MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 2a Cytoskeleton Cell Motility

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This deck focuses on 2a Cytoskeleton Cell Motility, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the approximate diameter of an actin microfilament?

Answer: About $7\ \text{nm}$. Actin microfilaments are the thinnest cytoskeletal elements, composed of polymerized G-actin subunits.

Flashcard 2: What is the approximate diameter of a microtubule?

Answer: About $25\ \text{nm}$. Microtubules are the largest cytoskeletal filaments, formed from tubulin dimers, aiding in rigidity and transport.

Flashcard 3: What is the approximate diameter of an intermediate filament?

Answer: About $10\ \text{nm}$. Intermediate filaments have an intermediate size, providing mechanical strength without polarity.

Flashcard 4: What is the monomeric subunit of actin microfilaments?

Answer: G-actin (globular actin). G-actin monomers polymerize to form filamentous F-actin, the structural basis of microfilaments.

Flashcard 5: What is the monomeric subunit of microtubules?

Answer: $\alpha$-tubulin and $\beta$-tubulin heterodimers. Tubulin heterodimers assemble into protofilaments that form the hollow tubular structure of microtubules.

Flashcard 6: What is treadmilling in actin filaments?

Answer: Plus-end addition with minus-end loss at steady state. Treadmilling maintains constant filament length through net assembly at one end and disassembly at the other.

Flashcard 7: Which end of a microtubule typically elongates faster in cells?

Answer: The plus end. The plus end exhibits faster polymerization due to favorable kinetics of tubulin addition.

Flashcard 8: Which end of an actin filament typically elongates faster in cells?

Answer: The plus (barbed) end. The plus end has a lower critical concentration for actin addition, promoting faster polymerization.

Flashcard 9: What nucleotide is hydrolyzed during microtubule polymerization dynamics?

Answer: GTP (tubulin is a GTPase). GTP hydrolysis by tubulin drives dynamic instability, enabling rapid microtubule remodeling.

Flashcard 10: What nucleotide is hydrolyzed during actin filament polymerization dynamics?

Answer: ATP (actin is an ATPase). ATP hydrolysis by actin subunits regulates filament dynamics, including treadmilling and critical concentration differences.

Flashcard 11: Identify the property of microtubules that creates a plus and minus end.

Answer: Polarity from head-to-tail tubulin dimer assembly. Head-to-tail arrangement of tubulin dimers creates inherent polarity, directing growth and motor movement.

Flashcard 12: Which motor protein generates ciliary and flagellar bending by sliding microtubule doublets?

Answer: Axonemal dynein. Axonemal dynein arms produce sliding forces between microtubule doublets, causing ciliary bending.

Flashcard 13: Identify the actin-based cell motility structure that drives membrane protrusion at the leading edge.

Answer: Lamellipodia (branched actin network). Lamellipodia use Arp2/3-mediated actin branching to push the plasma membrane forward during cell migration.

Flashcard 14: Identify the property of actin filaments that creates a plus and minus end.

Answer: Polarity due to asymmetric actin subunit orientation. Asymmetric orientation of actin subunits imparts structural polarity, influencing assembly and motor protein directionality.

Flashcard 15: Which motor protein moves toward the plus end of actin filaments?

Answer: Myosin (most myosins are plus-end directed). Myosins interact with actin to generate force, with most types walking toward the barbed end.

Flashcard 16: Which motor protein moves toward the minus end of microtubules?

Answer: Dynein. Dyneins facilitate retrograde transport, moving cargo toward the microtubule minus end at the cell center.

Flashcard 17: Which motor protein moves toward the plus end of microtubules?

Answer: Kinesin. Kinesins are ATP-dependent motors that transport cargo anterogradely along microtubules toward the cell periphery.

Flashcard 18: Which microtubule structure nucleates microtubule growth at the centrosome?

Answer: $\gamma$-tubulin ring complex ($\gamma$-TuRC). γ-TuRC acts as a template for tubulin dimer addition, initiating microtubule polymerization from the minus end.

Flashcard 19: What is the microtubule-organizing center in most animal cells?

Answer: The centrosome. The centrosome serves as the primary site for microtubule nucleation and organization in animal cells.

Flashcard 20: What is dynamic instability in microtubules?

Answer: Switching between growth and rapid shrinkage (catastrophe/rescue). Dynamic instability allows microtubules to explore cellular space and reorganize rapidly during processes like mitosis.

Flashcard 21: What is the classic $9+2$ microtubule arrangement found in eukaryotic cilia and flagella?

Answer: Nine outer doublets surrounding two central singlets. The 9+2 axoneme structure enables coordinated bending for motility in cilia and flagella.

Flashcard 22: What are the three major cytoskeletal filament systems in eukaryotic cells?

Answer: Microfilaments (actin), microtubules, and intermediate filaments. These filaments provide structural support, enable motility, and facilitate intracellular transport in eukaryotic cells.

Flashcard 23: Which cytoskeletal filament type forms the contractile ring in cytokinesis?

Answer: Actin microfilaments with myosin II. Actin-myosin interactions generate the contractile force to divide the cytoplasm in cytokinesis.

Flashcard 24: Which cytoskeletal filament type is the main component of the mitotic spindle?

Answer: Microtubules. Microtubules form the spindle apparatus to separate chromosomes during cell division.

Flashcard 25: What is the primary cytoskeletal role of intermediate filaments?

Answer: Tensile strength and mechanical stability. Intermediate filaments resist mechanical stress, maintaining cell integrity under tension.