A. Prokaryotes: Simplest organisms; unicellular. Bacteria,
Cyanobacteria (bluegreen algae ---> photosynthetic), and
Archaebacteria ...Figure
4.4
1. No internal membranes --> no nucleus
a) plasma membrane surrounded by cell wall; plasma membrane segregates cell from its environment but is selectively permeable allowing some molecules to cross
b) DNA coiled into region called nucleoid
2. External Structures
a) pili --> cell-cell attachment and communication
b) flagella --> cell movement
B. Eukaryotes: larger and more complicated; both unicellular and multicellular
1. Internal Membranes create compartments or organelles for specific functions
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2. Cytoskeleton: maintain cell shape, motility, intracellular transport
3. Plasma Membrane: selectively permeable boundry between cell interior and environment; also a component of the Endomembrane System
4. Cell Wall: in plants
1. Procaryotes are small: a) mycoplasma -- 0.1 - 1 µ b) bacteria -- 1 - 10 µ
2. Eukaryotes are larger: 10 - 100 µ
3. What limits cell size -- Surface/Volume
a) must take up nutrients and O2 by diffusion across plasma membrane
b) cell volume increases as (cell dia)3 ==> as cells become too large their plasma membrane surface area becomes inadequate to supply cell needs.
4. Eukaryotes take some functions of the plasma membrane of bacteria and segrate them in internal membranes of organelles.
1. Light Microscopes -- use visible light
a) resolution -- cannot be better than approximately the wavelength of light ==> 4000 - 6000 Å (0.4 - 0.6 µ); can be much worse depending upon quality of optics
b) best light microscope has a resolution of approx. 0.2 µ One can see smaller things, but can't separate them if they're closer together than the resolution limit of the microscope.
c) magnification is how big you make the image of the object; this is unrelated to the resolution.
2. Contrast -- how to make different parts of the cell look different:
a) stains -- can be specific for different substances; e.g. fluorescent antibody stains
b) phase contrast -- more dense regions appear darker
c) DIC (differential interference contrast) -- similar to phase contrast but image looks different.
3. Electron Microscope -- electrons moving at high speed have characteristics of a wave
a) accelerate in electric potential of 100,000 volts ==> wavelength = 0.01 Å. Resolution actually only 2 - 3 Å (still very good) because lenses used to focus electrons are very poor.
b) TEM (transmission electron microscope) - analogous to light microscope (has objective lens, condenser lens system, lenses for magnification): increase contrast with stains and by creating a type of phase contrast.
c) SEM (scanning electron microscope) - move a small (20 - 50 Å diameter) beam of electrons across specimen and measure how many electrons are emitted at each point; display on TV. Shows specimen surfaces with great depth of focus but lower resolution than TEM
d) TEM specimen preparation - most common is to remove water and embed in plastic and slice very thin section (specimens must be thin for TEM).
e) SEM specimen preparation - remove water (critical point dry) and coat with gold or platinum to make surface electrically conductive.
1. Break open the cells --> release organelles, macromolecules etc.
2. Separate components --> differential centrifugation
1. Large assemblies of RNA + dozens of different proteins, synthesized in nucleolus in eukaryotes
2. Location:
polysomes - in both cases several - many ribosomes are frequently found together bound to the same RNA molecule all synthesizing the same protein.
internal membranes of eukaryotic cells related by direct physical contact or by transfer of membrane segments via small vesicles; derived from Endoplasmic Reticulum.
1. Nuclear Envelope
a) double membrane surrounding the nucleus separating it from the rest of the cell
b) nuclear pores - pores through both membranes; formed by protein molecules; controls traffic of molecules between the cytoplasm and the nucleus
c) nuclear lamina - layer of proteins inside the nuclear membrane; may be responsible for stabilizing the nuclear membrane.
d) Nucleolus--where ribosomes are made
2. Chromosomes
a) DNA + proteins (histones) ==> chromatin which is dispersed through most of the nucleus. The proteins keep the very long DNA molecules organized. Chromatin is divided into individual units called chromosomes.
b) during cell division chromatin segregates into individual chromosomes
c) number of chromosomes depends upon type of cell; e.g. what organism and whether it is somatic or germ (reproductive) form.
3. Nucleolus -- part of the nucleus responsible for synthesis of ribosomes needed for protein synthesis in the cytoplasm. Nucleolar Organizers are regions of chromosomes which contain multiple copies of genes for ribosome synthesis.
half of the total membrane in many cells--produces membrane for others
1. Lumen -- space enclosed by ER; separate from the cytoplasm; contains proteins targeted for
a) secretion; b) other organelles
2. ER is continuous with the Nuclear Envelope -- nuclear memb. is part of the endomembrane syst.
3. Rough ER -- contains many bound ribosomes which bind to the ER after beginning synthesis of proteins which are to be secreted into the lumen. These proteins are identified by a short signal sequence at their N-terminus which causes the ribosomes to bind to the ER. The proteins are then threaded through the ER membrane as they are synthesized.
