Cellular Parts and Functions of Eukaryotic Cells

 

Part I: Introduction

A. History of the Cell:

      1. Antonie van Leeuwenhoeck (1632-1723)  built the first microscope and was probably the first person to see cells.

      2. Robert Hooke (1635-1703)  became a microscopist and was the first person to use the term cell.

B. Cell Theory

1.      The cell theory states that all organisms are composed of cells, that cells are the structural and functional unit of organisms, and that cells come only from preexisting cells.

2.      The Cell Theory is based on the work of three scientist

a.      Mathias Schleiden: all plants are composed of cells.

b.      Theodor Schwann: declared that all animals are composed of cells.  Cytology: study of cells was began.   Cell is the smallest unit of living matter.

c.      Rudolph Virchow:  viewed the human body as a state in which each cell was a citizen.  Cell is the basic unit of structure and function in organisms.  First to declare that cells reproduce and cells come from preexisting cells.

C.     Cell Size

                                                             1.      Cells range in size from one millimeter down to one micrometer.

                                                             2.      Cells need a surface area of plasma membrane large enough to adequately exchange materials.

                                                             3.      The surface‑area‑to‑volume ratio requires that cells be small.

a.       As cells get larger in volume, surface area relative to volume decreases.

b.       Size limits how large the actively metabolizing cells can become. Cells needing greater surface area utilize membrane modifications such as folding, microvilli, etc

    Part II.   Prokaryotic Cells

      A.  The Structure of Bacteria

1.   Prokaryotic cells lack a nucleus and are smaller and simpler than eukaryotic cells (which have a nucleus).

      a.  Bacteria are extremely small; average size is 11.5 μm wide and 26 μm long .

2.   Bacteria occur in three basic shapes: spherical coccus, rod-shaped bacillus, and spiral spirillum (if rigid) or spirochete (if flexible).

3.   Cell Envelope

a.   Includes the plasma membrane, the cell wall, and the glycocalyx. The plasma membrane is a lipid bilayer with imbedded and peripheral proteins; it regulates the movement of substances into and out of the cell.

             4.  Cytoplasm

a.   The cytoplasm is a semifluid solution containing water, inorganic and organic molecules, and enzymes.

b.   The nucleoid is a region that contains the single, circular DNA molecule.

c.   Plasmids are small accessory (extrachromosomal) rings of DNA; they are not part of the bacterial genetic material.

d.   Ribosomes are particles with two RNA-  and protein-containing subunits that synthesize proteins.

e.   Cyanobacteria (also called blue-green bacteria) are bacteria that photosynthesize; they lack chloroplasts but have thylakoids containing chlorophyll and other pigments.

 Part III. Eukaryotic Cells

A. Introduction to Eukarytoic Cells

                                                             1.      Eukaryotic cells are members of the protists, fungi, plants, and animals.

                                                             2.      A membrane‑bounded nucleus houses DNA; the nucleus may have originated as an invagination of the plasma membrane.

                                                             3.      Eukaryotic cells are much larger than prokaryotic cells, and therefore have less surface area per volume.

                                                             4.      Eukaryotic cells are compartmentalized; they contain small structures called organelles that perform specific functions.

                                                             5.      Some eukaryotic cells (e.g., plant cells) have a cell wall containing cellulose; plasmodesmata are channels in a cell wall that allow cytoplasmic strands to extend between adjacent cells.

B.    The Structure of Eukaryotic Cells

               1.   The nucleus communicates with ribosomes in the cytoplasm.

                                 2.   The organelles of the endomembrane system communicate with one another; each organelle contains its   own set of enzymes and produces its own products, which move from one organelle to another by transport vesicles. 

3.   The energy-related mitochondria (plant and animal cells) and chloroplasts (plant cells) do not  communicate with other organelles; they contain their own DNA and are self-sufficient.

4.   The cytoskeleton is a lattice of protein fibers that maintains the shape of the cell and assists in   movement of the organelles.

 

More Information:

1)      Cytoplasm - the intercellular fluid containing the organelles (except the nucleus).

