Cell Organelles

Cellular Organelles
Chapter Three
11/24/2022 By: Asmamaw Menelih 1

Objectives
Describe the structure and function of a cell’s plasma membrane.
Summarize the role of the nucleus.
List the major organelles found in the cytosol, and describe their roles.
Identify the characteristics of mitochondria.
Describe the structure and function of the cytoskeleton.
List three structures that are present in plant cells but not in animal cells.
Compare the plasma membrane, the primary cell wall, and the secondary cell wall.
Explain the role of the central vacuole.
Describe the roles of plastids in the life of a plant.
Identify features that distinguish prokaryotes, eukaryotes, plant cells, and animal cells.

Cell Organelles
Eukaryotic cells have many membrane systems. These membranes divide cells
into compartments that function together to keep a cell alive.
An organelle is a specialized subunit within a cell that has a specific function.
In eukaryotes an organelle is a membrane bound structure found within a cell.
Prokaryotic cells do not have membrane bound organelles.
Organelle= “little organ”
All the stuff in between the organelles is cytosol
Everything in a cell except the nucleus is cytoplasm

Con’t
There are two kinds of cell organelles on the basis membrane covering. These are:
Membranous and Non-membranous organelles.
Membrane bound organelles
Endoplasmic reticulum (Rough and Smooth)
Golgi bodies
Mitochondria
Chloroplasts
 Nucleus
Lysosomes
Peroxisomes and
Vacuoles
 Non-membrane organelles
 ribosomes (70s and 80s),
 centrosomes,
 cilia and flagella,
 microtubules,
 basal bodies and microfilaments.

Animal cells

Cell Membrane
Boundary of the cell
Made of a phospholipid bilayer

Cytosol
Cytoplasm refers to the jelly-like material with organelles in it.
If the organelles were removed, the soluble part that would be left is
called the cytosol.
It consists mainly of water with dissolved substances such as
amino acids in it.
Cytoplasm is a viscous liquid found within the cell membrane that
houses the organelles and is the location of most of the action
happening in a cell.

Nucleus
The nucleus is the control center of the cell.
It is the largest organelle in the cell and it contains the DNA of the cell.
DNA (Deoxyribonucleic Acid) contains all the information for cells to live, perform
their functions and reproduce.
Inside the nucleus is another organelle called the nucleolus.
The nucleolus is responsible for making ribosomal RNA.
The circles on the surface of the nucleus are the nuclear pores.
These are where ribosomes, and other materials move in and out of the cell

Nucleus

 The average diameter of the nucleus is approximately 6 micrometers in mammalian cells,
which occupies about 10% of the total cell volume.
 The viscous liquid within it is called nucleoplasm.
 It appears as a dense, roughly spherical organelle.
 Nucleus has three important parts. These are:
I. Nuclear envelope and Nuclear pore
II. Nucleolus
III. Chromatin material
Structure of Nucleus
11
11/24/2022 By: Asmamaw Menelih

Nuclear envelope
Act as barriers that prevent the free passage of molecules
Two concentric membranes (phospholipid bilayers)
outer nuclear membrane is continuous with the ER
space between the inner and outer membrane is termed as perinuclear space
Under the inner membrane you have the nuclear lamina (fibrous meshwork that
provides structural support to the nucleus)

Nuclear Envelope
The nucleus is surrounded by a double membrane called the nuclear envelope
 Nuclear envelope has many nuclear pores through which mRNA, and proteins can
pass.
Perinuclear space

Internal Organization of the Nucleus
Nucleus has internal structure that organizes the genetic material and localizes some nuclear
functions to discrete sites.
Nucleolus - site at which the rRNA genes are transcribed and ribosomal subunits are
assembled
DNA of eukaryotes is quite complex
It is organized into morphologically distinct units called chromosomes
Each chromosome contains a single enormous DNA molecule
DNA is made compact by a hierarchy of different types of folding, each of which is mediated
by one or more protein molecules.

Con’t
The term chromatin means "colored material"
This refers to the material that is easily stained for viewing
with microscope.
It is composed mainly of coils of DNA bound with
proteins called histones.
The DNA is so long that it has to be packed in an
organization manner.

