Download Chapter 7: A tour of the cell

Chapter 7: A tour of the cell
How we study cells
Microscopes provide windows to the world of the cell
Brightfield (unstained and stained)
Phase contrast and Nomarski optics
Fluorescence
Confocal
TEM and SEM
Cell biologists can isolate organelles to study their functions
Homogenization
Differential centrifugation
Low speed centrifuge
Ultracentrifuge
A panoramic view of the cell
Prokaryotic and eukaryotic cells differ in size and complexity
All cells have
Plasma membrane
Cytosol which makes up the cytoplasm
Chromosomes
Ribosomes
Cells differ in
Size (mycoplasma are the smallest at .1 to 1 micron; bacteria at 110 micron; eukaryotes at 10-100 micron)
Geometric relationship between surface area and volume explains
why most cells are microscopic
Presence or absence of a nucleus and membrane defined organelles
Internal membranes compartmentalize the functions of the eukaryotic cell
Familiarize yourself with the diagrams of a representative animal cell
(page 114) and a representative plant cell (page 115)
The nucleus and ribosomes
The nucleus contains a eukaryotic cells genetic library
Nucleus
Contains MOST of the genes (excludes mitochondrial and
chloroplastic DNA)
Nuclear membrane aka nuclear envelope
Envelope is a double (2-lipid bilayers) membrane
Perforated by nuclear pores
Pores are lined with proteins, pore-complex proteins
Nuclear side of envelope is line with a mesh of protein filaments
aka nuclear lamina
A nuclear matrix (like a cytoskeleton for the nucleus) of protein
filaments maintains the shape of the nucleus
Chromatin
DNA is organized as chromatin (condenses prior to cell division as
chromosomes)
Each eukaryotic species has a characteristic # of chromosomes
Nucleolus
Assembles ribosome subunits
Pass out to cytoplasm through the nuclear pores
Ribosomes build a cell’s proteins
Made of ribosomal RNA and protein
Two pieces: large subunit and small subunit
Two structurally and functionally identical types: free (located in the
cytoplasm) bound (attached to rough ER)
Free type makes proteins to be used in the cytoplasm (glycolysis enzymes)
Bound type makes proteins to be inserted into membrane; or moved to and
used in organelles like lysosomes; or secreted from the cell
The endomembrane system
Consists of: nuclear envelope, endoplasmic reticulum, Golgi apparatus,
lysosomes, various vacuoles, and plasma membrane
They are NOT identical in structure or function
The ER manufactures membranes and performs other biosynthetic functions
The inside of the ER is called the cisternal space and is continuous with
the space between the two nuclear envelope layers
Smooth ER
No ribosomes attached
Responsible for diverse metabolic functions
Synthesis of lipids
Metabolism of carbohydrates
Detoxification of drugs (enzymes for adding OH)
Storage of Ca++ in muscle cells
Rough ER
Polypeptides assembled on ribosomes and extend into
cisternal space
Protein folds into native structure in the cisternal space
Protein modification (glycosylation) done by enzymes in
the cisternal space
Secretory proteins are isolated inside transport vesicles
from the cytosol
RER also makes phospholipids and assembles membranes
Membrane proteins are inserted into the membrane
The Golgi apparatus finishes, sorts and ships cell products
RER transport vesicles travel to the Golgi
Golgi membranes have a polarity, the cis face and the trans face
Cis face is located near RER, receiving side of the Golgi
Glycosylation is modified in the golgi
Molecules are modified in stages as they pass through the golgi
Trans face buds off vesicles with the finished product for transport
to the plasma membrane
Golgi also synthesizes many cellular polysaccharides
Lysosomes are digestive compartments
Membrane bounded sac of hydrolytic enzymes
Can hydrolyze all the macromolecule groups
Optimal pH for these enzymes is 5
Lysosomal membrane pumps H+ into the organelle to keep
pH acidic
Explain why enzymes are not active if one lysosome breaks
open
Why doesn’t a lysosome digest itself
Programmed destruction of cells by its own lysosomes is
important in the development of many multicellular
organisms.
