Actin Filaments and Polymerization Quiz

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the diameter of actin filaments?

5 nm

15 nm

10 nm

7 nm (correct)

Which type of actin is involved in the polymerization to form actin filaments?

Contractile (C) actin

Globular (G) actin (correct)

Fibrous (F) actin

Filamentous (F) actin

What is required for the polymerization of globular (G) actin into actin filaments?

ADP

NADPH

GTP

ATP (correct)

Actin filaments have specific ends that exhibit different rates of activity. What is the characteristic of the + (plus) end?

Rapid growth (A) Signup and view all the answers

What is the process called that leads to the disassembly of actin filaments?

Depolymerization (B) Signup and view all the answers

Which of the following statements is true regarding the organization of actin and myosin filaments?

They are unordered. (D) Signup and view all the answers

Which end of the actin filament is associated with slower disassembly?

<ul> <li>(minus) end (B)</li> </ul> Signup and view all the answers

How are actin filaments classified based on their structure?

Helical polymers (C) Signup and view all the answers

What is the structure of filaments described in the content?

Two protofilaments wrapped in a helix (D) Signup and view all the answers

How is regulation of contraction achieved according to the information provided?

Through the phosphorylation of myosin light chains (A) Signup and view all the answers

What characteristic of actin filaments is mentioned in the content?

They are flexible and can form bundles or networks (A) Signup and view all the answers

What is indicated about the contraction mechanism in the provided information?

It depends on the level of Ca2+ (B) Signup and view all the answers

How frequently do the protofilaments repeat in their helical structure?

Every 37 nm (D) Signup and view all the answers

Which component is NOT involved in the regulation of muscle contraction as mentioned?

Troponin (C) Signup and view all the answers

What property does the cell utilize from actin filaments as stated in the content?

Their flexibility for forming various structures (D) Signup and view all the answers

What does the flexibility of actin filaments allow them to do?

Adapt to various cellular structures and functions (C) Signup and view all the answers

What role does ATP play in the myosin-actin interaction during muscle contraction?

It binds to myosin, causing it to detach from actin. (D) Signup and view all the answers

What is the sequence of events after ATP hydrolysis in muscle contraction?

Myosin head propels actin, releases ADP, and then returns to rigor state. (B) Signup and view all the answers

What happens when Ca2+ ions are released from the sarcoplasmic reticulum?

They displace the tropomyosin/troponin complex, exposing myosin-binding sites. (B) Signup and view all the answers

What is the primary function of tropomyosin in muscle contraction?

To block myosin-binding sites on actin. (A) Signup and view all the answers

Which of the following correctly describes the 'rigor state' in muscle contraction?

It is the condition where myosin is tightly bound to actin with no nucleotides attached. (B) Signup and view all the answers

What is the consequence of ADP release from the myosin head during muscle contraction?

It allows myosin to return to the rigor state. (D) Signup and view all the answers

How is the movement of actin filaments towards the center of the sarcomere achieved?

Through the release of energy from ATP hydrolysis in myosin. (B) Signup and view all the answers

What is the effect of myosin head repositioning after ATP hydrolysis?

It enhances the binding affinity of myosin for actin. (A) Signup and view all the answers

What initiates muscle contraction at the binding site of myosin?

Binding of calcium to troponin (D) Signup and view all the answers

What is the immediate effect of an action potential arriving at the T tubules?

Opening of calcium channels in the sarcoplasmic reticulum (B) Signup and view all the answers

What occurs after intracellular Ca2+ levels increase?

Sliding of actin and myosin filaments (D) Signup and view all the answers

How is relaxation of the muscle achieved?

By reabsorbing calcium into the sarcoplasmic reticulum (C) Signup and view all the answers

What role does the Ca2+ pump play in muscle contraction?

It restores calcium levels for muscle relaxation (A) Signup and view all the answers

What happens to the calcium released from the sarcoplasmic reticulum after muscle contraction?

It is reabsorbed by the sarcoplasmic reticulum (B) Signup and view all the answers

What is the first step in excitation-contraction coupling?

Arrival of an action potential at the presynaptic motor neuron (C) Signup and view all the answers

What triggers the opening of a calcium release channel in the sarcoplasmic reticulum?

The opening of calcium channels in the T tubules (B) Signup and view all the answers

What is the role of myosin light chain kinase (MLCK) in smooth muscle contraction?

It phosphorylates myosin light chains. (D) Signup and view all the answers

In muscle contraction, what happens when the light chain is phosphorylated?

The myosin head can interact with actin filaments. (C) Signup and view all the answers

Which type of muscle is characterized by involuntary contractions and intercalated discs?

Cardiac muscle (B) Signup and view all the answers

What stabilizes actin filaments in muscle cells?

Tropomyosin (D) Signup and view all the answers

What is the critical concentration (Cc) at the positive end of actin filaments?

0.1 µM (A) Signup and view all the answers

Which of the following proteins is involved in severs actin filaments?

Gelsolin (A) Signup and view all the answers

What mechanism allows for the steady maintenance of actin filament length?

