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Lecture 5: Cellular Biology
I. Cell Theory
Concepts:
1. Cells are the functional and structural units of living organisms
2. The activity of an organism is dependent on both the individual and collective activity of the cells
that comprise that organism
3. Subcellular structures determine the biochemical activities of cells (i.e., principle of
complementarity)
4. Continuity of life has a cellular basis
II. Generalized Cell
A. Major parts
1. Nucleus
2. Cytoplasm
a. Contain organelles (see below)
3. Plasma membrane (To be discussed in Cellular Transport Lecture)
B. Types of Cells
1. Prokaryotic
a. Lack nuclear membrane
b. Genetic material in nucleoid region
c. Lack membrane-bound organelles
d. Found only in bacteria and archaebacteria
2. Eukaryotic
a. Have nucleus bounded by a nuclear membrane
b. Genetic material in nucleus
c. Have cytoplasm
i. Cytosol
ii. Membrane-bound organelles
III. Nucleus: control center for cellular function; contains genetic material
A. Number of nuclei
1. Most cells have a single nucleus
2. Large cells (those with a large amount of cytoplasm) have to be multinucleate
3. Red blood cells—only cell lacking a nucleus
B. Structures
1. Nuclear envelope
a. Double membrane—inner and outer
i. Outermembrane is continuous with ER
ii. Nuclear pores
iii. Selectively permeable
b. Perinuclear cisterna—fluid between membranes
2. Nucleoli
a. No membrane
b. Ribosomes subunits are assembled here
i. Large in growing cells
c. Associated with chromatin region associated with DNA coding for rRNA
i. Nuclear organizing regions (DNA regions)
3. Chromatin—DNA + globular histone
a. Nucleosome—fundamental unit of chromatin
i. Units of eight wrapped by DNA molecule
b. Chromosomes: prior to cell division, chromatin condenses to form chromosomes
IV. Cytoplasmic Organelles
A. Cytoplasm—cellular material inside cell
1. Most cellular activities occur here
2. Comprised of:
a. Cytosol—fluid in which other components are suspended
b. Organelles (see below)
c. Inclusions—non-functioning chemicals substances that may be unique to a given cell type
B. Ribosomes—site of protein synthesis
1. Complexes of RNA and protein
2. Free in cytosol
a. Suspended in the cytosol
b. Make proteins that function in the cytosol
3. Bound to endoplasmic reticulum
a. Make proteins that are either exported or integrated into the plasma membrane
C. Endomembrane System—interactive system of membranes that interact either directly through
physical contact or indirectly through vesicles
1. Vesicle—membrane-enclosed sacs that are pinched off portions of membranes moving from one
membrane to another
2. Endomembrane system includes:
a. Nuclear envelope
b. Endoplasmic reticulum
c. Golgi apparatus
d. Lysosomes
e. Vacuoles
3. Endoplasmic reticulum—network of membranous tubules and sacs (cisternae) within the cytoplasm
1. Continuous with the nuclear envelope
2. Two regions—smooth ER and rough ER
3. Smooth ER—lacks ribosomes
a. Synthesis of lipids, phospholipids and steroids
b. Carbohydrate metabolism
c. Detoxifies drugs
d. Calcium storage
4. Rough ER—protein synthesis (has ribosomes)
a. Ribosomes attached to ER synthesize secretory proteins
b. Growing polypeptide is threaded through ER membrane (receptor site) into cisternal space
c. Protein folds into native conformation
d. If a glycoprotein, oligosaccharides are enzymatically added to the secretory protein
e. Protein departs in a transport vesicle pinched off from the ER
5. Golgi apparatus—modifies, concentrates and packages rough ER products
a. Organelle of stacked, flattened membranous sacs (cisternae)
b. Has polarity
i. Cis face—receives transport vesicles from rough ER
ii. Trans face—pinches off vesicles
c. The rough ER products are modified as they move through Golgi apparatus
