3.

9: EUKARYOTE ORGANELLES

 RIBOSOME REVIEW
Figure 3.9.1 represents an important structure in living cells. It
is a component of a ribosome, the cell structure where proteins
are synthesized. Large ribosomal subunit (50S) of Haloarcula
marismortui, facing the 30S subunit. The ribosomal proteins are
shown in blue, the rRNA in ochre (a shade of brown and
yellow), the active site in red. All living cells contain
ribosomes, whether they are prokaryotic or eukaryotic cells.
However, only eukaryotic cells also contain a nucleus and
several other types of organelles.

Figure 3.9.2: This closeup of a cell nucleus shows that it is

surrounded by a structure called the nuclear envelope, which
contains tiny perforations, or pores. The nucleus also contains a
dense center called the nucleolus.

As you can see from the model in Figure 3.9.2 , the membrane
enclosing the nucleus is called the nuclear envelope. This is
actually a double membrane that encloses the entire organelle and
isolates its contents from the cellular cytoplasm. Tiny holes, called
nuclear pores, allow large molecules to pass through the nuclear
envelope with the help of special proteins. Large proteins and RNA
molecules must be able to pass through the nuclear envelope so
proteins can be synthesized in the cytoplasm and the genetic
material can be maintained inside the nucleus. The nucleolus shown
Figure 3.9.1: Ribosomal subunit
in the model below is mainly involved in the assembly of ribosomes.
An organelle is a structure within the cytoplasm of a eukaryotic cell After being produced in the nucleolus, ribosomes are exported to the
that is enclosed within a membrane and performs a specific job. cytoplasm where they are involved in the synthesis of proteins.
Organelles are involved in many vital cell functions. Organelles in
MITOCHONDRIA
animal cells include the nucleus, mitochondria, endoplasmic
The mitochondrion (plural, mitochondria) is an organelle that
reticulum, Golgi apparatus, vesicles, and vacuoles. Ribosomes are
not enclosed within a membrane but are still commonly referred to makes energy available to the cell (Figure 3.9.3 ). This is why
mitochondria are sometimes referred to as the power plants of the
as organelles in eukaryotic cells.
cell. They use energy from organic compounds such as glucose to
THE NUCLEUS make molecules of ATP (adenosine triphosphate), an energy-
The nucleus is the largest organelle in a eukaryotic cell and is carrying molecule that is used almost universally inside cells for
considered to be the cell’s control center. It contains most of the energy.
cell’s DNA, which makes up chromosomes and is encoded with the Scientists think that mitochondria were once free-living organisms
genetic instructions for making proteins. The function of the nucleus because they contain their own DNA. They theorize that ancient
is to regulate gene expression, including controlling which proteins prokaryotes infected (or were engulfed by) larger prokaryotic cells,
the cell makes. In addition to DNA, the nucleus contains a thick and the two organisms evolved a symbiotic relationship that
liquid called nucleoplasm that is similar in composition to the benefited both of them. The larger cells provided the smaller
cytosol found in the cytoplasm outside the nucleus (Figure 3.9.2 ). prokaryotes with a place to live. In return, the larger cells got extra
Most eukaryotic cells contain just a single nucleus, but some types energy from the smaller prokaryotes. Eventually, the smaller
of cells, such as red blood cells, contain no nucleus. A few other prokaryotes became permanent guests of the larger cells, as
types of cells, such as muscle cells, contain multiple nuclei. organelles inside them. This theory is called the endosymbiotic
theory, and it is widely accepted by biologists today

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 different functions, some of which include lipid synthesis,
calcium ion storage, and drug detoxification. The smooth
endoplasmic reticulum is found in both animal and plant cells
and it serves different functions in each. The SER is made up of
tubules and vesicles that branch out to form a network. In some
cells, there are dilated areas like the sacs of RER. Smooth
endoplasmic reticulum and RER form an interconnected
network.

Figure 3.9.3: Mitochondria, organelles specialized to carry out

aerobic respiration, contain an inner membrane folded into cristae,
which form two separate compartments: the inner membrane space
and the matrix. The Krebs Cycle takes place in the matrix. The
electron transport chain is embedded in the inner membrane and
uses both compartments to make ATP by chemiosmosis.
Mitochondria have their own DNA and ribosomes, resembling those
of prokaryotic organisms.

