1

Overview of Prokaryotic and Eukaryotic Cells

Objectives

After studying this chapter, the reader should be able to:

I. The Cell Theory

  1. General information
    1. Early cytologists analyzed the cell by light microscopy
    2. Cells are the basic units of life

  2. History of cell theory
    1. The historical basis for cell biology is microscopy
    2. The term "cell" was coined by the Englishman Robert Hooke (1673-1703)
      1. Using a compound microscope, Hooke observed small compartment-like units in the woody tissues of plants and called them "pores" or "cells"
      2. He also reported that living plant tissues were "filled with juices," substances we identify collectively as protoplasm
      3. He published his findings in 1665, in the book "Micrographia"
    3. The Dutchman Anthony van Leeuwenhoek (1632-1723) observed a variety of microscopic organisms
      1. Although a linen merchant by trade, he made his own single-lens microscope
      2. He saw bacteria, protozoa, sperm cells, blood cells and called them "animalcules" in 1674
      3. He communicated his findings in some two hundred letters to the Royal Society of London
    4. The current concept of the cell theory derives from the work of three Germans: Matthias Jakob Schleiden (1804-1881), Theodor Schwann (1810-1887), and Rudolph Virchow (1821-1902)
      1. In 1838, Schleiden argued that each plant cell "leads a double life," as an individual cell and as a part of a multicellular plant; he also recognized the importance of the nucleus in plant cells
      2. In 1839, Schwann observed that animal cartilage contains structures that "resemble... cellular tissues of plants"
      3. In 1855, Virchow extended the cell theory of Schleiden and Schwann to medicine, claiming that cells originate only from pre-existing cells
    5. A detailed version on the history of the cell is available, in the monograph "The Birth of the Cell", by Henry Harris (1999).
    6. The cell theory states that
      1. All living organisms are composed of nucleated cells
      2. Cells are the functional units of life
      3. Cells arise only from pre-existing cells

