📚 Cellular Motility and Membrane Biology: A Comprehensive Study Guide

This study material has been compiled and organized from a lecture audio transcript and supplementary copy-pasted text. It aims to provide a clear and structured overview of the molecular mechanisms underlying cellular movements and the fundamental principles of cell membrane biology.

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🔬 Chapter 5: Molecular Bases of Cellular Motility

5.1 General Features of Cellular Movements and Motor Proteins

Cellular movements are essential for life, enabling cells to perform diverse functions from muscle contraction to immune responses. These movements are powered by specialized proteins called motor proteins.

📚 Motor Protein Definition: An enzyme with ATPase activity that uses the energy released from ATP hydrolysis to change its spatial (3D) conformation, allowing it to "step" along a support structure and generate movement.

Cellular movements can be categorized based on their impact on cell position:

• I. Muscle Contraction: A specialized movement based on the actin-myosin system.
• II. Movements Modifying Cell Position in the Environment:
  1. Amoeboid Locomotion: Actin-myosin based (e.g., Amoeba proteus, neutrophils, fibroblasts).
  2. Locomotion with Flagella: Microtubule-dynein based (e.g., sperm cells).
  3. Movements of Cilia: Microtubule-dynein based (e.g., respiratory tract, oviduct).
• III. Movements Not Modifying Cell Position in the Environment (Intracellular):
  1. Movements from Microvilli: Actin-myosin based (e.g., enterocytes).
  2. Movements during Cell Division: Microtubule-dynein based.
  3. Cytoplasmic Currents: Both actin-myosin and microtubule-dynein based (e.g., organelle transport, cyclosis).

Two primary molecular systems drive these movements: ✅ Actin-Myosin System ✅ Microtubule-Dynein System

5.2 Movements Based on the Actin-Myosin Mechanism

5.2.1 Muscle Contraction

Muscle contraction relies on the interaction between thin actin filaments and the globular heads of myosin (thick filaments).

1. Calcium (Ca2+) Binding: Ca2+ binds to troponins, causing tropomyosin molecules to shift.
2. Myosin-Actin Linkage: This shift exposes binding sites on actin, allowing myosin heads to attach.
3. ATP Hydrolysis: Myosin's ATPase activity hydrolyzes ATP, releasing energy.
4. Conformational Change: Energy changes myosin's conformation, causing it to "step" along the actin filament.
5. Sliding Model: Repeated binding, pulling, and releasing shortens the muscle fiber.
6. Relaxation: All actin-myosin links are disrupted.

5.2.2 Amoeboid Movement

A highly active form of locomotion observed in single-celled organisms like Amoeba proteus and in human cells such as neutrophil leukocytes (immune response) and fibroblasts (wound healing).

The mechanism involves the actin-myosin system and dynamic sol-gel transitions of the cytoplasm:

• 1️⃣ Protrusion of a Leading Edge: The cell membrane is pushed forward by actin polymerization, forming extensions (pseudopodia in amoebae/neutrophils, lamellipodia in fibroblasts).
• 2️⃣ Attachment to the Substratum: Myosin II and integrins connect actin filaments to the extracellular matrix, creating tension.
• 3️⃣ Traction: Cytoplasm flows into the extension (sol-gel transition), pulling the entire cell body forward.

💡 Chemotaxis: Amoeboid movement is often directed by chemical cues (e.g., leukocytes attracted by bacterial peptides).

5.2.3 Movements from Microvilli

Microvilli are permanent, short extensions on cell surfaces (e.g., enterocytes, hepatocytes). They primarily enhance absorption.

• Passive Mechanism: Increase surface area (e.g., 20x in enterocytes).
• Active Mechanism: Interaction between actin microfilaments and myosin molecules.
  • Actin microfilaments (20-30) are organized into parallel bundles by fimbrin and villin.
  • They are anchored to the plasma membrane at the +end and laterally by myosin I arms.
  • Actin-myosin interaction causes the microvillus to shorten, pushing absorbed material into the cytoplasm.

5.2.4 Cytoplasmic Currents

Intracellular movements that transport organelles and mix cytoplasm.

• Examples: Jumping movements of mitochondria, cyclosis (continuous movement of cytoplasm and chloroplasts in plant cells).
• Influencing Factors: Light, pH, and temperature.

