Cellular Movement: Motility and Contractility

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Cellular Movement: Motility and Contractility

Introduction to Cellular Motility

Cellular movement, or motility, is a fundamental property of living cells, enabling them to move themselves, their environment, or intracellular components. Contractility is a specialized form of motility, most notably observed in muscle cells, where it results in cell shortening and force generation.

Motile Systems in Eukaryotic Cells

Levels of Motility

Tissue Level: Coordinated movement of groups of cells (e.g., muscle contraction).
Cellular Level: Movement of individual cells (e.g., sperm swimming, white blood cell migration).
Subcellular Level: Movement of organelles and vesicles within cells (e.g., chromosome separation during mitosis).

Motility is driven by the cytoskeleton, primarily microtubules (MTs) and microfilaments (MFs), which serve as tracks for motor proteins that convert chemical energy into mechanical work.

Two Eukaryotic Motility Systems

Microtubule-Based Motility

Microtubule-based motility is essential for processes such as axonal transport in neurons and the movement of cilia and flagella. The primary motor proteins involved are kinesins and dyneins.

Kinesins: Move cargo toward the plus end of microtubules (anterograde transport).
Dyneins: Move cargo toward the minus end of microtubules (retrograde transport).

Table of selected motor proteins of eukaryotic cells

Table: Selected Motor Proteins of Eukaryotic Cells

Microfilament-Based Motility

Microfilament-based motility is exemplified by muscle contraction and other actin-dependent processes. The main motor proteins are myosins, which generally move toward the plus end of actin filaments.

Myosin II: Responsible for muscle contraction.
Other myosins: Involved in vesicle transport, cell movement, and cytokinesis.

Molecular Motors: Structure and Function

Common Features of Motor Proteins

All motor proteins couple ATP hydrolysis to conformational changes that generate movement.
They undergo cycles of ATP binding, hydrolysis, and product release, resulting in processive movement along cytoskeletal filaments.
They have distinct domains for filament binding, cargo binding, and ATP hydrolysis.

Diagram of kinesin, dynein, and myosin V structures and domains

Figure: Domain organization of kinesin, dynein, and myosin V

Microtubule-Based Movement: Kinesins and Dyneins

Axonal Transport

Proteins and organelles are transported along axons by kinesins (anterograde) and dyneins (retrograde). This process is essential for neuronal function and survival.

Electron micrograph of vesicle transport along microtubules

Example: Vesicle transport along microtubules in neurons

Kinesin Structure and Movement

Kinesin I consists of two heavy chains and two light chains.
Heavy chains have globular heads (bind MTs and ATP), a coiled-coil stalk, and a tail (binds cargo via light chains).
Kinesin moves in a "walking" manner, with each head alternately binding and stepping along the microtubule.
Processivity allows kinesin to move long distances without detaching.

Diagram of kinesin structure Schematic of kinesin movement along a microtubule

Figure: Kinesin structure and movement mechanism

Dynein Structure and Function

Dyneins are large, multi-subunit proteins found in the cytosol and axonemes (cilia/flagella).
Cytoplasmic dynein associates with dynactin to link to cargo.
Axonemal dyneins drive the sliding of microtubules in cilia and flagella.

Diagram of dynein/dynactin structure and power stroke

Figure: Dynein structure and mechanism

Microtubule-Based Motility: Cilia and Flagella

Structure and Function

Cilia and flagella are motile appendages with a common internal structure called the axoneme. Cilia are short and numerous, moving with an oarlike beat, while flagella are longer and fewer, moving with a wave-like motion.

Cilia on mammalian tracheal cells Human sperm cell with flagellum

Examples: Cilia on tracheal cells and flagellum of a sperm cell

Axoneme and Basal Body Structure

The axoneme has a "9 + 2" arrangement: nine outer doublet microtubules and two central singlets.
Basal bodies anchor cilia/flagella and resemble centrioles (nine triplet microtubules).
Primary cilia have a "9 + 0" structure and are non-motile, serving sensory functions.

Longitudinal and cross-section of flagellum axoneme and basal body

Figure: Axoneme and basal body structure

Mechanism of Bending

Axonemal dynein arms generate sliding between adjacent microtubule doublets.
Interdoublet links (nexin) and radial spokes convert sliding into bending.

Diagram of doublet sliding and bending in axoneme

Figure: Sliding and bending mechanism in cilia and flagella

Beating Patterns

Cilia beat with a power stroke and recovery stroke, moving fluid parallel to the cell surface.
Flagella generate a propagated wave, moving the cell through fluid.

