CELLS

 

    Cell Theory tells us:

 

•         A cell is the basic unit of life

•         All living things are made up of cells

•         New cells arise from preexisting cells

 

Why are most cells small?

 

•         Consider the cell surface-area-to-volume ratio:

–        Small cells have a larger amount of surface area compared to the volume

–        An increase in surface area allows for more nutrients to pass into the cell and wastes to exit the cell more efficiently

–        There is a limit to how large a cell can be and be an efficient and metabolically active cell

What are the two major types of cells in all living organisms?

•         Prokaryotic cells

–        Thought to be the first cells to evolve

–        Lack a nucleus

–        Represented by bacteria and archaea

 

•         Eukaryotic cells

–        Have a nucleus that houses DNA

–        Many membrane-bound organelles

 
   Major differences between prokaryotes and eukaryotes

    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.

Eukaryotic Cells 

   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 µm 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

   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

            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 

animation of membrane structure

Selectively permeable?  The membrane allows some things in while keeping other substances out.

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

   Nucleus - control center of the cell

              
        1. Stores genetic information determining structure/function of cells by regulating sequences of amino acids
        2. Structures:
            a. Nucleus has a diameter of about 5 µm
            b. Chromatin - threadlike material that coils into chromosomes just before cell division occurs; contains DNA & protein
            c. Chromosomes - rod-like structures formed during cell division; coiled or folded chromatin
            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

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

    Endomembrane system - elaborate series of intracellular membranes that compartmentalize the cell

  Endoplasmic Reticulum (ER)
       
             

       Golgi apparatus 

          
    Lysosomes 

     

   Energy-Related Organelles       
      Mitochondria - cell powerhouses

            a. Sites of respiration - where chemical energy of carbohydrates is converted to energy
            b. Mitochondria are about 0.5-1.0 µm in diameter and 7 µm in length
            d. Mitochondria are bounded by a double membrane, the inner membrane has folds called cristae, increases surface are for reactions
            e. Mitochondria contain ribosomes and DNA arranged in a loop like that in prokaryotes

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

   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)