Week 3

Cytoskeleton

A network of fibers that organizes the structure and activities of the cell (Supports the cell and maintains its shape, Anchors organelles in specific location, Provides a ‘monorail system’ for vesicles to move on and reach their destination (platform and directionality, Used for cell motility and contractility)

Actin Microfilaments

Thinner, dynamic filaments made of actin that serve as tracks for vesicle transport, subunit is actin

Intermediate Filaments

Rope-like proteins providing structural support, composed of many different types of proteins (thin, permanent)

Microtubules

Hollow tubes made of tubulin dimers that serve as tracks for vesicle transport.

Monorail System

The mechanism provided by the cytoskeleton for vesicles to move towards their destination.

Keratin

A type of intermediate filament that assembles into filaments in epithelial cells (skin).

Desmosomes

Structures that connect keratin filaments between cells, enhancing tissue rigidity and flexibility, Stretching sheet of cells with intermediate filaments keep cells intact, without causes cells to rupture

Nuclear Lamins

Intermediate filaments forming a dense mesh inside the nuclear envelope that anchors chromosomes and defines the shape of the nucleus.

Kinesin

A motor protein that transports vesicles towards the plus end of microtubules using ATP.

Dynein

A motor protein that transports vesicles towards the minus end of microtubules.

Eukaryotic Flagella

Consist of several microtubules; move the cell by undulating back and forth.

Prokaryotic Flagella

Consist of a single helical rod made of flagellin; move the cell by rotating.

Secretory pathway hypothesis

• Protein enters ER while being synthesized by ribosomes + further processed

• Proteins exit er via vesicle

• Protein enters golgi apparatus (cis) then exists (trans) via vesicle

• Protein vesicle travels to plasma membrane to be secreted

Endocytosis

Process of bringing substances into the cell, can be non-specific or specific.

Clathrin-mediated endocytosis

clathrin assemble around plasma membrane creating clathrin-coated pit which pinches off and forms clathrin-coated vesicle, clathrin coat falls off to be reused so vesicle can fuse at destination

Free ribosomes

stay in cytoplasm  or get selectively imported into organelles that are not part of the dynamic ER / Golgi system connected by vesicles

Signal Recognition Particle (SRP)

A protein that binds to the ER signal sequence to facilitate protein entry into the ER.

Pulse-Chase Experiment

• Cells exposed to high concentration of radioactive amino acids to proteins synthesized will be radio-labelled

• Stops exposure by washing away radioactive amino acids to observe movement of the labelled

• Observed proteins inside rough ER then golgi then secreted

Dynamic/polarity of actin filament/microtubules

Constant assembly (“polymerization”) and disassembly (“depolymerization”) of globular subunits

They have structural polarity via +/- ends (Subunits can add to either end, but more rapid at “+” end)

Intermediate filament assembly

Wrapping of dimers into tetramer and tetramers into octamers and so on, generates a rope-like filament with tremendous tensile strength and no polarity, Projects from the nucleus to hold it in place or runs parallel to the cell surface and interact with proteins embedded in the plasma membrane

Actin Filament

Two strands coiled around one another; Pointed end is -, barbed end is +

Myosin attaches to + end and hydrolyzes ATP to ADP, and then contracts to pull itself along the actin filament, releasing ADP + P. These changes cause the actin and myosin to slide past each other. After repeated rounds of this attachment and contraction cycle, the myosin gradually moves toward the plus end of the actin filament

Microtubule

Made up of 2 subunits: a-tubulin (-) and b-tubulin (+) to form dimers

Polymerizes in head to tail fashion via noncovalent interactions to form thin filaments called protofilaments which interact with one another to form hollow tubes

Microtubule organizing center (MTOC): organization of microtubules, plus ends grow out, plants have many but animals/fungus have one near nucleus (centrosome/centrioles)

Involved in mitosis/meiosis anaphase to pull away

Squid experiment

observed large nerve cell in squid axon, found vesicle transport still occurred when cytoplasm was removed from axon, noticed vesicles move along microtubule tracks

