplasmic
reticulum and the golgi apparatus that each have specific functions within the cell.
• Cytoskeleton
Threadlike proteins that make up the cytoskeleton continually reconstruct to adapt to the
cells constantly changing needs. The network is composed of three elements:
microtubules, actin filaments, and intermediate fibers.
A photograph of microtubules.
• Microtubules
Microtubules function as the framework along which organelles and vesicles move
within a cell. They are the thickest of the cytoskeleton structures. They are long hollow
cylinders, composed of protein subunits, called tubulin. Microtubules form mitotic
spindles, the machinery that partitions chromosomes between two cells in the process of
cell division. Without mitotic spindles cells could not reproduce.
Microtubules, intermediate filaments, and microfilaments are three protein fibers of
decreasing diameter, respectively. All are involved in establishing the shape or
movements of the cytoskeleton, the internal structure of the cell.
• Microfilaments
Microfilaments provide mechanical support for the cell, determine the cell shape, and in
some cases enable cell movements. They have an arrow-like appearance, with a fast
growing plus or barbed end and a slow growing minus or pointed end. They are made of
the protein actin and are involved in cell motility. They are found in almost every cell,
but are predominant in muscle cells and in the cells that move by changing shape, such as
phagocytes (white blood cells that scour the body for bacteria and other foreign invaders).
A photograph of microfilaments.
Organelles
Organelles are bodies embedded in the cytoplasm that serve to physically separate the
various metabolic activities that occur within cells. The organelles are each like separate
little factories, each organelle is responsible for producing a certain product that is used
elsewhere in the cell or body.
Cells of all living things are divided into two broad categories: prokaryotes and
eukaryotes. Bacteria (and archea) are prokaryotes, which means they lack a nucleus or
other membrane-bound organelles. Eukaryotes include all protozoans, fungi, plants, and
animals (including humans), and these cells are characterized by a nucleus (which houses
the chromosomes) as well as a variety of other organelles. Human cells vary considerably
(consider the differences between a bone cell, a blood cell, and a nerve cell), but most
cells have the features described below.
The components of a cell.
• Nucleus
Controls the cell; houses the genetic material (DNA). The nucleus is the largest of the
cells organelles. Cells can have more than one nucleus or lack a nucleus all together.
Skeletal muscle cells contain more than one nucleus whereas red blood cells do not
contain a nucleus at all. The nucleus is bounded by the nuclear envelope, a phospholipid
bilayer similar to the plasma membrane. The space between these two layers is the
nucleolemma Cisterna.
The nucleus contains the DNA, as mentioned above, the hereditary information in the
cell. Normally the DNA is spread out within the nucleus as a threadlike matrix called
chromatin. When the cell begins to divide, the chromatin condenses into rod-shaped
bodies called chromosomes, each of which, before dividing, is made up of two long DNA
molecules and various histone molecules. The histones serve to organize the lengthy
DNA, coiling it into bundles called nucleosomes. Also visible within the nucleus are one
or more nucleoli, each consisting of DNA in the process of manufacturing the
components of ribosomes. Ribosomes are shipped to the cytoplasm where they assemble
amino acids into proteins. The nucleus also serves as the site for the separation of the
chromosomes during cell division.
A cross-sectional diagram of a cell.
• Chromosomes
Inside each cell nucleus are chromosomes. Chromosomes are made up of chromatin,
which is made up of protein and deoxyribonucleic acid strands. Deoxyribonucleic acid is
DNA, the genetic material that is in the shape of a twisted ladder, also called the double
helix. Humans have 23 pairs of chromosomes. Down Syndrome and Cri du Chat
Syndrome result from having an abnormal number of chromosomes.
• Centrioles
Rodlike structures composed of 9 bundles which contain three microtubules each. Two
perpendicularly placed centrioles surrounded by proteins make up the centrosome.
Centrioles are very important in cellular division, where they arrange the mitotic spindles
that pull the chromosome apart.
Centrioles and basal bodies act as microtubule organizing centers. A pair of centrioles
(enclosed in a centrosome) located outside the nuclear envelope gives rise to the
microtubules that make up the spindle apparatus used during cell division. Basal bodies
are at the base of each flagellum and cilium and appear to organize their development.
• Ribosomes
Ribosomes play an active role in the complex process of protein synthesis, where they
serve as the structures that facilitate the joining of amino acids. Each ribosome is
composed of a large and small subunit which are made up of ribosomal proteins and
ribosomal RNAs. They can either be found in groups called polyribosomes within the
cytoplasm or found alone. Occasionally they are attached to the endoplasmic reticulum.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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