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The cell (plasma) membrane
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The cell membrane
• The cell membrane (also known as the plasma
membrane or cytoplasmic membrane) is a biological
membrane, which is present in both eukaryotic and
prokaryotic cells
• The cell membrane is selectively permeable and works
as a barrier between the inner and outer surface of a cell
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Function of Plasma Membrane
• It separates the contents of the cell from its outside
environment and it regulates what enters and exits the cell.
• Plasma membrane plays a vital role in protecting the integrity of
the interior of the cell by allowing only selected substances into
the cell and keeping other substances out.
• It also serves as a base of attachment for the cytoskeleton in
some organisms and the cell wall in others. Thus the cell
membrane supports the cell and helps in maintaining the shape
of the cell.
• The cell membrane is primarily composed of proteins and lipids.
While lipids help to give membranes their flexibility and proteins
monitor and maintain the cell's chemical climate and assist in the
transfer of molecules across the membrane.
• The lipid bilayer is semi-permeable, which allows only
selected molecules to diffuse across the membrane
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Plasma Membrane Structure
• It is the phospholipids bilayer.
• Plasma membrane is an amphipathic, which
contains both hydrophilic heads and hydrophobic
tails.
• It is a fluid mosaic of lipids, proteins and
carbohydrate.
• It is lipid bilayer, which contains -two layers of
phospholipids, phosphate head is polar (water
loving), fatty acid tails non-polar (water fearing)
and the proteins embedded in membrane.
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The cell transport mechanisms
• Endocytosis: Endocytosis is the process in which cells
absorb molecules by engulfing them.
• Endocytosis is a pathway for internalizing solid particles
("cell eating" or phagocytosis), small molecules and ions
("cell drinking" or pinocytosis), and macromolecules.
Endocytosis requires energy and is thus a form of active
transport.
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The cell transport …
• Exocytosis: Exocytosis occurs in various cells to remove
undigested residues of substances brought in by endocytosis, to
secrete substances such as hormones and enzymes, and to
transport a substance completely across a cellular barrier eg.
Exocrine glands
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The cell transport …
• Exocrine glands secrete their products through a duct onto
an outer surface of the body. Secretion is directly onto the
apical surface
– Apocrine glands; Holocrine glands and Merocrine
glands
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The cell transport …
• Apocrine glands a portion of the secreting cell's body is lost during
secretion. e.g. mammary gland, sweat gland of arm pit, pubic region,
skin around anus, lips, nipples.
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The cell transport …
• Holocrine glands the entire cell disintegrates to secrete its substances
(e.g., sebaceous glands) sebaceous.
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The cell transport …
• Merocrine glands cells secrete their substances by exocytosis (e.g.,
mucous and serous glands). Also called "eccrine". e.g. the sweat gland,
goblet cells, salivary gland, tear gland, intestinal glands.
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Transport Proteins and
Carrier Proteins
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Transport protein/ Carrier proteins
• Carrier proteins facilitate the diffusion of different
molecules, while channel proteins are involved in the
movement of ions, small molecules, or macromolecules,
such as another protein, across a biological membrane
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Carrier proteins…
• Na+-K+ pump: A carrier protein is involved and it
pumps Na + out of the cell and K + into the cell. It
requires ATP to maintain this pump, where the
concentration of Na + outside the cell is 10-20 times
that in the cell, and likewise concentration of K + in
the cell is 10-20 times its concentration outside the
cell.
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CYTOPLASMIC ORGANELLES
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Cytoplasmic cell Organelles
• Membrane bound organelles that are generally found in eukaryotic
cells are rough ER, smooth ER, Golgi apparatus, mitochondria,
plastids and lysosomes
• Non-membrane bound organelles, which generally do not contain
membranes, are ribosomes, cytoskeletal structures, centrioles,
cilia, and flagella
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Cytoskeleton…
• Microfilaments - Any of the minute contractile protein fibers,
actin, located throughout the cytoplasm of cells and
functioning primarily in maintaining the shape and structural
integrity of a cell; they may contribute to changes in cell shape
in those cells capable of changing their shape.
