Introduction to Cell Structure II Flashcards
Forms the interface between the cell and its environment and also functions as a permeability barrier
Plasma Membrane
Consists of a lipid bi-layer with embedded proteins
Plasma Membrane
Have very highly defined RERs and show many features characteristic of cells
Plasma cells
Amphipathic molecules with polar hydrophilic groups and aliphatic side chains that are hydrophobic
Lipids
The hydrophobic groups of the lipids are positioned to the interior of the
Bilayer
There are several classes of lipids and sterols in the plasma membrane, including
-often asymmetrically distributed
Phospholipids, glycolipids, and cholesterol
The fine structure of the plasma membrane is visible only with the
Electron microscope
The cell-cell boundaries seen at the light microscopic level consists of both plasma membranes of adjacent cells with the
Extracellular matrix between them
There are also proteins in the plasma membrane, many of which are
Glycosylated
What are the two classes of plasma membrane proteins?
- ) Integral membrane proteins
2. ) Peripheral membrane proteins
Traverse the entire width of the cell membrane and have three domains: Cytoplasmic, membrane spanning, and external
Integral membrane proteins
Attached to the cytoplasmic/extracellular regions of the lipid bilayer non-covalently
Peripheral membrane proteins
Cell membranes are fluid structures. Many, but not all proteins and lipids can move through the membrane in which way?
Laterally
Distribution of proteins in cell membranes can be studied by
Freeze-fracture electron microscopy
A frozen piece of membrane is split mechanically between the outer and inner lipid layers
Freeze-fracture EM
The outter face is called the
E (external face)
The inner leaflet is called the
P (protoplasmic or cytoplasmic) face
More integral membrane proteins are associated with the P-face because the P-face is stabilized by attachment to the
Cytoskeleton
Plasma membrane proteins can be stabilized by
-Best studied in RBCs
Cytoskeletal proteins
An inherited disorder characterized by defects in the membranes of erythrocytes. RBCs are more spherical, less deformable, and more susceptible to destruction by phagocytes in the spleen
Hereditary spherocytosis
Hereditary Spherocytosis is caused by a mutation in the cytoskeletal protein
Spectrin
Cytoskeletal proteins that associate with several membrane proteins to stabilize the biconcave shape of normal erythrocytes
Spectrins
The study of hereditary spherocytosis shows that interactions between the cell membrane and intracellular cytoskeletal proteins are needed to maintain normal
RBC morphology
Serve as linker/anchor proteins, transporters, receptors, and enzymes
Membrane proteins
The tall columnar cells making up the epithelium of the small intestine that functions in the absorption of nutrients
Enterocytes
Enterocytes have modifications to their surface to increase the surface area available for absorption. These modifications of the outer (apical or lumenal) surface are called the
Brush border (when viewed in LM) or Microvilli (EM)
Many cells exhibit microvilli at their surface, but few cells have them organized as a dense
Brush border
EM reveals that the brush border of enterocytes consists of envaginations of the apical cell membrane that are called
Microvilli
Extend as minute finger-like projections from the top (apical surface) of the cells and increase the absorptive surface
Microvilli
A “fuzzy coat” seen at the tips of the microvilli
Glycocalyx
An internal cytoskeleton of actin filaments supports the
Membrane Microvilli
Extends upward at the apical surface and encloses each microvilli
Plasma membrane
Within the core of each microvillus are filamentous structures composed primarily of the structural protein
Actin
These actin filaments are only visible by EM and extend downward to terminate in a filamentous complex at the top of the cells called the
Terminal Web (TW)
Each microvilli has a core of actin filaments and a glycocalyx at the
Surface
In intestinal epithelial cells, enzymes, especially those involved in the terminal stages of protein and carbohydrate digestion, are part of the
Glycocalyx
The glycocalyx consists of
Glycolipids, glycoproteins, and prteoglycans
In the core of each microvillus are actin, actin cross-linking proteins that stabilize the structure, and a motor protein that moves material along the actin filaments. What is the motor protein called?
Myosin
The terminal web at the base of each microvillus contains
Actin and intermediate filaments
Cells interact via several modifications of their lateral borders that together form a
Junctional Complex (JC)
Also contribute to junctional complexes that mediate interactions between epithelial cells
Membrane proteins
In LM, the locations of junctional complexes are identified by
Terminal bars (reddish dots at the tips of a cell junction)
Under EM, we see that cells maintain contact with each other by specializations of the lateral surface. These include
- ) Tight or occluding junctions (Zona occludens)
- ) Belt desmosomes (Zona adherens)
- ) Spot desmosomes (Macula adherens
- ) Communicating (gap) junctions
The zone of a junctional complex closes to the surface of the epithelial cells is called the
Tight (occluding) Junction (Zona Occludens)
For most cells (except RBCs) there are what two major cellular compartments?
- ) Nucleus
2. ) Cytoplasm
The cytoplasm has which two main components?
