Living Things: Prokaryotes and Eukaryotes

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.