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The Cell
By Dr Rehab Omer
Faculty of Medicine, U of D
Definition of Cell
A cell is the smallest unit that is
capable of performing life
functions.
Two Types of Cells
•Prokaryotic
•Eukaryotic
Prokaryotes vs Eukaryotes
Eukaryotes:
A.Have organelles:
Organelles are subcellular
distinct compartments
which have distinct roles.
e.g. the nucleus contains
the DNA (stored genetic
information.
Organelles are covered
with membranes.
B. Larger in size.
C. DNA is organized into
multiple linear units called
chromosomes.
D. Plants, fungi and animals
are all eukaryotes.
Prokaryotes:
A. Have no organelles.
1. No nucleus.
2. DNA in cluster (less
DNA ).
B. Smaller in size.
C. DNA is a single circular
chromosome.
D. Bacteria .
Structure:
- A typical cell as seen by light
microscope has 2 major
parts, a nucleus & a
cytoplasm. The nucleus is
separated from the cytoplasm
by a nuclear membrane and
the cytoplasm is separated
from the surrounding by a
cell membrane.
Major Subcellular organelles
Cell membrane.
Nucleus.
Mitochondria.
Endoplasmic reticulum.
Golgi apparatus.
Ribosomes (no membrane)
Lysosomes.
Cell Membrane
Made up of lipids &
proteins.
Selectively permeable,
bilayer, allowing some
substances to pass
excluding others.
Lipids are : - phospholipids
bilayer such as
phosphatidylcholine &
phosphatidylethanolamine
Head end of the
phospholipids molecule
contains phosphate
portion, which is
relatively soluble in water
(polar, hydrophilic).
The tail is insoluble (
nonpolar, hydrophobic).
Proteins:
- Uncharged hydrophobic
part usually located in the
interior of the membrane
whereas charged
hydrophilic part on the
surface.
- Proteins carry out
functions such as:
- Pumps; actively
transporting ions across
the cell membrane.
- Carriers, transporting
substances down
electrochemical gradients
by facilitated diffusion
Ion channels, permit
passage of ions into and
out of the cell when
activated.
Receptors, bind
neurotransmitters and
hormones.
Enzymes, catalyzing
reactions at surface of
membrane.
Certain glycoproteins create
the tight junctions.
Nucleus
Made up in large part of
chromosomes
Each chromosome is
made up of a giant
molecule of DNA.
The nuclear envelope is a
double membrane
structure.
Numerous pores occur in
the envelope allowing RNA
and other chemicals to
pass.
All DNA is restricted to the
nucleus except the
mitochondrial and plastid
DNA.
RNA is formed in the
nucleus by coding of the
DNA bases.
RNA moves out to the
cytoplasm.
Nucleolus (usually 2
nucleoli per nucleus),
don’t have surrounding
membrane and are rich in
RNA.
Mitochondria
Sausage-shaped structure,
made of outer membrane &
inner membrane folded to
form shelves ( cristae ).
Power generating units of
the cell.
Outer membrane is
studded with enzymes
concerned with biologic
oxidation for reactions
occurring inside
mitochondria.
Inner membrane and the
matrix which it surrounds
contain enzymes that
convert products of CHO,
proteins, fat metabolism to
Co2 & water via Citric Acid
Cycle and the electron
transport chain
Their number increases in
cells where there is high
energy requiring
processes
Mitochondria have their
own genome. There is
much less DNA in
mitochondrial genome
than nuclear genome.
Lysosomes
Relatively large, irregular
structures surrounded by
membrane.
Interior of these structures is
more acidic than the rest of the
cytoplasm.
These acid hyrolases
enzymes allow lysosomes
to break down intracellular
substances, especially
damaged cells and
bacteria.
Usually, the membrane
surrounding the
lysosomes prevent the
enclosed enzymes from
coming in contact with
the surrounding
substances & therefore
prevent their digestive
action.
What will happen if one of the
lysosomal enzymes is
congenitally absent?
Defective lysosomes lead to
one of lysosomal storage
diseases.
Endoplasmic Reticulum
A complex series of
tubules.
The walls are made up of
membranes.
Rough: granules attached
to the cytoplasmic side of
the membrane.
Rough ER connects to the
nuclear envelope through
which mRNA travels to the
ribosomes (the site of
protein synthesis).
Smooth : granules are
absent it is the site of lipid
synthesis.
Ribosomes
Sites of protein synthesis.
Not membrane-bound.
Structurally, consists of a
small and larger subunits.
Biochemically, consists of
ribosomal RNA and
structural proteins.
Often cluster on the
Endoplasmic reticulum or
found free in the cytosol.
Golgi Apparatus
Has membrane similar to the a
smooth ER.
More prominent in secretory
cells and functions in
association with ER.
Involved in intracellular sorting
of proteins, glycosylation and
sulfation reactions.
Peroxisomes
Surrounded by membrane
which contains number of
specific proteins that are
concerned with transport of
substances into and out of
their matrix.
It is important in getting rid of
oxygen radicals and has a
metabolic role in fatty acids
oxidation
Cytoskeleton
System of fibers that
maintains the structure of
the cell.
Permits the cell to change
it’s shape and move.
Made up of :
1. Microtubules: provide
track for transport of
vesicles.
2. Intermediate filaments:
some connect the nuclear
membrane to cell
membrane.
3.Microfilaments: made up
of actin, a protein by it’s
interaction with myosin
brings about contraction
of muscle.