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Posted by Jim True on February 10, 2004 6:41 AM. Last Updated October 22, 2006 9:23 PM

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CH 07: A Tour of the Cell

Cells

• For some life forms, the cell and the organism are one and the same (unicellular).
• For multicellular forms, the number of cells may range into the tens of trillions, with various cells having different shapes and functions.
• Regardless, ALL CELLS HAVE SEVERAL COMMON CHARACTERISTICS (biological diversity is variation on common themes):
  • Cell (Plasma) Membrance -- A specialized series of molecules that separates the cell from its environment.
    • Cell membranes possess numerous connections between the interior and exterior of the cell for the transport of nutrients, communications, etc.
    • Why are cells so small? Because volume increases as a cubic function, whereas surface area increases as a square.
    • This means that as cells enlarged, eventually there would not be enough surface area to allow enough movement of materials in/out of the cell for life.
    • Thus, cell size is pretty uniform among various life forms.
    • Bigger organisms simply have MORE CELLS.
  • DNA -- the hereditary information passed from one generation to the next.
    • In one of the two major types of cells (prokaryotic), the DNA is formed as a single large loop and is not bound by a special membrane.
    • In the second type (eukaryotes), the DNA is membrane-bound and is linear. The number of DNA molecules will vary. Humans have 46, dogs 78, ducks 80).
    • In both cell types, DNA is held within one or more chromosomes ("chrom" - color; "som-" -- body).
  • Cytosol ("cyt" - cell; "sol" is short for solution). This is the semi-fluid material inside the cell. Typically use 'cytoplasm'; cytoplasm is cytosol with other large molecules and structures with in the fluid.
    • The cytosol plus its numerous dissolved molecules, ions and various specialized structures that perform specific functions for the cell (organelles) together is called the cytoplasm.
  • Ribosomes -- Organelles that function in the production of protein. Cellular equivalent of protein factory.
    • They are present in all cells, but differ somewhat in construction between the two major cell types.
• While all cells have these four basic components, there is tremendous variation on the basic theme, resulting in the high degree of biodiversity that we observe.
• All of the variety of life results from modifications of two basic cell types:
  • Prokaryotic organisms or prokaryotes are based on the prokaryotic cell type.
  • Eukaryotic organisms or eukaryotes are based on the eukaryotic cell type.
  • Prokaryotic cell ("pro" -- before; "karyon" -- kernel)
    • These are the simplest cell type.
    • Cells are typically very small (1 to 10 um)
    • Most have a cell wall, which differs in composition from eukaryotic types. Simplicity and complexity do not equal inferiority and superiority. Simplicity is not inability... most unicellular organisms have to be able to do all functions.
    • Very little internal complexity.
    • Virtually no organelles in cytoplasm.
    • The DNA is present in a single large loop (nucleoid) and there may also be numerous small loops present as well.
    • No membrane around the DNA, i.e. there is no nucleus. THIS IS THE DEFINING characteristic that gives the Prokaryotic cells it's name (before the core, pre nucleus).
    • Many forms are photosynthetic. Pigments are embedded in the cell membrane. Entire cell acts like a chloroplast.
    • All currently known prokaryotic organisms are unicellular and are referred to as bacteria.
  • Eukaryotic Cell -- ("eu" -- true) The True Kernel, or core.
    • Larger and more complex internally than prokaryotes.
    • Contain numerous internal, specialized structures called organelles ("little organs"), which have specific functions.
    • Some organelles are thought to have originally been independent bacterial cells incorporated into other larger cells. (Chloroplasts, Mitochondria and Centrioles, through a process known as phago cytosis, engulfing and bringing within).
    • Highly compartmentalized internally, mainly because of organelles.
    • Cell walls in some groups (fungi, plants, and some protistans) absent in others (animals), but totally different in construction than in prokaryotes.
    • DNA arranged as variable number of linear molecules (has a beginning and and end), bound by its own membrane, ie, nucleus is present. This is what defines these cells as eukaryotic.

