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Cells: Life's Basic Units

  1. Patterns that relate to all cells
    1. All cells face common problems. Cellular function is related to cellular structure. The ability to relate structure and function shows the common set of problems and selective pressures that organisms of all types must deal with. Unity of life functions and structures is evident amongst life's diversity.
    2. Each cell's specialization is related to its shape, size, and organelles.
    3. The life of a cell depends on the organizational pattern and development of its parts and their integrated activities.
    4. Linnaeus was one of the first to recognize that patterns of structure and development indicated rela tionships between groups of organisms.
    5. The development of the electron microscope and the ultracentrifuge enabled biologists and chemists to see patterns within the structure and chemistry of cells and thus to develop generalizations regarding the relationships between and among cells and organisms.
    6. Chemists discovered that the chemistry of living organisms did not differ in principle from the chemistry of the inorganic world. As Pasteur recognized, Biochemistry was thus a complex and fascinating branch of chemistry as applied to living systems.

  2. Fundamental features of cellular organization
    1. Compartmentalization - a property which allows the "effective" size of an object to increase without unduly affecting the surface area to volume relationship (S2V3)
      1. allows for increased surface area by subdividing sections of the cell
      2. limits certain cell constituents to certain parts of cell (destructive hydrolytic enzymes)
      3. makes it possible to concentrate substances in specific regions where specialized biochemical activities can occur
    2. Self Assembly - spontaneous arrangement of molecules into specific structures; requires no additional energy input and is often the result of chemical bonding; e.g. phospholipids into bilayers in mem- branes; proteins into tubules and filaments
    3. Repitition - cells from pre-existing cells; thus patterns of cellular organization are repeated from one generation to another
    4. An Inherent Genetic Program - embodied in cell's DNA each gene represents a segment of a DNA strand: DNA--->RNA--->polypeptides (proteins) is usual sequence; The program of an organism represents a plan for action - a blueprint which provides a set of instructions to be used under specific circumstances, in a sequence that may be triggered by the operation of the program itself
    5. Redundancy - represented by multiple copies of mole- cules' (e.g. enzymes); multiple copies of the genetic information; All body cells of the organism contain identical genetic information. Given the correct chemical signals and environmental conditions, the cell is capable of differentiation in several possible directions.

    Interesting concept map for studying cells.

  3. Early forms of microscopes were developed in 16th and 17th century Europe. Robert Hooke looked at cork and gave us the name "cells" as a result of his view of the structures. Other scientists developed a series of generalizations that came to be known as the Cell Theory. They concluded that all living things are made of cells and that living cells arise only from preexisting cells. Curious (and eccentric) non scientists like Anton von Leeuwenhoek made important contributions.

  4. The most fundamental differences in the living world are differences between prokaryotic and eukaryotic cells. These two types of cellular organization are found within the three Domains, or general categories of living things. See additional material on DOMAINS in WebUnit 1.
    1. The Kingdom Monera includes prokaryotes.
      1. Prokaryotes lack a membrane-bound nucleus; the chromosome is a naked strand of circular, double stranded DNA. It is localized in a region called the nucleoid.
      2. Prokaryotes lack other membrane-bound ORGANELLES.
    2. The other four kingdoms, Protista, Fungi, Plantae, and Animalia, are eukaryotic.
      1. Like prokaryotes, eukaryotic cells are surrounded by a CELL MEMBRANE.
      2. Eukaryotic cells have internal membranes that surround the nucleus and other internal structures.
      3. Eukaryotes have a highly organized CYTOPLASM with an internal latticework, the CYTOSKELETON, that contri butes to structure and movement within the cell.
      4. In multicellular eukaryotes, cells may specialize for given tasks.
        1. Plant cells may be surrounded by a CELL WALL and may be largely filled by a storage organelle, the VACUOLE.
        2. Plant cells may contain CHLOROPLASTS, which carry out photosynthesis.
        3. Cellular specialization ultimately led to specialization of tissues, organs, and systems within multicelled organisms.

  5. Cells vary in size and number in organisms
    1. Most cells are small. Prokaryotes are the smallest cells. The largest animal cells include ostrich eggs and giraffe nerve cells.
    2. The surface-to-volume ratio (S/V) determines a cell's ability to exchange materials with its environment. A cell's shape affects it S/V.

  6. All cells must perform certain tasks.
    1. The cytoplasm must be separated from its environment so appropriate homeostatic internal conditions can be maintained.
    2. The cell must take in raw materials and expel wastes through the membrane and other barriers that may separate it from its environment.
    3. It must take in energy and convert it to a form useful for powering the cellular machinery.
    4. It must synthesize molecules and cell parts for repair growth and replacement (reproduction).
    5. It must coordinate and regulate its activities.