Mitochondria
Mitochondria are membrane-enclosed organelles distributed through the cytosol of most
eukaryotic cells.
Their main function is the conversion of the potential energy of food molecules into ATP.
Every type of cell has a different amount of mitochondria.
There are more mitochondria in cells that have to perform lots of work
Other cells need less energy to do their work and have less mitochondria

Mitochondria
Large organelles with a double membrane. Kidney bean shape.
Contain their own DNA, RNA, and ribosomes
Function: Produce adenosine triphosphate ATP. ATP is the energy source for most cellular
functions.
Place where cellular respiration happens.
Mitochondria use chemical reactions that use oxygen in the breakdown of food to form
carbon dioxide and water.
Found in both animals and plants

Mitochondria
• Mitochondria have:
An outer membrane that encloses the entire
structure
The inner membrane that encloses a fluid-filled
matrix
The inner membrane is elaborately folded with shell
like cristae projecting into the matrix.

Con’t
The cristae project into an inner cavity filled with a
jelly-like matrix
These cristae contain proteins that convert much of
the energy in food into a usable form (the electron
transport protein).
The enfolding increase the surface area available
for keeping these important proteins.

Con’t
The matrix contains a mixture of hundreds of different dissolved enzymes (Citric acid cycle
enzymes)
These enzymes are important in preparing nutrient molecules for the final extraction of usable
energy by the cristae proteins.
Mitochondria are unique organelles in two ways:
1. In the matrix they have their own unique DNA called mitochondrial DNA.
2. Mitochondria have the ability to replicate themselves even when the cell to which they belong
is not undergoing cell division.

Endoplasmic reticulum (ER)
It is a network of membranes throughout the cytoplasm of the cell.
There are two types of ER.
When ribosomes are attached it is called rough ER and
Smooth ER when there are no ribosomes attached.
The rough endoplasmic reticulum is where most protein synthesis occurs in the cell.
The function of the smooth endoplasmic reticulum is to synthesize lipids in the cell.
The smooth ER is also helps in the detoxification of harmful substances in the cell.

Smooth Endoplasmic Reticulum (SER)
Smooth endoplasmic reticulum is a continuous tubular system with rough
endoplasmic reticulum.
It is devoid of ribosomes and is used for the formation of transport vesicles.
Since it does not have ribosomes, it looks “smooth” and does not produce proteins.
It serves a variety of other functions that differ in cell types.
In most cells, the smooth ER is sparse and serves in packaging and discharging site
for protein molecules that are to be transported from the ER.

Con’t
All new proteins and fats pass from ER gathered in the
smooth ER.
Portions of the smooth ER then bud off/pinch off, giving rise
to transport vesicles.
Transport vesicles move to the Golgi complex for further
processing of their cargo.
The smooth endoplasmic reticulum also has functions in
several metabolic processes.
It including synthesis of lipids and steroids, metabolism of
carbohydrates, regulation of calcium concentration, drug
detoxification.

Endoplasmic reticulum (ER)

Golgi complex
It is organelle in the cell that is responsible for sorting and correctly shipping the proteins produced in the ER.
The golgi complex processes, sorts, modifies, and packages proteins.
The Golgi Complex has a distinct polarity
• The cis face of the golgi {closest to the nucleus} is the site where membrane-bound vesicles from the ER
first fuse.
• The trans face of the golgi (closest to the plasma membrane) serves as the site where membrane-bound
vesicles bud and then exit the golgi complex.
 The cis face of a Golgi stack is the end of the organelle where substances enter from the endoplasmic
reticulum for processing, while the trans face is where they exit in the form of smaller detached vesicles

Golgi complex
• They have three functions:
• A. Modify lipids and proteins. Puts the finishing touches on these.
• B. Package wastes to be removed from the cell.
• C. Package lysosomes.
• Relation to factory: Central shipping department
• Found in both plants and animals

Con’t
• It performs the following important functions.
1. Processing the raw material into finished products.
The raw proteins from the ER are modified into their
final state.
This is a very elaborate, precisely programmed activity,
specific for each final product.