MITOCHONDRIAL COMPARTMENTS
The double membrane nature of the mitochondria results in five
distinct compartments, each with an important role in cellular
respiration. These compartments are:
1. the outer mitochondrial membrane, Figure 3.9.4: The ER is a winding network of thin membranous sacs
found in close association with the cell nucleus. The smooth and
2. the intermembrane space (the space between the outer and inner
rough endoplasmic reticula are very different in appearance and
membranes), function (source: mouse tissue). (b) Rough ER is studded with
3. the inner mitochondrial membrane, numerous ribosomes, which are sites of protein synthesis (source:
mouse tissue). EM × 110,000. (c) Smooth ER synthesizes
4. the cristae (formed by infoldings of the inner membrane), and
phospholipids, steroid hormones, regulates the concentration of
5. the matrix (space within the inner membrane). cellular Ca2+, metabolizes some carbohydrates, and breaks down
certain toxins.
ENDOPLASMIC RETICULUM
The endoplasmic reticulum (ER) (plural, reticuli) is a network of
GOLGI APPARATUS
phospholipid membranes that form hollow tubes, flattened sheets, The Golgi apparatus (Figure 3.9.5 ) is a large organelle that
and round sacs. These flattened, hollow folds and sacs are called processes proteins and prepares them for use both inside and outside
cisternae. The ER has two major functions: the cell. It was identified in 1898 by the Italian physician Camillo
Golgi. The Golgi apparatus modifies, sorts, and packages different
Transport: Molecules, such as proteins, can move from place to
substances for secretion out of the cell, or for use within the cell.
place inside the ER, much like on an intracellular highway.
The Golgi apparatus is found close to the nucleus of the cell where it
Synthesis: Ribosomes that are attached to the ER, similar to
modifies proteins that have been delivered in transport vesicles from
unattached ribosomes, make proteins. Lipids are also produced in
the Rough Endoplasmic Reticulum. It is also involved in the
the ER.
transport of lipids around the cell. Pieces of the Golgi membrane
There are two types of endoplasmic reticulum, rough endoplasmic pinch off to form vesicles that transport molecules around the cell.
reticulum (RER) and smooth endoplasmic reticulum (SER): The Golgi apparatus can be thought of as similar to a post office; it
Rough endoplasmic reticulum is studded with ribosomes, packages and labels "items" and then sends them to different parts of
which gives it a “rough” appearance. These ribosomes make the cell. The Golgi apparatus tends to be larger and more numerous
proteins that are then transported from the ER in small sacs in cells that synthesize and secrete large quantities of materials; for
called transport vesicles. The transport vesicles pinch off the example, the plasma B cells and the antibody-secreting cells of the
ends of the ER. The rough endoplasmic reticulum works with the immune system have prominent Golgi complexes.
Golgi apparatus to move new proteins to their proper The Golgi apparatus manipulates products from the Rough
destinations in the cell. The membrane of the RER is continuous Endoplasmic Reticulum (ER) and also produces new organelles
with the outer layer of the nuclear envelope. called lysosomes. Proteins and other products of the ER are sent to
Smooth endoplasmic reticulum does not have any ribosomes the Golgi apparatus, which organizes, modifies, packages, and tags
attached to it, and so it has a smooth appearance. SER has many them. Some of these products are transported to other areas of the

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cell and some are exported from the cell through exocytosis. type of transport vesicle in the figure above. Some vesicles are used
Enzymatic proteins are packaged as new lysosomes. as chambers for biochemical reactions. Other vesicles include:
Lysosomes, which use enzymes to break down foreign matter
and dead cells.
Peroxisomes, which use oxygen to break down poisons.
Transport vesicles, transport contents between organelle as well
as between cell exterior and interior.

CENTRIOLES
Centrioles are organelles involved in cell division. The function of
centrioles is to help organize the chromosomes before cell division
occurs so that each daughter cell has the correct number of
chromosomes after the cell divides. Centrioles are found only in
animal cells and are located near the nucleus. Each centriole is made
mainly of a protein named tubulin. The centriole is cylindrical in
shape and consists of many microtubules, as shown in the model
pictured below.