5.3 Movements Based on the Microtubule-Dynein Mechanism

5.3.1 Axoplasmic Transport

A specialized type of cytoplasmic current in neurons, transporting materials along axons.

• Anterograde Transport (Forward): From cell body (perikaryon) to axon terminal.
  • Motor Protein: Kinesin (dimer with globular heads, ATPase activity).
  • Movement: Steps 8 nm from the minus (-) end to the plus (+) end of microtubules.
  • Cargo: Neurotransmitter vesicles, mitochondria, lipids, proteins for axonal growth and membrane maintenance.
  • Speeds: Fast (3 µm/sec for synaptic vesicles), intermediate (mitochondria), slow (cytoskeletal molecules).
  • 💡 At the +end, myosin can continue transport along peripheral actin microfilaments.
• Retrograde Transport (Backward): From axon terminal back to the cell body.
  • Motor Protein: Dynein (moves from +end to -end of microtubules).
  • Cargo: Old organelles, vesicles for recycling.
  • Types: Cytosolic dyneins (transport cargo), axonemal dyneins (cilia/flagella).

5.3.2 Movements of Cilia and Flagella

Cilia and flagella are permanent, larger extensions with a common internal structure called the axoneme.

• Cilia: Found in respiratory tract, oviduct. Coordinated, wave-like movement.
  • Active Step (Power Stroke): Cilium is straight, whips like a paddle.
  • Passive Step (Recovery Stroke): Cilium bends, recovers position.
  • Function: Displaces mucus, foreign materials (dust, bacteria) to prevent infections.
• Flagella: Found in sperm cells. Continuous helical movement.

Axoneme Structure:

• Consists of 9 peripheral doublets of microtubules surrounding 2 central microtubules (9+2 arrangement).
• Each doublet has a complete A subfiber (13 protofilaments) and an incomplete B subfiber (11 protofilaments).
• Dynein Arms: Two protein arms on the A subfiber, with globular heads interacting with the adjacent doublet. They are arranged clockwise.
• Nexin: Links peripheral doublets, preventing sliding.
• Radial Spokes: Connect peripheral doublets to the central microtubules.

Mechanism of Movement: Dynein arms form and disrupt lateral bridges with tubulins of the adjacent doublet, using ATP. Because doublets are linked by nexin, they don't slide but instead cause the axoneme to bend, generating movement.

⚠️ Clinical Relevance:

• Immotile Cilia Syndrome (Primary Ciliary Dyskinesia): Absence or deficiency of axonemal dynein leads to immotile cilia and flagella.
  • Symptoms: Repeated respiratory infections, male sterility.
• Kartagener Syndrome: Immotile cilia syndrome associated with situs inversus (internal organs are inverted). This highlights the role of ciliary movement in embryonic development for proper organ placement.

5.4 Microtubule-Organizing Centers: Centrioles and Basal Bodies

Centrioles and basal bodies are structurally identical and can interchange roles.

• Centrosome (Cell Center): Located near the nucleus in animal cells. Contains two L-shaped centrioles surrounded by pericentriolar material (containing pericentrin and γ-tubulin).
  • Structure: Cylindrical, 9 triplets of microtubules (one complete, two incomplete) with an empty central region.
  • Function: Coordinates microtubule network in interphase, forms poles of mitotic spindle during cell division. Microtubules have their -end in the pericentriolar material and +end distal.
• Basal Bodies: The base of cilia and flagella.
  • Structure: Axonemal doublets become triplets; central microtubules stop at this level.
  • Function: Coordinate movements of cilia/flagella, polymerize tubulin within the axoneme. Microtubules have their -end in the basal body and +end distal.
  • 💡 Functional Plasticity: Basal bodies can take on centriole functions (e.g., Chlamydomonas alga: flagella retract, basal bodies become centrioles during mitosis). They also coordinate flagella reassembly after damage.

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🧬 Chapter 6: Molecular Biology of Cell Membranes

6.1 Definition and Functions

Biological membranes are two-dimensional structures composed of proteins and lipids, characterized by selective permeability.