Con’t
2. Sorting and directing finished product to their final
destination.
According to their function and destination, different types
of products are segregated by the Golgi complex.
It sorts Molecules as:
 Molecules that are destined for secretion to the exterior
 Molecules that will eventually become part of the plasma
membrane, and
 Molecules that will become incorporated into other
organelles.

Con’t
3.The smooth ER of the liver and kidney cells are responsible
for the detoxification and inactivation of drugs.
Enzymes within the smooth ER can inactivate or destroy a
variety of chemicals such as
 Alcohol
 Pesticides and
 Carcinogens.
4. In skeletal muscle cells, a modified form of smooth ER
stores Ca2+ to be released for muscle contraction.
Finally, all these things are Modified by the Golgi
Complex
General Functions Nucleus, ER and Golgi body

Golgi complex

Interaction of ER and Golgi

Ribosomes
Organelles that help in the synthesis of proteins.
Ribosomes are made up of two parts, called subunits.
One unit is larger than than the other so they are called large and small subunits.
Both these subunits are necessary for protein synthesis in the cell.
When the two units are docked together with a special information unit called messenger RNA,
they make proteins.
Some ribosomes are found in the cytoplasm, but most are attached to the endoplasmic
reticulum.
While attached to the ER, ribosomes make proteins that the cell needs and exported from the cell
for work elsewhere in the body.

Ribosomes = Site of protein synthesis; function in the cytoplasm, but
are assembled in the nucleolus
a. Free = unbound in the cytoplasm;
produce proteins for use inside the
cell
b. Bound = attached to the
endoplasmic reticulum; produce
proteins for export and for the
plasma membrane

Ribosomes

Vesicles
This term literally means "small vessel".
Cells contain several types of vesicles, which perform various roles.
Helps to store and transport products produced by the cell.
The vesicles are the transport and delivery vehicles like our mail and trucks.
Some vesicles deliver materials to parts of the cell and others transport materials outside the
cell in a process called exocytosis

Con’t
• Cells must be able to move molecules, digest particles, and secrete materials in order to survive.
• For many cellular functions, vesicles are used.
• It is a small, spherical compartment that is separated from the cytosol by at least one lipid bilayer.

Lysosomes
The word Lysosome is a Greek meaning 'Digestive Body’
Lysosomes were discovered by the Belgian cytologist Christian de Duve.
 Lysosomes are cellular organelles that contain acid hydrolase enzymes to break down waste materials
and cellular debris.
They are found in animal cells.

Con’t
 They are vesicles that bud from the Golgi apparatus and that contain digestive enzymes.
Function as the cell's recycling compartment
Lysosomes receive cellular and endocytosed proteins and lipids that need digesting.
The metabolites that result are transported either by vesicles or directly across the membrane.
Sac of hydrolytic (digestive) enzymes
Function: Help break down food, unneeded substances, damaged cell components, and viruses.
Relation to factory: Recycling department
Found in animals.

Con’t
Lysosomes vary in size and shape in a cell.
Surrounding membrane confines these enzymes, preventing from
destroying the cell that houses them.
Extrinsic material to be attacked by lysosomal enzymes is taken
into the interior of the cell through the process of endocytosis.
When the fluid is internalized by endocytosis, the process is
called pinocytosis.
Endocytosis is also accomplished by phagocytosis.

Lysosomes

Peroxisomes
Peroxisomes are similar to lysosomes but contain different enzymes and are not produced by the Golgi
apparatus.
Peroxisomes are abundant in liver and kidney cells, where they neutralize free radicals and detoxify alcohol
and other drugs.
Peroxisomes also break down fatty acids, which the mitochondria can then use as an energy source.
Function: break down fatty acids and proteins. Catalase in the peroxisomes breaks down hydrogen peroxide.
They also break down alcohol.
Factory relation: Quality assurance
Found in both animal and plant cells.

Con’t
• Peroxisomes closely interact with
other organelles in the cells.
• The biomolecules are transported into
peroxisomes for specific chemical
reactions.
• The products are also exported to other
organelles for biological functions.

