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The Cellular Level of Organization


An Introduction to Cells

The cell theory states:

      Cells are the building blocks of all plants and animals

      Cells are produced by the division of preexisting cells

      Cells are the smallest units that perform all vital physiological functions

      Each cell maintains homeostasis at the cellular level

    Homeostasis at higher levels reflects combined, coordinated action of many cells


Cell biology

      Cytology, the study of the structure and function of cells

    The human body contains both somatic and sex cells


A typical cell

      Is surrounded by extracellular fluid, which is the interstitial fluid of the tissue

      Has an outer boundary called the cell membrane or plasma membrane

The Cell Membrane

Cell membrane functions include:

      Physical isolation

      Regulation of exchange with the environment

      Structural support

      The cell membrane is a phospholipid bilayer with proteins, lipids and carbohydrates.


Membrane proteins include:

      Integral proteins

      Peripheral proteins

      Anchoring proteins

      Recognition proteins

      Receptor proteins

      Carrier proteins



Membrane carbohydrates form the glycocalyx





The Cytoplasm

The cytoplasm contains:

      The fluid (cytosol)

      The organelles the cytosol surrounds


      Nonmembranous organelles are not enclosed by a membrane and always in touch with the cytosol

    Cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes

      Membranous organelles are surrounded by lipid membranes

    Endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria


Cytoskeleton provides strength and flexibility


      Intermediate filaments


      Thick filaments



        increase surface area



      Direct the movement of chromosomes during cell division

      Organize the cytoskeleton

      Cytoplasm surrounding the centrioles is the centrosome



      Is anchored by a basal body

      Beats rhythmically to move fluids across cell surface



      Are responsible for manufacturing proteins

      Are composed of a large and a small ribosomal subunit

      Contain ribosomal RNA (rRNA)

      Can be free or fixed ribosomes



      Remove and break down damaged or abnormal proteins

      Require targeted proteins to be tagged with ubiquitin


Endoplasmic reticulum

      Intracellular membranes involved in synthesis, storage, transportation and detoxification

      Forms cisternae

      Rough ER (RER) contains ribosomes

     Forms transport vesicles

      Smooth ER (SER)

     Involved in lipid synthesis


Golgi Apparatus

      Forms secretory vesicles

    Discharged by exocytosis

      Forms new membrane components

      Packages lysosomes


Lysosomes and Peroxisomes

      Lysosomes are

    Filled with digestive enzymes

    Responsible for autolysis of injured cells


    Carry enzymes that neutralize toxins


Membrane flow

      Continuous movement and recycling of membranes



    Golgi apparatus

    Cell membrane


      Responsible for ATP production through aerobic respiration

      Matrix = fluid contents of mitochondria

      Cristae = folds in inner membrane



The Nucleus

The nucleus is the center of cellular operations

      Surrounded by a nuclear envelope

    Perinuclear space

      Communicates with cytoplasm through nuclear pores


Contents of the nucleus

      A supportive nuclear matrix

      One or more nucleoli


    DNA bound to histones



The genetic code

      The cells information storage system

      Triplet code

      A gene contains all the triplets needed to code for a specific polypeptide


Gene activation and protein synthesis

      Gene activation initiates with RNA polymerase binding to the gene

      Transcription is the formation of mRNA from DNA

    mRNA carries instructions from the nucleus to the cytoplasm


Translation is the formation of a protein

      A functional polypeptide is constructed using mRNA codons

    Sequence of codons determines the sequence of amino acids

    Complementary base pairing of anticodons (tRNA) provides the amino acids in sequence

How Things Get Into and Out of Cells


      The ease with which substances can cross the cell membrane

    Nothing passes through an impermeable barrier

    Anything can pass through a freely permeable barrier

    Cell membranes are selectively permeable



      Movement of a substance from an area of high concentration to low

      Continues until concentration gradient is eliminated



      Diffusion of water across a semipermeable membrane in response to solute differences

      Osmotic pressure = force of water movement into a solution

      Hydrostatic pressure opposes osmotic pressure

      Water molecules undergo bulk flow



      The effects of osmotic solutions on cells

      Isotonic = no net gain or loss of water

      Hypotonic = net gain of water into cell


      Hypertonic = net water flow out of cell




      Carrier mediated transport

    Binding and transporting specific ions by integral proteins



      Facilitated diffusion

    Compounds to be transported bind to a receptor site on a carrier protein


Active transport

      Active transport

    Consumes ATP

    Independent of concentration gradients

      Types of active transport include

    Ion pumps

    Secondary active transport


Vesicular transport: material moves into or out of cells in membranous vesicles

      Endocytosis is movement into the cell

     Receptor mediated endocytosis (coated vesicles)


     Phagocytosis (pseudopodia)

      Exocytosis is ejection of materials from the cell


The transmembrane potential

      Difference in electrical potential between inside and outside a cell

      Undisturbed cell has a resting potential


The Cell Life Cycle

cell division

      Cell division is the reproduction of cells

      Apoptosis is the genetically controlled death of cells

      Mitosis is the nuclear division of somatic cells

      Meiosis produces sex cells


      Most somatic cells spend the majority of their lives in this phase

      Interphase includes





Mitosis, or nuclear division, has four phases






Mitotic rate and cancer

      Generally, the longer the life expectancy of the cell, the slower the mitotic rate

    Stem cells undergo frequent mitoses

    Growth factors can stimulate cell division

    Abnormal cell division produces tumors or neoplasms


   Malignant (invasive, and cancerous)

   Spread via metastasis



      Process of specialization

      Results from inactivation of particular genes

      Produces populations of cells with limited capabilities

      Differentiated cells form tissues