Exam #1: Cell Biology III Flashcards
What are the five endocytotic pathways?
1) Macropinocytosis
2) Clathrin- mediated endocytosis
3) Non-coat-mediated endocytosis
4) Caveolae-mediated endocytosis
5) Phagocytosis
Macropinocytosis
- Actin based process
- Nonspecific ingestion of fluids & solutes
- Can be triggered by bacteria e.g. Salmonella
- Examples: thyroid cells & dendritic cells of the immune system
Clathrin-mediated endocytosis
- Occurs at clathrin coated pits
- Can be mediated by receptors e.g. LDL/cholesterol
- Dynamin (GTPase) pinches off vesicles
- Coated pits contain cargo receptors & adaptin protein
- Coated vesicles form and then quickly become uncoated
Non-coat-mediated endocytosis
- Simple invagination of the plasma membrane
- Associated with endocytosis of cholera toxin & Shiga (Dysentary)
Caveolae-mediated endocytosis
- Caveolin protein mediates pinocytosis
- Associated with endocytosis of Simian Virus 40
Phagocytosis
- Ingestion of large particles that is mediated by receptors
- Actin-dependent
- Results in formation of a phagosome
- Phagosomes fuse with lysosomes
- Associated with endocytosis of non-biologic material e.g. latex or asbestos
What are the two requirements for endocytosis?
1) Energy (ATP)
2) Ca++
Dynamin
GTPase required to pinch off the vesicle in clathrin mediated endocytosis
Adaptin
Protein that associates with the intracellular portion of the cargo receptor in clathrin mediated endocytosis
What is non-receptor clathrin mediated endocytosis used to bring into the cell?
Neurotransmitters
How does an LDL receptor defect lead to atherosclerosis?
- LDL receptors are required for clathrin-mediated endocytosis of cholesterol
- LDL mutation impairs binding to adaptin/cargo
- Less cholesterol is brought into the cell
What are the three modes of secretion from the cell?
- Exocytosis
- Porocytosis
- Exosomes & Exosome-like vesicles
Exocytosis
- Bulk secretion of cellular synthetic products e.g. proteins/enzymes, hormones, & neurotransmitters
- Secretory vesicles are formed by Golgi that fuse with plasma membrane
- COP (Coat Protein) directed inside the cell
Porocytosis
Quantal release of neurotransmitters
Exosomes
- Secretory products released into the extracellular environment, surrounded in plasma membrane
- Can function to discard unneeded membrane proteins
- E.g. reticulocyte (RBC) discarding transferrin
What are the two pathways of exocytosis?
- Constitutive
- Regulated
Constitutive exocytosis
- Continuous secretory process
- Product not stored in secretory granules
Regulated exocytosis
- Secretory product IS STORED in granules
- Released in response to signal
Describe the staining of ribosomes.
- Ribosomes are basophilic b/c of negatively charged phosphate groups
- Use Hematoxylin
- Stains blue
Where are ribosomes synthesized?
Nucleoli
What are the two subunits of the eukaryotic ribosome?
- Large, 60S
- Small, 40S
Where do ribosomes synthesize proteins?
Cytoplasm
What are the two different forms of a ribosome?
1) Individual granule i.e. discrete ribosomal subunits
2) Polyribosomes, mRNA + ribosome
Where are polyribosomes found?
- Free in the cytoplasm (polysome)
- Bound to the ER
Where are ribosomes not found?
Mature RBCs
Polysome (free ribosome)
- Free ribosome in cytoplasm with pearl on a necklace appearance
- Function in protein synthesis for use within the cell
rER Characteristics
- Rough Endoplasmic Reticulum
- Basophilic
- Forms intracellular network of cisternae
- Continuous with nuclear envelope
- Polyribosomes attached
How are exported proteins produced?
- Signal recognition sequence
- Signal recognition peptide, SRP, recognizes sequence
- Binds to SRP Receptor & places ribosome in translocon
- Signal peptidase removes signal sequence
rER Functions
1) Synthesis of proteins for secretion, membrane proteins & lysosomal proteins
2) Synthesis of enzymes associated with sER
3) Modification of proteins
ER Stress
- Unfolded/misfolded proteins in ER cisternae
- Protein must be folded properly to traffic to the golgi
Unfolded Protein Response
- Chaperone synthesis increased
- Decreased synthesis of proteins
- Misfolded proteins exported from ER & polyubiquinated in cytosol for proteosome degradation
- Caspases activated for apoptosis if needed
Alpha1-Antitrypsin Deficiency
- Enzyme forms but is misfolded & accumulated in the ER
- Implicated in emphysema
sER Characteristics
- Lacks polyribosomes (hence smooth)
- Acidophilic
- Continuous with rER
- Cisternae have more tubular appearance than rER
Functions of sER
1) Cholesterol Homeostasis
2) Steroid Synthesis
3) Synthesis of Phospholipids
4) Glycogenolysis
5) Detoxification of Drugs
6) Storage, release, & uptake of Ca++ in striated muscle
Von Gierke’s Disease
- Defect in glucose 6-phosphatase or glucose 6-phosphate transporter in sER that leads to glycogen accumulation
- Sx= hepatomegaly, hypoglycemia, & increased lactate
Effect of protracted drug and/or alcohol use on the sER?
Increase in cytochrome p450 enzymes
Malignant Hypertermia
- Mutant ryanodine-1 receptors that allow excessive Ca++ movement from sER into cytoplasm in response to anesthesia
- Increases sER-Ca++ pump that consumes large quantities of ATP
- ATP consumption gives off heat
Pharmacologic treatment for Malignant Hyperthermia
- Dantrolene
- Ryanodine-1 Receptor Antagonist
