home‎ > ‎Biology‎ > ‎2. Cells and Cellular Processes‎ > ‎Cell2‎ > ‎

C5 - Prokaryotic and Eukaryotic Cells

I can compare and contrast prokaryotic and eukaryotic cells

Diversity of Cells

Today, we know that all living cells have certain things in common. For example, all cells share functions such as obtaining and using energy, responding to the environment, and reproducing. We also know that different types of cells—even within the same organism—may have their own unique functions as well. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary in size as well as shape, but all cells are very small. In fact, most cells are much smaller than the period at the end of this sentence. If cells have such an important role in living organisms, why are they so small? Even the largest organisms have microscopic cells. What limits cell size?

Cell Size

The answer to these questions is clear once you know how a cell functions. To carry out life processes, a cell must be able to quickly pass substances into and out of the cell. For example, it must be able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. It is this need to pass substances across the surface that limits how large a cell can be. Look at the two cubes in Figure below. As this figure shows, a larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells; a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen and produces more wastes. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the harder it will be for the cell to get rid of its wastes and take in necessary substances. This is what limits the size of the cell.

Surface Area to Volume Comparison. A larger cube has a smaller surface area (SA) to volume (V) ratio than a smaller cube. This also holds true for cells and limits how large they can be.

Cell Shape

Cells with different functions often have different shapes. The cells pictured in Figure below are just a few examples of the many different shapes that cells may have. Each type of cell in the figure has a shape that helps it do its job. For example, the job of the nerve cell is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail-like projections on the algae cells? Algae live in water, and their tails help them swim. Pollen grains have spikes that help them stick to insects such as bees. How do you think the spikes help the pollen grains do their job? (Hint: Insects pollinate flowers.)

As these pictures show, cells come in many different shapes. Clockwise from the upper left photo are a nerve cell, red blood cells, bacteria, pollen grains, and algae. How are the shapes of these cells related to their functions?

Parts of a Cell

Although cells are diverse, all cells have certain parts in common. The parts include a plasma membrane, cytoplasm, ribosomes, and DNA.

  1. The plasma membrane (also called the cell membrane) is a thin coat of lipids that surrounds a cell. It forms the physical boundary between the cell and its environment, so you can think of it as the “skin” of the cell.
  2. Cytoplasm refers to all of the cellular material inside the plasma membrane. Cytoplasm is made up of a watery substance called cytosol and contains other cell structures such as ribosomes.
  3. Ribosomes are structures in the cytoplasm where proteins are made.
  4. DNA is a nucleic acid found in cells. It contains the genetic instructions that cells need to make proteins.

These parts are common to all cells, from organisms as different as bacteria and human beings. How did all known organisms come to have such similar cells? The similarities show that all life on Earth has a common evolutionary history.

A nice introduction to the cell is available at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/33/Hmwvj9X4GNY (21:03).

Two Types of Cells

There is another basic cell structure that is present in many but not all living cells: the nucleus. The nucleus of a cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear membrane) and contains DNA. Based on whether they have a nucleus, there are two basic types of cells: prokaryotic cells and eukaryotic cells. You can watch animations of both types of cells at the link below. http://www.learnerstv.com/animation/animation.php?ani=162&cat=biology

Prokaryotic Cells

Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm rather than enclosed within a nuclear membrane. Prokaryotic cells are found in single-celled organisms, such as bacteria, like the one shown in Figure below. Organisms with prokaryotic cells are called prokaryotes. They were the first type of organisms to evolve and are still the most common organisms today.

Prokaryotic Cell. This diagram shows the structure of a typical prokaryotic cell, a bacterium. Like other prokaryotic cells, this bacterial cell lacks a nucleus but has other cell parts, including a plasma membrane, cytoplasm, ribosomes, and DNA. Identify each of these parts in the diagram.

Bacteria are described in the following video http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/16/TDoGrbpJJ14 (18:26).

Eukaryotic Cells

Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is shown in Figure below. Eukaryotic cells are usually larger than prokaryotic cells, and they are found mainly in multicellular organisms. Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to people. Eukaryotic cells also contain other organelles besides the nucleus. An organelle is a structure within the cytoplasm that performs a specific job in the cell. Organelles called mitochondria, for example, provide energy to the cell, and organelles called vacuoles store substances in the cell. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can.

Eukaryotic Cell. Compare and contrast the eukaryotic cell shown here with the prokaryotic cell. What similarities and differences do you see?

Viruses: Prokaryotes or Eukaryotes?

Viruses, like the one depicted in Figure below, are tiny particles that may cause disease. Human diseases caused by viruses include the common cold and flu. Do you think viruses are prokaryotes or eukaryotes? The answer may surprise you. Viruses are not cells at all, so they are neither prokaryotes nor eukaryotes.

Cartoon of a flu virus. The flu virus is a tiny particle that may cause illness in humans. What is a virus? Is it a cell? Is it even alive?

Viruses contain DNA but not much else. They lack the other parts shared by all cells, including a plasma membrane, cytoplasm, and ribosomes. Therefore, viruses are not cells, but are they alive? All living things not only have cells; they are also capable of reproduction. Viruses cannot reproduce by themselves. Instead, they infect living hosts, and use the hosts’ cells to make copies of their own DNA. For these reasons, most scientists do not consider viruses to be living things.

An overview of viruses can be seen at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/17/0h5Jd7sgQWY (23:17).

Lesson Summary

  • All cells are very small because they need to pass substances across their surface. Their small size gives them a relatively large ratio of surface area to volume, facilitating the transfer of substances. The shapes of cells may vary, and a cell’s shape generally suits its function.
  • Cells are diverse, but all cells contain a plasma membrane, cytoplasm, ribosomes, and DNA.
  • Prokaryotic cells are cells without a nucleus. They are found in single-celled organisms. Eukaryotic cells are cells with a nucleus and other organelles. They are found mainly in multicellular organisms.

Review Questions

Recall

1. List the four parts that are found in all living cells.

Think Critically

2. Why are all cells very small? Explain what limits the size of cells.

3. Compare and contrast prokaryotic cells and eukaryotic cells.

4. Explain why viruses are not considered to be living.