Cell structure

د. أديب عبد العالي الازبجي Histology Lecture :- 2

Cell structure

Cell

The basic structural and functional units of all multicellular organisms.
Cells can be divided into two major compartments: the cytoplasm and the nucleus.
Cells structure
Cells have a common basic structure
An outer membrane surrounds each cell and separates it from its environment and from other cells.
They are composed of a solution of proteins, electrolytes and carbohydrates (cytosol), divided up into specialized functional compartments (organelles) by inner membrane systems .
Plasma Membrane
The plasma membrane (cell membrane or plasma lemma) is the lipid bilayer with embedded proteins that surrounds a cell and is seen only with the TEM.
The lipid bilayer forms from amphipathic phospholipids, stabilized by cholesterol, and contains many embedded (integral) proteins and many peripheral proteins on its cytoplasmic surface.
Endocytosis is cellular uptake of macromolecules or fluid by plasma membrane engulfment or invagination, followed by the “pinching off” of a filled membranous vesicle in the cytoplasm.
Major types of endocytosis include phagocytosis (uptake of particulate material), pinocytosis (uptake of dissolved substances), and receptor-mediated endocytosis (uptake of specific molecules bound to integral membrane receptor proteins).
Exocytosis is a type of cellular secretion in which cytoplasmic membrane vesicles fuse with the plasma membrane and release their contents to the extracellular space.
Ribosomes
The two ribosomal subunits, each a complex of rRNA and many proteins, attach to mRNA and translate that message into protein.
Endoplasmic Reticulum
The ER is a convoluted network of membrane enclosing continuous spaces called cisternae and extending from the nucleus to the plasma membrane.
Rough ER has a granular, basophilic cytoplasmic surface due to the presence of polysomes making most membrane proteins, proteins in certain other organelles, or for exocytosis; RER is always well developed in cells actively secreting proteins.
Smooth ER (SER) lacks ribosomes, but includes enzymes for lipid and glycogen metabolism, for detoxification reactions, and for temporary Ca2+ sequestration.
Golgi Apparatus
The Golgi apparatus is a dynamic organelle consisting of stacked membranous cisternae in which proteins made in RER are processed further and packaged for secretion or other roles.
Modified proteins leave the Golgi apparatus after packaging in vesicles with coat proteins that direct movement to lysosomes, the plasma membrane, or secretion by exocytosis.
Lysosomes
■ Primary lysosomes emerge from the Golgi apparatus containing
inactive acid hydrolases specific for degrading a wide variety of cellular
macromolecules.
■Secondary lysosomes are more heterogeneous, having fused with
vesicles produced by endocytosis that contain material to be digested
by the hydrolytic enzymes.final condensed vesicles containing any indigestible molecules are called residual bodies.
Mitochondria
■ Mitochondria are the major sites of ATP synthesis and are abundant
in cells or cytoplasmic regions where large amounts of energy
are expended.
■Mitochondria are usually elongated organelles and form by fission
of preexisting mitochondria.
■ The inner membrane includes enzyme assemblies of the electron transport system and ATP synthase.
■Mitochondria of stressed cells may release cytochrome c from the
inner membrane, triggering a regulated series of events culminating
in cell death (apoptosis).
Cytoskeleton
■The cytoskeleton contains three types of polymers:
(1) microtubules (2) microfilaments (3) intermediate filaments.
■ Microtubules are important in maintaining cell shape and as tracks
for transport of vesicles and organelles by the motor proteins kinesin
and dynein.
■ Microfilaments are short, flexible, highly dynamic filaments of actin
subunits, in which changes in length and interactions with binding
proteins regulate cytoplasmic viscosity and movement.
■Myosins are motor proteins that bind and move along actin filaments,
carrying vesicles or producing cytoplasmic movement.
■ Movements of cytoplasm produced by actin filaments and myosins are important for endocytosis, cell cleavage after mitosis, and cell locomotion on substrates.
■Intermediate filaments are the most stable cytoskeletal component,
conferring strong mechanical stability to cells.
■ Intermediate filaments are composed of various protein subunits in different cells; they include vimentin; nuclear lamins; neurofilament proteins; and keratins, which are especially important in epithelial
cells.
Inclusions
■ Unlike organelles, inclusions are not metabolically active and are
primarily storage sites, such as lipid droplets, glycogen granules,
pigment granules, or residual bodies (also called lipofuscin).
Nuclear Envelope
■ Cytoplasm is separated from nucleoplasm by the nuclear envelope,
a double set of membranes with a narrow perinuclear space; the
outer membrane binds ribosomes and is continuous with the RER.
■The nuclear envelope is penetrated by nuclear pore complexes, through which proteins and protein-RNA complexes move in both directions.
■The nuclear envelope is supported internally by a meshwork, the
nuclear lamina, composed of intermediate filament subunits called
lamins.
Chromatin
■Chromatin is the combination of DNA and its associated proteins.
■Chromatin with DNA that is active in transcription stains lightly and
is called euchromatin; inactive chromatin stains more darkly and is
called heterochromatin.
■The DNA molecule initially wraps around complexes of basic proteins
called histones to form nucleosomes, producing a structure
resembling beads on a string.
Nucleolus
■The nucleolus is a very basophilic or electron-dense area of chromatin
localized where rRNA transcription and ribosomal subunits
assembly occur.
The Cell Cycle
■The cell cycle is the sequence of events that controls cell growth and
division.
■The G1 phase, the longest part of the cycle, begins immediately after
mitosis and includes all preparations for DNA replication.
■The period of DNA (and histone) synthesis is the S phase.
■ In a short G2 phase the cell prepares for division during mitosis (M).
■ Cell cycling is controlled by the sequential appearance of key cytoplasmic
proteins, the cyclins, which bind cyclin-dependent kinases (CDKs).
■CDKs phosphorylate and activate the enzymes and transcription
factors whose functions characterize each phase of the cell cycle.
■Progress through the cell cycle stages is monitored at checkpoints,
including the G1 restriction point; only when each phase’s activities
are completed are the cyclins changed to trigger those of the next
phase.
Mitosis
■Stages of mitotic cell divisions include prophase, when chromosomes
condense, the nuclear envelope disassembles, and the microtubular
spindle forms; metaphase, when chromosomes are aligned;
anaphase, when they begin to separate toward the two centrosomes;
and telophase, when nuclear envelope re-forms around the separated
chromosomes.
■Telophase ends with cytokinesis or cell cleavage into two daughter
cells by a contractile ring of actin filaments and myosin.
Meiosis
■ Meiosis is the process by which two successive cell divisions produce
cells called gametes containing half the number of chromosomes
found in somatic cells.
■ Prophase of the first meiotic division is a unique, extended period in
which homologous chromosomes pair and undergo genetic recombination
during the process called synapsis.
■Synaptic pairs separate toward two daughter cells at the first meiotic
division.
■ The second meiotic division occurs with no intervening S phase and
separates the sister chromatids into two final cells that are haploid.