Main Functions:

• a) ✅ Barrier Function: Separate cell contents from the environment or compartmentalize organelles.
• b) ✅ Compartmentalization: Create distinct environments within cells (organelle membranes).
• c) ✅ Metabolic Functions: Host membrane-bound enzymes for complex processes (e.g., oxidative phosphorylation, photosynthesis).
• d) ✅ Control of Information Flow: Receive (receptors) and emit (signals) information for intercellular communication.
• e) ✅ Signal Transduction: Convert external signals into intracellular responses.
• f) ✅ Immunity: Role in defense against infections.
• g) ✅ Adaptive Modifications: Can undergo functional changes.

Types of Membranes in Eukaryotic Cells:

1. Cell Membrane (Plasmalemma): Barrier, individuality, communication.
2. Organelle Membranes: Nuclear, ER, Golgi, mitochondria, peroxisomes, lysosomes, vesicles. Compartmentalization and metabolic roles.
3. Special Membranes: Myelin sheath (barrier), retinal rod cell discs.

6.2 Chemical Composition of Biological Membranes

Membranes are primarily composed of proteins and lipids, with attached carbohydrate groups.

• Protein:Lipid Ratio: Varies with function.
  • High Metabolic Function: Higher protein percentage (up to 75% in inner mitochondrial membrane, chloroplast inner membrane, bacterial membrane).
  • Barrier Function: Higher lipid percentage (up to 80% in myelin).

Lipids

Membrane lipids are organized into a lipid bilayer and possess fluidity. They are categorized into three main types:

1. Phospholipids:

   • Phosphoglycerides: Glycerol backbone, two fatty acids, and a polar head group (e.g., choline in lecithin, ethanolamine, serine, inositol). Fatty acid chains vary (12-24 C atoms), often one saturated and one unsaturated.
   • Sphingolipids: Sphingosine backbone (amino alcohol), one fatty acid (amide linkage), and a polar head group. Structurally similar to phosphoglycerides.
     • Sphingomyelin: Most important sphingolipid, with a polar group like lecithin.

2. Glycolipids:

   • Based on sphingosine, but with glucidic residues instead of a phosphorylcholine polar group.
   • Cerebrosides: Simplest glycolipids, with a single sugar residue (e.g., glucose, galactose). Galactocerebroside is a major component of myelin.
   • Gangliosides: More complex, contain one or more sialic acid residues, conferring a negative electrical charge. Abundant in neuron plasmalemma.

3. Cholesterol:

   • Major lipid in eukaryotic cell membranes.
   • Distribution: Higher in plasmalemma and myelin (barrier function), lower in intracellular membranes.
   • Role in Fluidity: Increases fluidity in saturated lipid bilayers, decreases fluidity in unsaturated lipid bilayers.

Amphiphilic Nature and Lipid Bilayer

• Amphiphilic Structure: All membrane lipids have a hydrophilic (polar) head and a hydrophobic (nonpolar) tail (fatty acid chains).
• Self-Assembly: In water, they spontaneously form micelles or, more stably, a lipid bilayer, which is the structural basis of biological membranes.

Fluidity of the Lipid Bilayer

Fluidity is a crucial characteristic, making membranes suitable for their functions.

• Phase Transition: An artificial lipid bilayer can transition from a gel (crystalline) to a fluid (liquid crystal) state at a specific temperature.
• Factors Affecting Fluidity:
  • Fatty Acid Chain Length: Shorter chains = higher fluidity, lower transition temperature.
  • Fatty Acid Saturation: More unsaturated (more double bonds) = higher fluidity, lower transition temperature.
  • Cholesterol: Modulates fluidity (increases with saturated lipids, decreases with unsaturated lipids).
• Physiological State: Membrane lipids are in a fluid, liquid crystal state at physiological temperatures. Cells adapt fatty acid composition to maintain fluidity if temperature changes.

Types of Lipid Movements within the Bilayer:

1. Movements within the Phospholipid Molecule:
   • a) Flexion movements of C atoms in fatty acid chains (more mobile towards the middle of the bilayer).
   • b) Movements of atoms in the polar head group.
2. Movements of the Entire Phospholipid Molecule:
   • a) Lateral Diffusion: Very rapid; a molecule can cover ~2 µm in 1 second.
   • b) Rotation Movement: Rapid rotation around the longitudinal axis.
   • c) Transversal Diffusion (Flip-Flop): Very slow; requires specific proteins called flipases to move from one layer to another.

📊 Summary: The dynamic nature of both cellular motors and membrane components allows cells to perform a vast array of essential functions, from movement and transport to communication and barrier maintenance.