General Cellular Structure

1. Cell membranes
   1. Plasma membrane or outer boundary of cell regulates permeability
   2. Internal membranes have same structure

   Internal membrane systems consist of intracellular membrane bound organelles & vesicles, partition cell into functionally distinct areas

2. Cytosol
   1. Intracellular fluid exclusive of organelles
   2. Colloidal, composed of 70-90% water
   3. Organic compounds: particularly proteins & lipids, some carbohydrates
   4. Inorganic compounds: high K, low Na, & Cl, PO₄, HCO₃
3. Nucleus
   1. Large membrane-bound structure that contains hereditary information in form of DNA
   2. Has double membrane with large pores, outer layer is connected to RER
   3. Nucleolus - spherical regions of RNA & protein, where ribosomal subunits formed
4. Ribosomes
   1. Small round bodies composed of ribonucleic acid & protein, consist of light & heavy subunits
   2. Free clusters or attached to RER membrane
   3. Responsible for protein synthesis
5. Endoplasmic reticulum
   1. System of interconnected flattened sacs & cisternae continuous with nuclear membrane
   2. Synthesize protein, lipid & carbohydrates
      1. Smooth ER (SER) - no ribosomes, synthesizes lipids including phospholipids, cholesterol, steroids
         1. SER in liver - detoxifies, synthesizes glycogen
         2. SER in muscle store Ca⁺²
      2. Rough ER (RER) - outside covered with ribosomes that synthesize proteins which enter lumen of ER. Proteins are glycosylated or folded, surrounded by vesicles & transferred to Golgi complex
6. Golgi complex
   1. Stacks of saccules
   2. Receives vesicles from RER on larger forming face, modifies molecules & selectively exports (maturing face) them in vesicles to appropriate locations
7. Lysosomes
   1. Vesicle containing about 40 acidic hydrolytic enzymes
   2. Enzymes produced in RER & vesicles formed in Golgi
   3. Used for controlled intracellular digestion of macromolecules
   4. Primary lysosome carrying inactive enzymes fuses with endocytotic vesicle or damaged organelle (autophagic), digest contents, clean-up & recycling
8. Peroxisomes
   1. Vesicle buds from RER
   2. Contains catalase which oxidizes substrates to hydrogen peroxide or breaks down H₂O₂
   3. Protect cells from free radicals
9. Inclusions
   1. Substances in cytoplasm but not in vesicles
   2. ex. droplets, granules, pigment
10. Mitochondria
    1. Double membrane
       1. Outer membrane is sac with large aqueous channels (transmembrane protein) permeable to molecules up to10,000 Dalton
       2. Inner membrane is folded into cristae & contains specialized enzymes for respiration
    2. Location of cellular respiration including production of ATP, convert energy to forms useable for vital cell functions
    3. Contains own DNA & ribosomes, self-replicating protein synthesis
11. Cytoskeleton
    1. Complex network of protein filaments in cytoplasm
    2. Responsible for changes in cell shape & movement, scaffolding for organelles
    3. Types listed by diameter:
       1. Microfilaments or actin filaments
          1. thin rods of varying length
          2. provide intracellular structural support for microvilli, podia, & hair cells
          3. involved in muscle contraction
       2. Intermediate filaments
          1. ropes of fibrous protein intermediate in size
          2. structural & tension-bearing, ex. Neurofilaments
       3. Thick filaments or myosin filaments in muscle
       4. Microtubules
          1. hollow tubes composed of tubulin
          2. support cell shape & change in shape
          3. form spindle fibers, centrioles, cilia, & flagella
12. Centrioles
    1. Cylindrical bodies located near nucleus
    2. Contain 9 groups (9+0) of microtubular triplets arranged in circle
    3. Involved in spindle formation during mitosis
    4. Have own DNA & are self-replicating
13. Flagella & Cilia
    1. Cytoplasmic projections of cell containing microtubules (9+2)
    2. Flagella are long & function in locomotion
    3. Cilia are shorter & used in transport
       1. Not to be confused with microvilli which are very short & contain actin filaments

Ends survey of cell structure & begins discussion of some cellular functions.