در روزهای اولیه میکروسکوپ الکترونی، زیست شناسان تصور می‌کردند که اندامک سلول یوکاریوتی آزادانه در سیتوزول شناور است. اما پیشرفت های انجام شده در تکنولوژی ساخت میکروسکوپ‌های نوری و الکترونی نشان داده است که شبکه‌ای از فیبرها در سراسر سیتوپلاسم گسترش یافته است و آنها آن را اسکلت سلولی نامیده اند. -------------------------------------------------------------------------- In the early days of electron microscopy, biologists thought that the organelles of a eukaryotic cell floated freely in the cytosol. But improvements in both light microscopy and electron microscopy have revealed that a network of fibers is extended throughout the cytoplasm and they named it the cytoskeleton.

1. A Tour of the Cell
PART TWO
UNIVERSITY OF TEHRAN
Ahmad V.Kashani, PhD

2. Outlines
• Cells, Fundamental units of life
• Tools and Techniques
• Eukaryotes vs. Prokaryotes
• Animal cell
• Plant cell
• Genetic instructions
• Nucleus
• Ribosomes
• Endomembrane system
• ER network
• The Golgi Apparatus
• Lysosomes
• Vacuoles
• Energy Transfer
• Mitochondria
• Chloroplast
• Peroxisomes
• The Cytoskeleton
• Microtubules
• Actin filaments
• Intermediate filaments

3. Energy Transfer

4. Energy Transfer
• Mitochondria are rod-shaped organelles: (1-10 μm)
• Considered the power generators of the cell
• Cellular Respiration: converting oxygen and nutrients
into adenosine triphosphate (ATP).
• By extracting energy from sugar, fat, etc.
• Chloroplasts are lens-shaped organelles: (3-6 μm)
• Found in plants and algae
• Site of photosynthesis
• The process of using sunlight to drive the synthesis of
organic compound such as sugar from carbon dioxide and
water

5. Energy Transfer
• Endosymbiont Theory and Evolution origin of
Mitochondria and Chloroplast
• Endosymbiont is a cell living within another cell
• Widely accepted theory, consistent with many
structural features:
• Two membrane surrounding them unlike
members of endomembrane system
• Ribosomes and circular DNA molecules
• Autonomous organelles
• Independent and grow and reproduce
in the cell

6. Energy Transfer - Mitochondria
• Found in almost all forms of life: Plants,
animals, fungi, etc.
• Population: depending on the level of
metabolic activity of the cell
• Structure:
• Phospholipid bilayer with specific proteins
• Soft outer membrane and inner membrane is convoluted
with infoldings (Cristae)
• Intermembrane space
• Mitochondrial matrix
• Full of different enzymes and DNA and ribosomes
• Enzymes catalyze some of the steps of cellular respiration
• Other enzymes are build into the inner membrane
• Cristae infoldings => larger surface area => enhanced
cellular respiration
• Other properties: moving, changing shape, fusing and
dividing in two

7. Energy Transfer - Chloroplast
• Contains: Chlorophyll and enzymes and
molecules of photosynthesis of sugar
• Structure:
• Chloroplast envelope
• Outer membrane
• Intermembrane space
• Inner membrane
• Thylakoids
• Thylakoid lumen
• Thylakoid membrane
• Granum (pl. Grana)
• Stroma (the fluid outside the thylakoids):
• DNA
• Ribosomes
• Enzymes
• Three compartments:
• Intermembrane space
• Stroma
• Thylakoid space
• The chloroplast is a specialized member of a
family of closely related plant organelles called
plastids.