Cell Organelles

• In the eukaryotic cell, there are a wide array of cellular organelles that provide specific funcitons for the cell. These include:
  • Membrane bound structures that create barriers that prevent or allow certain things to cross, or to contain specific substances (inter-related in transport into, around or out of the cell):
    • Cell Membrane (7.9) (covered more in Chapter 8)
    • Nucleus (7.10) - Corporate Headquarters, control all cellular functions. Most cells have a singular nucleus; human skeletal muscle cells have numerous nucleous. Red blood cells (erhythrocytes) have no nucleus. Surrounded by nuclear membrane (or nuclear envelope)
      • Nucleolus -- shown in the model as a structure (sphere within the nucleus). Nucleolus is not a structure, it's a region, cluster of molecules -- RNA, DNA and proteins. Primarily RNA and proteins. Raw materials for making cellular machinery is found; ribosomes are built here.
      • Chromatin -- Molecular structure - proteins, RNA and DNA to create molecule called Chromatin. Long, unravelled molecule. During specific phases of cell development and when the cell is getting ready to divide, the chromatin (all of them) coil up and is referred as a Chromosome. 'Chromo' 'some' - color body. Only when the cell is getting ready to divide.
      • Ribosomes - machines that are designed to take a program that tells them what to build and in exactly what order. The ribosomes build amino acids, thus they build proteins.
    • Endoplasmic reticula (7.11) -- Main factory complex; network inside the cytoplasm. "Endo means into, plasm - in the plasma, reticulum - network". Interwoven channels inside the cytosplasm spreading out from the nucleus. Two kinds:
      • Rough Endoplasmic Reticulum -- Ribosomes on the surface, proteins are built here. Rough ER. Highly compartmentalizes the cell.
      • Smooth Endosplasmic Reticulum -- these machines build lipids not proteins. Lipid synthesis.
    • Golgi apparatus (7.12) -- May be multiples of these in the cell. One by itself is called a 'golgi body'; several together is the 'golgi complex'. Package and shipping for the factory. (Golgi is the name of an Italian chemist). Materials released from the ER (especially proteins) are transported to the Golgi apparatus and packaged for shipment outside or around the cell.
    • Lysosomes (7.13) -- Lyso - to loosen, some - body. Small inclusions that contain digestive enzymes and move to the site of another organelle that needs to be broken down and they surround the organelle or raw component and break it down. Also involved in cell destruction; when a cell is ready to die, the lysosomes release and break down the cell body to reuse the chemicals elsewhere. Plants do not have lysosomes.
    • Vacuoles (7.15) -- small membrane bound inclusions are called 'vesicles'; large ones are called 'vacuoles'. Both plants and animals have vesicles and vacuoles. Plants have a LARGE CENTRAL VACUOLE, used to store the food created for storage by the plant by the chloroplasts. "Endomembrane transport system" (transport process from nucleus to Er to golgi to plasma membrane.
    • Peroxisomes (7.19) -- Peroxide bodies. Water can form a peroxide of oxygen, hydrogen peroxide. Very, very destructive substance; tears molecules apart. Peroxides incredibly corrosive substance. Peroxisomes release substances that will convert peroxides back to h2o and oxygen. Glycoxisomes (in plants) convert lipids to carbohydrates in plants
  • Organelles (also membrance bound) involved in energy transformations (generator plant, generate raw materials to be used for fuel and power generating plants):
    • Chloroplasts (plants) (7.18) - only in plants and photsynthetic eukaryotes. Manufacture the raw materials to be put in the generator plants, the mitochondria. Plants can manufacture their own raw materials. Outer membrane surrounding a highly folded inner membrane (pigmented structure). When you see a leaf changing color, chloroplast production is slowing down.
    • Mitochondria (animals & plants) (7.17). Generator plants in the cell, generate ATP. Some cells have one, some have many, depending on the amount of ATP production needed by the cell. Have an outer membrane and a highly folded inner membrane. Powerhouse of the cell, their job to manufacture ATP. Sole source of energy for life. Mitochondria can duplicate themselves without the cell having to replicate.
  • Cytoskeletal organelles (skeletal just means a framework) that provide structural support, locomotion and regulation of cell division in some cells; only category of organelles that have structures OUTSIDE the cell. All of the cytoskeletal organelles are constructed of proteins:
    • Microtubules, microfilaments and intemediate filements (all in Table 7.2) -- Microfilaments are solid, microtubules are hollow like a straw.
    • Centrosomes and Centrioles (7.22) -- Region, optically dense, called the centrosome (process to build protein structures, microfilaments, microtubules). Can also construct pairs of short structures called Centrioles (9 triplets), play a very important role in the division of the nuclei. Plants and prokarotes do not have centrioles.
    • Cilia and Flagella (7.23 & 7.24) -- only extracellular organelles, manufactured by microtubules. Flagellum (latin word for 'whip'), Cilium (latin for 'eyelash'). Flagella are rarely very numerous per cell; cilia are never in numbers less than hundreds of thousands. All form movement; if cell is detached, it moves the cell; if attached, it moves the fluid over the cell. All constructed of internal framework of microtubules. 9 pairs of microtubules around a pair of microtubules (9 + 2 arrangement). More reinforcement at the cell membrane of 9 sets of triplets of microtubules.