- ) Cytosol
2. ) Organelles
The cytoplasm outside of the organelles
-contains many enzymes and reactions of intermediate metabolism
Cytosol
Free ribosomes/polysomes are located in the
Cytosol
Temporary constituents without membranes including glycogen granules and lipid droplets, which are located in the cytosol
Inclusions
The cytosol also contains a structure made up of actin, intermediate filaments, and microtubules, known as the
Cytoskeleton
Localized in bundles of microfilaments that extend throughout the cell and are visualized through IHC
-also located in muscle cells and microvilli
Actin
Small cylindrical structures that are widely distributed through the cell and are composed of the protein tubulin
-component of the cytoskeleton
Microtubules
Microtubules not only extend through the cytoplasm, but also contribute to
Centrioles, Cilia, and Mitotic Spindles
Complex membrane-bound structures within cells
-includes the ER, Golgi, mitochondria, lysosomes, and peroxisomes
Organelles
An extensive cytoplasmic membrane system containing sites of protein and lipid synthesis and calcium storage
Endoplasmic reticulum
Consists of an extensive array of membrane tubules and their enclosed cisterns or spaces
-Does not have ribosomes
Smooth ER
Enzymes in the smooth ER function in
- ) Lipid and steroid synthesis
- ) Detoxification
- ) Drug metabolism
- ) Calcium ion storage
A cell with an extensive SER and a lot of lipid droplets is likely making
Steroid hormones
The outer nuclear membrane contains ribosomes and is continuous with the
Rough ER
Basically a ribosome studded membrane-bounded space
Rough ER
The rRNA of ribosomes contributes to the basophilic staining properties of
-Readily observed via LM
Secretory Cells (basophilia)
The Rough ER is abundant in cells synthesizing
Secretory proteins (i.e. plasma cells)
A few other proteins synthesized by the RER are not secreted by rather are retained in the cell, including
Lysosomal proteins and membrane proteins
Function in the synthesis, packaging, and secretion of digestive enzymes, and have linear arrays of rough ER that function in secretory protein synthesis
Pancreatic Acinar cells
Proteins synthesized in the rough ER are then transported to the
Golgi Complex
Contains several individual subdivisions composed of flattened sacs of membrane
-made up of cis and trans faces
Golgi Apparatus
Closest to the rough ER
cis-Golgi network (forming face)
Furthest from the rough ER
trans-Golgi network (maturing face)
What is between the cis- and trans- golgi networks?
cis-, medial, and trans-cisternae
Used to transport material between the Golgi cisternae
Vesicles
Bud from the trans Golgi network
Secretory Vesicles
Proteins enter the Golgi at the
cis-face
Associated with the cis-Golgi is a series of tubules (the cis-Golgi network), which transport vesicles carrying newly synthesized protein from the
Rough ER
The trans-Golgi is the maturing face, directed toward the secretory granules and the apical or peripheral cytoplasm. The trans-Golgi is where proteins
Leave for sorting
In some cells, proteins will accumulate in the Golgi and be sorted prior to secretion. This occurs in
Secretory Granules
What are the two mechanisms by which release of granule contents from the cell (exocytosis) is regulated?
Constitutive secretion or Regulated secretion
Proteins are packaged into secretory granules for export from the cell in the
trans-Golgi
The continual secretion of proteins that are not stored
Constitutive secretion
What are two examples of cells that perform constitutive secretion?
Plasma cells and fibroblasts
Continually secrete immunoglobins for the entire two weeks of their lives
Plasma cells
Continually secrete ECM components
Fibroblasts
Few, if any, secretory granules accumulate in
Constitutive Secretion
A large number of granules accumulate in many secretory cells that undergo
Regulated Secretion
During a meal, the exocrine pancreas receives a signal from cholecystokinin (CCK) and the acinar cells secrete digestive enzymes into the duct system for transport to the duodenum. This is an example of
Regulated Secretion
In addition to exocytosis (secretion), cells can also take in material in a process called
Endocytosis
Receptors in the cell membrane (usually integral membrane proteins) can bind to signaling molecules or nutrients. The receptor-ligand complexes can then enter the cell via vesicle formation that is initiated when coat proteins associate with the
Cytoplasmic lipid layer
The ligand-receptor complexes and the coat proteins then form a
-rapidly detaches from the surface to form a coated vesicle
Coated pit
This process of endocytosis is termed
Receptor-mediated endocytosis
The internalized vesicles then fuse with the
Early Endosomes
Distinct organelles consisting of vesicles containing material that has entered the cell
Endosomes
Receptors and ligands are separated, and the receptors are often recycled, in
Early endosomes
Route nutrients such as cholesterol to the lysosome where nutrients can be made available to the cell
Late endosomes
A cell that is undergoing regulatory cell secretion is characterized by well defined
Rough ER and Secretory Granules
A cell that is undergoing constitutive cell secretion is characterized by well defined
Roguh ER but NO secretory Granules
Mitochondria can move through the cytoplasm with the aid of
Cytoskeletal proteins
On the cristae of the inner mitochondrial membrane are spherical protein particles which couple oxidation to phosphorylation and
ATP synthesis
Mitochondria also participate actively in
Apoptosis
The numbers of mitochondrial cristae vary in different cells depending on
Energy needs (more energy = more cristae)
The liver is contains dark clumps in EM called
Glycogen granules
Mitochondria in adrenal cells are different because their cristae are
Tubular
Mitochondrial morphology can vary between
Cell types
Mitochondrial DNA does encode a few peptides, however, most mitochondrial proteins are encoded by
-enter from the cytosol, not from the rough ER and Golgi pathway
Nuclear DNA
Membrane bound packets of hydrolytic enzymes that become active at acidic pH and are the sites of intracellular digestion
Lysosomes
An ATP-driven proton pump in the lysosomal membrane maintains the
pH
Lysosomes process molecules that enter the cell from the
-Ex: cholesterol, antigens
External environment
Also function in the removal of worn-out intracellular organelles (mitochondria, RER) as well as entire cells (erythrocytes and bacteria) following phagocytosis
Lysosomes
Which dye will allow you to see lysosomes?
Trypan Blue
Lysosomes that are in eosinophils can be detected in EM because they have a
Dark line down the center
Membrane bound organelles involved in hydrogen peroxide metabolism and multiple other important metabolic processes
Peroxisomes
Because of high enzyme concentrations, peroxisomes usually have a dense material in their matrix that forms a
Crystalline core