6. Lysomes—digestive compartments; membranous sac containing hydrolytic enzymes
a. Digest all major classes of macromolecules
b. Acidic
i. Pump H+ in from cytosol
c. Sequesters destructive enzymes from the cytosol
d. Functions:
i. Intracellular digestion—phagocytosis
ii. Recycle cellular organic material
iii. Programmed cell death
e. Role in disease—storage diseases
i. Lack specific lysosomal enzymes which causes substrate accumulation
ii. Pompe’s—glycogen in liver
iii. Tay-Sachs—lipid accumulation in brain
7. Vacuole
a. Membrane-enclosed sac
b. Diverse functions
i. Food vacuole
ii. Contractile vacuole
iii. Central vacuole in plants
D. Mitochondria—transduce energy into useable cellular work
1. Double membrane—structure similar to plasma membrane
a. Outer membrane—permeable to small solutes
b. Inner membrane—contains embedded proteins involved in cellular work
i. Cristae—folds of the inner membrane
c. Intermembrane space
d. Mitochondrial matrix—contains enzymes for metabolic steps of cellular respiration
E. Chloroplasts
1. Plastid—plant and algal membrane-bound organelles
a. Amyloplasts
i. Starch storage in roots
b. Chromoplasts
i. Pigments other than chlorophyll
c. Chloroplasts
i. Site of photosynthesis
2. Functional compartments
a. Intermembrane space
i. Space between double membrane that surrounds the chloroplast
b. Thylakoid space
i. Membrane surrounds chlorophyll
ii. Photosynthetic steps that convert light into chemical energy
c. Stroma
i. Interior of chloroplasts not surrounded by the thylakoid membranes
ii. Photosynthetic steps that use chemical energy to convert carbon dioxide to sugar
F. Peroxisomes
1. Specific metabolic pathways requiring peroxide-producing oxidases
a. Produce hydrogen peroxide
b. Converted to water by catalase
c. Breakdown of fatty acids
d. Detoxification of alcohol and other harmful compounds
2. Single membrane
V. Cytoskeleton—network of fibers throughout the cytoplasm that form a framework for support and
movement and regulation
A. Functions:
1. Mechanical support to maintain shape
2. Allows cell to change shape when environment changes
3. Associated with motility (has motor molecules—see later lectures)
4. Regulatory role in transmitting signals from cell’s surface to its interior
B. Constructed from three types of fibers:
1. Microtubules (thickest)
a. Radiate from cell’s center
b. Determine cell shape
c. Provide tracks for organelle movement
d. Involved in separation of chromosomes during cell division
e. Organized to form complex cellular components
i. Centrosomes and centrioles
ii. Cilia and flagella
2. Microfilaments (thinnest)
a. Made up of contractile protein (actin)
b. Attach to cytoplasmic side of plasma membrane
c. Participate in muscle contraction
d. Localized cell contraction
3. Intermediate filaments—most stable and permanent cytoskeletal element
a. Act as guy wires to resist pulling forces on the cell
b. Fix organelle position
VI. Specialized Surface Structures and Functions
A. Cell walls
1. Unique to plants
2. Basic design
a. Cellulose fibers embedded in a matrix of protein and polysaccharide
b. Plasmodesmata
i. Channels connecting neighboring cells
B. ECM—extracellular matrix
1. Found in animals
2. Elaborate network of macromolecules outside the plasma membrane
a. Secreted by cells
b. Glycoproteins embedded in a matrix
i. Collagen is the most abundant fiber
ii. Proteoglycans
3. Cells are attached to the ECM
a. Directly to the collagen or proteoglycans
b. Via fibronectins
i. Fibronectins bind with transmembrane receptors
ii. Integrins
C. Intercellular junctions
1. Tight junctions—impermeable junctions
2. Desmosomes—anchoring junctions; plaques and linkers; intermediate filaments
3. Gap junctions—movement of chemicals between adjacent cells; connexons