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Prokaryotic and Eukaryotic Cells

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Prokaryotic and Eukaryotic Cells

Are bacteria cells like our cells?

Yes and no. Bacteria cells are similar to our cells in some ways. Like our cells, bacteria cells have DNA and a plasma membrane. But bacteria are simpler and unique in their own ways. The term prokaryote comes from the Greek "pro" meaning "before" or "first", and "karyon" meaning "nut" or "kernal".  These are the first cells to appear in the fossil record - over three and half billion years ago.  It took another two billion years for complicated cells like our own to evolve.  Let's take a look at the two types of cells that all living things on Earth are made of.

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Fossilized stromatolites from Shark Bay, Australia. These are some of the oldest fossils found on Earth - dating back 3.6 billion years. The layers are formed from successive layers of ancient prokaryotic bacteria. [Figure1]

Prokaryotic and Eukaryotic

There are two basic types of cells, prokaryotic cells and eukaryotic cells . The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus . The nucleus is where cells store their DNA , which is the genetic material. The nucleus is surrounded by a membrane.  The nuclear-membrane is very similar to the cell membrane - evidence that this membrane evolved from a modified cell membrane sometime between 3.6 billion and 1.5 billion years ago.   Organisms with eukaryotic cells are called   eukaryotes . Animals, plants, fungi, and protists are eukaryotes. All multi-cellular organisms are eukaryotes. Eukaryotes may also be single-celled.

Prokaryotic cells do not have a nucleus. Instead, their DNA floats around inside the cell. Organisms with prokaryotic cells are called prokaryotes . All prokaryotes are single-celled organisms. Bacteria and Archaea are the two groups of prokaryotes.  Archaea were orginally classified as bacteria or "archaeabacteria", but recent evidence has led scientists to give them their own distinct grouping: genes more closely related to eukaryotes, distinct biochemical processes, etc.  Archaea are common extremophiles in hotsprings, in the oceans, and in a wide range of terrestrial habitats.

Both prokaryotic and eukaryotic cells have structures in common. All cells have a plasma membrane, ribosomes, cytoplasm, and DNA. The plasma membrane , or cell membrane , is the phospholipid layer that surrounds the cell and protects it from the outside environment.  Ribosomes are found in all cells, both prokaryote and eukaryote, and are relatively small, non-membrane bound organelles where proteins are made (a process called protein synthesis). The cytoplasm is all the contents of the cell inside the cell membrane, not including the nucleus.  It is about 80% water, salts, and other organic compounds (carbs, amino acids, proteins, lipids).

Eukaryotic Cells

Eukaryotic cells usually have multiple chromosomes , composed of DNA and protein. Some eukaryotic species have just a few chromosomes, others have close to 100 or more. These chromosomes are protected within the nucleus. In addition to a nucleus, eukaryotic cells include other membrane-bound structures called organelles . Organelles allow eukaryotic cells to be more specialized than prokaryotic cells. Pictured below are the organelles of eukaryotic cells ( Figure below ), including the mitochondria , endoplasmic reticulum , and Golgi apparatus . These will be discussed in additional concepts.

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Eukaryotic cells have many specialized features. Organelles are parts of the cell that enclosed in their own membrane and have specific jobs. The nucleus of a eukaryotic cell is the largest organelle - enclosing the cell's DNA is a special double -lipid membrane (nuclear membrane). [Figure2]

Prokaryotic Cells

Prokaryotic cells ( Figure below ) are usually smaller and simpler than eukaryotic cells. They do not have a nucleus or other membrane-bound organelles. In prokaryotic cells, the DNA, or genetic material, forms a single large circle that coils up on itself. The DNA is located in the main part of the cell.

Prokaryotes do not have a nucleus. Instead, their genetic material is located in the main part of the cell.

Feature Prokaryotic cells Eukaryotic cells
Nucleus No Yes
DNA Single circular piece of DNA Multiple chromosomes
Membrane-enclosed organelles No Yes
Examples Bacteria Plants, animals, fungi

Vocabulary

  • cytoplasm : Entire contents of the cell inside the plasma membrane, excluding the nucleus.
  • deoxyribonucleic acid (DNA) : Nucleic acid that is the genetic material of all organisms.
  • endoplasmic reticulum : Organelle that is the site of lipid synthesis and protein modification.
  • eukaryote : Organism with cells containing a nucleus and membrane-bound organelles.
  • eukaryotic cell : Cell that contains a nucleus and membrane-bound organelles.
  • Golgi apparatus : Organelle that processes and packages proteins.
  • mitochondrion ( plural mitochondria ): Organelle of the cell in which energy is generated.
  • nucleus : Cell structure that contains the genetic material, DNA.
  • organelle : Structure within the cell that has a specific role.
  • plasma membrane : The lipid barrier that surrounds the cell; known as the cell membrane.
  • prokaryote : Organism that lacks a nucleus; i.e. bacteria.
  • prokaryotic cell : Cell without a nucleus or membrane-bound organelles.
  • ribosome : Organelle in which proteins are made (protein synthesis).

Summary

  • All cells have a plasma membrane, ribosomes, cytoplasm, and DNA.
  • Prokaryotic cells lack a nucleus and membrane-bound structures.
  • Eukaryotic cells have a nucleus and membrane-bound structures called organelles.

Practice

Use the resource below to answer the questions that follow.

  1. What does "naked" DNA mean? What kinds of organisms have "naked" DNA?
  2. Where do you find membrane bound organelles? Are plasmids membrane bound organelles?
  3. What is the size of mitochondria in prokaryotes?

Review

  1. What do all cells have in common?
  2. What are organelles?
  3. Compare the location of the genetic material of eukaryotic cells and prokaryotic cells.
  4. What are some examples of eukaryotes?

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0

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