Other Vesicles
Specialized peroxisomes, called glyoxysomes, can be found in the seeds of some plants.
They break down stored fats to provide energy for the developing plant embryo.
Some cells engulf material by surrounding it with plasma membrane.
The resulting pocket buds off to become a vesicle inside the cell.
Food vacuoles are vesicles that store nutrients for a cell.
Contractile vacuoles are vesicles that can contract and dispose of excess water inside a cell.

Centrosome
The centrosome, also called the "microtubule organizing
center", is an area in the cell where microtubules are produced.
Centrosomes are made up two centrioles

Con’t
• Centrosomes are Microtubule Organizing Centers contain - tubulin
• The Microtubules grow in from this - tubulin

Con’t

Centrioles
Centrioles are barrel-shaped organelles found in the
cells of animals and most protists.
They occur in pairs, usually located at
right angles to each other near the nuclear membranes
Found only in animal cells.
Self-replicating
Made of bundles of microtubules.
Help in organizing cell division.

Cytoskeleton
The cytoskeleton is a network of thin tubes and
filaments that crisscrosses the cytosol.
The cytoskeleton also acts as a system of internal tracks
on which items move around inside the cell.

Cytoskeleton
Microtubules
Microtubules are hollow tubes made of a protein called tubulin.
Each tubulin molecule consists of two slightly different subunits.
Microtubules hold organelles in place, maintain a cell’s shape, and act as tracks
that guide organelles and molecules as they move within the cell.
Structure of Microtubules

Con’t
Vesicles can be transported along microtubules using motor
proteins that use ATP to generate force.
The vesicles are attached to motor proteins by connector
molecules, such as the dynactin complex shown here.
The motor protein dynein moves the connected vesicle along
microtubules.

Cytoskeleton
Microfilaments
Finer than microtubules
microfilaments are long threads of the beadlike protein actin and are linked end to end and
wrapped around each other like two strands of a rope.
Microfilaments contribute to cell movement, including the crawling of white blood cells
and the contraction of muscle cells.
Microfilaments

Cytoskeleton
Intermediate Filaments
Intermediate filaments are rods that anchor the nucleus and some other organelles to their
places in the cell.
They maintain the internal shape of the nucleus.
Hair-follicle cells produce large quantities of intermediate filament proteins.
These proteins make up most of the hair shaft.

Cilia and Flagella
External appendages from the cell membrane that aid in
locomotion of the cell.
Cilia also help to move substance past the membrane.
Eukaryotic cells have a flagellum, consisting of a circle of nine
microtubule pairs surrounding two central microtubules.
This arrangement is referred to as the 9 + 2 structure

Con’t
A eukaryotic flagellum originates directly from a basal body.
The flagellum has two microtubules in its core connected by
radial spokes to an outer ring of nine paired microtubules with
dynein arms (9 + 2 structure).
The basal body consists of nine microtubule triplets connected
by short protein segments.
The structure of cilia is similar to that of flagella, but cilia are
usually shorter.

Motion of Flagella
• A flagellum usually undulates, its snakelike
motion driving a cell in the same direction as
the axis of the flagellum.
• Propulsion of a human sperm cell is an
example of flagellate locomotion (LM)

Motion of Cilia
Cilia have a back-and-forth motion.
The rapid power stroke moves the cell in
a direction perpendicular to the axis of the cilium.
Then, during the slower recovery stroke,
The cilium bends and sweeps sideways, closer to
the cell surface.

Chloroplasts
Chloroplasts are useful organelles among plastids as they
highly participate in the process of photosynthesis
Chloroplasts are green because they contain the chlorophyll
pigment.
The word chloroplast is derived from the Greek words chloros
means green and plastis means "the one who forms".

Con’t
Structurally it is very similar to the mitochondrion.
It contains:
permeable outer membrane
less permeable inner membrane
intermembrane space, and
an inner section called the stroma.
However, the chloroplast is larger than the mitochondria.
 It needs to have the larger size because its membrane is not
folded into cristae.

Con’t
The cell organelle in which photosynthesis takes place.
In this organelle the light energy of the sun is converted into chemical energy.
Chloroplasts are found only in plant cells not animal cells.
The chemical energy that is produced by chloroplasts is finally used to make
carbohydrates like starch, that get stored in the plant.
Chloroplasts contain tiny pigments called chlorophylls.
Chlorophylls are responsible for trapping the light energy from the sun.

