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Review premed cell biology
Nucleus
- Surrounded by double membrane (nuclear envelope).
- Nuclear pores let things travel to and from the nucleus.
- The outer nuclear membrane is continuous with the ER.
- Nucleolus: region in the nucleus where rRNA is transcribed.
Endoplasmic Reticulum
- Site of membrane synthesis.
- Rough ER: ribosomes synthesize peptides into the lumen. Adds N-linked sugars to glycoproteins.
- Smooth ER: steroid synthesis in adrenal gland, detoxification in liver, calcium storage/release in muscle.
Golgi Complex
- Modifies proteins and lipids made in the ER. Adds O-linked sugars to glycoprotein, modifies N-linked sugars.
- Packages proteins for either secretion, incorporation to membrane, or for the lysosome (addition of mannose-6-phosphate targets proteins to be lysosomal proteins).
- I-cell disease = can't add mannose-6-phosphate, lysosomal enzymes secreted, damages tissues results in course facial features, clouded corneas, restricted joint movement.
- Cis faces ER. Trans faces outward.
- Anterograde = ER → Golgi. Retrograde = Golgi → ER.
Lysosome
- Principle site of intracellular digestion.
- Acidic lumen ~pH 5.
- Contains a variety of hydrolytic enzymes.
- Protection against autodigestion by hydrolytic enzymes: the enzymes requires the acidic pH of lysosomes, so if they spill into the cytosol, they become inactive.
- Glycosylation of membrane proteins facing the lumen protects them from degradation.
- Things taken in by phagocytosis are stored in phagosomes, which fuse with lysosomes for digestion.
- Endosomes either deliver their content to lysosomes, or gets converted to lysosomes themselves.
- Autophagosomes are membranes enclosing aged cell organelles, to be fused with lysosomes.
Peroxisome
- Oxidizes toxic molecules.
- Hydrogen peroxide converted to H2O and O2 by catalase and peroxidase.
Endosome
- Endocytosed vesicles are endosomes.
- Some endosomes re-fuse with the plasma membranes.
- Some endosomes may convert to lysosomes.
Ribosome
- rRNA is made in the nucleolus.
- Polysomes is when multiple ribosomes transcribe one piece of mRNA simultaneously.
Centrosome
- Region in the cell where microtubules originate.
- Contains centrioles.
Plasma Membrane
- Diffusion
- Simple diffusion: gas, water (osmosis), lipid soluble stuff.
- Facilitated diffusion: glucose (carrier), amino acids, ions (channels).
- Active transport
- Na-K pump: 3 Na out, 2 K in. Gives resting potential of -70 mV.
- Exocytosis: v-SNAREs bind t-SNAREs.
- Endocytosis:
- Clathrin-coated vesicles: clathrin on the cytoplasmic side of the membrane form a coated pit.
- Non-clathrin coated vesicles: caveolae formed by caveolin.
Cilia and Flagella
- Cilia: sweeps things across cell surface.
- Flagella: tail for cell motility.
- Basal bodies: centrioles at the base of cilia and flagella.
Cytoskeleton
- Intermediate filaments
- Protect the cell from mechanical stress.
- Forms network across cytoplasm, also in desmosomes.
- Forms nuclear lamina (nuclear lamins).
- Includes keratin filaments, vimentin, neurofilaments, and nuclear lamins.
- Epidermolysis bullosa simplex: mutation in keratin gene, causes skin to blister.
- Plectin cross-links intermediate filaments to other cytoskeleton members.
- Microtubules
- Grows out of the centrosome (or spindles during mitosis).
- Provides anchoring of organelles and tracks for vesicle transport.
- Motor proteins: Kinesins move outward (toward plus end), dyneins move inward (toward minus end).
- Gives cilia and flagella their motility.
- Tubulin (α β heterodimer) polymerizes at the nucleation site (γ tubulin ring at the centrosome) and grows outward.
- Centrioles are present in animal cells. They are the same as basal bodies at the base of cilia and flagella. They organize microtubules.
- Dynamic instability: GTP-bound tubulin promotes polymerization. Hydrolysis of GTP makes GDP-tubulin, which depolymerizes. If addition of GTP-tubulin is faster than GTP hydrolysis, then the microtubule grows. Otherwise, it shrinks
- Colchicine prevents tubulin addition. Causes mitotic spindle to disappear.
- Taxol prevents tubulin disassembly. Stalls mitosis.
- Actin filaments (microfilaments)
- Found in microvilli, lamellipodia, filopodia, contractile bundles and contractile rings.
- Involved in cell crawling.
- Actin-myosin sliding filament in muscles.
- GTP-actin monomer promotes polymerization. GDP-actin monomer promotes depolymerization.
- cytochalasins prevent polymerization, jasplakinolides prevent depolymerization. They stall cell movement.
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