This document summarizes the structures and functions of organelles in eukaryotic cells. It describes how the cell is made up of numerous membrane-bound organelles that carry out specialized functions like protein production, energy generation, waste disposal, and more. Organelles include the nucleus, which houses genetic material and controls the cell, mitochondria and chloroplasts that generate energy and food, and the endoplasmic reticulum, Golgi apparatus, vacuoles, and cytoskeleton that transport materials and provide structure.

2. Introduction
All living organisms are composed of one
or many cells. The cell is the building
block of life and is able to control and
execute several functions in all types of
organisms. Cell structures can only be
observed under a high magnification
electron microscope and are separated
internally into numerous membranous
compartments called organelles (little
organs).

3. Organelles perform a variety of functions
like the production of proteins, storage of
important materials, harvesting of energy,
repairing of cell parts, digestion of
substances, and maintenance of the
shape and structure of the cell. Since the
organelles are compartmentalized, one
system does not interfere with the
functions of the other systems, but their
roles are interdependent.
Organelles

4. (Illustration of the cell membrane showing the peripheral and
embedded proteins and carbohydrate molecule in the lipid bilayer.)

5. Within the cell, there exist a jelly-like
substance which is mainly composed of
water and dissolved substances, such as
salt and proteins, called cytoplasm. The
cytoplasm is responsible for the fluid
nature of the cells’ internal environment. It
is also responsible for the dynamic
suspension of organelles. It is the
substance that fills the cells, excluding the
nucleus.
Cytoplasm

6. The nucleus, which is the control center of
the cell, is enclosed in a double
membrane that selectively permits certain
materials or compounds (e.g., mRNA) to
exit. The double membrane is separated
by approximately 50 nm. The outer
membrane is continuous with the
endoplasmic reticulum.
During the interphase and prior to cell
division, the DNA is loosely coiled inside
the nucleus and is called chromatin.
Nucleus

7. Within the nucleus, a suborganelle known
as nucleolus is present. It is where the
subunits of the ribosome assemble and
where ribonucleic acid (RNA) are
synthesized and mature for the release to
the cytoplasm. This is also where protein
synthesis occurs. Around 10,000
ribosomes per minute are needed by the
cell, meaning the nucleolus would need to
produce huge amounts of ribosomal
subunits to sustain protein synthesis in the
cell.
Nucleus

8. The endoplasmic reticulum (ER) is a
membranous system of interconnected
tubules that serves as the transport
system of the cell. It is composed of two
types based on the presence or absence
of attached ribosomes: the smooth ER
and the rough ER.
Endoplasmic Reticulum (ER)

9. The rough ER appears grainy in structure,
it is responsible for the production of
secretory proteins such as glycoproteins.
The smooth ER lacks ribosomes on its
outer surface. Various metabolic
processes such as metabolism of
carbohydrates, lipid synthesis, and
detoxification of drugs, chemicals and
poisons are performed in this organelle.
Endoplasmic Reticulum (ER)

10. Golgi bodies (or Golgi apparatus/Golgi
complex) consist of stacks or piles of
flattened sacs called cisternae which are
responsible for the packaging, sorting,
and refining of the products that the cell
are making. In short, the products of the
ER will be modified by the Golgi
apparatus and then transported to other
destinations in the cell.
Golgi Bodies

11. The vacuole is one organelle that is larger
than in animal cells. The vacuole has the
ability to enlarge as plant cells absorb
water. The central vacuole or tonoplast
serves as the main compartment for
storing essential organic and inorganic
compounds. It is also a disposal site for
the substances that are not needed by the
cell or that could harm the cell.
Vacuole

12. Eukaryotic cells can have one or many
mitochondria and/or chloroplasts
depending on the function, activity, and
nature of the cell. The mitochondrion is
the powerhouse of the cell where cellular
respiration takes place. Adenosine
triphosphate (ATP) is the usable form of
energy for the cell to perform its functions.
The mitochondrion has outer and inner
membranes. The outer is smooth,
whereas the cristae (inner membrane) has
infoldings, providing a large surface area
for metabolic activities.
Eukaryotic Cells

13. The chloroplast is only found in plant cells
and other photosynthetic eukaryotic
organisms. It is the site for food
production. The chloroplast contains the
green pigment chlorophyll which functions
in glucose production. The chloroplast’s
internal structure is composed of
flattened sacs called thylakoids and each
stack is called granum (pl. grana).
Stroma is the fluid outside the thylakoid
where enzymes, the chloroplast’s DNA,
and ribosomes are found.
Chloroplast

14. The cytoskeleton is composed of
microfilaments, microtubules and
intermediate filaments.
• Microfilaments are made up of action
molecules that form fine fibers of
approximately 7-8 nm diameter.
• Microtubules are composed of
tubulins that form a hollow cylinder 25
nm in diameter.
• Intermediate filaments are around 8-
10 nm, making it “intermediate” in size.
Cytoskeleton

15. Some cells also possess cilia and flagella
which are cellular projections that are
used for the movement of a cell over a
surface. Flagellar range from 20-100 µm
in length and are fewer in number on the
cell, whereas cilia range from 2-20 µm in
length and are numerous and found all
over the cell surface. Flagella and cilia are
covered by a plasma membrane and are
connected to the cell by an array of
microtubules arranged as axonemes.
Axonemes are composed of nine pairs or
doublet of microtubules arranged in
cylindrical form.
Cilia and Flagella

16. Fine fibers connect the doublet and each
doublet contains dynein arms (a protein
that appears as the two short arms of the
doublet) . Flagella and cilia movements
are caused by the sliding of adjacent
microtubule doublets. The dynein arm
initially attaches to the adjacent doublet,
changes its conformation, and then
dissociates.
Some cells develop specialized structures
such as root hair and/or microvilli, while
other cells change their shape to fit their
function. These are present in animal
cells, plant cells, fungal cells, and even
bacterial cells.
Cilia and Flagella