THE CELL WALL
Structure
As a plant divides, a gluey material forms between the two cells on an area called the *middle
lamella. * The primary cell wall contains cellulose molecules bundled together in microfibrils that
are laid down in a matrix of gluey polymers.
Commented [mm1]: made of pectin and
polysaccharides.
THE CELL SURFACE MEMBRANE
Structure
The cell membrane regulates the passage of materials into and out of the cell, and it is about 7-9
nm thick. Eukaryotes cells consist of phospholipid and cholesterol molecules arranged such that
their hydrophobic tails are clustered inwards, forming filling of molecular sandwich, which
globular proteins embedded throughout.
Short carbohydrates chains called
the glycolipid are attached to the globular protein, and the phospholipids on the external
environment of the cell. Some additional proteins are embedded on the side facing the cytoplasm
as well.
THE NUCLEUS
Description of the nucleus.
It is a spherical prominent structure within the cells, that is surrounded by two lipoproteins, which
make up the nuclear envelope. The lipoproteins are separated by a gap of 20-40 nanometres.
Activity within the Nuclear Envelope in the Nucleoplasm
At intervals the lipoproteins and the nucleus fuse and forms pores that allow passage of material
between the cytoplasm and the nucleus. The inner and outer membranes fuse and assemble
nucleoporins, that make up the pore complex that is arranged in an octagonal pattern, and forms
a narrow channel through the fused lipid bilayers.
Functions of the Nucleus
Commented [mm2]: these are large, spherical and
soluble proteins
•
•
Carries hereditary information and it is the only link between father and offspring (in a
sperm cell)
Hammerling's work indicates that the nucleus exerts influence on the activities of the
cell, ensuring that complex molecules are synthesized in the numbers and kind needed
by the cell.
The Nucleolus;
It is a structure within the nucleus of eukaryotes cells and it is a site for ribosome biogenesis.
Synthesis and assemble of Ribosomal RNA and ribosomal proteins
Commented [mm3]: the process by which cells
synthesize ribosomes, which are essential for protein
synthesis
STRUCTURE
Centrioles are cylindrical, microtubule-based structures in animal cells. They are present in
pairs, nestled in a region called the centrosome. Each centriole are consists of 9 sets of 3
separate microtubules arranged in cylindrical fashion, giving them a barrel like structure.
.
The mother centriole is the template of the daughter centriole, which together forms the
centrosome.
Functions of Centrioles
•
They help organize the microtubules in the cytoplasm, which forms the cytoskeleton and
help in intracellular transport.
Commented [mm4]: centrosomes are made of two
centrioles, arranged at 90 degrees of each other. They are
the microtubule organizing centres that contain the
gamma tubulin. Microtubules grow out of this gamma
tubulin
•
They help organize the mitotic spindle, which separates chromosomes during cell
division. Each pair duplicates before division, and the two centrosomes (with centrioles)
move to opposite sides of the cell to form the poles of the spindle.
•
They give rise to basal bodies, which act as anchors to cilia and flagella.
Clinical Links
Inhibition with spindle poison leads to arrest of mitosis and cessation of cell division, as well
vinca alkaloids and taxemes.
The Rough Endoplasmic reticulum
Description of the Rough ER
The rough ER is a network of interconnected flattened sacs, tubes and tunnels found in
eukaryotes cells. The rough er has a membrane (the membrane is continuous with the outer
membrane of the nuclear envelope) embedded with ribosomes and flattened sacs called the
cisternae.
Commented [mm5]: A "spindle poison" is ==a
substance that interferes with the function of the mitotic
spindle==, a structure essential for cell division. This
interference disrupts the process of mitosis, leading to
cell cycle arrest and potentially cell death. Spindle
poisons are often used as anti-cancer drugs because
cancer cells, which divide rapidly, are particularly
susceptible to these agents.
Functions of the cisternae
Predominate cells in making copious amounts of protein for export, so they are abundant in cells
responsible for protein synthesis.
Activities of the Rough ER.
The initial synthesis of proteins in the rough er includes the synthesis of 'leader' hydrophobic
ammino acids that navigates the transport of protein through the lipid bilayer membrane, to the
interior of the endoplasmic reticulum. The newly synthesized proteins will move from the rough
er, into the smooth endoplasmic reticulum and into the Golgi apparatus. During the progression
between the cell organelles, the protein undergoes further processing by cleavage of the
sequence of hydrophobic ammino acids and addition of carbohydrate group to the pattern.
The Smooth Endoplasmic Reticulum
Just like the rough er, it is also a network of interconnected flattened sacs, tubes and tunnels
found in eukaryotes cell. However, it has no ribosomes.
Functions of the Smooth ER
•
They synthesize lipids and are found steroid hormones secreting glands.
•
The smooth er is found in the liver cells and it appears to be involved in different
detoxification process, which is one of the many functions of the liver.
•
They are active in the breakdown of glycogen to glucose in the liver cells
RIBOSOMES
Description of the ribosomes
Ribosomes are small organelles that are either free in the cytoplasm or attached to the rough
endoplasmic reticulum. They formed from ribosomal RNA. which has some enzyme properties.
Function
Ribosomes are a site of protein synthesis. They read RNA to make polypeptides in a process
called translation.
