32 4.6 Cell Organelles

Created by: CK-12/Adapted by Christine Miller

Image shows a large 3D work of art displayed at the Cold Spring Harbor Laboratory. It is a representation of ribosomes attached to a ribbon of metal meant to represent a strand of messenger RNA.
Figure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.  This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.

Ribosome Review

The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.  This artwork brings to life an important structure in living cells: the , the cell structure where  are synthesized. The slender silver strand is the messenger (mRNA) bringing the code for a out into the cytoplasm.  The purple and green structures are ribosomal subunits (which together form a single ), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.  All living  — whether they are or — contain , but only eukaryotic cells also contain a  and several other types of .

What Are Organelles?

An  is a structure within the of a that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the , , and .  are not enclosed within a membrane, but they are still commonly referred to as organelles in cells.

The Nucleus

The  is the largest organelle in a cell, and it’s considered the cell’s control center. It contains most of the cell’s (which makes up chromosomes), and it is encoded with the genetic instructions for making . The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called , which is similar in composition to the found in the cytoplasm outside the nucleus. Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red blood cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.

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.
Figure 4.6.2 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 in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the . This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called  allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and  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 in the model below is mainly involved in the assembly of . After being produced in the , ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.

Mitochondria

The mitochondrion (plural, ) is an organelle that makes  available to the cell. This is why mitochondria are sometimes referred to as the “power plants of the cell.” They use energy from organic compounds (such as ) to make molecules of  (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy.

Image shows a diagram of a mitochondrion. Labelled are the inner and outer membranes, the intermembrane space, the matrix, DNA and ribosomes.
Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!

Mitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.  In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.  Does this sound similar to the requirements to be considered a cell?  That’s because they are!

Scientists think that mitochondria were once free-living organisms because they contain their own . They theorize that ancient prokaryotes infected (or were engulfed by) larger cells, and the two organisms evolved a relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as  inside them. This theory is called , and it is widely accepted by biologists today. (See the video in section 4.3 to learn all about endosymbiotic theory.)

Endoplasmic Reticulum

The  (ER) is an organelle that helps make and transport and . There are two types of endoplasmic reticulum: (rER) and (sER). Both types are shown in Figure 4.6.4.

  • rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.
  • sER looks smooth because it does not have ribosomes. sER makes lipids, stores substances, and plays other roles.
Image shows a diagram of the organelles included in the endomembrane system, inclduing: nuclear envelope, rough ER, smooth ER, golgi body, cell membrane, and vesicles.
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.

The Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.

Golgi Apparatus

The  (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.

Vesicles and Vacuoles

Both  and  are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport and molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.

There are some vesicles which are specialized to carry out specific functions.  Lysosomes, which use enzymes to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.  Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins. 

Centrioles

Image shows a diagram of a centriole, made up of microtubules. There are nine bundles of microtubules arranged in a circle to form the tube-shaped centriole.
Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.

 are organelles involved in . The function of centrioles is to help organize the  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 in Figure 4.6.5.

Image shows a diagram of a ribosome. It is made up of two sub-units, a smaller sub-unit shown in blue and a larger sub-unit shown in red.
Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.

Ribosomes

Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in  Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.

4.6 Summary

  • An is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job. Although  are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.
  • The is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls gene expression, including controlling which proteins the cell makes.
  • The mitochondrion (plural, ) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted , mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
  • The (ER) is an organelle that helps make and transport proteins and lipids. (rER) is studded with ribosomes. (sER) has no ribosomes.
  • The is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
  • Both  and  are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.
  • are organelles located near the that help organize the  before  so each daughter cell receives the correct number of chromosomes.
  •  are small structures where proteins are made. They are found in both  and cells. They may be found alone or in groups within the or on the rER.

4.6 Review Questions

  1. What is an organelle?
  2. Describe the structure and function of the nucleus.
  3. Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.
  4. Why are mitochondria referred to as the “power plants of the cell”?
  5. What roles are played by vesicles and vacuoles?
  6. Why do all cells need ribosomes — even prokaryotic cells that lack a nucleus and other cell organelles?
  7. Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?

4.6 Explore More

Biology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.

David Bolinsky: Visualizing the wonder of a living cell, TED, 2007.

Attributes

Figure 4.6.1 

Ribosomes at Work by Pedrik on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

Figure 4.6.2

Nucleus by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 4.6.3 

Mitochondrion_structure.svg by Kelvinsong; modified by Sowlos on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 4.6.4

Endomembrane_system_diagram_en.svg by Mariana Ruiz [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.6.5

Centrioles by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 4.6.6

Ribosome_shape by Vossman on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

References

Blausen.com staff. (2014). Nucleus – Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014

Blausen.com staff (2014). Centrioles – Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014

Nucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https://www.youtube.com/watch?v=URUJD5NEXC8&feature=youtu.be

TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https://www.youtube.com/watch?v=Id2rZS59xSE&feature=youtu.be

 

 

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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