Cell Structure 2 Flashcards
What are the parts of the endoplasmic reticulum?
Rough ER
Smooth ER
What does each part of the ER synthesize?
SER: components of the membrane
What are the purposes of SER?
- Membrane synthesis
- Synthesis of steroid hormones
- Detox of lipid-soluble drugs, metabolic waste, and ingested toxins.
- Calcium homeostasis
What is the mechanism by which SER detoxifies?
Cytochrome P450 enzyme system oxidizes toxins to an inactive form or into parts that can be excreted (in liver: either in bile or excreted by kidneys)
In which cells is SER particularly abundant?
Cells that produce steroid hormones like cells in adrenocorticoids, testes, etc.
How is SER involved in Ca homeostasis?
Ca pumps used to sequester Ca from cytoplasm and stored in SER; proteins in membranes act as channels and can be stimulated to release Ca into cytoplasm.
In muscle: Ca regulates muscle contraction (sarcoplasmic reticulum)
What does RER synthesize?
Integral membrane and secreted proteins via ribosomes (many ribosomes = polyribosomes)
Qualities of RER
- Studded w/ ribosomes on cytoplasmic surface
- Continuous with nuclear envelope
- Initial site of posttranslational modification of proteins and lipids
How do plasma cells synthesize antibodies?
Massive proliferation of RER, which makes antibodies (which are a kind of protein).
Where are ribosomes found?
Free in the cytosol or bound to the RER
What do ribosomes do?
Catalyze synthesis of proteins (i.e. protein translation of mRNA in cytoplasm); can have polysomes, which is translation of the same mRNA by several ribosomes
Where will protein synthesis occur if mRNA is coding for a soluble protein?
Cytoplasm
What is different if mRNA is coding for a secreted or transmembrane protein?
The bit of protein already synthesized will have a signal sequence on the end.
Also, synthesis will occur at the membrane as the ribosome binds to the membrane and synthesizes into the cell.
What is a signal sequence, and what does it do?
A chain of ~20 very hydrophobic amino acids that allows the synthesized protein to gain access into the cell membrane by binding to the signal recognition particle, which in turn also binds to the SRP receptor on the ribosomes located on the surface of the RER.
Describe the process by which ribosomes start synthesis.
- mRNA released from nucleus into cytoplasm.
- mRNA is bound by the ribosomes’ small subunit.
- After small subunit binds, large subunit binds to the complex.
- Two subunits together now begin to translate mRNA into proteins.
Describe the protein synthesis pathway by RER ribosomes.
- Protein synthesis in cytoplasm begins.
- Protein synthesis inhibited by binding of signal recognition particle to signal sequence.
- Protein synthesis starts up again once the SRP binds to SRP receptor on RER surface and forms pore into RER.
- Signal sequence is cleaved once inside the RER.
- Protein synthesis continues to completion in the RER, including any post translational modification (e.g. addition of carbohydrates to susceptible amino terminal).
- Ribosome dissociates from RER surface while protein folds and is then released into the cytoplasm.
Why does the RER determine plasma membrane polarity?
Enzymes that add carbohydrates to the protein also modify the heads of phospholipid molecules (to form glycolipids) inside the ER lumen-facing side.
Where is the default place for material synthesized in the RER?
Golgi apparatus - stacks of flattened membranes near the nucleus
What are the functions of the Golgi apparatus?
- Further post-translational modification of proteins
- Assembly of multi-subunit proteins
- Protein sorting and packaging into secretory vesicles
What types of post-trans mod occur in the Golgi?
Adding sugars, getting phosphorylated, getting sulfated
What are the parts of the Golgi?
- Cis (forming) face - vesicles from RER fuse here
- Medial (intermediate) face
- Trans (exit) face - sorting of proteins
Each layer involved in post-translational modification of proteins in vesicles.
What is the ERGIC?
Endoplasmic reticulum/Golgi intermediate compartment: mediates traffic b/w ER and Golgi complex, sorting cargo.
What role does the trans face play in lysosomal protein packaging?
Here, proteins are modified to add mannose 6-phosphate, which binds to M6P receptors located at that part of the Golgi, causing the lysosomal enzymes to concentrate there. Clathrin mediates budding of vesicles, which then lose the clathrin coat and remain in the cytoplasm as lysosomes.
What are the 3 destinations of proteins leaving the Golgi apparatus?
- Clathrin-coated vesicles that bud off into secretory granules that will fuse to the cell membrane and release contents into the extracellular matrix.
- Non-clathrin coated vesicles, which fuse to the membrane and deposit the proteins to become part of the plasma membrane.
- Lysosomes free in the cytoplasm.
What is a primary lysosome?
Vesicle that buds off the Golgi apparatus with lysosomal enzymes (acid hydrolases) inside.
What is a secondary lysosome?
A primary lysosome that fuses with an endosome containing something that requires breaking down.
What is inside a lysosome?
Hydrolytic enzymes only active at acidic pH’s; can break down anything in an endosome.
What does a protease break down?
Proteins
What does a nuclease break down?
Nucleic acids
What does a glycosidase break down?
Polysaccharides/carbohydrates
What does an aryly sulfatase break down?
Organic-linked sulfates
What does a lipase break down?
Lipids
What does a phospholipase break down?
Phospholipids
What does a phosphatase break down?
Organic-linked phosphates
What is heterophagy?
Digestion within the cell of material ingested via phagocytosis or pinocytosis (i.e. endocytosis).