8. Energy Transfer - Peroxisome
• Peroxisomes contain enzymes that remove
hydrogen atoms from various substrates and
transfer them to oxygen (O2), producing hydrogen
peroxide (H2O2) as a by-product in different
functions:
• Uses oxygen to break fatty acids down into
smaller molecules that are transported to
mitochondria and used as fuel for cellular
respiration.
• In the liver, they detoxify alcohol and other
harmful compounds by transferring hydrogen
from the poisonous compounds to oxygen.
• Contains an enzyme that cleaves H2O2
• Grow larger by proteins made in cytosol and
ER
• Increase in number by splitting in two at a
certain size

9. The Cytoskeleton
Microtubules

10. The cytoskeleton
• The eukaryotic cytoskeleton
• Major Roles:
• Mechanical support
• Motility (both cell location and cell movement)
• Manipulating the plasma membrane is
phagocytosis
• Composed of three types of molecular
structures:
• Microtubules
• Microfilaments
• Intermediate filaments

11. The Cytoskeleton

12. The cytoskeleton - Microtubules
• Composed of tubulin dimers:
• α Tubulin
• Β Tubulin
• Functions:
• Shape and support (compression resisting)
• Main tracks for organelles’ movement
• Guide vesicles from ER to Golgi system
• Chromosome separation during cell
division

13. The cytoskeleton - Microtubules
• In animal cells, they grow out of
Centrosomes
• Centrosome is a region near the nucleus
• Structure:
• A pair of centrioles
• Maternal centriole
• Daughter centriole
• Each centriole is composed of nine sets
of triplet microtubules arranged in a ring

14. The cytoskeleton - Microtubules
• Cilia and Flagella are microtubule-
containing extensions
• Functions:
• They can act as locomotor appendages,
e.g. the sperm of animals have flagella
• In a tissue, they can move fluid over the
surface of the tissue e.g. the ciliated
lining of the trachea (windpipes) sweeps
mucus, containing trapped debris out of
the lungs.

16. The cytoskeleton - Microtubules
• Cilia and Flagella differences
• Motile cilia usually occur in large
numbers on the cell surface.
• Flagella are usually limited to just one or
a few per cell
• Flagella are longer than cilia.
• Flagella and cilia differ in their beating
patterns.
• A flagellum has an undulating motion like the
tail of a fish.
• Cilia have alternating power and recovery
strokes, much like the oars of a racing crew
boat
• Cilia and Flagella Similarity
• A common structure
• A group of microtubules sheathed in an
extension of the plasma membrane
• Nine doublets of microtubules are
arranged in a ring with two single
microtubules in its center. “9+2” pattern.
• The microtubule assembly of a cilium
or flagellum is anchored in the cell by
a basal body, which is structurally
very similar to a centriole, with
microtubule triplets in a “9 + 0”
pattern.

17. The cytoskeleton - Microtubules
• Cilia and Flagella Similarity
• Flagella and motile cilia bending involves large
motor proteins called dynein
• Dynein and Kinesin are motor proteins for
organelle and vesicle transport along
microtubules in the cell, each with specific
activity

18. The Cytoskeleton
Microfilaments

19. The cytoskeleton – Microfilament
• AKA Actin Filaments because of their
globular actin subunits
• A microfilament is a twisted double chain
of actin subunits
• Formation:
• Linear filaments
• Network filaments in association with
certain proteins
• Their structural is to bear tension (pulling
forces)
• The 3D structure (cortical microfilaments)
helps support the cell’s shape.

20. The cytoskeleton – Microfilament & Motility
• In animals actin Filaments and thicker
filaments called myosin interact to cause
muscle contraction.
• In Amoeba and some WBCs localized
contractions brought about by actin and
myosin are involved in the amoeboid
(crawling) movement of the cells
• The cell crawls along a surface by extending
cellular extensions called pseudopodia and
moving toward them
• In plant cells, actin-protein interactions
contribute to cytoplasmic streaming, a
circular flow of cytoplasm within cells

21. The Cytoskeleton
Intermediate Filaments

22. The cytoskeleton – Intermediate Filaments
• Named after their diameter
• Structures larger than actin filaments and
smaller than microtubules
• Only found in some cell of some animals,
including vertebrates
• Specialized for bearing tension (like
microfilaments)
• A diverse class of cytoskeletal elements
• Unlike microtubules and microfilaments, each
type is constructed from a particular
molecular subunit belonging to a family of
proteins whose members include the keratins
• Involve in diverse functions:
• Reinforcing cell shape and fixing the position of
organelles
• Nuclear lamina

23. The cytoskeleton

24. Questions…?
THANK YOU!