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					    Cell

     By
Dr OJ Tsotetsi
                              CELL
Cells are the basic unit of life made from macromolecules.
Cells serves as the structural building block to form tissues and
organs
      Each cell is functionally independent- it can live on its own
        under the right conditions.
      1. it can define its boundaries and protect itself from external
       changes causing internal changes
      2. it can use sugars to derive energy for different processes
       which keep it alive
      3. it contains all the information required for replicating itself
       and interacting with other cells in order to produce a
       multicellular organism
      4. It is even possible to reproduce the entire plant from almost
       any single cell of the plant
              Membrane Functions

A. Form selectively permeable barriers
B. Transport phenomena
• Passive diffusion
• Active transport
C. Cell communication and signaling
D. Cell-cell adhesion and cellular attachment
E. Cell identity and antigenicity
F. Conductivity
                      Types of cells
A. Prokaryotic cells- eg. bacteria
          1. very simple-there are no organelles and most
   everything functions in the cytoplasm
B. Eukaryotic cells
           1. all contain the organelles that subcompartmentalize the
               cell
           2. includes unicellular algae and protists (e.g. ameba) that
               live alone or in colonies
           3. includes multicellular organisms - animals, plants,
               fungi - where cells work together
              a. plant cells are unlike animal cells in that plant cells
               have chloroplasts and cell walls.
              Animal cells have neither of these. Plant cells also have
   relatively large vacuoles.
         Parts of the eukaryotic cell
A. Outside (boundary) of the cell
       1. cell wall
          a. protects and supports cell
          b. made from carbohydrates- cellulose and pectin-
                polysaccharides
          c. strong but leaky- lets water and chemicals pass
                through- analogous to a cardboard box


Cell membrane made up of lipids, protein and to a less extend
carbohydrates.
Major function is protection and support of the structure
   Fluid Mosaic Model of Membrane Structure
  A. Mosaic: an object comprised of bits and pieces embedded in a
                        supporting structure
1. membrane lipids form the supporting structure
2. membrane proteins provide the bits and pieces
3. both lipids and proteins may be mobile or 'fluid'
B. Membrane lipids: the supporting structure
1. phospholipids
2. glycolipids
3. cholesterol
C. Membrane proteins: the bits and pieces
1. integral (intrinsic) proteins
2. peripheral (extrinsic) proteins
              The Membrane Lipids
A. phospholipids
1. most abundant of the lipids in membranes: form a lipid bilayer
2. phospholipid composition
a. glycerol backbone covalently linked to:
b. two long, non-polar fatty acid hydrocarbon chains
c. variable phosphate-containing polar group
3. phospholipids are amphiphilic (amphipathic) molecules:
a. hydrophobic ('water fearing') end: fatty acid chains
orient toward the interior of the membrane
b. hydrophilic ('water 'loving') end: phosphate group end
orients towards the extracellular space or cytoplasm
some common membrane phospholipids
a. choline containing phospholipids
(1) phosphatidycholine
(2) sphingomyelin
b. non-choline containing phospholipids
(1) phosphatidylserine
(2) phosphatidylethanolamine
(3) phosphatidylinositol
5. synthesis occurs in the membranes of the endoplasmic reticulum
All of the phospholipids are initially synthesized on the cytoplasmic
side of the lipid bilayer. The phospholipids containing choline tend
to get flipped to the opposite face of the lipid bilayer (which is
topologically equivalent to the extracellular space) by enzymes
known as 'flippases'. Once 'flipped', further flip-flopping is rare.
                       Glycolipids
1. least common of the membrane lipids (~2%)
2. always found in outer leaflet of plasma membrane*
3. general structure of a glycolipid is a variation on the
phospholipid theme
a. two long hydrocarbon chains
(1) hydrophobic, non-polar part of molecule
b. carbohydrate component: one or more sugars
(2) hydrophilic, polar part of molecule
4. synthesis of glycolipids
a. starts in membranes of endoplasmic reticulum
b. carbohydrates added in Golgi apparatus
              Cholesterol

1. steroid;lipid soluble; found in both
leaflets of lipid bilayer
2. amphiphilic: -OH group forms the polar
end of the molecule
3. synthesized in membranes of
endoplasmic reticulum
Summary
                 Membrane Proteins