Figure 3.9.5: The rough ER is continuous with the nuclear envelope

and has ribosomes on it's surface. The ribosomes produce proteins
such as the one shown which remains bound to the membrane of the
rough ER. The membrane of the rough ER pinches off to form a
transport vesicle containing the protein. The vesicle fuses with the
cis face of the Golgi apparatus. The protein is now found on the
membrane of the Golgi apparatus and travels along the cisternae. Figure 3.9.6: Centrioles are tiny cylinders near the nucleus, enlarged
Once it reaches the trans face of the Golgi apparatus, it gets here to show their tubular structure.
packaged into a secretory vesicle that sends the protein to the plasma
membrane. RIBOSOMES
The stack of cisternae has four functional regions: the cis-Golgi Ribosomes are small structures where proteins are made. Although
network, medial-Golgi, endo-Golgi, and trans-Golgi network. they are not enclosed within a membrane, they are frequently
Vesicles from the ER fuse with the network and subsequently considered organelles. Each ribosome is formed of two subunits, like
progress through the stack from the cis- to the trans-Golgi network, the one pictured at the top of this section. Both subunits consist of
where they are packaged and sent to their destination. Each cisterna proteins and RNA. RNA from the nucleus carries the genetic code,
includes special Golgi enzymes which modify or help to modify copied from DNA, which remains in the nucleus. At the ribosome,
proteins that travel through it. Proteins may be modified by the the genetic code in RNA is used to assemble and join together amino
addition of a carbohydrate group (glycosylation) or phosphate group acids to make proteins. Ribosomes can be found alone or in groups
(phosphorylation). These modifications may form a signal sequence within the cytoplasm as well as on the RER.
on the protein, which determines the final destination of the protein.
For example, the addition of mannose-6-phosphate signals the REVIEW
protein for lysosomes. 1. Define organelle.
2. Describe the structure and function of the nucleus.
VESICLES AND VACUOLES 3. Explain how the nucleus, ribosomes, rough endoplasmic
Both vesicles and vacuoles are sac-like organelles that store and reticulum, and Golgi apparatus work together to make and
transport materials in the cell. Vesicles are much smaller than transport proteins.
vacuoles and have a variety of functions. The vesicles that pinch off 4. Why are mitochondria referred to as the power plants of the cell?
from the membranes of the ER and Golgi apparatus store and 5. What roles are played by vesicles and vacuoles?
transport protein and lipid molecules. You can see an example of this 6. Why do all cells need ribosomes, even prokaryotic cells that lack
a nucleus and other cell organelles?

3.9.3 https://bio.libretexts.org/@go/page/92022
 7. Explain endosymbiotic theory as it relates to mitochondria. What ATTRIBUTIONS
is one piece of evidence that supports this theory? 1. 50S-subunit of the ribosome by Yikrazuul, licensed CC BY-SA
8. Lysosomes and peroxisomes are types of: 3.0 via Wikimedia Commons
A. A. Organelles 2. Cell nucleus by Blausen.com staff (2014). "Medical gallery of
B. B. Vesicles Blausen Medical 2014". WikiJournal of Medicine 1 (2).
C. C. Vacuoles DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY
D. D. Both A and B 3.0 via Wikimedia Commons
9. Which of the following organelles fits best with each description 3. Animal mitochondrion by LadyofHats, released into the public
of function? Choose only one organelle for each answer: Golgi domain via Wikimedia Commons
apparatus, centrioles, nucleolus, nucleus, rough endoplasmic 4. Endoplasmic reticulum by OpenStax, licensed CC BY 4.0 via
reticulum Wikimedia Commons
A. a. Contains the genetic instructions for the production of 5. Golgi Apparatus by Openstax, licensed CC BY 4.0 via
proteins Wikimedia Commons
B. b. Organizes chromosomes before cell division 6. Centrioles by Blausen.com staff (2014). "Medical gallery of
C. c. Provides a framework for ribosomes Blausen Medical 2014". WikiJournal of Medicine 1 (2).
D. d. Packages and labels proteins DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY
E. e. Assembles ribosomes 3.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-
10. True or False. All eukaryotic cells have a nucleus.
NC 3.0
11. True or False. The outer surface of the nucleus of a eukaryotic
cell is not completely solid. This page titled 3.9: Eukaryote Organelles is shared under a CK-12 license
and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep
EXPLORE MORE Grewal via source content that was edited to the style and standards of the
https://bio.libretexts.org/link?16744#Explore_More
LibreTexts platform.
5.6: Cell Organelles by Suzanne Wakim & Mandeep Grewal is licensed
CK-12. Original source: https://www.ck12.org/book/ck-12-human-
biology/.

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