2)      Nucleus

a)      Structure

i)        Usually most prominent feature of cell

ii)       Has a double membrane called the  nuclear envelope

(1)   Opening in the nuclear envelope are called nuclear pores

(a)    Control the movement of substances between the nucleus and the cytoplasm

iii)     nucleoli: spherical bodies inside the nucleus which function in producing ribosomes

(1)   very important in cells that produce large amounts of protein

b)      function

i)        Nucleus houses most of cell's DNA

(1)   DNA is what makes genes

(2)   Organized into chromosomes.

3)      ORGANELLES FOUND IN THE CYTOPLASM

a)      RIBOSOMES - tiny granules that contain ribosomal RNA and proteins.

i)        Function:  to synthesize protein.

(1)   FREE RIBOSOMES - float in the cytosol.  Make protein for use within the cell

(2)   ATTACHED RIBOSOMES - are attached to the endoplasmic reticulum and make protein destined for insertion in the membrane or for export.

b)      ENDOPLASMIC RETICULUM - (ER) a system of membrane enclosed channels called cisternae that connect the nuclear membrane to the plasma membrane.

i)        Two types:

(1)   Rough endoplasmic reticulum (granular) - have ribosomes attached. 

(a)    Function:

(i)      Rough ER serves as a temporary storage area for newly synthesized molecules and may add sugar groups to certain proteins forming glycoproteins. 

(ii)    The rough ER also synthesizes phospholipids.  

(iii)   Together the rough ER and Golgi apparatus synthesize and package molecules that will be secreted from the cell.

(2)   Smooth endoplasmic reticulum (agranular) - have no ribosomes attached.

(a)    Function:

(i)      Smooth ER is the site of fatty acid, phospholipid and steroid synthesis.

(ii)    Enzymes within the smooth ER can inactivate or detoxify a variety of chemicals, including alcohol, pesticides, and carcinogens.     

c)      GOLGI COMPLEX - consists of flattened sacs called cisternae, stacked upon each other like a pile of plates with expanded bulges at their edges.  There are Golgi vesicles along the expanded edges of the cisterns. 

i)        Function:

(1)   Process, sort, package and deliver proteins and lipids to the plasma membrane, and forms lysosomes and secretory vesicles. 

(2)   All proteins destined for export follow a similar route: 

(a)    ribosomes --> rough ER --> transport vesicles --> Golgi complex -->secretory vesicles --> release to exterior of cell by exocytosis.

(b)    Some vesicles that leave the Golgi complex are loaded with digestive enzymes intended for use within the cell.  They become organelles called lysosomes.

d)      LYSOSOMES - membrane-enclosed vesicles that contain powerful digestive enzymes capable of breaking down a wide variety of molecules.

i)        Function:

(1)    Lysosomes digest bacteria and other substances that enter the cell through phagocyosis or pinocytosis.  These digested substances may be used by the cell.  RECYCLES.

e)      MITOCHONDRIA - (powerhouse of cell) are the main sites for generation of ATP.  The mitochondria consist of 2 membranes, a smooth outer membrane that gives the mitochondria a cigar shape and an inner membrane arranged in a series of folds called cristae.  The folds of the cristae provide a tremendous surface area for chemical reactions known as cellular respiration.  Mitochondria self-replicate; they divide to increase their numbers.  The  replication is controlled by genes found in mitochondrial DNA.

f)         CYTOSKELETON - cell shape and the capability to carry out a variety of coordinated cellular movements depend on a complex internal network of filamentous proteins in the cytoplasm called the cytoskeleton.

g)       FLAGELLA AND CILIA

i)         flagella - a whip-shaped structure that assists in the movement

ii)        Cilia - short, numerous, hair-shaped structures that wave back and forth to move substances along the external surface of the cell.

h)      CENTROSOME AND CENTRIOLES

i)        centrosome - a dense area of cytoplasmic material located near the nucleus;functions as a center for organizing microtubules in nondividing cells and for      forming the mitotic spindle during cell division. 

ii)       Centrioles - a pair of cylindrical structures within the centrosome.  Centrioles play a role in the formation and regeneration of flagella and cilia.