Lysosomes budding off the Golgi either…
Release hydrolytic enzymes into the extracellular matrix or fuse with a late endosome to digest its contents.
What is autophagy?
Degradation of a cell’s own components through the lysosomal machinery in order to clean up debris from breakdown or damage of other parts within the cell.
What is lipofuscin?
A pigment-filled granule that is the condensation of material that can be broken down no further; typically cannot be released into extracellular matrix (except by macrophages); usually products of autophagy are kept as this.
What activates the enzymes in lysosomes?
Protein pumps in the lysosomal membranes that trigger hydrolyzation of acid hydrolases.
What is the problem with lipofuscin in neurons?
Accumulation of insoluble material b/c the cell cannot dilute it out (unlike epithelial cells, which constantly divide).
What are lysosomal storage disorders?
Buildup of substrate that would normally be broken down by enzymes.
What enzyme deficit occurs in Tay-Sachs? What accumulates as a result? What is most affected?
- Hexosaminidase
- Gangliosides (complex lipids)
- CNS neurons
What enzyme deficit occurs in Pompe’s disease? What accumulates as a result? What is most affected?
- Glucosidase (releases simple sugars from glycogen, a carbohydrate store)
- Glycogen
- Hearts are massively enlarged b/c of huge storage of glycogen.
What enzyme deficit occurs in Hurler’s disease? What accumulates as a result? What is most affected?
- L-Iduronidase
- Dermatan sulfate, and molecule found in extracellular material in cartilage and bone
- Skeletal system
What enzyme deficit occurs in Niemann-Pick disease? What accumulates as a result? What is most affected?
- Sphingomyelinase
- Sphingomyelin
- CNS oligodendrites
What is the main function of mitochondria?
To produce ATP for energy for the cell’s processes
Describe the mitochondria membrane.
Double membrane with:
- smooth outer membrane
- inner membrane with cristae (folds) that penetrate into interior space
What is inside the mitochondrial matrix?
- Enzymes of the Krebs cycle
- Matrix granules, which are important for binding calcium.
- Circular DNA, which makes mRNA
- Also ribosomes, tRNA responsible for translating mRNA into proteins.
Where do proteins in the mitochondria come from?
Either synthesized in the mitochondria itself, or synthesized normally in the nucleus.
What does the inner mitochondrial membrane do?
Contains ATP synthase protein complex, which converts ADP to ATP using protons generated by electron transport chain to provide energy for that transformation.
Contains enzymes of the electron transport chain.
What is cardiolipin?
Phospholipid located in the inner mito membrane that has 4 fatty acid chains (instead of 2) to make membrane extremely impermeable to protons and electrons
How does the mitochondrion replicate?
Fission, as it contains its own genome.
Where does an individual’s mitochondria come from?
Mothers
What would a mitochondrial cytopathy affect most?
Muscle cells because they require a lot of energy (and, therefore, have a lot of mitochondria) and neurons. Neither dies out or divides, which means they don’t have the ability to dilute out damaged mitochondria. These are terminally differentiated, so they have the mitochondria that they inherit.
What happens to epithelial cells that inherit damaged mitochondria?
They die out because they don’t have as much energy and don’t divide as fast as other cells w/ normal mitochondria cells, so they are weeded out.
What is the result of damaged mitochondria?
Buildup of lactic acid because of reliance on anaerobic metabolism–glycolysis instead of oxidative phosphorylation. Presents as neurological deficits, muscle weakness.
What are inclusions?
“non-living” components of the cells: things that are being stored or about to be secreted, things accumulated but not gotten rid of.
What are some inclusions and their functions?
Glycogen granules, lipid droplets (storage of lipids in cells for later use), pigment granules (lipofuscin and melanin), crystalline inclusions (purpose unknown)
What are the components of the nucleus?
- Nuclear envelope (double membrane around nucleus)
- Chromatin
- Nucleolus
What is inside the nuclear envelope?
- Inner nuclear membrane, outer nuclear membrane continuous with RER & studded w/ ribosomes.
- Nuclear pores that enable transport of materials of things synthesized in nucleus to cytoplasm, like mRNA, ribosomes; and things in cytoplasm (DNA polymerases, RNA polymerases) to gain access to nucleus, where they’re functional.
What is the function of the nucleolus?
Ribosomes synthesized here; most BASOPHILIC structure in the cell.
Nucleus contains all the cell’s chromosomal DNA. How is it stored, and what are the types?
It is stored as chromatin: a complex of DNA and proteins. 2 types of chromatin are:
- Heterochromatin
- Euchromatin
What is heterochromatin?
Chromatin that is condensed, inactive; dormant cells will have much more of this and a smaller nucleus
- Will be coiled/condensed much more tightly because unused (300-700 nm wide).
- Usually toward the outside of the nucleus
What is euchromatin?
Chromatin that is dispersed and transcriptionally active.
- Will be much more loosely coiled in preparation for use (11 nm-30 nm in girth)
- Usually toward the inside of the nucleus
- Nucleus will be larger if filled w/ euchromatin and active
What is the nuclear pore complex?
The place in the nuclear envelope at which things can flow in and out of the nucleus.
- Allows free passage of ions and molecules over a certain size (60 kDa)
- Filled w/ proteins on inner and outer surface of membrane and a basket-like structure on the inside membrane.
What happens with larger molecules going in and out of the nucleus?
They must be guided through the nuclear pore complex by active transport (requires ATP). 2 different signal sequences:
- importins: move things into the nucleus
- exportins: move things out of the nucleus