Treadmilling (B) Signup and view all the answers

Which muscle type is responsible for voluntary movements?

Skeletal muscle (A) Signup and view all the answers

What structure connects cardiac muscle cells and allows rapid propagation of action potentials?

Intercalated discs (B) Signup and view all the answers

Which actin-binding protein prevents actin polymerization by binding to actin subunits?

Thymosin (A) Signup and view all the answers

What are the basic units of contraction in striated muscle?

Sarcomeres (A) Signup and view all the answers

What is the primary function of actin filaments in muscle cells?

Enabling muscle contraction (D) Signup and view all the answers

The I band in a sarcomere consists primarily of what?

Thin actin filaments (B) Signup and view all the answers

Which of the following proteins connects actin to cell membranes?

Spectrin (C) Signup and view all the answers

What is the primary role of nebulin in the sarcomere?

To regulate the length of actin filaments (A) Signup and view all the answers

Which accessory protein caps the ends of actin filaments?

Tropomodulin (D) Signup and view all the answers

What structure is primarily responsible for muscle contraction?

Sarcomere (A) Signup and view all the answers

What is the function of the lamellipodia and pseudopodia during cell migration?

To contribute to cell movement and adhesion (A) Signup and view all the answers

What happens to the sarcomere during muscle contraction?

It shortens (D) Signup and view all the answers

What is the primary role of myosin II in muscle cells?

To bind to actin filaments (B) Signup and view all the answers

What are stress fibers primarily involved in?

Adhesion and migration (A) Signup and view all the answers

What structural feature characterizes actin filaments?

Bipolar arrangement (A) Signup and view all the answers

Which protein is responsible for the mechanical sensing in muscle cells?

Stress fibers (C) Signup and view all the answers

Which protein serves to stabilize the structure of the sarcomere?

Nebulin (B) Signup and view all the answers

What is the primary purpose of Cap Z in muscle cells?

To cap the ends of actin filaments (B) Signup and view all the answers

What role do focal contacts play in cellular processes?

Support adhesion (A) Signup and view all the answers

What drives the protrusion of the lamellipod during cell migration?

Actin polymerization (C) Signup and view all the answers

How is the shortening of the sarcomere primarily achieved?

Through tension generated by myosin filaments (D) Signup and view all the answers

What type of structure do thick and thin filament arrangements form in muscle cells?

Bipolar arrangements (B) Signup and view all the answers

Study Notes

Muscle Contraction Mechanisms

Skeletal Muscle:
- Striated appearance due to sarcomeres
- Voluntary movements
- Actin slides over myosin, shortening the sarcomere
- Myosin heads bind to actin, then detach and re-attach
- ATP hydrolysis powers the movement
Cardiac Muscle:
- Striated but involuntary
- Responsible for continuous heart contractions
- Intercalated discs synchronize contractions
- Branched cells
Smooth Muscle:
- Non-striated
- Involuntary, slower contractions
- Found in intestines and other organs
- Regulation by phosphorylation of myosin light chains
- Ca2+ dependent contraction

Muscle Structure and Organization

Sarcomeres: Basic units of contraction
- Contain actin and myosin filaments
- Z-lines define the boundaries
- Titin: Elastic protein for sarcomere stability
- Nebulin: Binds to actin, regulating length
Actin Filaments:
- Helical polymers of G-actin
- Polarized, one end grows faster (+ end)
- Steady-state balance between assembly and disassembly
Myosin:
- Myosin II: Involved in muscle contraction (Sliding Filament Model)
- Myosin I: Involved in vesicle transport
Accessory Proteins:
- Tropomyosin: Stabilizes actin filaments
- Troponin: Links tropomyosin and regulates contraction
- Cap Z and Tropomodulin: Maintain actin length
- Spectrin: Links actin to membranes
- Fimbrin and Filamin: Organize actin into bundles or networks
- Gelsolin: Severs actin filaments

Actin Toxins and Binding Proteins

Actin-binding proteins (ABPs):
- Regulate actin polymerization and depolymerization
- Examples: fimbrin, filamin, gelsolin, thymosin

Excitation-Contraction Coupling

Action potential triggers Ca2+ release
Ca2+ binds to troponin, exposing myosin-binding sites
Myosin heads bind and power the sliding filament

Calcium Role

Released from the sarcoplasmic reticulum (SR) in response to action potentials
Binds to troponin, initiating contraction
Reabsorbed by SR for muscle relaxation

Smooth Muscle Mechanism

Contraction dependent on Ca2+ and myosin light chain kinase (MLCK)
Ca2+ binds to calmodulin, activating MLCK
Phosphorylation of myosin light chains allows myosin to interact with actin

Microtubules and Microfilaments

Cytoskeleton components involved in various cell processes (e.g., cytokinesis, cell movement)
Actin and myosin play vital roles in cell movement and cytokinesis (cell division)

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Test your knowledge on actin filaments, their structure, and polymerization processes. This quiz covers various aspects including the characteristics of actin filament ends, their assembly, and disassembly mechanisms. Perfect for students studying cell biology and molecular biology.