Cells and Cell Structure

All living things are made up of cells. Some organisms, such as bacteriaA microscopic organism. Bacteria lack a nucleus. They are found in very large numbers in almost all locations, including the human body. While most bacteria are harmless or necessary, some can cause disease and death., may exist solely as single-celled creatures. Others, including humans, are comprised of countless cells all working together to form a single living being. Humans are comprised of trillions of cells that are organized into tissues such as muscle and skin or organs like the liver or lung. The animation below depicts the relationship between an organ (the liver) and the cells from which it is constructed. The final image is a close-up view of a single cell.

 

The proper function of human bodies is dependent on smaller structures, or organs, such as the heart or lungs. The tiny cells that make up these organs actually contain within them smaller structures called organelles. These organelles help the cells to perform their jobs. In cancer, changes to these organelles can cause the individual cells and ultimately the entire organism to have serious problems. To get a better understanding of how cells work, we will now spend some time examining some of these subcellular structures.

The organelles that we will discuss are involved in the information flow within cells and in energy production. In addition, we will look at a structure that gives cells their shape and allows them to reproduce themselves. All of the organelles and processes to be discussed have direct relevance to cancer because these are the cellular structures/activities that become disturbed in the disease.

The image below shows two living mouse cells. The mitochondria Mitochondria are subcellular organelles responsible for extracting the bulk of the energy we use from the food we eat. As a byproduct, oxygen radicals (reactive chemicals) are produced that may contribute to cancer formation by damaging DNA. A mitochondrion (the singular version of the word) is filled with inner membranes upon which the last stages of energy generation take place. are colored red and the nuclei (with brightly stained chromosomes) are colored blue. The green colored region near the nuclei of the cells represents the golgi apparatusA series of sac-like, membrane enclosed areas of the cytosol. The golgi apparatus is responsible for processing proteins secreted from the endoplasmic reticulum into vesicles. Vesicles leaving the golgi can go to other organelles or may fuse with the plasma membrane, releasing the contents into the extracellular space around the cell., an organelleA subcellular collection of biomolecules that performs a specific function in the cell. For example, the nucleus acts like a brain for the cell, issuing commands and controlling cellular activities. Organelles are often enclosed by a lipid bilayer membrane. involved in the processing and packaging of molecules within the cell.

Stained Cells

The image above was used with the permission of the copyright owner, Molecular Probes.

Further information on the topics on this page can also be found in most introductory Biology textbooks, we recommend Campbell Biology, 11th edition.1

The organelles responsible for maintaining proper cellular function are described in the following sections:

Organelles Overview

The functions performed in the body are divided up and performed by different organs and tissues. Food is digested in the stomach and intestines, bones provide structure and strength, and the brain acts as a central location for processing information and issuing commands to other parts of the body.

In much the same way, the functions of individual cells are divided amongst well organized combinations of biomolecules. These structures are analogous to the organs in the body and are called organelles.

The organelles are suspended in a viscous water-based liquid. The fluid is known as the cytosolThe semi-liquid portion of the cell outside the nucleus, excluding the organelles. Compare to cytoplasm.. The fluid and the organelles that are located outside the nucleus are collectively called the cytoplasmThe portion of a cell that is located outside the nucleus. This includes many organelles such as ribosomes and mitochondria. The cytoplasm also contains many fibers of the cytoskeleton..  The cytoplasm of a cell is highly organized and the positions of the organelles actively controlled.

View the image below to get a brief description of the functions of some of the organelles present in a typical cell.

cellrollover.png

Nucleus

 

The nucleus can be thought of as the brains of a cell. Our genetic material (DNAAbbreviation for deoxyribonucleic acid. Composed of very long strings of nucleotides, which are abbreviated as A, C, G and T. DNA is the storage form of our genetic material. All of the instructions for the production of proteins are encoded in our DNA. ), in the form of chromosomes, is stored in this organelle. The nucleus (plural-nuclei) is roughly spherical and is surrounded by two membranes. As stated in the previous chapter, cellular membranes are made up of two sheets of lipids facing one another.

 

As shown above, the nucleus is home to chromosomes. Chromosomes are composed of long strings of DNA. As shown in the animation below, the DNA in a chromosomeA long DNA molecule containing genetic information (genes). Humans have 46 chromosomes. One set of 23 is inherited from each parent. A full set of chromosomes is present in the nucleus of each human cell. is highly organized and looped. The X shaped chromosome shown in the two animations on this page actually represents a chromosome that has been copied or replicated in preparation for cell division. An unreplicated chromosome consists of a single DNA molecule which can contain thousands of genes. The DNA in chromosomes acts as a kind of blueprint to guide all of the other activities in the cell.

