Biochem Quiz 3 Study: Prokaryotes & Eukaryotes

Prokaryotes and eukaryotes differ at the cellular level. How so?

A glance through a microscope should be enough to help you determine whether an organism is a prokaryote or a eukaryote. As you know, prokaryotes don't have a nucleus or membrane-bound organelles, while eukaryotes do. How would you expect prokaryotic cells to appear under a microscope, as opposed to eukaryotic cells?

Provide a summary about what the two types of cells are.

All life on earth is either prokaryotic or eukaryotic. Life on earth is divided into two broad categories based on the types of cells an organism is composed of. Prokaryotes are single-cell organisms that lack a nucleus and membrane-bound organelles such as mitochondria. Eukaryotes can be unicellular or multicellular, and are made of cells that contain a nucleus and membrane-bound organelles.

What do archaeans have in common?

Archaeans share several unusual characteristics. The sulfur hot springs of Yellowstone National Park, with temperatures near the boiling point, don't look as though they could support any life at all. Yet they are home to some of the most unusual life-forms on earth—prokaryotes in the Domain Archaea. Archaeans can live in extreme habitats such as hot springs, very salty lakes, and high-acid environments. In many members of the Domain Archaea, the cell membrane consists of a phospholipid monolayer, or only one layer of lipid molecules. That feature sets them apart from bacteria and eukaryotic cells. Scientists didn't recognize archaeans as an entirely different category of life until about 30 years ago. Before then, scientists classified them as bacteria. Improvements in genetic techniques enabled scientists to discover that archaeans are about as different from bacteria as human beings are.

What are the most abundant organisms on earth?

Bacteria are the most abundant organisms on earth. Prokaryotes are divided into two broad categories of life called domains. One of these domains contains unusual forms of life called archaeans. The other includes all of the bacteria on earth. Many human diseases are caused by some types of bacteria. If you've ever had strep throat or tonsillitis, you've had a bacterial infection. Tooth decay and many forms of food poisoning also are caused by bacteria. Most people probably associate bacteria with disease, but disease-causing, or pathogenic, bacteria are actually only a minority of all the bacteria on earth. Without bacteria, life on the planet would be very different. Many types of bacteria carry out photosynthesis and play important roles in producing atmospheric oxygen. Others recycle nutrients. Still others are key components of industries like food processing, pharmaceuticals, and genetic engineering. Bacteria live inside the bodies of nearly every organism on earth. In fact, your own digestive tract contains trillions of bacteria that help you digest food.

What are eukaryotes?

Eukaryotes are organisms made of cells with a nucleus and membrane-bound organelles. How many living things are around you right now? How many living things did you encounter in the last week? Your list might be long and varied, but all of those organisms are eukaryotes. One of the most fundamental divisions of life on earth is the division between prokaryotes and eukaryotes. Eukaryotes are organisms that have eukaryotic cells, or cells that contain a nucleus and membrane-bound organelles, such as mitochondria, chloroplasts, Golgi bodies, and lysosomes. The nucleus in eukaryotes contains the genetic material DNA.

Some eukaryotes are single-cell organisms. Explain.

If you've ever baked a batch of bread, you know that the ingredient yeast plays an important role in making bread rise. Yeast is a single-cell eukaryote that produces carbon dioxide. By giving off carbon dioxide gas, yeast creates air bubbles in bread dough, causing it to rise. Yeast is just one example of a single-cell eukaryotic organism. Unlike people, plants, or insects, single-cell eukaryotes do not have specialized cells and tissues. They carry out all of the functions of life inside their single cell. Other single-cell eukaryotes include the amoeba, the paramecium, and a tiny photosynthetic organism called a diatom.

What are prokaryotes?

Prokaryotes are single-cell organisms. From the digestive systems of termites and cows to the depths of the hot springs at Yellowstone National Park, prokaryotic organisms, or prokaryotes, have evolved ways to survive in nearly every habitat on earth. Prokaryotes are amazingly diverse, but they are all single-cell organisms. Despite having neither a nucleus nor membrane-bound organelles such as mitochondria, prokaryotes still carry out all of the functions of life—growth and development, reproduction, heredity, homeostasis, metabolism, response, and cellular organization—inside a single cell. The lack of a nucleus and membrane-bound organelles is one way in which prokaryotes differ from the cells of all multicell organisms, called eukaryotes. The DNA of prokaryotes is also organized differently from that of eukaryotes. In prokaryotes, DNA usually exists as a looped molecule.

What were the oldest life forms?

The earliest life forms were prokaryotes. In 1993, geologist J. William Schopf made a groundbreaking discovery on the coast of Western Australia. There he found fossilized, cylinder-shaped rock formations. He thought they looked similar to structures associated with a modern-day bacteria, and he brought samples back to his lab. By analyzing the chemical composition of the rocks and the structures inside them, Schopf determined that the rocks were 3.5 billion years old, and that they contained carbon compounds—a sign that those objects were at one time living organisms. Most scientists today consider these rock deposits, called stromatolites, the oldest evidence of life on earth. These life-forms, and all life on earth for the next 1.5 billion years, were all single-cell organisms without a nucleus and membrane-bound organelles. This type of cell is called a prokaryotic cell. It is from these earliest life-forms that the diversity of life on earth today originated.

Most scientists suggest that eukaryotes arose from prokaryotes. Explain why.

The key difference between eukaryotes and prokaryotes is that eukaryotes have a nucleus and membrane-bound organelles such as mitochondria. But where did organelles come from? More than 30 years ago, biologist Lynn Margulis championed the idea that organelles originated as independent prokaryotic cells and were engulfed by other prokaryotic cells. According to that idea, called the endosymbiotic theory, one prokaryotic cell may have engulfed another, forming a union in which both cells worked cooperatively together. Over millions of years, that kind of partnership may have evolved, producing the eukaryotic cells we see today. The mitochondria and chloroplasts in today's eukaryotes have their ancestry in prokaryotic cells millions of years ago. Genetic evidence supports this idea. Both mitochondria and chloroplasts contain their own DNA, which is separate from the DNA in the cell's nucleus. In addition, organelle DNA shares many more traits with bacterial DNA than with the eukaryotic DNA found in the nucleus. Both lines of evidence suggest that those organelles did indeed have a prokaryotic ancestor.

The eukaryotes you see every day are what?

They are multicellular. From your dog to the tomato plants in your neighbor's garden, all of the living things you can see around you are multicell eukaryotes. Like you, each organism is made of trillions of cells. All of those cells share the characteristics of eukaryotic cells: They all contain genetic material inside a nucleus, and they all have membrane-bound organelles. Multicell organisms, however, also contain specialized cells that may function together as tissues and organs. Muscle cells, for example, are specialized for movement, and are arranged together into muscle tissue. Specialized cells, tissues, and organs form some of the levels of biological organization characteristic of multicell life.