CELL COMPONENTS


Cells are the structural and functional units of life. The smallest organisms are composed of only a single cell while the largest are made up of billions of cells.

Even when comparing the most diverse and complex organisms, at the cellular level they are remarkably similar. Even though the human body has over 100 different cell types, they all share certain features and they even have many characteristics in common with plants.

   A.  Cell Theory  
        1. all living organisms are composed of cells 
        2. cells are the fundamental units of all organisms and the chemical reactions of life take place within cells 
        3. all cells come from preexisting cells 

    B. Cells Are Small
        1. Cells range in size from a frog's egg (one millimeter) down to one micrometer

                               
        2. Surface-area-to-volume ratio requires that cells be small:
            a. As cells get larger in volume, relative surface area actually decreases
            b. Limits how large actively metabolizing cells can become 
            c. Cells needing greater surface area use modifications such as folding

II. Two types of cells distinguish two fundamentally distinct groups of organisms 

    A. Prokaryotes (before, nucleus) - no nucleus, i.e. DNA not membrane bound
    B. Eukaryotes (true, nucleus) - have a nucleus, i.e. DNA membrane bound
    C.  Major differences between prokaryotes and eukaryotes

 

Prokaryotes Eukaryotes
1. Nucleus no yes
2. Membrane bound organelles no yes
3. DNA (hereditary material) loop (no proteins) chromosomes (DNA + protein)

    D. Prokaryotes perform most of the metabolic functions that Eukaryotes do but the reactions do not take place in distinct compartments called organelles. Prokaryotes have an outer plasma membrane and some also have a cell wall. Only a relatively small number of organisms are prokaryotes (3,000 species). The majority of organisms are Eukaryotes.

III. Eukaryotic Cells 

                

    A. Eukaryotic Cells
        1. Include cells of all organisms except bacteria
        2. Membrane-bounded nucleus houses DNA in threadlike structures called chromatin
        3. Most are between 10-100 Ám in diameter, or ten to 100 times larger than prokaryotic cells
        4. More complex than prokaryotic cells: organelles, true nucleus, and a lattice of protein filaments called cytoskeleton

    B. Plasma membrane 

               
        1. Semi fluid, selectively permeable lipid bilayer embedded with proteins, carbohydrates and other chemicals 
        2. Called fluid mosaic model because phospholipids move about freely in the plane of the membrane and the proteins scattered about like a mosaic
        3. Plasma membrane: 


           

            a. keeps cells distinct from the environment 
            b. mediates transport of substances into and out of cell 
            c. translates hormonal and environmental signals involved in the control of cell growth and differentiation 



    C. Cytoplasm - semi fluid ground substance of the cell
        1. Forms most of the cell mass
        2. About 70% water

    D. Nucleus - control center of the cell

              
        1. Stores (isolates)  genetic information (DNA) which contains the code for protein assembly
        2. Structures:
            a. Nucleus has a diameter of about 5 Ám
            b. Chromatin - threadlike material that coils into chromosomes just before cell division occurs; total collection of DNA and associated proteins
            c. Chromosomes - single molecule of DNA and its associated proteins; may take on different appearances depending on current events happening within cell; series of organelles in which lipids are assembled and new polypeptide chains are modified into final proteins
            d. Nucleoplasm - semi fluid medium of nucleus
            e. Nucleoli - spherical bodies in nucleus; sites ribosomal rRNA formation
            f. Nuclear envelope - a double lipid bilayer
            g. Nuclear pores (100 nm) - permit passage of materials in and out

    E. Ribosomes - sites of protein synthesis
        1. May be attached to endoplasmic reticulum

    F. Endomembrane system - elaborate series of intracellular membranes that compartmentalize the cell
        1. Endoplasmic reticulum


            a. Endoplasmic Reticulum (ER) - system of membranous channels continuous with outer membrane of the nuclear envelope and wind through cytoplasm 
            b. Serves as a transport system, detoxification, storage and synthesis of a variety of materials 
            c. Rough ER - has ribosomes attached
            d. Smooth ER - lacks ribosomes; site of various synthetic processes, and storage; smooth ER forms transport vesicles. 

        2. Golgi apparatus 


            a. Golgi apparatus consists of a stack of 3-20 slightly curved sacs whose edges break away as secretory vesicles
            b. Proteins and lipids undergo final processing, sorting, and packaging
            

        3. Lysosomes (vesicles)


            a. Vesicles produced by Golgi apparatus that contain digestive enzymes 
            b. Function as cellular recycling centers & for autodigestion during development, e.g. tadpole tail absorption
            c. White blood cells that engulf bacteria use lysosomes to digest bacteria. 
        4. Vacuoles
            a. membrane bound sacs            
            b. Vacuoles in protozoa include digestive vacuoles and water-regulating contractile vacuoles

    G. Energy-Related Organelles       
        1. Mitochondria - cell powerhouses


            a. Sites of respiration - where chemical energy of carbohydrates is converted to ATP energy under oxygen plentiful conditions
            b. Mitochondria are bounded by a double membrane, the inner membrane has folds called cristae, increases surface are for reactions
            c.  Mitochondria contain own ribosomes and DNA arranged in a loop like that in prokaryotes

    H. Cytoskeleton - 3-D network of protein fibers which provides structural framework for the cell and suspends the organelles


    I. Cilia and Flagella
        1. Cilia - short, usually numerous hair like projections that can move in an undulating fashion (e.g., Paramecium, lining of human upper respiratory tract)

        2.. Flagella - longer, usually fewer, whip-like projections that move in whip-like fashion (e.g., sperm cells)