A. Integral (intrinsic) proteins
1. penetrate the bilayer or span the membrane entirely
2. can only be removed from membranes by disrupting the
phospholipid bilayer
3. types:
covalently linked to membrane phospholipids or glycolipids
a. transmembrane proteins
(1) single-pass
(2) multiple-pass
Trans-membrane proteins have membrane spanning portions
containing alpha helically arranged sequences of 20-25
hydrophobic amino acids. Short strings of hydrophilic amino acids
separate the hydrophobic sequences from each other: These
hydrophilic stretches tend to be found exposed to the more aqueous
environments associated with the cytoplasm or the extracellular
space.
b. covalently tethered integral membrane proteins
Tethered integral membrane proteins may be largely exposed to
either the cytoplasm or aqueous extracellular space, but are
4. many integral proteins are glycoproteins
a. covalently linked via asparagine, serine, or threonine to sugars
The sugars of glycoproteins are exclusively found on the
extracellular side of the membrane or topological equivalent
5. synthesis of integral proteins:
a. occurs in the rough endoplasmic reticulum
b. many integral proteins wind up as glycoproteins
(1) glycosylation begins in lumen of er
(2) carbohydrates are modified in Golgi
6. integral proteins often form protein complexes having multiple
subunits
7. functions
a. enzymatic
b. receptors
c. transport
d. communication
e. adhesion
      Peripheral (extrinsic) proteins
1. do not penetrate the phospholipid bilayer
2. are not covalently linked to other membrane components
3. form ionic links to membrane structures
a. can be dissociated from membranes
b. dissociation does not disrupt membrane integrity
4. located on both extracellular and intracellular sides of the
membrane
a. often link membrane to non-membrane structures
5. synthesis of peripheral proteins:
a. cytoplasmic (inner) side: made in cytoplasm
b. extracellular (outer) side: made in er and exocytosed
              Membrane Dynamics
Lipid Asymmetry
1. phospholipids are asymmetrically distributed within the lipid
bilayer
a. outer leaflet of plasma membrane
(1) phosphatidylcholine
(2) sphingomyelin
b. inner leaflet of plasma membrane
(1) phosphatidylethanolamine
(2) phosphatidylserine

flippase helps establish the phospholipid asymmetry
2. glycolipids exclusively found on outer half of membrane
                      Lipid Mobility
mobility of membrane lipids:
a. rotational movement
b. lateral movement
The lipids (especially the phospholipids) are mobile within their half
of the lipid bilayer. Flip flop to the opposite side of the membrane is
rare. Mobility of the phospholipids tends to increase as the number
of double bonds ('kinks') between adjacent carbon atoms in the fatty
acid chains increase.
2. cholesterol effects on membrane fluidity:
At high temperatures cholesterol tends to reduce membrane fluidity,
probably by interacting with the hydrocarbon tails of the
phospholipid and glycolipid molecules. At low temperatures
cholesterol helps prevent membranes from freezing and thus tends to
maintain membrane fluidity.
              Protein Asymmetry
1. many different kinds of proteins are in the cell membranes
a. each type has a unique conformation and orientation
b. flip flop of proteins does not occur
c. conformational changes of protein can occur
2. carbohydrates of glycoproteins always at outer surface
a. help form the 'glycocalyx' (along with glycolipids)
                 Protein Mobility
 rotational mobility
lateral diffusion
Protein mobility can vary greatly. Some proteins are free to move.
Others may be tethered to structures in the cytoplasm or
extracellular spaces, thus restricting their movement. Some types
of cell junctions (e.g., tight junctions) can restrict protein
movements to a specific membrane domain.
             Membrane Protein
           Conformational Changes
1. help explain many many membrane functions
2. concept:
The binding of a membrane protein/glycoprotein to some other
cellular or extracellular substance, molecule, ion, etc. can/will result
in a 3-dimensional conformational change in that membrane
protein.
That conformational change can/will, in turn, drive or inhibit some
other cellular event.
Example 1: facilitated diffusion

Example 2: signaling
                         Cytoplasm
1. cytosol - watery inside of cell composed of salts, proteins which
act as enzymes
 2. microtubules and microfilaments - cables made out of protein
which stretch around the cell
           a. provide structure to the cell, like cables and posts on a
        suspension bridge
          b. provide a structure for moving cell components around
        the cell -sort of like a moving conveyer belt.-
 3. organelles - sub-compartments within the cell which provide
different functions. Each organelle is surrounded by a membrane
that makes it separate from the cytosol
mitochondrion - Cell powerhouse. Converts compounds into energy
through aerobic respiration. Also, plays a role in cellular response to
toxic stimuli from the environment
golgi apparatus - membranous hollow sacs arranged in a stack
      i. modifies proteins, lipids, and other substances from the ER
     ii. packets of these materials move to the edge of the golgi
         where the golgi membrane is pinched off to make vesicle
         (package); this new vesicle moves to the plasma membrane
         where it leaves the cell, or it goes to other sites within the
         cell
     iii. builds primary cell walls between newly divided nuclei
Lysosomes – proteins are synthesized by rough ER and transported to
the golgi apparatus for where enzymes are packed into vesicles
Function: destruction of foreign substances
chloroplast (50-100 per cell) - site of photosynthesis.
      i. allows production of sugars from sunlight and carbon
        dioxide
      ii. only found in plants and algae- other cells have to find
        sugar from outside the cell


vacuole or tonoplast- stores compounds that may interfere with other
things in the cell.
           Dominates the inside of a plant cell.
             - sugars, salts, pigments (e.g. red pigment in beets and
               purple onions, acids (lemon acids)


