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Prokaryotic and Eukaryotic Cells

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Cell Types: Structure & Function
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Lesson Objectives

  • Differentiate characteristics of prokaryotic and eukaryotic cells.

  • Describe the structure and function of 10 parts of eukaryotic cells.

  • Compare and contrast plant and animal cell structures.

Vocabulary

  • chloroplast

  • endosymbiosis

  • eukaryote

  • multicellular

  • mitochondrion

  • nucleus

  • organelle

  • plasma membrane

  • plastids

  • prokaryote

  • ribosome

  • unicellular


 

How many different types of cells are there?

There are many different types of cells. For example, in you there are blood cells and skin cells and bone cells and even bacteria. Here we have drawings of bacteria and human cells. Can you tell which depicts various types of bacteria? However, all cells - whether from bacteria, human, or any other organism - will be one of two general types. In fact, all cells other than bacteria will be one type, and bacterial cells will be the other. And it all depends on how the cell stores its DNA.

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 the  Figure below . Organisms with prokaryotic cells are called prokaryotes . They were probably the first type of organisms to evolve and are still the most common organisms today.

A diagram of a typical prokaryotic cell and its structure

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/watch?v=TDoGrbpJJ14 (18:26).

Eukaryotic Cells

Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is shown in the  Figure below . Eukaryotic cells are usually larger than prokaryotic cells, and they are found in both unicellular and 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 life functions more efficiently than prokaryotic cells can. This allows eukaryotic cells to have greater cell specificity than prokaryotic cells. Ribosomes, the organelle where proteins are made, are the only organelles in prokaryotic cells.

A diagram of the parts of a typical eukaryotic cell

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

In some ways, a eukaryotic cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and other organelles. You can also explore the structures of an interactive animal cell at this link: http://www.cellsalive.com/cells/cell_model.htm .

 

Cell Structures Found in All Cells

The Plasma Membrane

The plasma membrane forms a barrier between the cytoplasm inside the cell and the environment outside the cell.  It protects and supports the cell and also controls everything that enters and leaves the cell. It allows only certain substances to pass through, while keeping others in or out. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability. The plasma membrane is discussed at http://www.youtube.com/watch?v=-aSfoB8Cmic (6:16). The cell wall (see below) is also discussed in this video.

Cytoplasm and Cytoskeleton

The cytoplasm consists of everything inside the plasma membrane of the cell. It includes the watery, gel-like material called cytosol, as well as various structures. The water in the cytoplasm makes up about two thirds of the cell’s weight and gives the cell many of its properties.

The cytoplasm has several important functions, including:  1. suspending cell organelles  2. pushing against the plasma membrane to help the cell keep its shape  3. providing a site for many of the biochemical reactions of the cell

Crisscrossing the cytoplasm is a structure called the cytoskeleton , which consists of thread-like filaments and tubules. You can see these filaments and tubules in the cells in the  Figure below . As its name suggests, the cytoskeleton is like a cellular “skeleton.” It helps the cell maintain its shape and also holds cell organelles in place within the cytoplasm.

License: CC BY-NC 3.0

Cytoskeleton. The cytoskeleton gives the cell an internal structure, like the frame of a house. In this photograph, filaments and tubules of the cytoskeleton are green and red, respectively. The blue dots are cell nuclei. [Figure1]

 

Ribosomes

Ribosomes are small organelles where proteins are made. They contain the nucleic acid RNA, which assembles and joins amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm as well as on the rough endoplasmic reticulum in eukaryotic cells.

 

 Structures Found in Eukaryotic Cells

Eukaryotic cells contain a nucleus and several other types of organelles. These structures are involved in many vital cell functions.

Nucleus

The nucleus is the largest organelle in a eukaryotic cell and is often considered to be the cell’s control center. This is because the nucleus controls which proteins the cell makes. The nucleus of a eukaryotic cell contains most of the cell’s DNA, which makes up chromosomes and is encoded with genetic instructions for making proteins.