Con’t
In addition to the two bounding membranes, they contain internal membranes called
thylakoids which, in plants, form stacks called grana
The thylakoids contain the proteins and other molecules responsible for light capture.
The dark reactions of photosynthesis , on the other hand, takes place in the matrix, called
the stroma, which also contains the DNA and ribosomes.

Chloroplast

Vacuoles
Vacuoles are essentially larger Vesicles, and they are formed by
the joining together of many Vesicles.
Vacuoles and vesicles are storage organelles in cells.
They are membrane bound organelles that have no specific
shape and contain water with a number of different compounds
within it.
Their function varies greatly depending on the type of cell.
In plant cells they are important in maintaining Turgor Pressure.

Vacuoles
Vacuoles and vesicles are storage organelles in cells.
Vacuoles are larger than vesicles.
Either structure may store water, waste products, food, and other
cellular materials.
In plant cells, the vacuole may take up most of the cell's volume
The membrane surrounding the plant cell vacuole is called the
tonoplast.

Cell walls
The cells of plants, fungi, and many types of protists have cell walls, which protect
and support the cells.
The cell walls of these eukaryotes are chemically and structurally different from
prokaryotic cell walls.
In plants and protests, the cell walls are composed of fibers of the polysaccharide
cellulose, whereas in fungi, the cell walls are composed of chitin.

Plant cell wall
One of the most important distinguishing features of plant cells is the presence of a cell wall, in which serves
for variety of functions.
The cell wall
protects the cellular contents
gives rigidity to the plant structure
provides a porous medium for the circulation and
distribution of water, minerals, and other small nutrient molecules; and contains specialized molecules
that regulate growth and protect the plant from disease.
A structure of great tensile strength, the cell wall is formed from fibrils of cellulose molecules, embedded in
a water-saturated matrix of polysaccharides and structural glycoproteins

Plant Cells

Plant Cell Wall
Primary and Secondary Cell Walls
The main component of the cell wall, cellulose, is made directly on the surface of the plasma
membrane by enzymes that travel along the membrane.
Growth of the primary cell wall occurs in one direction, based on the orientation of the
microtubules.
Some plants also produce a secondary cell wall.
When the cell stops growing, it secretes the secondary cell wall between the plasma membrane and
the primary cell wall.
The secondary cell wall is very strong but can no longer expand.

Plant cell wall
Plant cell walls are thick, strong, and rigid.
Primary cell walls are laid down when the cell is young.
Thicker secondary cell walls may be added later when
the cell is fully grown.

Cell wall & Plasmodesmata
In addition to cell membranes, plants have cell walls.
Cell walls provide protection and support for plants.
Unlike cell membranes materials cannot get through cell walls.
This would be a problem for plant cells but they do have special openings called
plasmodesmata.
These openings are used to communicate and transport materials between plant cells and
the environment.

Plasmodesmata
 Plant cells can communicate through specialized
openings in their cell walls, called plasmodesmata
 which connect the cytoplasm of adjoining cells.

Bacterial cell wall
Prokaryotic cells have relatively simple organization.
Prokaryotic cells contain DNA and ribosomes, but they lack a nucleus, an internal membrane system, and
membrane-bounded organelles.
A rigid cell wall surrounds the plasma membrane
Bacterial cell walls consist of peptidoglycan.
Peptidoglycan is composed of carbohydrate cross-linked with short peptides.

Fungal cell wall
The fungal cell wall is an essential structure with great plasticity that is vital to maintaining cellular integrity
and viability.
The cell wall plays an important role in different biological functions such as controlling cellular permeability
and protecting the cell from osmotic and mechanical stress.
The cell wall is made up of:
1. Chitin (polymers of acetylated amino sugar N-acetyl-glucosamine)
2. Glucan (polymers of glucose)
3. Proteins (polymers of amino acids).
Glucan and chitin are components of the primary wall.
Proteins are components of the secondary wall.