Some key features of our  genetic make-up:

  • We have two sets of chromosomes; one contributed by each parent via the gameteA mature reproductive cell: sperm and egg. Gametes contain only half the genetic information of a typical cell. When two cells of this type fuse via the process of fertilization, the offspring has the full complement of genetic information. In the case of humans, each gamete contains 23 chromosomes and the embryo has 46 chromsomes. (sperm or egg). Human cells normally contain 46 chromosomes, 23 from each parent.
  • Chromosomes are made up of a complex between DNA and proteins. This complex is called chromatinA combination of DNA and proteins. Our chromosomes are composed of DNA that is twisted around proteins much like thread on a spool. .
  • Genes are sections of DNA that contain the information for the production of a specific molecule such as a proteinOne of the four basic types of biomolecule. Proteins are polymers made up of strings of amino acids. Proteins serve many functions in organisms including transport of molecules, structure, cell adhesion and as signaling molecules such as hormones. Many transcription factors, including p53 and Rb are proteins.. Important in the development of cancer, small changes in the nucleotideA monomer building block of the nucleic acids DNA and RNA. There are approximately 3 billion nucleotides in the entire human genome. Each chromosome contains about 50-250 million nucleotides. Four nucleotides are used to form DNA abbreviated as A, C, G and T. RNA contains four nucleotides as well, but uses U instead of T, so RNA contains A, C, G and U. Some chemotherapy agents, like 5-Fluorouracil ( 5-FU) and ARA-C are chemically very similar to one of the nucleotides and work by interfering with DNA function. sequence of a geneA stretch of DNA that leads to the production of an RNA. The RNA is produced during the process of transcription. This RNA can be used to guide the formation of a protein via translation or can be used directly in the cell. may result in the altered behavior of a cell.

Changes to the genetic material are essential for the development of cancer.

Mitochondria

Mitochondria (singular- mitochondrion) are the energy powerhouses of cells. Much of the energy that cells (and therefore individuals) require to function is harvested from biomolecules such as sugars and fats obtained from food. Mitochondria carry out the final steps of converting the food to energy. Like the nucleus, mitochondria are surrounded by a double membrane.

Like the burning of gasoline in an automobile engine, the energy production process is not completely efficient and produces by-products that often have undesirable effects. Energy production in mitochondria leads to the production of chemicals that may damage DNA and therefore cause genetic changes. These dangerous side products are thought to contribute to the mutations seen in cancer cells.

A diagram of a mitochondrion, showing the two separate membranes and the inner compartment that is the site of energy production is shown below.

Mitochondria Diagram

 

In the image below, the mitochondria in mouse cells have been stained red. The nuclei and chromosomes of the cells are blue. Note the wide distribution, large number and the somewhat irregular shapes of the mitochondria. The green regions near the nucleus in each cell are organelles known as the golgi apparatus, they are involved in the modification and shipment of biomolecules such as proteins.

 

Stained Cells

 

The image above was used with the permission of the copyright owner, Molecular Probes.

Ribosome

Ribosomes are made up of two large complexes comprised of RNAAlso: ribonucleic acid. RNA is a polymer comprised of the nucleotides A, C, G and U. RNA is the working form of our genetic information. RNA is produced via the process of transcription. Some RNA is used to help build ribosomes (rRNA) and some (mRNA) are used to guide the formation of proteins. Other forms of RNA are used to perform specialized functions in the nucleus. and protein. Ribosomes are located in the cytosol and are quite numerous. They are responsible for reading RNA and using the encoded information to produce proteins in a process called translationThe formation of protein by a ribosome reading a messenger RNA (mRNA). The RNA is produced in the nucleus via the process of transcription and shipped to the cytoplasm via the nuclear pores. The RNA is bound by ribosomes and the catalytic activity of the ribosomes allows the reading of the RNA and the formation of a protein based on the information encoded in the mRNA.. Translation is discussed in more detail in the Gene Function section.

The diagram below shows the two ribosomal subunits (large and small) bound to a messenger RNA (mRNAMessenger RNA, an RNA molecule is a copy of a particular gene that is used in the production of a protein. Messenger RNA is produced in the nucleus via the process of transcription and is exported through holes or pores in the nuclear envelope into the cytoplasm. mRNA then attaches to a ribosome where the encoded message is read to produce a protein in the process termed translation.).