Cell Wall

• The presence of a cell wall, a thick reinforcing structure is actually common to most organisms, being found in 5 of the 6 kingdoms of living organisms. All other kingdoms have cell walls (including most protistas).
• Only members of the animal kingdom always lack cell walls.
• Plants are the best known for having cell walls. The basic construction is the same for all, although the exact chemical composition is variable.
• The basic construction is cellulose fibers embedded in other polysaccharides and proteins. The cellulose fibers act like reinforcing rods in concrete.
• Many plants actually have TWO cell walls.
• All plant cells possess a primary cell wall.
• This is a thinner, more flexible wall that grows with the plant cell.
• Between primary walls of adjacent cells is a sticky layer of polysaccharides called pectin referred to as the middle lamella. Cells in plants glue themselves together using pectin. (Lamella means sheet). (figure 7.28, p. 132)
• This helps the cells to literally "stick" together.
• When the cell is mature, one of two things can happen:
  • the primary cell wall thickens and hardens. In this case, there is still transport between the cell and its environment and the cell remains alive within the wall.
  • A secondary cell wall of much thicker, harder material is secreted, often in several layers. This wall isolates the cell, which dies, leaving a hollow "box". A tree's bark contains secondary cell walls; dead cells.

Extracellular Matrix

• As mentioned, animal cells lack cell walls, but they do have an extracellular matrix. Prefixes to understand to know where items relate to the cell: Extra - outside, Intra - within, Inter - between.
• The ECM is a complex arrangement of several glycoproteins (CHO (Carbohydrate) bonded to a protein molecule), which are interwoven and connected to the outer surface of the cell membrane.
• These have numerous functions including cell communications and recognition ("I.D. tags"). Example, blood types, four blood types -- A, B, O, and AB. Differentiated by the presence of glycoproteins in the extracellular matrix. (figure 7.29, p.133). Extracellular matrix structures are also used to meld cell structures together.

Physical Connections

• In multicellular organisms, certain cell types are physically bound together in various ways.
  • Animal -- Three types of connections:
    • Desmosomes -- ("bonding" + "body") -- A close binding of cells by the interlocking of button-like proteins from each cell membrane, which are internally anchored by protein filaments ("spot weld"). Connection is attached between two layers at a specific point.
      • Substances can still pass between cell membranes (around cells), e.g., skin, also heart fibers.
    • Tight Junctions -- Here, the proteins cross both sets of cell membranes.
      • Binds cells tightly enough to eliminate space between cells.
      • Substances cannot cannot move between cells but must cross cell membranes to move through the cells, e.g. intestinal lining. If somethings wants to cross it has to pass THROUGH the cell and be processed by the cell.
    • Gap Junctions -- also have proteins which cross both sets of membranes. To assist in faster travel from both sets of membranes, very rapid transmission of substances and signals to travel from one cell to the next, etc.
      • However, proteins are twisted around each other in such a fashion that they can be opened or closed.
      • Allows for materials to pass from one cell's cytoplasm to another. Allow rapid chemical and electrical communications, e.g heart muscle cells. Rapid meaning milliseconds or microseconds. (figure 7.30, p.134)
  • Plant -- only a single connection type:
    • Plasmodesmata -- Allows for inter-cell communications.
      • Channels that pass through the cell walls, thus connecting the cytoplasm of adjacent plant cells. Don't need the other structures because the cell walls protects the individual cells. However, only having a plasmodesmata also opens the cell up to a widespread illness.
      • The cell membrane is continuous between adjacent plants as is the ER.
      • Functionally similar to gap junctions. (figure 7.28, p.132)

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