II. Prokaryotes and Eukaryotes

  1. General information
              1. Based on the absence or presence of a distinct nucleus, living things are divided into prokaryotes and eukaryotes
    1. The ratio of surface area to volume is an important consideration in determining cell size
  2. Prokaryotes
    1. Prokaryotes lack the organelle nucleus, which is the characteristic feature of the eukaryotes
      1. "Pro" is before; "eu" is true or typical; "karyon" is nut or nucleus
      2. These terms are also spelled procaryotes and eucaryotes
      3. An organelle is a membrane-bound structure that compartmentalizes functions within a eukaryotic cell; each organelle contains a specific enzyme complement and chemical composition related to its function
      4. Nucleus is the major organelle of the eukaryotic cell, in which the chromosomes are separated from the cytoplasm by the nuclear envelope
    2. Prokaryotes include archaebacteria, eubacteria and cyanobacteria (also called blue-green algae)
      1. These unicellular organisms live in a variety of environments, including soil and water; they are also found within plants and animals
      2. Traditionally archaebacteria (or Archaea) are classified as members of the prokaryotes with unique characteristics
        1. They live in extreme environments, such as hot springs and ocean depths
        2. Examples include: methanogens, which reduce carbon dioxide to methane; thermoacidophiles, which live in hot acid springs; and halophiles, which inhabit salt waters and brines
        3. Analysis of archaebacteria suggests that they are as far removed from other prokaryotes as prokaryotes are from eukaryotes; it has been suggested that archaebacteria should be classified into a separate group
        4. A genome is the total genetic information carried by a cell or an organism
        5. The complete genome of one species, Methanococcus jannaschii, has been published in 1996; among the 1738 predicted protein-coding regions, genes similar to eukaryotes such as histones have been found
    3. Prokaryotes have a simple cell organization
      1. The living material of a cell is called protoplasm
      2. The protoplasmic content of a prokaryotic cell is enveloped by the plasma membrane
      3. The genetic material of the prokaryotic cell is a circular DNA located in a region called the nucleoid in the cytoplasm
      4. The mesosome is an invagination of the prokaryote plasma membrane that may fuction during respiration, photosynthesis and cell division
      5. A cell wall may be present to provide support and shape
    4. Although they may be structurally simple, prokaryotes are metabolically complex
      1. All of the major metabolic pathways can be found in them
      2. Glycolysis is the energy-yielding process whereby glucose is metabolized anaerobically (not oxygen-requiring) to generate ATP
      3. Respiration is the aerobic (oxygen-requiring) metabolism of glucose to produce ATP
      4. Photosynthesis is the conversion of CO2 to organic compounds using energy derived from sunlight
  1. Eukaryotes
    1. Eukaryotes have a true nucleus that is surrounded by a double nuclear membrane
      1. The nucleae membrane has structures known as nuclear pores for communication with the cytoplasm
      2. The genetic material of the eukaryote is stored in multiple, linear chromosomes
      3. Chromosome, the structural unit of genetic material of the eukaryotic cell, consists of a DNA molecule in association with histones and other proteins
      4. The eukaryotic nucleus separates RNA synthesis (in the nucleus) and protein synthesis (in the cytoplasm) to different compartments
      5. In addition to the nucleus, an eukaryotic cell has other subcellular organelles and internal membranes
    2. Eukaryotes include the protozoa, fungi, plants, and animals
    3. Thinking of the eukaryotic cell as a factory is a helpful analogy
      1. The cell membrane, with its pores, is the factory's outside wall, with its gates
      2. The nucleus is the executive office
      3. The endoplasmic reticulum is the production facility
      4. The Golgi complex is the packaging division
      5. The secretory granules (which are found in some cell types) are the storage areas
      6. The mitochondria are the energy supply system
      7. The lysosomes are the waste-management system
    4. Lynn Margulis has popularized the endosymbiont theory for the origin of the eukaryotic cell in her books "Origin of Eukaryotic Cells" (1970) and "Symbiosis in Cell Evolution" (1993)
      1. There are similarities between prokaryotes and the organelles of eukaryotes (mitochondria and chloroplasts)
      2. For example, both mitochondria and chloroplasts contain circular DNA, similar to that of prokaryotes
      3. Organelle ribosome size and protein synthesis are similar to that of prokaryotes and not to that of the eukaryotic cytoplasm
      4. The prokaryote Prochloron contains chlorophyll b, which is also seen in plant chloroplasts
    5. The entire genome of the eukaryote, yeast, Saccharomyces cerevisiae, has been completed in 1996
      1. Genome size is 12,067,266 bp from 16 chromosomes
      2. The genome codes 5885 genes (with 50% as potential novel proteins), about 140 ribosomal RNA genes, 40 genes for small nuclear RNA molecules, and 275 transfer RNA genes
    6. These data suggest that a mammal may have 50,000 to 100,000 genes
  2. Living organisms and cell size
    1. The simplest living cells are the mycoplasmas
      1. Members can be free-living or parasitic; e.g., Thermoplasma acidophilum lives in coal refuse piles, Spiroplasma citrii attacks plant cells, and Mycoplasma pneumoniae causes respiratory diseases
      2. Mycoplasmas have a size range of 0.2 to 0.3 μm in diameter
      3. The complete genome of one species, Mycoplasma genitalium, has been published in 1995: the mycoplasma genome codes only 470 proteins, thus this is the minimum number of proteins that a cell needs to survive
    2. Whereas mycoplasmas represent the simplest free-living cells, even simpler parasitic organisms are known
      1. Common examples include viruses of animals, plants, and bacteria, e.g. the human polio virus, the tobacco mosaic virus, the bacteriophage T4
      2. Viroids and prions are the smallest known agents of disease
      3. Viroids, which consist of naked, circular RNA of 300 nucleotides, cause diseases in plants
      4. A nucleotide is a molecule containing a purine or pyrimidine, ribose or deoxyribose, and a phosphate group; polymers of nucleotides or polynucleotides are the nucleic acids (DNA or RNA), genetic material of a cell
      5. Prions, or proteinaceous infective particles, are thought to consist only of proteins; they have been implicated in neurodegenerative diseases, such as kuru in humans and scrapie in sheep
