Parts of Cell

Smooth endoplasmic reticulum

Biology 42 — Human Biology (Spring 2007)

Parts of Cell

[ Students and professors, please read. ]

Plasma membrane:

Composition:  selectively permeable phospholipid bilayer with embedded proteins – surrounds cell as the boundary between outside and inside of cell; proteins can move inside this membrane which is liquid at room temperature.

Function:  regulates selective passage of molecules into and out of cell.

Molecules get in and out of the cell through the plasma membrane, either through diffusion, osmosis, facilitated transport, active transport, endocytosis, or exocytosis.  There are four types of plasma membrane proteins, which function in facilitated and active mechanisms by binding to molecules that can’t get through on their own.  In active transport, the proteins are often referred to as pumps (as in the sodium-potassium pump, which leads to the symptoms of cystic fibrosis if the chloride-ion channels in the pump malfunction, as they do in persons with cystic fibrosis).  Though the text only covers the carrier protein, the lecture mentions there are four types of plasma membrane proteins: channel, carrier, cell recognition, and receptor.  Channel proteins allow a molecule or ion to cross the membrane freely.  Carrier proteins selectively interact with a specific molecule or ion so it can pass through the membrane.  Cell recognition proteins, like the MHC glycoprotein, are different for every person, and are attacked by blood cells responsible for immunity in persons who have received transplants (there are prescriptions which counteract this).  Receptor proteins are shaped to bind to a particular molecule, like a puzzle piece.  Perhaps this is what the text is referring to when it mentions that some proteins, called glycoproteins, have short chains of sugars (carbohydrates) attached to them, specific to each cell, enabling the protein to act as a receptor for a chemical messenger, such as a hormone.  Receptor proteins also aid in receptor-mediated exocytosis (only mentioned in the lecture). 

The lecture notes mention that proteins function also in cell-cell recognition (I’m guessing the glycoproteins serve this function, which may be what the text is referring to when it says it is believed the carbohydrates attached to glycoproteins “help mark it as belonging to a particular individual”), communication between cells, and sticking cells together (cell adhesion molecules).

Nucleus:

Composition:  membrane-bounded organelle; nuclear envelope surrounding nucleoplasm, chromatin, and nucleolus.

Function:  storage of genetic information; contains chromosomes and controls the structure and function of the cell.

Nuclear envelope:

Composition:  double membrane surrounding nucleus, connected to ER (below).

Function:  its pores allow substances to pass between nucleus and cytoplasm.

Nuclear pore:

Composition:  opening in nuclear envelope.

Function:  permits passage of proteins into nucleus, and ribosomal subunits (produced in nucleolus) out of the nucleus.

Chromatin:

Composition:  network of fine threads in the nucleus.

Function:  these threads are stored DNA and proteins, and will serve the functions of DNA and proteins.

Nucleolus:

Composition:  spherical body; concentrated area of chromatin, RNA, and proteins.

Function:  produce ribosomal subunits.

Cytoplasm:

Composition:  contents of a cell between the nucleus and the plasma membrane.

Function:  contains the organelles.

Note:  the part of the cytoplasm which is not the organelles is called cytosol.

Organelles:

Composition:  small membranous structures in the cytoplasm, each with a particular structure and function; small compartments like rooms in a house.

Function:  depends on the particular organelle, see below…

Ribosome:

Composition:  protein and rRNA in two subunits.

Function:  site of protein synthesis (protein workbench) in the cytoplasm and on rough ER.

Polyribosome:

Composition:  string of ribosomes.

Function:  translate regions of the same mRNA strand during protein synthesis.

Endoplasmic reticulum (ER):

Composition:  system of membranous saccules and canals/channels in cytoplasm.

Function:  synthesis and/or modification of proteins and other substances, and transport by vesicle formation.

Rough ER:

Composition:  studded with ribosomes; continuous with nuclear membrane.

Function:  makes membrane, protein synthesis and secretion, attaches carbohydrate (to what?).

Smooth ER:

Composition:  having no ribosomes (an increase of smooth ER signifies tolerance in alcoholics).

Function:  various; lipid and phospholipid synthesis in some cells; detoxifies poisons in the liver, such as phenol barbital and alcohol.

Golgi apparatus:

Composition:  organelle; stack of membranous saccules and vesicles.

Function:  processing, packaging, and distribution of molecules about or from the cell; secretion of proteins.

Vacuole and vesicle:

Composition:  membranous sacs.

Function:  storage and transport of substances.

Lysosome:

Composition:  membrane-bound vesicle containing digestive (hydrolytic) enzymes.

Function:  intracellular (macromolecule) autodigestion (like webs between fingers during development, old organelles, and, in the case of WBC, bacteria).

Note:  leprosy and TB are bacteria that can avoid fusion with the Lysosome.

Also note:  Tay Sachs results when the lysosomes can’t break down lipids in the nerve cells.

Peroxisome:

Composition:  from the text, it looks like a lysosome.

Function:  detoxify poisons like alcohol; digest fats.

Note:  if you don’t have any, you’re dead.

Mitochondrion:

Composition:  membrane-bound organelle with inner membrane (with cristae).

Function:  power house of cell where aerobic cellular respiration occurs, during which ATP (cell energy) molecules are produced.

Note:  oxygen goes into mit. and carbon dioxide leaves; carbohydrate and oxygen turn into carbon dioxide and water and in the process ATP is made.

Also note:  the lecture note mentions endosymbiotic hypothesis, but I don’t recall it from the lecture, nor is it mentioned in the text (I hope it won’t be on quiz/test).

Cytoskeleton:

Composition:  microtubules, actin filaments, and intermediate filaments.

Function:  shape (internal framework) of cell and movement of its parts.

Actin filament:

Composition:  long, thin fibers; the protein actin.

Function:  involved in movement; cytoskeleton and muscle-cell framework; cytokinesis (division of cytoplasm), cell movement of neutrophils and other White Blood Cells.

Microtubule:

Composition:  cylindrical, containing 13 rows of the protein tubulin around an empty central core.

Function:  in cytoplasm, centrioles, cilia and flagella; form tracts for organelle and protein vesicle movement.

Note:  the microtubule organizing center, or MTOC, assists in forming spindle fibers and directs the movement of chromosomes.  Cancer drugs target microtubules, halting cell division by dismantling microtubule assembly.

Cilia (pl., cilium) and flagella (pl., flagellum):

Composition:  9+2 pattern of microtubules; cilium:  short, hair-like projections from plasma membrane, usually occurring in large numbers; flagellum:  slender, long extension; grow from basal bodies.

Function:  movement of cell; flagellum propels a cell through a fluid medium.

Note:  sperm cells have a flagella to help them swim; cilia are on the cells in the respiratory tract and sweep debris up the throat to clean the lungs (when immobile, can get bronchitis or sinusitis); cilia also line the ovaries (when immobile, will be infertile).

Centriole:

Composition:  9+0 (text) pattern of microtubules; 9 sets of 3 (lecture) microtubules; cellular structure, existing in pairs, located in centrosomes outside the nucleus; part of MOTC.

Function:  formation of basal bodies; possibly organizes mitotic spindle for chromosomal movement during mitosis and meiosis.

Basal bodies:

Composition:  cytoplasmic structure located at the base of cilia or flagella; same organization as centrioles that function to form them.

Function:  may organize (direct formation of) cilia or flagella.

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