  7. All eukaryotic cells have certain structures.
    1. The PLASMA OR CELL MEMBRANE separates the cell from its environment and controls the movement of substances into and out of the cell. You may review the information in WebUnit 5 regarding membrane structure and function.
      1. The membrane is a LIPID BILAYER composed of PHOSPHO LIPID molecules with hydrophilic heads and hydro phobic tails. It is SEMIPERMEABLE.
      2. The membrane includes proteins that stabilize the lipid bilayer and act as gates, pumps, markers, or signal receptors.
      3. The membrane may fold inward to import materials (endocytosis) or outward to expel materials from the cell (exocytosis).
    2. The NUCLEUS is the largest organelle and contains the genetic material, DNA.
      1. The genetic information passes from DNA to RNA to proteins, which carry out the work of the cell.
      2. The NUCLEOPLASM of the nucleus is surrounded by a double-layer membrane, the NUCLEAR ENVELOPE OR MEMBRANE, which is perforated by pores. Each pore is a cluster of proteins that form a channel. Flow of molecules between the interior of the nucleus and the cytoplasm is regulated by a protein called Ran which directs molecular traffic.(Science, Vol. 279, February 20, 1998, p.1129-1131.)
      3. RIBOSOMAL RNA is formed on the ends of certain chromosomes and thus creates the dark-stained regions known as NUCLEOLI. After being exported to the cytoplasm, this RNA forms RIBOSOMES, which build protein according to the genetic blueprints encoded in messenger RNA.
    3. The CYTOSKELETON is a 3-dimensional latticework composed of proteins which are arranged into MICROFILAMENTS,MICROTUBULES, and INTERMEDIATE FILAMENTS that maintain the cell's shape and move materials within the cell.
    4. A system of INTERNAL MEMBRANES is involved in the manufacture, storage, transport, and export of proteins and raw materials.
      1. The ENDOPLASMIC RETICULUM (ER) is a series of membrane channels that may be studded on the outside with ribosomes (rough ER) for protein synthesis or without ribosomes (smooth ER) and involved in the synthesis of non-proteins such as lipids.
      2. After proteins are constructed on the ribosomes they come into contact with the ER and a channel is formed to allow entrance of these proteins into the ER. Once the protein has entered, a portion of one end (the signal portion) is enzymatically removed. The channel is disassembled.(Hanein, D., Matlack, K.E.S., Jingmikel, B., Plath, K., Kalies, K-U, Miller, K.R., Rapoport, T.A., and Akey, C.W. "Oligomeric rings of the Sec61p complex induced by ligands required for protein transloation."Cell,87. 721-732, November 5, 1996.)
      3. At the ends of the ER channels, membrane sacs (vesicles) pinch off and carry the products to their destination, which may be another membrane system, the GOLGI COMPLEX, which sorts, modifies and packages proteins, lipids, and other substances and exports most of them from the cell. (See also Science, "Coming to Grips with the Golgi," Vol. 282, December 18, 1998, p. 2172-2174.)
      4. LYSOSOMES contain digestive enzymes that break down ingested food or, if broken open, digest the cellular components. The acidic pH requirement of the enzymes of the lysosome prevents immediate damage to the cell if the contents were to enter the cytosolic compartment.
    5. MITOCHONDRIA provide chemical fuel for cellular processes by converting the energy in carbon-containing molecules into the energy of ATP molecules. This process is called AEROBIC RESPIRATION when oxygen is present in adequate supplies for the cell. Additional information will be found in WebUnit 8. Mitochondria and Chloroplasts have a prokaryotic origin and represent an example of endosymbiosis. Each have their own DNA and ribosomes and are capable of semiautonomous growth and reproduction. (See also Scientific American, "Mitochondrial DNA in Aging and Disease," August 1997, p. 40-47.)
    6. Plastids harvest solar energy and produce and store food or pigments.
      1. CHLOROPLASTS are the organelles of photosynthesis, converting solar energy into chemical energy in the form of carbon compounds. Chloroplasts contain CHLOROPHYLL and other light-absorbing pigments embedded in membranous sacs called THYLAKOIDS.
      2. CHROMOPLASTS are plastids that store yellow, red, or orange energy-trapping pigments and give color to fruits and flowers.
    7. Many plant cells have a CENTRAL VACUOLE that contains water and various storage products including pigments.
      1. This reduces the volume of cytoplasm and thus increases the cell's S/V ratio.
      2. The pressure of the water in the vacuole keeps the cell inflated. A plant wilts when the vacuole pressure drops because of drought.
    8. Many protists that live in fresh water have a CONTRACTILE VACUOLE that collects and pumps out water that could otherwise build up and burst the cell. This occurs when the water environment outside of the cell is hypotonic to the interior of the cell.
    9. Organelles of movement are made of protein and include CILIA, FLAGELLA, and microfilaments and microtubules.
      1. Cilia and flagella have the same internal structure, with 9 pairs of microtubules in a circle surrounding 2 inner microtubules.
      2. Using ATP energy, the microtubules slide past one another, causing the structure to bend.
    10. The plasma membranes of most cells are surrounded by cell coverings that protect the delicate membrane.
      1. An EXTRACELLULAR MATRIX, a meshwork of secreted molecules, protects many cells that live within multicellular organisms.
      2. CELL WALLS are made largely of cellulose and surround plant cells.
      3. Virtually all animal cells secrete a meshwork of molecules that surrounds them. These molecules are mostly fibrous proteins. COLLAGEN is the most common of the fibrous proteins.