Examples tadpole tail cells, finger web cells, insect
metamorphosis
Lysosomal storage diseases are inherited disorders of
lysosomal metabolism (Pompe’s disease, Tay Sachs)
Vacuoles have diverse functions in cell maintenance
Vacuoles are larger than vesicles
Many types
Food vacuoles
Contractile vacuoles
Central vacuole
Content of a vacuole is different than the Cytosol
Other membranous organelles
Mitochondria and chloroplasts are the main energy transformers of cells
These organelles are semi-autonomous
Grow and reproduce inside the cell (binary fission)
Contain DNA and ribosomes
Proteins for these are NOT made in RER but in cytosol ribosomes
and their own ribosomes
Mitochondria
Sites of cellular respiration
Found in nearly all eukaryotic cells (1-10 microns long)
Number per cell is correlates with the cells metabolic rate
Two membranes
Outside is smooth
Inner convoluted
(cristae)
Gives rise to intermembrane space
And the matrix
Respiratory enzymes spatially separated some in the matrix
some on the inner membrane
Chloroplasts
Sites of photosynthesis
Found in green plants and algae (2-5 micron long, lens shaped)
Two membranes
Outside is smooth
Inner is smooth, small intermembrane space
Thylakoids stacked inside the inner membrane
Fluid around the thylakoid is the stroma
Peroxisomes generate and degrade H2O2 in performing various metabolic
functions
Single membrane encloses enzymes that produce H2O2 and destroy
H2O2.
Metabolic functions
Use O2 to break fatty acids to smaller molecules
Detoxify alcohol (OH) by transferring H to O2
In plants (glyoxysomes) covert fats to sugar for a seeding
before it has a leaf
The cytoskeleton
Providing structural support for the cell and function in cell motility and
regulation
Network of proteins throughout the cell to:
Provide structure, shape and support
Motility for the cell and cell organelles
Regulation of biochemical activities
Three main types of fibers (based on diameter)
Microtubules (largest diameter, 25 nm)
Composed of dimer protein tubulin (alpha and beta tubulin)
Shape and support cell, move chromosomes during mitosis, tracks
for motor molecules
Specialized organelles are made of microtubules
Centrosomes and centrioles
Microtubules scan grow out of a centrosome
Centrioles are only in animal cells
Cilia and Flagella
Responsible for cell motion (sperm, move fluid over
cell surfaces)
Cilia short and numerous, flagella longer and fewer
Ultrastructure is common
9 doublets of tubulin in a ring with a central pair ( 9
+ 2 pattern)
Doublets are attached to each other by a pair of
motor molecules, the protein dynein and protein
cross links
Anchored to the cell by the basal body, ring of 9
triplet tubulin, no central pair
Microfilaments (smallest diameter, 7 nm)
Composed of protein actin
Function to bear tension (pulling) in the cell
Major role in cell motility
Example is muscle cells (page 131)
Make the pseudopodia in amoeba
Function in cytoplasmic streaming in plants
Intermediate filaments (diameter 8-12 nm)
More permanent structure is cells
Holds the nucleus in place
Cell surfaces and junctions
Plant cells are encased by cell walls
Prevents excessive uptake of water
Primary cell wall is thin, followed by secretion of the middle lamella made
of pectin
Secondary cell way is thicker and glued to the middle lamella
Wood is secondary cell wall
The ECM of animal cells functions in support, adhesion movement and regulation
Animal cells have extensive extracellular matrix (ECM)
Mostly glycoproteins, most abundant is collagen
Collagen is embedded in proteoglycan another glycoprotein
Fibronectin attaches the ECM proteins to the plasma membrane
Integrins are receptor proteins that attach fibronectin to the microfilaments
in the cytoplasm
Intercellular junctions help integrate cells into higher levels of structure and
function
Cytosol of adjacent plant cells are connected through plasmodesmata,
openings in the cell wall, plasma membrane is continuous
Three types of cell junctions in animal cells
Tight junctions: membranes of cells fused to prevent leakage of
extracellular fluids across a layer of epithelial cells (intestinal
epithelium)
Desmosomes: attach cells together into sheets
Gap junctions: allow cytosol to flow between cells
The cell is a living unit greater than the sum of its parts