• Microtubules - The cylindrical hollow cytoskeletal protein
complexes (structures) which are distributed throughout the
cytoplasm of eukaryotic cells, providing structural support
and assisting in cellular locomotion, internal transport and
forming the spindle apparatus at cell
• Centrioles - The two cylindrical, non-membrane bound cell
organelles. Centrioles are involved in the organization of the
mitotic spindle and in the completion of cytokinesis
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Endoplasmic Reticulum
• Endoplasmic reticulum - A complex membrane-bound cell
organelle which consists of a network of interconnected
membranous channels and compartments (cisternae) of
various shapes which are continuous with both the
nuclear envelope and the Golgi apparatus located within
the cytoplasm of cells which are involved in the synthesis,
modification, and transport of cellular materials.
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Endoplasmic Reticulum (Rough ER )…
• Rough ER - A cell organelle with a network of membranous
channels and compartments (cisternae) of various shapes
which are continuous with both the nuclear envelope and the
Golgi apparatus; it always has ribosomes which are attached
to the cytoplasmic face of the channels which synthesize
proteins for storage in vesicles and perhaps export from the
cell.
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Endoplasmic Reticulum (Smooth ER )…
• Smooth ER - A cell organelle with a network of membranous
channels and compartments (cisternae) of various shapes
which are continuous with both the nuclear envelope and the
Golgi apparatus; ribosomes are not attached to the
cytoplasmic face of the channels.
• It synthesizes lipids, phospholipids, and steroids.
Cells which secrete these products, such as those in
the testes, ovaries, and skin oil glands have a great
amount of smooth endoplasmic reticulum.
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Endoplasmic Reticulum (Smooth ER )…
• Metabolism of carbohydrates
• It also carries out the metabolism of carbohydrates,
drug detoxification (they have enzymes cytochrome
P-450 which is involved in detoxification of drugs)
Golgi apparatus
• The Golgi apparatus is an integral in modifying,
sorting, and packaging these macromolecules for
cell secretion (exocytosis) or use within the cell.
• It primarily modifies proteins delivered from the
rough endoplasmic reticulum, but is also involved in
the transport of lipids around the cell, and the
creation of lysosomes.
•
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Golgi apparatus…
• The GA is involved in the synthesis of lipoproteins and
proteoglycans (sulphated mucopolysaccharides).
• Production of mucopolysccharides is particularly prominent in
mucus secreting cells such as Goblet cells.
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Ribosomes
• Ribosomes are made up of 35% ribosomal
protein and the 65% ribosomal RNA (rRNA).
• There are more than 50 different proteins that
unite with rRNA to form a ribosome.
• The formation of ribosomes begins in the cytosol
where all cell proteins are synthesized.
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Mitochondria
• The mitochondrion (plural mitochondria) is a
membrane bound organelle found in most eukaryotic
cells
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Inheritance of mitochondrial DNA
• About 99% of mitochondrial DNA (mtDNA) are inherited
from the mother, but about 0.1-1% of persons
mitochondria can be inherited from the father.
• The reason for such a low percentage from the father is
that the mitochondria of the spermaotozoa are normally
thrown out during fertilization.
• It is therefore possible to tress the uterine (maternal)
lineage far back in time. Unlike the nuclear DNA which
changes by 50% in each offspring.
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Lysosomes
• Is the membrane-bound cytoplasmic organelles
which
contain
various
hydrolytic
enzymes
(proteases, lipases, nucleotidases, etc.,
• Synthesized at the rER and packaged by the Golgi
apparatus) which function in intracellular digestion
by merging with a phagosome and are suspected of
contributing to aging when they become "leaky;"
• Lysosomes are often large enough to be seen in the
light microscope; they are especially prominent in
leukocytes, and liver and kidney cells.
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Peroxisomes
• Peroxisomes contain oxidative enzymes, such as
catalase, D-amino acid oxidase, and uric acid oxidase.