Diagram of ciliary and flagellar beating patterns

Figure: Beating patterns of cilia and flagella

Microfilament-Based Motility: Muscle Contraction

Muscle Types and Structure

Muscle contraction is the most familiar example of microfilament-based motility. There are three muscle types: skeletal (voluntary), cardiac (heart), and smooth (involuntary).

Skeletal muscle structure Smooth muscle structure

Figure: Skeletal and smooth muscle cell structure

Myosin Structure and Function

Myosins are ATP-dependent motors that move along actin filaments.
All myosins have a globular head (binds actin and ATP) and a tail (varies in length and function).
Type II myosins form thick filaments in muscle and mediate contraction by pulling actin filaments together.

Diagram of myosin types and domains

Figure: Myosin II, I, V, and VI structures

Muscle Cell Organization

Muscle fibers contain myofibrils, which are divided into repeating units called sarcomeres.
Sarcomeres contain thin (actin, troponin, tropomyosin) and thick (myosin) filaments.
Filaments are aligned, creating a pattern of dark (A bands) and light (I bands) regions.

Sarcomere structure in muscle Diagram of sarcomere bands and lines

Figure: Sarcomere structure and banding pattern

Sliding-Filament Model of Contraction

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere without changing filament length. Myosin heads form transient cross-bridges with actin, powered by ATP hydrolysis.

Sliding filament model and length-tension diagram

Figure: Sliding-filament model of muscle contraction

The Contraction Cycle

Myosin binds actin in a high-energy state (ADP + Pi bound).
Release of Pi triggers the power stroke, moving the thick filament toward the Z line.
ATP binding causes myosin to detach from actin; hydrolysis resets the head for another cycle.

Contraction cycle of myosin and actin

Figure: The contraction cycle in muscle

Regulation of Muscle Contraction by Calcium

At low Ca2+, tropomyosin blocks myosin-binding sites on actin.
At high Ca2+, Ca2+ binds troponin C, shifting tropomyosin and exposing binding sites.
Muscle contraction is initiated by nerve impulses that trigger Ca2+ release from the sarcoplasmic reticulum.

Calcium regulation of contraction and relaxation

Figure: Calcium-dependent regulation of muscle contraction

Cardiac and Smooth Muscle

Cardiac muscle: Cells are joined by intercalated discs with gap junctions for electrical coupling; contraction is coordinated by pacemaker cells.
Smooth muscle: Cells are long, thin, and non-striated; contraction is slower and regulated by Ca2+-calmodulin activation of myosin light-chain kinase (MLCK).

Smooth muscle cell structure and contraction

Figure: Structure and contraction of smooth muscle cells

Regulation of Smooth Muscle Contraction

Increased Ca2+ binds calmodulin, activating MLCK.
MLCK phosphorylates myosin light chains, enabling cross-bridge cycling with actin.
Relaxation occurs when Ca2+ levels fall and myosin light-chain phosphatase removes the phosphate.

Phosphorylation of myosin by MLCK in smooth muscle

Figure: Regulation of smooth muscle contraction by Ca2+ and MLCK

Microfilament-Based Motility in Nonmuscle Cells

Cell Migration and Amoeboid Movement

Nonmuscle cells move by extending lamellipodia or filopodia, attaching to the substrate via integrins, and generating tension to pull the cell forward.
Amoeboid movement involves cycles of actin gelation (solidification) and solation (liquefaction) to form pseudopodia.

Cytoplasmic Streaming

Actomyosin-dependent movement of cytoplasm, known as cytoplasmic streaming or cyclosis in plants, helps distribute nutrients and organelles within large cells.

Summary Table: Selected Motor Proteins of Eukaryotic Cells

Motor Protein	Typical Function
Cytoplasmic dynein	Moves cargo toward minus ends of MTs
Axonemal dynein	Activates MT sliding in flagella and cilia
Kinesin 1	Direct, moves cargo toward plus ends of MTs
Kinesin 2	Direct, movement of organelles within cilia and flagella
Kinesin 5	Bidirectional, sliding of antiparallel MTs in mitosis
Kinesin 13	Depolymerizes MT ends
Myosin II	Muscle contraction, cytokinesis
Myosin V	Organelle and vesicle transport
Myosin I	Membrane association, endocytosis

Table: Main functions of selected motor proteins

Additional info: This guide covers the core concepts of cellular motility and contractility, including the molecular mechanisms, structural organization, and regulation of movement in eukaryotic cells. It integrates microtubule- and microfilament-based systems, highlighting their roles in both muscle and nonmuscle cells.