Additional research found that vesicle movement requires ATP and kinesin (protein) converts the energy into movement towards plus end (dynein to minus end)

Motor proteins

carries cargo vesicles/organelles bound to tail of motor transported across cell

Kinesin

head section, tail of small polypeptides, stalk connecting the two (all together makes one large subunit, two large subunits make most of kinesin)

Nuclear lamins

intermediate filaments, form a dense mesh inside the nuclear envelope that anchors chromosomes, defines the shape of the nucleus, and stabilizes the envelope. By controlling the interactions between these lamins, many eukaryotic cells will break down and reform the nuclear envelope during cell division.

Bound ribosomes

protein will be in the lumen of the endomembrane system of cell

Snares

proteins that help transport vesicles fuse with plasma membrane

Recycling via lysosome

• Receptor-mediated endocytosis forms vesicle to deliver cargo (macromolecules0 to early endosome (acidifies then matures into late endosome then lysosome)

• Phagocytosis: brings in food/smaller molecules via phagosome which fuses with lysosome to digest its contents

• Autophagy: encloses damaged organelle forming autophagosome which delivers to lysosome for digestion

• Lysosome releases small molecules from digestion into cytosol

In golgi apparatus:

• Proteins have diff tags for diff destinations

• Proteins sorted in trans-golgi binding to diff receptors

• Transport vesicles bud off trans-golgi to respective destinations

• Transport vesicles attach/fuse at destinations

Signal hypothesis

• ER signal sequence synthesized by ribosome

• ER signal sequence binds to signal recognition particle (SRP) and halts synthesis

• SRP binds to receptor in ER membrane

• SRP released and protein enters ER via translocon

• ER signal sequence removed + protein synthesis continues

Prokaryotes:

• No Nucleus 

• One circular chromosomes supercoiled in nucleoid

• Plasmids: circular supercoiled DNA (not always expressed)

• Cytoskeleton

• Organelles

• Internal membrane complexes (can increase SA for photosynthesis)

• Moves via flagellum (assembled from many diff protein at cell surface) vs fimbriae (needle like projection extending from plasma membrane and attached to other cell surface)

Cytoplasm is hypertonic

• Causes water to enter cell via osmosis increasing cell volume

• Stiff cell wall prevents this

Eukaryotes

organelles compartmentalized which allows incompatible chemical reactions to be separated, making them more efficient

Nucleus

Center for information storage/processing

Enclosed by nuclear envelop[e (double membrane) 

Nuclear lamin stiffens it to keep shape

Nucleolus: manufactures/processes RNA molecules into RNA

Rought/smooth ER

rough has ribosomes attached to surface to translate mRNA to proteins while smooth synthesizes lipids

Golgi apparatus

 cis (receives vesicles or cargo from ER) to trans (delivers to other organelles)

Lysosome

 animal cells, hydrolytic enzymes that breaks down macromolecules in acidic environment

Vacuoles

storage deposit for water/nutrient

Peroxisomes

site of oxidation reaction originates when empty vesicles from ER filled with peroxisome specific enzymes from cytosol

Mitochondria/chloroplast

enzymes for ATP synthesis, fusion/fission, has mDNA / third membrane with thylakoids in interior arranged in interconnected stacks called grana with stroma (fluid filled space around grana) containing enzymes to convert chemical energy to sugar

Nuclear envelope

separates nucleus from cell but has openings called nuclear pore complex which extends through inner/outer nuclear membrane + connects inside of nucleus with cytosol (DNA does not leave through pores but RNA molecules produced in nucleus can)

Nucleoplasmin experiment:

when nucleoplasmin is injected into cytosol, they quickly move into the nucleus. Used proteases to cut nucleoplasmin into core/fail, results showed core remains in cytosol while tail moves into nucleus. Hypothesis: nuclear proteins contain zip code (nuclear localization signal) to pass into nucleus)