Topic 6: CELLS: STRUCTURE & FUNCTIONAL COMPONENTS (lecture 8)
OBJECTIVES:
1. Be able to differentiate prokaryotic vs. eukaryotic cells.
2. Know the classes of major chemical components in the cytosol.
3. Know the general structures, properties and general function of the major
organelles- nucleus, ER (both rough and smooth), Golgi apparatus, lysosomes and
perxisomes.
4. Be able to compare and contrast the structure and functions of mitochondria vs.
chloroplasts.
5. Be able to differentiate between the various types of cell junctions.
Cell- the ultimate functional unit of life (self-replicating, energy-transducing etc.);
however, viruses/viroids and prions (potentially infectious proteins) push this definition
further (more later…)
Fig. 7.1- cells vary in terms of their size; from microscopic to macroscopic (large).
Prokaryotic vs. eukaryotic cells
(1) prokaryotic (fig. 7.4) - no distinct nucleus; genetic material is not present in a
nucleus but rather is aggregated in a nucleoid; lack membrane-enclosed
internal structures (organelles); characteristically have a capsule (which may
be the target of antibiotics). BACTERIA.
(2) eukaryotic - distinct nucleus and membrane-bound organelles; yeast, animal
and plant cells.
Typical animal cell (fig. 7.7)Typical pant cell (fig. 7.8)- unique structures are chloroplast, presence of
rigid cell wall, central vacuole surrounded by a tonoblast, plasma
membrane is penetrated with plasmadesmata.
Cytoplasm - the material within the area bounded by the plasma membrane excluding
the nucleus; includes organelles and the semi-fluid material referred to as cytosol.
Cytosol is actually a term which refers to an experimental product of cell isolation. If
you gently break open cells and then centrifuge the material, the cytosol is the semifluid material that does not sediment. Cytosol consists mostly of the following:
(1) water
(2) inorganic ions- mostly K+, Cl-; some Na+, PO42- and Mg2+; trace amounts of Ca2+
(3) low molecular weight organic compounds- glucose, free amino acids, nucleotides
(4) macromolecules- proteins (enzymes), complex carbohydrates
(Note: the cytosol differs radically in chemical composition form the fluid that bathes the
cells, the so-called extracellular or interstitial fluid; we’ll talk about this later)
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There is considerable controversy as to the exact nature of the cytoplasm. The
traditional view is that it is simply a random mixture of the above components which
passively diffuse throughout the intracellular space. Others view the cytoplasm as a
structured and organized space in which the above components may associate with
various regions of the cell. In other words, distinct compartments may exist even in the
absence of membrane barriers. Scientific opinion is beginning to move towards this
latter point of view.
Membrane-bound compartments- Organelles
Nucleus- contains the bulk of genes (note: mitochondria and chloroplasts have their
own genome!)
fig. 7.9- nuclear envelope; double membrane perforated by pores; inner region is
supported by the nuclear lamina which consists of a dense network of protein fibers.
chromatin- DNA/protein complexes
chromosomes- condensed chromatin structures prior to cell division
nucleolus- organizing center
Endoplasmic reticulum (ER)- extensive double membrane system spread throughout
the cell; according to your text it represents 50% of total membrane in the cell. This
structure is extensively involved in protein synthesis and packaging as well as other
processes including membrane biosynthesis, detoxification and storing of certain
materials. There are two basic types of ER- (1) smooth ER and (2) rough ER (see fig.
7.11).
(1) smooth ER- diverse array of functions
-biosynthesis of various lipids including steroids, fatty acids & phospholipids
-conversion of glucose-6-phosphate to glucose ( G6Pase; glucose-6 phophatase)
-detoxification of foreign substances; converts them into more polar and easily
excretable compounds by action of enzymes known as mixed function
oxidases
-sarcoplasmic reticulum (SR) in muscle; stores and releases calcium (more later)
(2) rough ER- studded with ribosomes (ribosomes = macromolecular complexes
consisting of RNA and protein; site of translation of genetic message into
protein); primary site of synthesis of proteins bound for the cell and those that
will be secreted to the outside (usually are glycoproteins and are wrapped in
membrane vesicles). Membrane fragments are also made here.
Golgi Apparatus (fig. 7.12)- packages vesicles of protein for transport to the exterior.
Two sides to the structure: (1) cis side or face (receives materials from the ER;
reassembles them into laminar type structure) and (2) trans side or face ( new vesicles
pinch off form Golgi for transport elsewhere). Chemical constituents of membranes are
altered during residence in this structure.
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Lysosomes (figs. 7.13 & 7.14)- membrane bound structures that contain degradative
enzymes that hydrolyze the major classes of organic compounds; engulf and digest
materials acquired from the outside by phagocytosis and internal fragments
(autophagy). Formed by the Golgi apparatus.
Vacuoles- membrane bound structures that have a diverse array of functions; particles
engulfed during phagocytosis become food vacuoles; plants have central vacuoles (fig.
7.15) which are used to create storage sites for materials such as citric acid.
Peroxisomes- specialized organelles which have enzyme catalyzed reactions that
ultimately produce hydrogen peroxide (H2O2) as a product. H2O2 is highly reactive but
there is an enzyme (peroxidase) that detoxifies H2O2.
Mitochondria and Chloroplasts- extremely specialized organelles; very likely derived
in an evolutionary sense from early prokaryotic symbionts of other larger prokaryotic
cells (Endosymbiontic theory of origin of eukaryotic cells);
(1) both mitochondria and chloroplasts have their own genetic info and protein synthetic
machinery.
(2) however, a large fraction of proteins in mitochondria and chloroplasts are coded for
by genes in the nucleus; they are syhthesized on ribosomes in the cytoplasm.
Mitochondria (fig. 7.17)- 1- 10 m long (except for giant mitochondria from insect flight
muscle cells); two double membrane systems
- outer double membrane penetrated by pores which are very selective
- inner double membrane which is folded to form cristae (inner membrane has a large
number of peripheral and integral proteins); membrane is extremely selective in terms
of permeability.
- between the two membranes is the intermembrane space
- the interior of the organelle is known as the matrix
- functional role is energy conversion
Chloroplasts (fig. 7.18)- also have two double membrane systems
-outer double membrane
- internal double membrane structures known as thylacoids which are stacked together
to form grana (membranes have a high concentration of peripheral and integral proteins
too)
- the fluid region outside the thylacoids is known as the stroma
- functional role is conversion of light energy into chemical and reducing energy for
biosynthesis of sugar from carbon dioxide
Intercellular junctions
some cells are intimately connected to each other
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(1) plants (fig. 7.28)- cell wall are perforated with holes or plasmodesmata that literally
allow the cytoplasm of one cell to connect with the cytoplasm of another cell.
(2) animals (fig. 7.30)-tight junction: form a seal as you might find in epithelial cells
- desmosomes: form a secure junction which resists shearing forces due to the
presence of intermediate filaments
- gap junctions: proteins form a pore which allow continuity of cytoplasm; often
found in electrically excitable cells that must act synchronously (e.g.
muscle cells of the heart)
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