4. Smooth ER -- connected to rough ER; no ribosomes
a) formation of vesicles to transport membrane and contents of lumen; vesicles bud off ---> Golgi
b) modified in special cells for various functions - e.g. sarcoplasmic reticulum in muscle cells.
c) lipid synthesis
d) drug detoxification
1. Intermediate step for vesicles produced by smooth ER transporting membrane and proteins
a) flattened discs of stacked membranes enclosing a lumen
b) cis face - end of Golgi where vesicles arrive; very close to or in contact with ER
c) trans face - opposite end from forming face; where processed vesicles are leaving
2. Golgi modifies products of ER
1. Compartment surrounded by membrane which contains enzymes to digest (hydrolyze) macromolecules, proteins polysaccharides, fats, nucleic acids.
a) membrane keeps enzymes from digesting functional parts of the cell
b) maintains low pH (approx. 5) where these digestive enzymes are most active
2. Used to digest:
a) external material by engulfing into vesicles (Phagocytosis) which then fuse with lysosomes.
b) digest and recycle cellular materials -- engulfs damaged organelles
1. Food Vacuoles -- produced by phagocytosis (plasma membrane enveloping large particles)
2. Plant Central vacuoles ...Figure 4.20
a) storage of organic compounds (including proteins) and inorganic ions
b) serves the functions of lysosomes in plant cells
They have evolved from prokaryotes engulfed by primitive eukaryotes ==> symbiotic relationship where each benefits
1. component proteins are synthesized on free cytoplasmic ribosomes
2. Have their own genetic system, DNA and ribosomes
3. They grow and divide independently of the other organelles
1. Cellular Respiration -- oxidize intermediate products of metabolism to CO2 and H2O; excess free energy transformed into synthesis of ATP
2. Structure
a) Approximately 1 µ in diameter; may be very long in some
cells
b) 2 membranes - inner membrane and outer membrane
c) inner membrane has large surface area and folds inward forming cristae; contains membrane proteins responsible for respiration and ATP synthesis
d) outer membrane is a sieve permeable to small molecules; intermembrane space is similar to cytoplasm in concentration of small molecules
e) matrix - contains enzymes responsible for many steps of metabolism, DNA, ribosomes, etc.
1. Photosynthesis
a) light energy used to make ATP via mechanism similar to that in miotochodnria
b) ATP is used to make CH2O from CO2 and H2O
2. Structure
a) 2 - 5 µ in diameter
b) 3 membranes - outer membrane, inner membrane, and thylakoid membrane
c) stroma - within inner membrane; contains enzymes for metabolism, DNA, ribosomes, etc.
d) thylakoids - disc-like sacks which stack producing grana. Contain membrane proteins which absorb light and make ATP.
1. Fatty Acid Degradation
2. H2O2 converted to H2O + O2 by an enzyme, catalase
1. Mechanical Support -- maintain cell shape
2. Motility
a) movement of entire cell via pseudopods or flagella
b) movement of organelles within cells
c) separation of chromosomes
d) cytokinesis -- division of cytoplasm(c) and (d) are essential for cell division
1. Hollow rods approx. 25 nm in diameter
a) constructed from globular proteins.
b) Grow, usually, from addition of subunits to one end.
2. MT's cause:
a) separation of chromosomes during Mitosis (cell division)
b) movement of organelles and vesicles
c) movement of cells -- cilia and flagella
3. Centrioles -- bundles of 9 triplets of MT's at center of MT organizing centers; these act as the focal point for MT formation ==> MT's radiate from these organizing centers and separate the chromosomes during mitosis. Centrioles are not essential; plants don't have them and mitosis still occurs.
4. Cilia and Flagella -- Figure 4.24
a) central pair of MT's surrounded by 9 double MT's
b) bend by action of dynein, and enzyme which gets energy from ATP; causes cellular motion or motion of fluid past tissues
c) attached to cell via basal body which is identical in structure centriole
1. 7 nm diameter -- helical fiber made of globular actin monomers
2. Movement caused by another protein, myosin (makes thick filaments in muscle)
3. Found in all types of eukaryotic cells and is responsible for
a) constriction of plasma membrane during cell division -- cytokinesis
b) pseudopod extension in ameboid movement
c) contraction of muscles
1. One end attaches to "Cargo" which the motor molecule carries along "Rails" of either Microtubules or Microfilaments
2. Fuel which powers the movement is a chemical called ATP which is produced by cellular respiration.
1. Integrin: A membrane protein of the plasma membrane connects the cytoskeleton with ...
2. Collagen and Fibronectin: extracellular glycoproteins
1. Tight Junctions: Join neighboring cells and prevent diffusion between them
2. Gap Junctions: Join the membranes of neighboring cells and allow diffusion of small molecules from one cell to the other ==> Communicating Junctions