    3. The entire genome or nucleotide sequence of a free-living prokaryote has been completed in 1995 (as the first achievement of genomics research)
      1. The circular chromosome of the bacterium Haemophilus influenzae has a genome size of 1,830,137 base pairs of nucleotides: this is the first species whose genome has been completely analyzed
      2. The genome codes for 6 ribosomal RNA operons, 54 transfer RNA genes, and 1,743 proteins
      3. Among the 1,743 proteins, 736 or 42% are novel proteins, the rest have assigned roles such as translation, transport and energy metabolism
    4. Complete microbial genomes are reported in recent years
      1. The genome of Helicobacter pylori, the peptic ulcer pathogen, with 1,667,853 bp coding for 1,594 genes (with 42% potential novel genes), is reported in 1997
      2. The genome of Escherichia coli has also been completed in 1997: with 4,639,221 bp, including 4,288 protein-coding genes (38% novel proteins)
      3. The genome of Treponema pallidum, the syphilis spirochete, was completed in 1998; it has 1,138,006 bp containing 1041 coding sequences
    5. Prokaryotes and eukaryotic cells come in a variety of sizes
      1. A bacterium may be 2-5 μm long and 1 μm wide; and a typical animal cell has a diameter of 20 μm
      2. A yeast cell is 5 μm long, but the protozoan Euglena may be 50 μm long
      3. On a larger scale, a nerve cell may be 1 m in length; the unicellular alga Acetabularia is 8 cm; then there are chicken, ostrich and fossil dinosaur eggs
    6. The largest known bacterium is probably the "sulfur pearl of Namibia", or Thiomargarita namibiensis, reported in 1999
      1. Diameters can be 100 to 750 mm
      2. The bacterium is isolated from marine sediments
      3. It has a huge fluid-filled sac or vacuole
      4. It stores nitrate and uses it to oxidize sulfur to generate energy
    7. Another large bacterium is Epulopiscium fishelsoni, which was reported in 1993
      1. Individual Epuloposcium can be as large as 600 μm x 80 μm
      2. They live as symbionts in the intestinal tract of surgeonfish in the Red Sea and the Great Barrier Reef of Australia
      3. Originally classified as protozoa, analysis of their ribosomal RNA suggests that they are prokaryotes
    8. The smallest are the nanobacteria reported in 1998
      1. The size of nanobacteria is 0.05-0.5 μm
      2. They may be involved in kidney and bladder stone formation
    9. Surface area-to-volume ratio is an important consideration in determining cell size
      1. When a cell increases in size, the surface area increment always lags behind the volume increment
        1. Surface area is important because this is the location for the exchange of materials (metabolites, nutrients, waste products, etc.) between the cell and its environment
        2. The problem of maintaining adequate surface area arises because the volume of a cell increases with the cube of the cell's length or diameter, whereas its surface area only increases with the square
        3. As the linear dimension of the cell increases (i.e., the cell gets larger), the surface area-to-volume ratio decreases
        4. Theoretically, surface area-to-volume ratio can set a constraint for the upper limit of cell size
      2. An average eukaryotic cell is 10 times larger in linear dimension and 1,000 times greater in volume than a typical prokaryote such as Escherichia coli
        1. A typical eukaryotic cell may have non-spherical morphology, e.g. the flattened "pancake" shape of human fibroblast, and the extensive microvilli of human intestinal epithelial cell, both increase the surface area in these two cell types
        2. Eukaryotic cells can have internal membrane systems to increase surface area and to increase the "topologically equivalent extracellular space" of the cell
        3. "Topologically equivalent extracellular space" is an interior that is topologically equivalent to the exterior of the cell, e.g. the interior of a lysosome contains degradative enzymes, and the lysosome both receives and digests extracellular materials engulfed by the cell; in this sense the lysosome interior is the "topologically equivalent extracellular space"
  3. Chemical composition of cells
    1. The cell is composed of small molecules and macromolecules
      1. The elements carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up most of the cell; by weight, they may comprise 99% of the cell
      2. The most abundant small molecule of the cell is H2O; by weight, water comprises up to 70% of the cell
      3. Adenosine triphosphate (ATP) serves as the "money" of the cell, carrying chemical energy for various cellular metabolic processes
      4. Other common small molecules found in the cell include sugars, fatty acids, amino acids, nucleotides, metal ions, vitamins, etc.
      5. The macromolecules of the cell are based on carbon; these include nucleic acids, proteins, lipids and carbohydrates
    2. Nucleic acids play an important role in information storage and expression
      1. The expression known as the central dogma of biology states that DNA codes for RNA, which codes for protein
      2. DNA is involved in replication, RNA in transcription, and protein in translation
      3. In the genetic code, triplet codons code for each amino acid and stop signal
      4. In some instances, RNA species may have catalytic function; these RNA-based catalysts are called ribozymes
    3. Proteins are the most abundant cellular macromolecules
      1. About 18% of the cell's total weight is made up of proteins
      2. Proteins may combine with other substances to be functional
        1. Glycoprotein may have carbohydrate units
        2. Some proteins require prosthetic groups; a prosthetic group is a small organic molecule or metal ion component of an enzyme that plays an indispensible role in the catalytic activity of the enzyme
      3. Proteins perform many cellular functions; they act as structural components (such as cytoskeleton), as information processing components (transcription factors), and as catalysts
    4. The first completed genome of a multicellular eukaryote is the nematode Caenorhabditis elegans, with 19,099 ORF (open reading frames) from 97-megabase genomic sequence
      1. This was compared to the 6,217 ORFs of yeast
      2. For core biological functions, the distribution is: 28% for intermediary metabolism, 18% for DNA and RNA metabolism, 13% for protein folding and degradation, 11% for transport and secretion, another 11% for signal transduction, 8% unclassified, 6% for ribosomal proteins and 5% for cytoskeleton

 

 

 

Study Activities

  1. Describe the major features of the cell theory.
  2. List five differences between bacteria and mammalian cells.
  3. Discuss the importance of surface area-to-volume ratio within a cell.
  4. Explain the functions of nucleic acids and proteins.