Cell and Cell Structure

I. Overview

• Organization of the cell: Diagram
  
• Cell Membrane
• Cytoplasm
  • Cytosol
  • Organelles
    • Nonmembranous: Cytoskeleton, Microvilli, Centrioles, Cilia, Flagella, Ribosomes
    • Membranous: Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Peroxisomes, Vesicles

II. Plasma Membrane (Cell Membrane) Diagram

• "Fluid Mosaic Model" - plasma membrane is composed of a double layer (bilayer) of
      phospholipid molecules with proteins that float/move among the phospholipids, yet the
      plasma membrane is stable.
• Diagram of the cell membrane:
  
• Proteins function....
  1. As cell markers for recognition by immune system
  2. As receptors (e.g for hormones)
  3. As catalysts
  4. Transportation
• Proteins in the membrane...
  1. integral proteins (maintain selective transport)
  2. peripheral proteins (catalyst and mechanical function)
• The plasma membrane also contains a myriad of biological compounds such as glycoproteins,
      glycolipids, and proteoglycans (all referred to as glycocalyx) that extend outward from the
      plasma membrane.

III. Cytoplasm

• Cytoplasm is the material found inside the cell and is divided into two subdivisions: cytosol
      and organelles.
  • Cytosol (intracellular fluid) contains dissolved nutrients, ions, soluble and insoluble
        proteins, and waste products.
  • Organelles are structures that perform specific functions within the cell and are classified as
        membranous and non-membranous. Diagram

  Mitochondria	Rod-like, double membrane, inner membrane folded into projections called cristae; Site of ATP synthesis.
  Ribosomes	Dense particles consisting of two subunits, each composed of ribosomal RNA and proteins; can be free or it can be attached to ER; site of protein synthesis
  E. R. (rough)	Coiling membrane system with ribosomes attached; proteins synthesized are packaged into vesicles for transport to the golgi apparatus
  E. R. (smooth)	Coiling membrane system lacking ribosomes; synthesizes lipids and carbohydrates
  Golgi apparatus	Stack of smooth membrane sacs adjacent to the nucleus; modifies synthesized proteins, then packages the proteins (e.g. lysosomes & peroxisomes) in vesicles for transport around/out of cell
  Lysosomes	Membranous sacs containing hydrolytic enzymes used in cell digestion
  Peroxisomes	Membranous sacs containing oxidative enzymes (e.g. peroxidase) that degrade toxic compounds such as hydrogen  peroxide
  Vesicles	Membrane bound sac that transports cellular material
  Microfilaments	Filaments containing the contractile protein actin; part of the cytoskeleton and functions in intracellular movement
  Intermediate filaments	Protein fibers that provides strength, stabilize the position of organelles, and transport materials within the cytoplasm
  Microtubules	Hollow tubes composed of the globular protein tubulin; microtubules provide strength and rigidity and anchoring major organelles
  Thick filaments	Large and long strands of myosin protein found in muscle cells that interact with thin actin filaments to produce muscle contraction
  Centrioles	Cylindrical structure composed of nine triplets of microtubules; centrioles direct the movement of DNA during cell division as well as form the bases of cilia and flagella
  Microvilli	Small, finger-shaped projections of the cell membrane that actively absorb fluid and nutrients
  Cilia	Cell surface projections composed of microtubules; cilia move to propel substance across the cell surface
  Flagella	Larger and longer cilia that provides cellular locomotion (e.g. human sperm)
  Nucleus	Structure housing genetic information and is surrounded by a membrane (nuclear envelope)

IV. Membrane Transport Processes

• Transportation of materials across the cell membrane is determined by the components in the membrane that impart permeability.
• Most cell have selective permeability, free passage of some materials and restricts the passage of others
• Permeability may be based on size, electrical charge, molecular shape, solubility, etc... Passage across the membrane is classified as active (requiring energy) and passive (not requiring energy)
• Membrane transport processses:
  • Passive
    • Diffusion - net movement of particles from an area of higher concentration to an area of lower concentration. Diagram
    • Osmosis - diffusion of water through a selectively permeable membrane. Diagram
    • Facilitated diffusion - diffusion of a substance with the aid of a membrane carrier. Diagram
    • Filtration - movement of water and solutes through a semipermeable membrane from a region of higher hydrostatic pressure to a region of lower hydrostatic pressure
  • Active
    • Active transport - movement of a substance (with the aid of a membrane carrier) through a membrane against its concentration gradient.
    • Exocytosis - substances enclosed in a vesicle fuses with the plasma membrane, the vesicle then ruptures, releasing the substances outside the cell. 
    • Endocytosis (types): Diagram
      • Phagocytosis - the cell membrane extends outward and encloses large particles which are then transported into the cell
        