Peroxisomes – are membrane – bound organelles containing
enzymes that participate in oxidative reactions
ribosomes - site of protein synthesis-
       i. many different proteins have to be made by the cell- the
        proteins that a cell makes directs the cell's function and
        identity
         ii. ribosomes use the information coded in the DNA of the
        nucleus to produce proteins


 endoplasmic reticulum (ER) - a network of folded membranes
throughout the cytoplasm
     i. rough ER has attached ribosomes, active in protein synthesis
     ii. smooth ER lacks ribosomes and functions in the transport
        and packaging of proteins as well as the synthesis of lipids
                   Inside the cell

Cell is made up of cytoplasm and the nucleus
The two are separated by the nuclear membrane
                      NUCLEUS
Consist of chromosomes, chromatitin and nucleolus.
Surrounded by nuclear envelop which is porous and is a double
membraned.

nucleus- contains the genetic information which tells the cell
machinery which proteins, carbohydrates and lipids to make and
how they are assembled.
         i. this genetic information is coded in DNA
(deoxyribonucleic acid)
        What is a cell made from?
Four groups of biologically important molecules:
•             lipids
•             carbohydrates
•             nucleic acids
•             proteins
Water
                              Lipids
Composed of Carbon, Oxygen, Hydrogen atoms (COH) in building
                         blocks of Fatty acids
Fats (solid) and oils (liquid at room temperature)
       1. fats associated with animals - butter, lard
       2. oils associated with plants - corn oil, olive oil

Main characteristic of lipids - won't dissolve in water and is
repelled from water
Roles of lipids
        1. food- high energy (many C-H bonds), has more energy
        than any other molecule
        2. part of cell membranes
        3. also- waxes (cutin, suberin), hormones (testosterone,
        estrogen), certain vitamins, certain pigments (chlorophyll)

Basic form for energy storage- monoglycerides, diglycerides,
triglycerides
                       CARBOHYDRATES

Composed of COH -makes building blocks of monosaccharides
Roles
 1. energy storage (many C-H bonds) - sugar/starch energy source
 2. structural (especially in plants- cellulose) / animals with proteins
 3. carbon sources for making other building blocks (such as ribose
and deoxyribose for nucleic acids, amino acids)
3 main types
        1. monosaccharides (simple sugars)
        2. disaccharides
        3. polysaccharides (poly = many)
           a. polymers- composed of repeating subunits of
                monosaccharides -
                      Nucleic acids
DNA (deoxyribonucleic acid; master information carrying molecule
for the cell), RNA (ribonucleic acid; Copy of DNA molecule
Function- Contains the information for entire cell-expressed through
protein synthesis


Polymers of nucleotides- composed of:
       1. base- organic molecule with nitrogen- cytosine, guanine,
       thymine, adenine, and uracil (uracil is in RNA only)
       2. sugar- ribose, deoxyribose
       3. phosphate
Shape of DNA molecule- double helix (DNA
Other important nucleotide compounds- example- ATP (universal in
organisms; role- energy transfer or exchange)
                           Proteins
Composed of COH and Nitrogen (four main elements) -building
block is amino acids (20 different)
Roles
        1. basic building blocks of cell - much of cell structure
        2. part of cell membranes (help control entrance and exit of
        materials through membranes)
        3. important in animal structure: hair, nails, connective
        tissue (tendons, cartilage), muscles
        4. enzymes- facilitate chemical reactions
Composed of amino acids
1. repeating amino acids joined by the peptide bond forms
        a protein
2. 20 of them in proteins
3. 2 functional groups:-NH2 (amino group) & -COOH (acid group)
order of amino acids is important- order determines the 3-
dimensional shape of the molecule. This is significant because the
function follows form: the biological activity of a protein
depends largely on its 3-dimensional structure.
                             Water
Water is the most abundant and important molecules in cells.
Human are about two third water
Water is a solvent, reactant or product.
It is an important determinant of biological structure as lipid
bilayer, folded proteins and macromolecules are all stabilised by by
the hydrophobic effect
Properties of water is very important

				
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posted:3/13/2013
language:English
pages:43