The subunits of ribosomes are measured in sedimentation coefficient (S) which is an indirect
measure of the size and density of a particle. For eukaryotes, the large subunit is 60s and the
smaller subunit is 40s. Together they make 80s. For prokaryotes and mitochondria and cytoplasm
in eukaryotes, the ribosome is 70s. The large subunit is 50s and the small subunit is 40s.
S= 1 x 10^-13
The Protein Synthesis Process
Commented [mm6]: Polypeptides are chains of ammino
acids, assembled in a specific sequence
The A site
The aminoacyl site tunnel where tRNA enters the subunit of the ribosome carrying amino acid.
The anticodon of the mRNA pairs with the codon of the tRNA to ensure the correct amino acid is
added to the chain.
The P site
This is where the tRNA holds the growing polypeptide chain. A catalytic reaction (peptidyl
transferase activity) occurs here, forming a peptide bond between the amino acid in the P site
and the new amino acid brought into the A site.
E site
This is where the now uncharged (empty) tRNA exits the ribosome after its amino acid has been
added to the chain.
GOLGI BODIES
Structure;
The Golgi consists of flattened, membrane-bound sacs stacked loosely on one another and
surrounded by tubules and vesicles.
Function of the Golgi's
•
Package and distribution centre- They accept vesicles from the ER and modify the
membrane vesicles, further process their contents and distributes finished products to
other parts of the cell.
•
In plant cells, they bring together some components of the cell wall and export them to
the surface where they are assembled.
LYSOSOMES
Structure
They are small vesicles found in the Golgi and are essentially hydrolytic enzymes that are
enclosed in membranes that separate them from the rest of the cell.
Activities of Lysosomes
Through a process called phagocytosis, the lysosomes engulf bacteria in the human body. As
cells take up the bacteria, they are enclosed within a membrane sac (a vacuole called a
Commented [mm7]: "Hydrolytic" refers to ==anything
related to hydrolysis, a chemical process where a
molecule is broken down by the addition of water==. In
simpler terms, it's a reaction where water splits a bond
within a molecule, resulting in the formation of two new
molecules. Hydrolytic enzymes, also known as
hydrolases, are ==a class of enzymes that use water to
break down large molecules into smaller ones==. They are
formed in the ER.
phagosome). The lysosomes fuse with this membrane vacuole and release their hydrolytic
enzymes to break down the bacteria. The bacteria are quickly digested. The tenderness and
inflammation associated with rheumatoid arthritis and gout appears to be associated with the
escape of the hydrolytic enzymes from lysosomes.
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MITOCHONDRIA
Structure
The mitochondrion is among the largest organelles in the cell. They are almost spherical, potato
shaped or elongated cylinders. It has two membranes, and the inner membrane folds inwards.
These folds, or rather, cristae, are the working sites for mitochondria reactions. The more active
a mitochondrion is, the more cisternae it has.
! [[20250705-1118-29.6327625.mp4]]
CHROLOPLASTS
Structure
This double membrane organelle is responsible for photosynthesis in plant cells and
photosynthetic eukaryote algae. They move around the cell to position themselves for favourable
light conditions.
The thylakoids found in the interior of the cell constitutes a third membrane system. It is
surrounded by a dense solution called the stroma.
In the mitochondria, large energy-yielding organic molecules are broken down into small units,
which are vitally used for cell processes.
Mitochondria DNA
All the mitochondrial DNA come from the mother. It is round, not linear like the nucleus DNA and
completely separate from it. The DNA has 27 genes that code for specific proteins used by
mitochondria e.g. AT synthase. According to the endosymbiotic theory, it was once a single celled
organism engulfed by a cell.
Functions of the Mitochondria
•
•
They are the site for respiration in the cell, which generates fuel for cellular activities.
They are involved in cell division, cloning and cell death.
CILIA AND FLAGELLA
Structure
Cilia and flagella are long, thin hair-like structures extending from the surface of some eukaryotic
cells. When they are shorter and in large numbers, they are called cilia and when they long and
few they are called flagella.
Both cilia and flagella share a common structure. They have 9 pairs of microtubules arranged in
a circle with another pair of microtubules in the centre. We call this the 9+2 structure. Using the
energy from ATP, the pairs of microtubules can move relative to the pairs next to them, which
creates the bending movement of these organelles. Where a cilium and a flagella form depend
on a complex of proteins. A key part of the complex are centrioles, and the formation of cilia and
flagella is another function of centrioles.
Application of knowledge.
Cilia are found in cells that line that line the surfaces within our bodies. The cells of the respiratory
tract called the respiratory epithelium beat upward, propelling a current of mucus from the goblet
cells that sweeps foreign material of soot, dust, pollen, tobacco etc in our throats where they are
removed and prevented from entering the lungs.
! [[20250705-1330-41.4869849.mp4]]
Clinical Applications
Primary Ciliary Dyskinesia is rare genetic disorder in children where cilia are structurally
abnormal or immobile, impairing their function. Since cilia line the respiratory tract in the
respiratory epithelium, the middle ear and the reproductive system, children with PCD are often
have complications like
Chronic respiratory infections due to poor mucus clearance
Ear infections leading to hearing loss
infertility issues later in life due to the disfunction of the flagella of the sperm or the cilia in the
fallopian tube.
MICROVILLI
Microvilli are hairlike structures that increases the surface of the cells. They are abundant in cells
that specialize in absorption.
Function
Help increase surface area