• However the last enzyme is absent in humans, explaining
the disease known as gout, caused by the accumulation
of uric acid
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The Nucleus
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Nucleus
is the largest cellular
• The nucleus
organelle in
animal cells
• It contains most of the cell's genetic material,
organized as multiple long linear DNA molecules in
complex with a large variety of proteins, such as
histones, to form chromosomes
• The genes within these chromosomes are the cell's
nuclear genome
The following are the components
of the nucleus
Nuclear lamins
Nucleoplasm
Nucleolus
Chromatin
Ribosomes
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Nuclear envelope
• A nuclear membrane, also known as the nuclear
envelope, nucleolemma, is the double lipid bilayer
membrane which surrounds the genetic material and
nucleolus in eukaryotic cells.
• During the G2 phase of interphase, the nuclear
membrane increases its surface area and doubles its
number of nuclear pore complexes. The nuclear
membrane must break down during the prometaphase
state of mitosis to allow the mitotic spindle fibers to
access the chromosomes inside.
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Nucleolus
• The nucleolus is the nuclear subdomain that assembles
ribosomal subunits in eukaryotic cells. Ribosome assembly
begins with transcription of pre‐rRNA.
• The pre‐rRNA is modified and processed into rRNA with the
aid of non‐ribosomal proteins and small nucleolar RNAs.
• The nucleolus has numerous other functions including
assembly of signal recognition particles, modification of
transfer RNAs and sensing cellular stress
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Genetic materials
• The terms chromatin and chromosomes are used in relation
to genetic materials that are present in the nucleus.
Chromatin and chromosomes are similar structures in the
sense that they are made up of DNA and histone proteins.
• The use of these terms is based on morphological
appearance of genetic materials.
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Chromosome and chromatin
• The nucleus of a non-dividing cell contains chromatin;
they are responsible for giving the nucleus a
characteristic appearance of electron dense materials
under the electron microscope.
• There are two types of Chromatin; heterochromatin
(condensed) and euchromatin (extended) forms
• Euchromatin comprises the most active portion of the
genome within the cell nucleus. 92% of the human
genome is euchromat
• Heterochromatin is a tightly packed form of DNA, which
comes in different varieties. These varieties lie on a
continuum between the two extremes of constitutive and
facultative heterochromatin.
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Constitutive and facultative
heterochromatin
• The
genes
contained
within
the
constitutive
heterochromatin are poorly expressed.
• For example, all human chromosomes 1, 9, 16, and the Ychromosome contain large regions of constitutive
heterochromatin.
• In most organisms, constitutive heterochromatin occurs
around the chromosome centromere and near telomeres
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Facultative heterochromatin
• Facultative
heterochromatin:
contains
DNA
sequences that are not transcribed in one cell but
may be transcribed in other cells. An example of
facultative heterochromatin is X-chromosome: one X
chromosome is packed in facultative heterochromatin
and does not transcribe, while the other X
chromosome is packaged in euchromatin and
transcribed
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Centromere
• The centromere (centro- + -mere) is the part of a
chromosome that links sister chromatids. During mitosis,
spindle fibers attach to the centromere via the
kinetochore
• The chromosomes can be metacentric, subcentric,
acrocentric and telocentric
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Karyotyping
• A karyotype is the number and appearance of
chromosomes in the nucleus of a eukaryotic cell.
• Karyotyping involves: Counting the number of
chromosomes & Looking for structural changes in
chromosomes
Certain abnormalities can
be identified through the
number or arrangement of
the
chromosomes.
Therefore
useful
for
detecting genetic diseases
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Examples of Genetic diseases
• Down syndrome (Trisomy 21), is a genetic condition in
which a person has 47chromosomes instead of the
usual 46. Down syndrome occurs when there is an
extra copy of chromosome 21. Thus known as Trisomy
21. The extra chromosome causes problems and
interferes with normal development of the body and
brain
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Genetic diseases…
• The most common types of autosomal trisomy that survive
to birth in humans are:
• Trisomy 21 (Down's syndrome)
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)
• Trisomy of sex chromosomes can also occur and include:
• XXX (Triple X syndrome)
• XXY (Klinefelter syndrome)
• XYY
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