      • Pinocytosis - particles attach to the cell membranes which collapses, causing particles to be taken into the cell. 
      • Receptor-mediated - pinocytotic movement initiated by protein receptors on the plasma membrane. 
• Movement of particle may be....
  • Symport - movement of two or more different kinds of material in the same direction across the
                        cell membrane
  • Uniport - movement of one type of material in one direction across the cell membrane
  • Antiport - moving two types of material across the cell membrane in opposite directions

V. Cell Division (Cell Life Cycle)

Multicellular organisms develop from a zygote, which is formed by the fusion of a sperm and an egg (gametes). Each gamete has half a half compliment of chromosomes (haploid number) and when combined gives rise to a zygote with a complete set (diploid number) of chromosomes. In order for the zygote to develop into a multicellular organism, it must repeatedly undergo cellular divisions. The series of events a cell (or zygote) undergoes that ultimately produces a new cell is called the cell cycle.

The cell cycle is divided into two major stages: Mitosis and Interphase. Interphase is subdivided into three phases: S, G₁, and G₂ phases. During the S phase, DNA is duplicated in order to provide a full compliment for the new cell, called a daughter cell. The G phases are periods of growth and differentiation of a cell. The cell spends 90% of its time in interphase. Diagram

Mitosis, in comparison to interphase, is subdivided into four phases: Prophase, Metaphase, Anaphase, and Telophase. During prophase, chromosomes (consisting of DNA and proteins) become distinguishable in the nucleus. In early prophase, the nuclear membrane breaks down, and the chromosomes condense and become distributed throughout the cytoplasm. At high magnifications, sister chromatids may be detected. Chromatids, which are joined to each other in a region called the centromere, are identical copies made during DNA replication. The chromosomes may be sorted or arranged with the aid of contractile fibers called mitotic spindle fibers. By late prophase the chromosomes are drawn toward the middle of the cell.

In metaphase, sister chromatids become arranged toward the center of the cell (equatorial plate) in a plane at right angles to the long axis of the spindle. Once all chromatids are aligned at the equatorial plate, anaphase begins. The pair of chromosomes that comprise the chromatid are separated and transported to the polar (opposite) ends of the cell. Telophase will begin. During this stage in plant cells a cell plate will form and divide the original cell into two daughter cells. In animal cells, the cytoplasm pinches inward forming the cleavage furrow. Towards the end of telophase, in both plant and animal cells, the nuclei begin to reorganize, chromosomes uncoil, and the nuclear membrane reforms. Diagram and uncontrolled mitosis? Diagram

VI. Protein Synthesis Diagram Diagram

• Ribonucleic acid (RNA) links DNA's genetic instructions for making proteins to the process of protein synthesis
• It copies or transcribes the message from DNA and then translates that message into a protein.
• RNA, like DNA, is a nucleic acid or polymer of nucleotides
• RNA structure differs from DNA in the following ways:
  • The five carbon sugar in RNA nucleotides is ribose rather than deoxyribose
  • The nitrogenous base uracil is found in place of thymine
• The linear sequence of nucleotides in DNA ultimately determines the linear sequence of amino acids in a protein.
• Nucleic acids are made of four types of nucleotides which differ in their nitrogenous bases
• Hundreds or thousands of nucleotides long, each gene has a specific linear sequence of the four possible bases.
• Proteins are made of twenty types of amino acids linked in a particular linear sequence (the protein's primary structure).
• Information flows from gene to protein through two major processes, transcription and translation.
• Transcription - the synthesis of RNA using DNA as a template
  • A gene's unique nucleotide sequence is transcribed from DNA to a complimentary nucleotide sequence in messenger RNA (mRNA).
  • The resulting mRNA caries this transcript of protein-building instructions to the cell's protein-synthesizing machinery.
• Translation - synthesis of a polypeptide, which occurs under the direction of messenger RNA (mRNA)
  • During this process, the linear sequence of bases in mRNA is translated into the linear sequence of amino acids in a polypeptide.
  • Translation occurs on ribosomes, complex particles composed of ribosomal RNA (rRNA) and protein that facilitate the orderly linking of amino acids into polypeptide chains.
  • Signals are contained in the RNA to start and stop translation.