Cytoskeleton

The cytoskeleton is an intricate network of proteins that criss-cross the cytoplasm of cells. The cytoskeleton is composed of a wide variety of proteins. These proteins often form long twisted strands that look like electrical wire or the cables used to hold up bridges. Like these man-made components, the proteins that make up the cytoskeleton are both strong and flexible.

A main fiber type, actin, is made up of long strings (polymers) of the protein actin. The image below shows the actin fibers in a cow endothelial (blood vessel) cell. The yellow colored strings are the polymerized form of the protein and the red color indicates the presence of the single protein units.

 

actin fibers

 

Another critical cytoskeletal fiber is the microtubules. They are also polymers, and are comprised of the protein tubulin. The image below shows the microtubules in a cow endothelial cell.

 

microtubules

 

As can be seen from the images above, the cytoskeleton is distributed extensively throughout cells.

The images on this page were used with the permission of the copyright owner, Molecular Probes..

Cytoskeleton Function

The image below shows both the actin fibers (in red) and microtubules (in yellow) in cow endothelial cells. The nuclei of the cells have been stained blue.

 

actin fibers and microtubules

The cytoskeleton serves several key functions:

  • It supplies structure to cells and acts as a scaffolding for the attachment of many organelles.
  • It is responsible for the ability of cells to move.
  • It is required for the proper division of cells during cellular reproduction.

As we will see, changes in the cytoskeleton are observed in cancer cells. Cancer cells often show increased movement. In fact, metastaticThe term for a cancer that has spread beyond its point of origin. Metastatic disease is responsible for the majority of cancer deaths. spread of cancer is dependent on tumor cells that invade neighboring tissues.

The essential role of the cytoskeleton in the proliferationRefers to cell division. The proliferation rate is an indicator of how quickly a tumor is growing. The proliferation rate may be represented as a percentage, showing what fraction of the cells are actively involved in the division process. of cells has led to the use of drugs that inhibit the cytoskeleton as anti-cancer drugs. Examples of drugs that interfere with cytoskeletal function include Taxol® and vinblastine.

More on cytoskeletal inhibitors in cancer treatment.

The image on this page was used with the permission of the copyright owner, Molecular Probes.

 

Cell Structure Summary Sheet

Organelles

  • Organelles are structures that carry out different functions within a cell.
  • Organelles in a cell are analogous to the organs in a body.
  • Organelles are suspended in a water-based fluid called cytosol.

The Nucleus

  • The nucleus stores the genetic information (chromosomes) of eukaryotic cells.
  • The nucleus is roughly spherical and is surrounded by two membranes.
  • The nucleus is the 'brain' of a cell.

The Mitochondrion

  • Mitochondria are the power houses of a cell.
  • Mitochondria convert biomolecules (i.e. fats and sugars) into energy.
  • By-products of energy production in mitochondria may damage DNA and cause mutations.

The Ribosome

  • Ribosomes are made up of two large complexes comprised of RNA and protein.
  • Ribosomes are located in the cytosol. Their function is to read RNA and produce proteins in a process known as translation.

The Cytoskeleton

  • The cytoskeleton is an intricate network of proteins that criss-cross the cytoplasm of a cell.
  • Actin and tubulin are the proteins used to build main fibers of the cytoskeleton (microfilaments and microtubules, respectively).
  • The cytoskeleton serves several key functions:
    • Provides structure to cells and a place to anchor organelles
    • Cell motility
    • Control of cell division during mitosisThe portion of the cell cycle (M stage) during which a cell divides into two (usually identical) daughter cells.
  • Changes in the cytoskeleton that allow increased movement are observed in cancer cells.
  • Many anti-cancer drugs work by interfering with the activity of cytoskeletal proteins.

Know the Flow 

Know the Flow is an educational game for you to test your knowledge. To play:

  • Drag the appropriate choices from the column on the right and place them in order from smallest to largest in the boxes on the left. Note that you will only use five of the six choices to complete the game.
  • When done, click on 'Check' to see how many you got correct.
  • For incorrect answers, click on 'Description' to review information about the processes.
  • To try again, choose 'Reset' and start over.

Know the Flow: Cell Structure
Processes in order
  • 1
  • 2
  • 3
  • 4
  • 5
Processes
  • Organ
  • Organelle
  • Cell
  • Body System
  • Tissue
  • Skeleton

If you find the material useful, please consider linking to our website

  • 1. Further information and the topics on this page can also be found in most introductory Biology textbooks, we recommend Campbell Biology, 11th edition.