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@entropybliss

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Being up at 4-5 am is like loading in a level but the textures haven’t loaded all the way through yet.
i like this text post a lot because it’s a comparison between two extremely different styles of humor despite it, effectively, being the same joke
why fall in love when you could fall on the floor and never get up
Notes on Microfilaments
Non-Muscle actin filaments are abundant in eukaryotic cells
Microvilli
Bush boder epithelia of intestine
They increase surface area
Are hair cells of the inner ear (sound detection)
Stress fibers
Adhesion and Cell Shape (Fibroblasts growing in in vitro)
Filopodia and Lamellipodia
At the leading edge of the moving cells
Contractile Ring
Division of cytoplasm in animal cells
Cortical Actin
Just beneath plasma membrane of most Euks, anchors membrane
Intracellular Transport
Myosin-coated organelles moving along actin filaments in plants
Actin polypeptice assembles in microfilament (F-Actin)
ATP affects assembly dynamics
G- Actin
Tertiary structure
F- Actin
Quaternary structure
Actin filaments are polar. They are added at the plus end and are taken out at the minus end
Two-Stranded Helix shape
Actin Filaments in cells are often Dynamic
5% of cell protein is actin and about half is polymerized filaments
**** ATP binding to actin affects dynamics
C-actin with ATP bound adds + end filaments preferentially grow at plus end
Away from plus end, ATP is hydrolyzed. This induces conformational change in actin subunits.
ACTIN IS AN ATPASE
ADP actin subunits do not bind each other as strongly
G-actin with ADP bound is preferentially released at – end, shortens filament
DYNAMICS ALSO REGULATED BY ACTIN-BINDING PROTIENS
Filaments can grow and shorten rapidly
Treadmilling
This is the same addition to plus and subtraction at minus we just discussed
This is how the actin filaments crawl through the cell
Filaments move left to right but subunits don’t move
Dynamic F-actin can disassemble at one location ad reassemble elsewhere
Actin depolymerizes at sperm entry site and repolymerizes at new rhizoid pole
Actin marking the rhizoid pole is dynamic
Dynamic Actin at the surface of a moving cell
Lamellipodia
Ruffles- sheet-like extensions of the cells leading edge
Filopodia
Microspikes- finger-like
Motility is dependent upon actin assembly
Arrangment of actin filaments in lamellipodia and filopodia
Stress fibers
Contractile bundle
Antiparallel
Lamellipodium
Branched network
Filopodium
Parallel filaments
Actin assembly drives extension of lamellipodia and filopodia
Modulating the assembly/function of actin:actin-binding proteins
Regulate structure of F-actin array, and dynamics of array
Nucleating proteins
Crosslinked proteins
Bundling protiens
Motors
ARP 2/3 Complex (Nucleating Proteins)
In Euks
Nucleation of actin filaments from pure subunits is very slow
Actin nucleation-related proteins (ARPs 2 and 3)
And several other polypeptide in the macromolecular complex
ARP2/3 “nucleates” actin filament assembly by providing a template or “seed” that can be elongated by subunit addition
The ARP 2/3 complex caps minus-end of actin filaments
Capped ends are less dynamic
ARP 2/3 nucleates branching network in lamellipodia
Activated ARP 2/3 binds to side of existing actin filaments
And nucleates new filaments from the side (branches)
New, elongating filaments are not yet capped and push leading edge outward
Network depolymerizes in rear
COMMON IN ALL EUKS SO FAR
Actin filament assembly drives extension of lamellipodia
Actin filament assembly drives forward membrafe extension
Actin filaments disassemble behind leading edge
Filament Capped when finished elongating
Actin movement also drives movement of intracellular parasites
Initiation of actin assembly is regulated by Rho family GTPases
Part of Ras superfamily of G proteins that serve as molecular switches
Allow rapid assembly and disassembly of actin-based structures in response to enviro cues
Rho activation (GTP bound confirmation) causes formation of ‘stress fibers’
Rac (Rho Subfamily) activation causes formation of massive lamellipodium
Cdc42 (Rho Subfamily) Activation causes formation of filapodia
The Rho GTPase triggers activation of ARP2/3 complex (but form different arrays)
Bundling vs. Cross-linking: a-actin vs. filamin
Bundling
A-actin (dimer) is rod-shaped with two actin binding sites:
Forms loose parallel bundles of actin filaments
Z-line of striated muscle
Stress fibers and focal contacts
Filopodia
Cross-linking
Filamin (dimer) has actin binding sites on long flexible arms
Forms cross-linked actin gels
Stress fibers and focal adhesion
Smooth muscle
Actin filament bundles called “stress fibers” are common in cultured fibroblasts in animals
Stress fibers
i. Provide mechanical strength to cell
ii. Attach cell to substratum at focal point of adhesions
Polarity of actin filaments in bundle is not uniform
Organized by actin binding proteins
Bundling (a actin)
Crosslinking (filamin)
Type II myosin
How do stress fibers attach to substratum?
