Cells have evolved two basic architectural plans

1. Cells without a nucleus = Prokaryotes (can also be spelled procaryotes)
   • includes the Bacteria and Archaea
   • generally very small, unicellular
   • the earliest and still most abundant life forms; evolved ~ 4 billion years ago
   • some species highly evolved pathogens: e.g. Borrelia burgdorferi, the cause of Lyme disease.
   
   Image drawn by Thomas M. Terry for The Biology Place. Used with permission.
   
2. Cells with a nucleus = Eukaryotes (eucaryotes)
   • include the Animals, Plants, Fungi, and Protists
   • some unicellular, some multicellular forms
   • evolved ~ 1 billion years ago
   • size ranges from tiny yeasts to giant sequoias, dinosaurs

Eucaryotic Cell Structure and Cell Components

Diagram of a Eucaryotic cell -- see text for identification

• Compartments bounded by membranes (to be discussed in Ch. 8)
• Cytoplasm = central metabolic compartment, bounded by cell membrane. Other compartments inside cytoplasm are called organelles
• Compartmentation allows specialized functions to be carried out in different locations

The cytoplasm: site of protein synthesis and many metabolic events

• Contains many ribosomes = particles on which proteins are synthesized
  • View figures of ribosomes
  • Ribosome size measured in Svedberg (S) units; derived from sedimentation in ultracentrifuge (used before electron microscopes were available)
  • Prokaryotes: ribosomes made of 30S and 50S subunits, assemble into 70S ribosome
  • Eukaryotes: ribosomes made of 40S and 60S subunits, assemble into 80S ribosome
  • In bacteria, ribosomes occupy 25% of cell volume, use 90% of cell energy. Less in many specialized eukaryotic cells, but still the dominant activity of almost all cells.
• Contains many enzymes for general metabolism
• Compartment in which foodstuffs enter and from which wastes leave cell
• Contains fiber of the cytoskeletal system, which organize cytoplasmic structure
• Contains many different organelles
• View cartoon of the interior of a eukaryotic cell

• Contains chromatin = DNA-protein complexes. Chromatin can condense into chromosomes during cell division
• Site of RNA synthesis. 80% of RNA = ribosomal RNA. Remaining 20% leaves nucleus as t-RNA & m-RNA, directs protein synthesis (to be discussed Ch. 17)
• Review the role of the nucleus and ribosomes in protein synthesis (Campbell website activity)
• View diagram of cell nucleus
• Contains nucleolus = assembly plant for ribosomes. Ribosomal proteins are made in cytoplasm, must be transported back into nucleus. Ribosomal RNA is made in nucleus. These two elements are integrated inside nucleolus to create ribosomal subunits. These are then exported out of nucleus through nuclear pores.
• View diagram of Nucleolus
• Bounded by nuclear membrane = double layered structure. Contains many nuclear pores, allow material to move in and out of nucleus
• View nuclear pores
• Nuclear Pores have octagonal "doors" made of protein; open and close on either side depending on specific signals. Pore has diameter of about 10 nanometers (10 x 10^-9 m), smaller than diameter of a complete ribosome. Pore can open up to as much as 26 nm in response to certain signals. Some signals allow motion in but not out, other signals control reverse transport.
• View diagram of nuclear pore structure

• Endoplasmic Reticulum
• Rough ER: synthesizes proteins for export or movement to different cell compartments (but not to cytoplasm). View quicktime movie of rER [353 KB, 00:59 min, from Cells.De Online service for cell biology] .
• Signal hypothesis: certain mRNAs encode proteins designated for export. These carry a peptide signal at growing end, causes growing protein to move to ER ("docking"), insert peptide into membrane, translocate growing polypeptide chain across ER membrane. When protein synthesis is complete, polypeptide folds up inside ER, not in cytoplasm.
• View diagram showing ribosomes attaching to ER
• Smooth ER (sER): synthesizes lipids, detoxifies drugs and poisons (in liver).
  View smooth ER -- structure labeled "1".
• Golgi body
  • functions as intracellular "post office" for sorting new proteins made on rER.
  • Vesicles containing protein pinch off from ER, fuse with cis face of Golgi. Inside Golgi, oligosaccharide chains on proteins are modified. Vesicles pinch off from trans face of Golgi, carry proteins to several possible destinations: export (out of cell), lysosomes, peroxisomes, cell membrane, etc.
  • View diagrams showing how substance move through Golgi body
  • View animation of secretion from rER to Golgi to cell exterior (Campbell website activity)
• Lysosomes
  • compartments to break down old proteins, foreign materials, many wastes.
  • Contain ~40 hydrolytic enzymes: lipases, proteases, nucleases, etc. Break down organic polymers of all types.
  • "Suicide bags" if opened up on cell itself = apoptosis.
  • Lysosomes are used in phagocytosis, a process in which foreign materials are brought into the cell and "chewed up".
  • View animation showing phagocytosis. (Flash animation by Tom Terry, 2001)
  • View diagrams of lysosomes
  • View
  • animation of secretion from rER to Golgi to lysosome (Campbell website activity)
• Cell membrane (aka plasma membrane) - see Ch. 8
• Vacuoles
  • large membrane compartments (contrasted with small membrane bags called vesicles).
  • Plant cells have especially large vacuole called the central vacuole, can occupy most of the volume of a plant cell. Stores pigments, wastes, water, poisons, and more

• "Energy organelles" have unique properties:
  1. are enclosed by double membrane system
     
  2. contain DNA and ribosomes (70S, not 80S like cytoplasmic ribosomes)
     
  3. make some of their own proteins
  4. from their own genes
     
  5. divide by binary fission (but not autonomous, cannot
  6. grow or sustain life outside of cell)
     
