Dr. Basim Al-Ka'abi
Cellular Physiology
Second Lecture
1
Lecture Name: Cellular Structure and Organelles
Lecturer Name: Dr. Basim A. Al-Ka'abi
Department: Medical Physiology
Stage: First Stage Medical Students
Lecture Objectives:
By the end of this lecture students the should be able to know the following:
Description of a physical structure of the cell.
Cell membrane and its constituents.
Various cellular organelles.
The concepts of exocytosis and endocytosis.
References:
Barrett, K et al. (2018). Ganong's Review of Medical Physiology. Twenty
sixth edition. USA.
Guyton, A and Hall, J (2015). Text Book of Medical Physiology.
Thirteenth edition. Philadelphia, USA.
Dr. Basim Al-Ka'abi
Cellular Physiology
Second Lecture
2
Physical structure of the cell
-The cell is not merely a bag of fluid, enzymes and chemicals, but it also
contains highly organized physical structures, many of which are called
organelles, including the followings:
Cell membrane
-It is a thin elastic structure, generally about 7.5 nm thick. It also generally
known as plasma membrane.
-It is not only semipermeable, allowing some substances to pass through it
and excluding others, but its permeability can be varied.
-It is composed almost entirely of proteins (about 55%), phospholipids
(25%); cholesterol (13%), other lipid (4%) and carbohydrates (3%).
-Its basic structures are lipids that are bilayerd, interspersed with large
globular protein molecules.
-The head end of lipid bilayer molecule contains the phosphate portion and
is relatively soluble in water (polar hydrophilic). While the tails are
relatively insoluble (nonpolar hydrophobic) .
-The lipid bilayer membrane is highly permeable to lipid-soluble
substances such as oxygen, carbon dioxide and alcohol, but acts as a major
barrier to water-soluble substances such as ions and glucose.
Two types of membrane proteins are present
(1) The integral proteins, which protrude all the way through the
membrane.
-Many of the integral proteins provide structural channels (pores), through
which water-soluble substances, especially ions, can diffuse.
-Other integral proteins acts as carrier proteins for transporting substances.
(2) The peripheral proteins, which are attached to the inner surface of the
membrane and do not penetrate.
Dr. Basim Al-Ka'abi
Cellular Physiology
Second Lecture
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-The peripheral proteins are normally attached to one of the integral
proteins and usually function as enzymes that catalyze chemical reactions
in the cell.
The mitochondria
-The mitochondria are often called the "powerhouses" of the cell.
-The mitochondria contain large quantities of oxidative and other enzymes
that are responsible for supplying energy to the cells.
-The total number of them per cell varies from less than hundred to several
thousand depending on the amount of energy required by each cell.
-After entering the cell, the foods are split into smaller molecules, which
then enter the mitochondria, where other enzymes remove CO
2
and
hydrogen ions in the process called the citric acid cycle.
-Then an oxidative enzyme system causes progressive oxidation of the
hydrogen atoms.
-The products of the reaction of the mitochondria are water and CO
2
and
the energy liberated is used by the mitochondria to synthesize another
substance, adenosine triphosphate (ATP), a highly reactive chemical that
can diffuse through the cell to release its energy whenever it is needed for
performing cellular functions.
-Mitochondria are self-replicative, means that one mitochondrion can form
a second one, a third one and so on, since the mitochondria contain DNA
similar to that found in the nucleus.
Endoplasmic reticulum (ER)
-Is a large network of tubules and vesicles, penetrates almost all parts of
the cytoplasm.
-It provides an extensive surface area for manufacturing multiple
substances that can be used inside the cells and released from some cells.
Dr. Basim Al-Ka'abi
Cellular Physiology
Second Lecture
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-These substances include proteins, carbohydrates, lipids and structures
such as lysosomes, peroxisomes and secretory granules.
-In rough or granular endoplasmic reticulum, granules called ribosomes are
attached to the cytoplasmic side of the membrane. The granular ER is
concerned with protein synthesis.
-In the smooth or agranular ER, the granules are absent. Smooth ER is the
site of steroid synthesis in steroid-secreting cells and the site of
detoxification processes in other cells.
Ribosomes
-The ribosomes contain about 65% ribonucleic acid (RNA) and 35%
protein. They are the site of protein synthesis.
-Ribosomes attached to ER synthesize proteins in the form of hormones
and cell membrane proteins.
-While, the free ribosomes synthesize cytoplasmic proteins such as
hemoglobin.
Lysosomes
-The lysosomes provide an intracellular digestive system, contain digestive
enzymes that allow lysosomes to digest intracellular substances and
structures, especially damaged cell structures, food particles and unwanted
materials, such as bacteria.
-Lysosomes are surrounded by a typical lipid bilayer membrane and filled
with large numbers of small granules, which are protein aggregates of
hydrolytic (digestive) enzymes.
Golgi apparatus
-The Golgi apparatus is a collection of membrane-enclosed sacs that are
stacked like dinner plates.
Dr. Basim Al-Ka'abi
Cellular Physiology
Second Lecture
5
-There are usually about six sacs in each apparatus, but there may be more.
-One or more Golgi apparatus is present in all eukaryotic cells, usually near
the nucleus. It functions in association with the ER.
-Substances entrapped in the ER vesicles are transported from the ER to
the Golgi apparatus.
-These substances are then processed in the Golgi apparatus to form
lysosomes, secretory vesicles or other cytoplasmic components.
Cytoskeleton
-All cells have a cytoskeleton, a system of fibers that not only maintain the
structure of the cell but also permit it to change its shape and move.
-A cytoskeleton is made up primary of microfilaments, intermediate
filaments and microtubules.
Microfilaments
-They are made up of actin, the protein that by its interaction with myosin
bring about contraction of muscles. It presents in all types of cells.
-Myosin is difficult to observe in cells other than muscles but is also
present.
-Large numbers of actin filaments frequently occur in the outer zone of the
cytoplasm to form an elastic support for the cell membrane.
Microtubules
-The primary function of microtubules is to act as a cytoskeleton, providing
rigid physical structures of cells.
-They also provide the tracks for transport of vesicles, such as secretory
granules and organelles such as mitochondria from one part of the cell to
another.
Dr. Basim Al-Ka'abi
Cellular Physiology
Second Lecture
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Intermediate filaments
-They are about 8-10 nm in diameter, some of these filament connect the
nuclear membrane to the cell membrane.
Intercellular connections
-Two types of junctions form between the cells that make up tissues:
-Junctions that attach the cells to one another and to surrounding tissues
(tight junctions), and junctions that permit transfer of ions and other
molecules from one cell to another (gap junctions), which permit the
passage of ions, sugars, amino acids and some of other solutes.
Exocytosis
-Proteins that are secreted by cells move from the ER to Golgi apparatus
and then extruded into secretary granules or vesicles.
-These secretory granules and vesicles move to the cell membrane.
-Their membrane then fuses to the cell membrane and the area of fusion
breaks down.
-This leaves the contents of granules or vesicles outside the cell and the
cell membrane intact.
-This extrusion process is called exocytosis.
Endocytosis
-Endocytosis is the reverse of exocytosis.
-One form of endocytosis, called phagocytosis (cell eating) is the process
by which bacteria and dead tissues are engulfed by tissue macrophages and
some of the white blood cells.
-The second form, called pinocytosis (cell drinking) is essentially the same
process, the only difference being that the substances ingested are in
soluble form.