Living Things: Prokaryotes and Eukaryotes

Understanding cells is essential for scientists seeking to understand life. In this lesson, students are introduced to the two broad types of cells: prokaryotes and eukaryotes, using microscopes to investigate the structure and form of each.

• All living things, from microbes to humans, share certain characteristics, including the need for energy to grow and develop.
• All living things are made up of one or more cells, and that the cell is the smallest unit of life.
• All living things can be divided into two broad groups depending on whether their cells have membrane-bound organelles or not.
• There are certain differences between the two groups—prokaryotes and eukaryotes—that scientists can use to differentiate the two.

• Why are cells matter?
• How are living things different from nonliving things?
• What is the relationship between cells and a living thing such as a plant or an animal?
• How are cells alive?
• What structures do all cells—both prokaryotes and eukaryotes—have in common?
• What is the primary difference between prokaryotes and eukaryotes?
• How would you explain the relationship between the size of the cell and the presence or absence of membrane-bound organelles?

Misconception: Human cells are completely different from other animal cells, plant cells, and prokaryotic cells.

Fact: All cells, whether human, plant, or prokaryote, share certain similarities. All cells must perform essential life functions, and all cells share certain parts, including a cell membrane and cytoplasm.

Misconception: Individual cells do not need water to function.

Fact: Because cells are living things, cells need water to survive.

Cell : the smallest unit of life; makes up single-celled and multi-celled organisms; surrounded by a cell membrane and filled with cytoplasm

Eukaryotes :single or multi-celled organisms that have a nucleus and membrane-bound organelles

Nucleus : a membrane-bound organelle that holds an organism’s genetic material (DNA)

Organelle : small parts that carry out specific functions in a cell

Organism : a complete living thing that has the ability to carry out all functions for life.

Prokaryotes : single-celled organisms that do not have a nucleus or membrane-bound organelles

Discovering Life Under Ice

Almost 1 kilometer beneath the West Antarctic Ice Sheet, scientists have discovered a lake. This lake is so far beneath the surface that it hasn’t been exposed to sun or wind for millions of years. Despite its location, the lake is home to more than 4,000 different kinds of living things. These living things are all microorganisms. An organism is a complete living thing that has the ability to carry out all functions for life. Function is the normal action of something or how something works. A microorganism, also called a microbe, is an organism too small to see with the human eye.

The places where life is found continue to surprise scientists. For example, before 1977, scientists thought the sun was the primary source of energy for all living things, so life couldn’t exist where sunlight didn’t reach. Since then, scientists have continued to find life in surprising places, including some of the deepest parts of the ocean, and now, far below an Antarctic ice sheet.

The discovery of life underneath the West Antarctic Ice Sheet confirmed for the scientists something they had predicted years before. It took years to find evidence to confirm their prediction, however. Scientists began their project in 2009, and in 2013 they drilled down more than 0.8 kilometers (0.5 mile) beneath the ice sheet to Subglacial Lake Whillans.

Characteristics of Life

The scientists of the expedition to the West Antarctic Ice Sheet used microscopes to identify the different organisms they found. They then had to show that the organisms were alive and growing.

All living things, from microbes to humans, share certain characteristics. For example, all organisms need energy to survive. If they get enough energy, all organisms will grow and then eventually die. And all organisms are made up of cells. The cell is the smallest unit of life. Cells are alive because they use energy to carry out life functions. Life functions include growing, developing, and getting energy from food. Energy is required because it powers all of these actions.

Some organisms are made up of just one cell. Other organisms are made up of hundreds, thousands, or millions of cells. Cells come in a variety of shapes and sizes. Like all matter, cells are made up of molecules. Living things take in new molecules from air, food, and liquids. These molecules get into the cells and help them function.

Cells are systems because they are made up of different structures that help them carry out their life functions. In some cells, these individual parts are surrounded by a membrane, and they are called organelles. An organelle is a small part that carries out a specific function in a cell. In other cells, the smaller parts of the cell are not surrounded by a membrane.

All living things can be divided into two broad groups depending on whether their cells have membrane-bound organelles or not. These two groups are called prokaryotes (pronounced proh-kar-ee-otes) and eukaryotes (pronounced yoo-KAR-ee-otes).

Eukaryotic cells are many times larger than prokaryotic cells, between 2 and 10 micrometers in length. Because of their size, nutrients cannot rapidly get to all parts of the cell, so eukaryotes need many specialized organelles that deliver nutrients to where they are needed in the cell.

Eukaryotes can be either single- celled or multi-celled organisms. Animals and plants are multi-celled eukaryotes. In multi-celled organisms, all of the cells work together to keep the organism functioning. Animals are eukaryotic organisms that eat other organisms for energy, breathe oxygen, and undergo growth and reproduction.

Plants are eukaryotic organisms that capture energy from sunlight for growth and development, and they make their own food through photosynthesis. Protists are a third kind of eukaryotic organism that can be single- celled or multi-celled. Some protists perform photosynthesis like plants, while others move around like animals. Diatoms, amoebas, algae, and other hard-to- classify organisms are protists.

The other group of organisms is prokaryote, and these organisms were the earliest forms of life on Earth. The word prokaryote means “before nucleus.” They are called this because prokaryotes don’t have a nucleus or other membrane-bound organelles. All the internal structures in a prokaryotic cell are spread throughout its cytoplasm. Cytoplasm is the jelly-like liquid that makes up most of a cell’s volume. This structure is an advantage because it allows prokaryotes to absorb nutrients quickly. Prokaryotes are simple single-celled organisms. Because they are single-celled, that one cell carries out all of the life functions necessary for the organisms to survive.

In this lesson, students use microscopes to investigate the form and structure of prokaryotic and eukaryotic cells, the two broad types of cells. As they investigate, students collect and analyze observational data on prokaryotic and eukaryotic cells, comparing the form and structure of each to explain how similar and different they are.