Mitochondria

The mitochondrion (plural, mitochondria ) is an organelle that makes energy available to the cell. This is why mitochondria are sometimes referred to as the power plants of the cell. They use energy from organic compounds such as glucose to make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy. Scientists think that mitochondria were once free-living organisms because they contain their own DNA. They theorize that ancient prokaryotes infected (or were engulfed by) larger prokaryotic cells, and the two organisms evolved a relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the prokaryotes became permanent guests of the larger cells, as organelles inside them. This theory is called the endosymbiotic theory, and it is widely accepted by biologists today.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. There are two types of endoplasmic reticulum: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in the  Figure below .

  • RER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.
  • SER looks smooth because it does not have ribosomes. SER also makes lipids, stores substances, breaks down toxins and plays other roles.

Golgi Apparatus

The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is shown in Figure below . The Golgi apparatus is somewhat like a post office. It receives items (proteins from the ER), packages and labels them, and then sends them on to their destinations (to different parts of the cell or to the cell membrane for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell. At the link below, you can watch an animation showing how the Golgi apparatus does all these jobs. http://www.johnkyrk.com/golgiAlone.html

Vesicles and Vacuoles

 Both vesicles and vacuoles are sac-like organelles that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus (see Figure below ) store and transport protein and lipid molecules. Some vesicles are used as chambers for biochemical reactions. Other vesicles include:

  • Lysosomes, which use enzymes to break down foreign matter and dead cells.
  • Peroxisomes, which use oxygen to break down poisons.

License: CC BY-NC 3.0

This drawing includes the nucleus, RER, SER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins. [Figure2]

 

Structures Found in Animal Cells

Centrioles

Centrioles are organelles involved in cell division. They help organize the chromosomes before cell division so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells and are located near the nucleus (see Animal Cell  above ).

 Structures Found in Plant Cells

Plant cells have several structures that are not found in animal cells, including a cell wall, a large central vacuole, and organelles called plastids. You can see each of these structures in the Figure below . You can also view them in an interactive plant cell at the link below. http://www.cellsalive.com/cells/cell_model.htm

License: CC BY-NC 3.0

Plant Cell. In addition to the organelles and other structures found inside animal cells, plant cells also have a cell wall, a large central vacuole, and plastids such as chloroplasts. Can you find each of these structures in the figure? [Figure3]

 

Cell Wall

The cell wall is a rigid layer that surrounds the plasma membrane of a plant cell. It supports and protects the cell. Tiny holes, or pores, in the cell wall allow water, nutrients, and other substances to move into and out of the cell. The cell wall is made up mainly of complex carbohydrates, including cellulose.

Central Vacuole

Most mature plant cells have a large central vacuole . This vacuole can make up as much as 90% of the cell’s volume. The central vacuole has a number of functions, including storing substances such as water, enzymes, and salts. It also helps plant tissues, such as stems and leaves, stay rigid and hold their shape. It even helps give flowers, like the ones seen in the figure below , their beautiful colors.

 

License: CC BY-NC 3.0

These flowers are red because of red pigment molecules in the central vacuoles of their cells. The bright colors are an important adaptation. They help the flowers attract pollinators such as hummingbirds so the plants can reproduce. [Figure4]

 

 Plastids

Plastids are organelles in plant cells that carry out a variety of different functions. The main types of plastids and their functions are described below.

  • Chloroplasts are plastids that contain the green pigment chlorophyll. They capture light energy from the sun and use it to make food. A chloroplast is shown in the plant cell above .
  • Chromoplasts are plastids that make and store other pigments. The red pigment that colors the flower petals in  the figure above   was made by chromoplasts.
  • Leucoplasts are plastids that store substances such as starch or make small molecules such as amino acids.

Like mitochondria, plastids contain their own DNA. Therefore, according to endosymbiotic theory, plastids may also have evolved from ancient, free-living prokaryotes that invaded larger prokaryotic cells. If so, they allowed early eukaryotes to make food and produce oxygen.

 

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0

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