Exam 3: Topic 15 Cellular Motility and Contractility

3 Broad functions of cellular motility

Movement of a cell through its environment

The Environment moving past the cell

Movement of components within the cell

MT and MF act like tracks for

Motor Proteins to attach to and move along using energy/ This movement carries cargo or creates forces

Motor Proteins use energy from

ATP Hydrolisis

Motor Proteins have 3 major parts

Filament-binding domain/Head: Attaches to the cytoskeletal track

Stalk= Connects the head to other end

Cargo-binding domain/tail= Attaches to the material being transported

Drawing of 3 major motor protein parts

VESICLE/CARGO

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CARGO-BINDING TAIL

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STALK

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FILAMENT-BINDING HEAD

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CYTOSKELETAL TRACK

MT Motor Proteins

Kinesin and Dynein

Kinesin

Moves toward + end of MT/ Away from centrosome

Carries cargo outwards

Dynein

Moves towards - end of MT/ Towards centrosome

Carries cargo inward

Cillia and Flagella are structures that

Use MT to create movement

Cillia

Usually short and numerous, move fluid or material across the cells surface

Flagella

Usually longer and fewer, helps propel a cell through its environment

How does dynein create movement?

Axonemal Dynein attaches to neighboring MT doublets and use ATP to generate force

1. Dynein attaches to an adjacent MT

2. ATP hydrolysis powers a conformational changes

3. Dynein attempts to slide the doublets past one another

4. Structural connections restrict sliding

5. The sliding force is converted into bending

Myosin

Motor Proteins that bind to actin MF

Kinesis Vs Myosin molecules

Myosins moves short distances

Many myosin work together in arrays to create larger movements

Protrusion and movement of a lamellipodium in crawling cells

Contraction of a skeletal muscle cell

Cyotokensis

Sacomere

Basic Contractile unit of skeletal muscle cells

2 Major filament type of Sarcomere

Think Filament: Made of Myosin

Thin Filament: Made of actin, along with regulation, proteins tropoin and tropomyosin

How Muscles Contract

thin actin filaments slide past thick myosin filaments.

The filaments themselves do not need to shorten/ the sarcomere shortens as the filaments slide past each other.

When a muscle is relaxed:

• Tropomyosin blocks myosin-binding sites on actin.

• Myosin cannot effectively bind to actin.

When calcium is released:

• Calcium binds to troponin.

• Troponin changes shape.

• Tropomyosin moves away from the myosin-binding sites on actin.

• Myosin can bind actin.

• Cross-bridge cycling produces contraction.

Cross-bridge cycle

• Attachment: Myosin head binds to actin.

• Power stroke: Myosin pulls actin toward the center of the sarcomere.

• Detachment: ATP binds to myosin, causing it to detach from actin.

• Reactivation: ATP is hydrolyzed, re-energizing the myosin head.

What happens during relaxation?

Calcium is pumped back into the sarcoplasmic reticulum. Calcium leaves troponin, tropomyosin returns to blocking the binding sites, and cross-bridge formation stops.

Structure	What to know
Z line	Boundary of a sarcomere; anchors thin filaments
A band	Entire length of thick filaments
I band	Region containing thin filaments without thick filament overlap
H zone	Region with thick filaments but no thin filament overlap
M line	Middle of the sarcomere