Integrins
Integrins link stress fibers to extracellular matrix at ‘focal adhesions’
Integrins bound to linker proteins which are bound in actin filaments
Defects in integrins can cause muscular dystophe
Extracellular matrix is composed of secreted proteins and glycoproteins
Linkage from stress fiber to ECM attaches cell to substratum at focal adhesion
There are many other linkages from actin to ECM in animal cells
Actin filaments in plants are also linked to the ECM (cell wall) but are linking molecules are different
Non-muscle cells contain multiple myosins
17 subfamilies of myosins- gene duplication and divergence
Note** that some have 2 heads and some only have one
Myosin II = 2 headed, discovered first in muscle, also in non-muscle cells
N terminus- motor domain of heavy chains
Coiled-coil tail
C-terminus of heavy chains
Each head is independent of the other
Myosin is an ‘actin dependent’ ATPase that acts as a molecular motor
No nucleotide. Myosin head is tightly bound to actin
ATP binding releases myosin from actin
i. ATP binding site changes the conformation of myosin
ATP hydrolysis “cocks” myosin, myosin binds weakly to actin
Pi is released, strengthens binding of myosin to actin
Myosin bound tightly to actin undergoes “power stroke” and then ADP is released
i. Myosin head walks towards the plus end of the actin filament
ii. Mechanism is the same for muscle and non-muscle actin
Non-muscle myosins perform many roles
“type II myosin:” slides antiparallel actin filaments
i. Myosin walks towards the plus end
ii. Cell contraction and cell division
Single headed myosins
Vessicle transport –cytoplasmic streaming inalgae and plants
Links actin to membrane – Minus end leading. Myosin walks towards plus end
Myosins coat organelles and power cytoplasmic streaming in green algae and plants
- Stirs the cytoplasm, plant cells are often too large to rely on simple diffusion
- Myosins power cytoplasmic streaming in green algae and plants
- Vesicles, ER, Nucleus, and other organelles move along actin cables on subcortical cytoplasm
- Powered by myosin 6 speeds ip to 7 micrometers per second
Myosins and cell motility
- Type 2 mosins found in the tail (contractile)
- Type 1 founding in leading edge
Myosins work in conjunction with actin polymerization to drive movement
A model for motility using actin and myosin motors
- Focal adhesion
- Actin polymerization extends lamellipodium (myosin 1)
- Focal adhesion and actin disassembly
- Contraction of myosin II
Neutrophils are chemotactic- movement towards or away from chemical
-Change direction is mediated by activation of RHO GTPases
- Crawl towards N- formyl peptides
Plants have all the same categories of actin-binding proteins

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Notes on Intermediate Filaments
Cytoplasmic
Keratins- these are epithelial. So in your hair. Skin. Nails. Claws.
Vimentin- connective tissue, muscle cells, neuroglial cells
Neurofilaments- Nerve cells
Nuclear
Nuclear lamins- in animal cells
Characteristics-
~The are located between actins and myosins in the muscles
~They have a strong rope like structure, gives shape to nucleus euks and cytoplasm of vertebrate cells
~Provides Tensile Strength that allows layers of cells to stretch without rupture. (Resists mechanical stress)
Keratins Protect Skin Cells exposed to mechanical stress
Basal cells express keratins k5 and k14
If there is a point mutation in these it disrupts keratin filaments
So it causes things like epidermolysis bullosa simplex
Other ailments
Neurofilaments: ALS
Lamins: Muscular dystrophy and premature aging
Vimetins: Muscular Dystrophy
Let's talk about cell surface receptors. The ligands for these are hydrophobic. This makes sense because they are not polar and therefore cannot diffuse into the cell. The receptors for these are located on the cell surface. the ligand binds to the receptor and and intracellular signal is sent out. this signal goes into the nucleus. In the nucleus the signal gives the orders to translate and transcribe this signal.
There are three types of cell surface receptors. Ion-Mediated Channel G-Protien Coupled Receptors Enzyme Coupled Receptors (for growth and proliferation) If you have any specific questions let me know.
Literally me during finals. (Thanks to the magical powers of coffee) Have you ever wondered why coffee makes you jittery? Well lets talk about why. Coffee inhibits cAMP (Which is a secondary messenger) from turning off. So basically your body can't turn off its "fight or flight response system" Lets talk about this fight of flight response system, shall we? Adrenaline will serve as our ligand. Adrenaline binds to the surface receptor. The seven pass transmembrane receptor has something called an effector domain. The goal is to turn on the trimeric G Protein. The Inactive protein, with GDP bound, will go to the seven pass transmembrane protein. In the effecter domain of the membrane GEF (G Protein) will exchange the GDP for GTP so the trimeric G Protein becomes active. At this point the Alpha subunit splits from the Beta-Gamma subunit and they are each able to signal downstream targets. The alpha subunit with GTP still bound is able adenylyl cyclase. With ATP it forms cAMP (second messenger) cAMP then activates PKA. PKA with the help of ATP then activates Phosphorylase Kinase. Phosphorylase Kinase with ATP activates Glycogen Phosphorylase. Glycogen Phosphorylase is then able to signal Glycogen Breakdown. And this my friends is how you activate the fight or flight response. SO when cAMP levels are high, fight or flight response is stimulated. Coffee doesn't let cAMP deactivate so you, as a result, have energy.