• Mitochondria = centers for respiratory catabolism. Oxygen combined with chemicals to break down foods, generate cell energy. Contain outer and inner compartments, with many membranous cristae that "criss-cross" the internal space. Found in virtually every eukaryotic cell. Small structures similar to bacteria in some size.
  View mitochondria    (protected)
• Chloroplasts = centers for photosynthetic anabolism. Belong to group of plant organelles called plastids. Include chloroplasts (photosynthesis), amyloplasts (store starch), chromoplasts (store pigments). Trap light, convert energy to sugars (+ CO₂, water). Contain stacks of thylakoids, where green pigmented chlorophyll is embedded in membrane to trap light.
  View chloroplasts

Endosymbiont theory: All organelles seem to share many properties with bacteria: contain 70S ribosomes (whereas rest of eukaryote cells contain 80S ribosomes), divide by binary fission, contain circular DNA without nucleus, etc. Lynn Margulis proposed endosymbiont hypothesis: that organelles derived from ancient colonization of large bacteria (became the eucaryotic cell) by smaller bacteria (became the mitochondria, chloroplast, etc.) Symbiosis = "living together". Eventually, organelles lost ability to exist as separate organisms, cannot be separated from cell. Recent evolutionary taxonomy by comparing ribosomal RNA shows that this idea has lots of merit. Mitochondrial and plastid ribosomes are very similar to current bacteria, very different from eukaryotes.

• Microtubules
  • Largest diameter fiber. Found in cytoplasm of all eukaryotes.
  • Involved in many structures: cilia, flagella (9+2 arrangement); spindle fibers that polymerize from centrioles during mitosis/meiosis.
  • Made of tubulin protein; polymerizes into hollow tubules 25 nm diameter.
  • View cell treated with anti-tubulin fluourescent antibody
  • Cilia and flagella: organelles of locomotion. Contain 9 double rings of microtubules, 2 central microtubules.
  • View micrographs showing structure of cilia
  • View SEM of cilia on surface of epithelial tissue
  • Two motor proteins allow motion along microtubules
    1. Motor protein 1 -- Dynein
       • side arms allow one tubule to "walk up" along neighboring tubule, cause bending (powered by ATP).
       • animation of microtubule motion (Campbell website activity)
       • Causes motion from positive (+) end of the microtubule (where new tubulin is added to the microtubule) toward the minus (-) end of the microtubule.
       • When coordinated, this causes rotating or whip-like motion of entire cilium or flagellum --->motion.
       • View animation of dynein pulling vesicle along a microtubule
    2. Motor protein 2 -- Kinesin
       • Also powered by ATP, also allows protein to move along microtubule
       • Causes motion from negative (-) end of the microtubule toward the positive (+) end of the microtubule (where new tubulin is added to the microtubule).
       • pulls things toward outer reaches of cell
       • Example: in nerve cells, kinesin pulls vesicles away from center towards nerve endings.
       • View animation of kinesin pulling vesicle along a microtubule
       • View more sophisticated animation of kinesin moving along a microtubule. (Select either the Quicktime or MPEG movie titled "Structural Analysis of the Kinesin Motor Protein" ) )
• Microfilaments (= actin)
  • another kind of fiber. Found in cytoplasm of most eukaryotes.
  • nvolved in muscle contraction, cell support, pinching off of daughter cells after mitosis (in animals), cytoplasmic streaming (in plants).
  • View TEM of microvilli    (protected) of the striated border of epithelial tissue. Microfilaments within them can be seen extending down into the terminal web, an aggregate of fine filaments lying in the cell cytoplasm.
  • View animation showing "treadmilling" of actin -- subunits are added at one end, removed at the other, producing net migration of the filament in one direction.
  • View movie showing cell movement (left panel) and assembly of actin (right panel) (2.8 Meg file)
• Intermediate filaments
  • a third kind of fiber.
  • Made from keratin subunits. Not so quickly assembled and disassembled as microtubules or microfilaments.
  • May be involved in resisting tension, reinforcing cell shape, fixing location of nucleus.

• Found in plants, fungi, bacteria -- not in animal cells
• Allow cells to survive in plain water, rigid structure (tree towering 150 feet high!)
• Thicker than cell membrane. Made from cellulose (plants and fungi), other polysaccharides (bacteria).
• View plant cell walls    (protected)
• Cells maintain contact by plasmodesmata -- thin cytoplasmic connections, lined by membrane, that pass across cell wall junctions.

• Animal cells don't have walls, but do have ECM = meshwork of macromolecules outside plasma membrane. Consists mainly of glycoproteins (proteins with oligosaccharide chains), especially collagen.
• Some cells attached directly to ECM by bonding to collagen or fibronectin.
• View diagram of ECM   (protected)

• In multicellular organisms, adajcent cells are held together by several types of specialized junctions.
1. Tight junctions (found in animals): specialized "belts" that bind two cells tightly to each other, prevent fluid from leaking into intracellular space. View animation of tight junctions (Campbell website activity)
2. Desmosomes (found in animals): intercellular "rivets" that create tight bonds between cells, but allow fluids to pass through intracellular spaces.
   View animation of desmosomes (aka anchoring junctions) (Campbell website activity)
3. Gap junctions (found in animals): formed by two connecting protein rings embedded in cell membrane of adjacent cells. Allows passage of water, small solutes, but not macromolecules (proteins, nucleic acids).
   View animation of gap junctions (aka communicating junctions) (Campbell website activity)
4. Plasmodesmata (found in plants): channels connecting cells; allow free passage of water and small solutes, but not macromolecules (proteins, nucleic acids).
   View figure showing plasmodesmata (Campbell website activity)

• Cell-Tissue-Body Explorer - An interactive animated atlas of cells and functions of the human body. Note: 3-D renderings are very slow to load unless you have a very fast computer and connection.