Fungal cell wall function
Gives shape to fungi.
Gives strength to fungi.
Provides protection for the protoplasm from ultra-violet rays (presence of melanin's)
Ability to resist lysis by organic solvents such as enzymes, toxins, osmotic integrity.
Ability to bind with metal ions.
Secretes enzymes from their walls (invertase hydrolyses sucrose to glucose and fructose) and so assisting in
nutrition.

Extracellular Matrix
Although animal cells lack walls similar to those of plant cells, they do have an elaborate
extracellular matrix (ECM).
The main ingredients of the ECM are glycoproteins and other carbohydrate-containing molecules
secreted by the cells.
Glycoproteins are proteins covalently bonded with carbohydrates.
The most abundant glycoprotein in the ECM of most animal cells is collagen
Collagen forms strong fibers outside the cells
Collagen accounts for about 40% of the total protein in the human body.

Con’t

Con’t
The molecular composition and structure of the ECM vary from one cell type to another.
 In this example, three different types of ECM molecules are present:
Collagen
 Fibronectin and
Proteoglycans.
Integrins
proteoglycan
collagen
fibronectin
Plasma
Membrane
Integrin

Cell Junctions
 Cells in an animal or plant are organized into tissues, organs, and organ systems.
 Neighboring cells often adhere, interact, and communicate via sites of direct physical contact.
Plasmodesmata --------> in Plant Cells
Tight Junctions
Desmosomes in Animal Cells
Gap Junctions

Plasmodesmata
It might seem that the nonliving cell walls of plants would isolate plant cells from one another.
But in fact, as shown in many plant cell walls are perforated with plasmodesmata (singular, plasmodesma)
The word derived from the Greek desma, bond), channels that connect cells.
Plasma membranes of adjacent cells line the channel of each plasmodesma and thus are continuous.
Because the channels are filled with cytosol, the cells share the same internal chemical environment.
By joining adjacent cells, plasmodesmata unify most of the plant into one living continuum.
 The cytoplasm of one plant cell is continuous with the
cytoplasm of its neighbors via plasmodesmata.

Tight Junctions
• The plasma membranes of neighboring cells are very
tightly pressed against each other, bound together by
specific proteins.
• Forming continuous seals around the cells, tight junctions
establish a barrier that prevents leakage of extracellular
fluid across a layer of epithelial cells.
• For example, tight junctions between skin cells make us
watertight.

Desmosomes
• Desmosomes (one type of anchoring junction) function
like rivets, fastening cells together into strong sheets.
• Intermediate filaments made of sturdy keratin proteins
anchor desmosomes in the cytoplasm.
• Desmosomes attach muscle cells to each other in a muscle.
• Some “muscle tears” involve the rupture of desmosomes

Gap junctions
Gap junctions (also called communicating junctions) provide
cytoplasmic channels from one cell to an adjacent cell.
They are similar in their function to the plasmodesmata in
plants.
 Gap junctions consist of membrane proteins extending from the
membranes of the two cells.
These proteins create pores through which ions, sugars, amino
acids, and other small molecules may pass.
Gap junctions are necessary for communication between cells in
many types of tissues, such as heart muscle, and in animal
embryos.

Summery
Cell membranes are made of two phospholipid layers and proteins.
 The nucleus directs the cell’s activities and stores DNA.
 Mitochondria harvest energy from organic compounds and transfer it to ATP.
 Ribosomes are either free or attached to the rough ER and play a role in protein synthesis.
 The rough ER prepares proteins for export or insertion into the cell membrane. The smooth ER builds lipids
and participates in detoxification of toxins.
 The Golgi processes and packages proteins.
 Vesicles are classified by their contents.
 The cytoskeleton is made of protein fibers that help cells move and maintain their shape.

Summery
Plant cells have cell walls, central vacuoles, and plastids.
In plant cells, a rigid cell wall covers the cell membrane and provides support and protection.
Large central vacuoles store water, enzymes, and waste products and provide support for plant tissue.
Plastids store starch and pigments.
The chloroplast converts light energy into chemical energy by photosynthesis.
Prokaryotes, animal cells, and plant cells can be distinguished from each other by their unique features.

Cell Organelles

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