Ch. 7 part 2 (7.4-7.6)

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Cell Systems Overview

Introduction to Cell Systems

Cellular life is defined by the collaboration of internal structures, each contributing to the cell's overall function. The main systems discussed here are nuclear transport, the endomembrane system, and the dynamic cytoskeleton, which together orchestrate the movement, processing, and organization of molecules within eukaryotic cells.

Overview of cell systems and chapter structure

Nuclear Transport (7.4)

Structure and Function of the Nuclear Envelope

The nuclear envelope is a double-membrane structure that surrounds the nucleus, separating it from the cytosol. It is perforated with nuclear pores, which regulate the movement of molecules between the nucleus and cytoplasm.

Double Membrane: Consists of an inner and outer membrane.
Nuclear Pores: Complexes that allow selective transport of molecules.
Nuclear Lamina: Provides structural support to the envelope.

Cross-sectional view of nuclear envelope Diagram of nuclear envelope and pore complex

Mechanisms of Nuclear Transport

Transport through nuclear pores is highly selective and energy-dependent. Small molecules like nucleotides can diffuse freely, but larger molecules such as proteins, ribosomes, and RNA require specific signals.

Nuclear Localization Signal (NLS): Proteins destined for the nucleus must have an NLS, a molecular address tag.
Nuclear Export Signal (NES): Ribosomes and mRNA exiting the nucleus require an NES.
Selective Transport: Only molecules with the correct tags are actively transported.

Nuclear pore complex and transport

Endomembrane System (7.5)

Overview and Secretory Pathway

The endomembrane system is responsible for manufacturing, shipping, and recycling cellular cargo. It includes the endoplasmic reticulum (ER), Golgi apparatus, vesicles, and lysosomes. The secretory pathway is a model describing the stepwise movement of proteins from synthesis to secretion.

Rough ER: Site of protein synthesis and initial processing.
Golgi Apparatus: Further modifies, sorts, and packages proteins.
Vesicles: Transport proteins between organelles and to the plasma membrane.

Secretory pathway: protein synthesis in ER Secretory pathway: protein enters ER Secretory pathway: protein exits ER in vesicle Secretory pathway: protein travels to Golgi apparatus Secretory pathway: protein is secreted from cell

Vesicle Formation and Fusion

Membranes are fluid, allowing vesicles to form by pinching off and fuse with target membranes to deliver their contents. Vesicles move materials into, out of, and within the cell.

Endocytosis: Uptake of materials from outside the cell.
Exocytosis: Secretion of materials to the outside.
Intracellular Transport: Movement between organelles.

Vesicle formation and fusion

Pulse-Chase Experiment

The pulse-chase experiment is used to track the movement of proteins through the endomembrane system. Cells are exposed to radiolabeled amino acids (pulse), followed by unlabeled amino acids (chase), allowing visualization of protein trafficking.

Pulse: Short exposure to labeled amino acids.
Chase: Replacement with unlabeled amino acids.
Tracking: Proteins are followed as they move from ER to Golgi to vesicles.

Pulse-chase experiment setup Tracking pulse-labeled proteins during chase

Protein Entry into the Endomembrane System: The Signal Hypothesis

Proteins destined for the endomembrane system begin synthesis on free ribosomes. The first amino acids form an ER signal sequence, which is recognized by the signal recognition particle (SRP). The SRP binds to its receptor on the ER membrane, and the protein is translocated into the ER lumen.

ER signal sequence is synthesized by ribosome.
SRP binds to signal sequence and halts synthesis.
SRP binds to SRP receptor on ER membrane.
SRP is released; protein enters ER via translocon.
Signal sequence is removed; protein synthesis completes.

ER signal sequence synthesis SRP binds to ER signal sequence SRP binds to receptor in ER membrane Protein enters ER through translocon Signal sequence removed, protein synthesis completes

Protein Processing in the Golgi Apparatus

Proteins are further modified in the Golgi apparatus, where each cisterna contains different enzymes. Modifications occur as proteins move from the cis to the trans face, after which they are sorted and packaged for their final destination.

Stepwise Processing: Sequential modification in Golgi cisternae.
Protein Sorting: Proteins are tagged for specific destinations.
Vesicle Transport: Vesicles bud off and deliver proteins to their targets.

Protein sorting and vesicle transport in Golgi Protein sorting: binding to receptors in Golgi Transport vesicles bud off Golgi Vesicle fusion and delivery

Lysosome Formation and Recycling

Lysosomes are recycling centers formed by three mechanisms: phagocytosis, autophagy, and receptor-mediated endocytosis. They degrade and recycle cellular components.

Phagocytosis: Uptake of large particles into phagosomes.
Autophagy: Degradation of damaged organelles via autophagosomes.
Receptor-mediated Endocytosis: Uptake of specific molecules via vesicles.

Recycling via the lysosome

Dynamic Cytoskeleton (7.6)

Structure and Types of Cytoskeletal Filaments

The cytoskeleton is a dynamic network of protein filaments that controls the location of organelles and is essential for cell shape, movement, and division. There are three main types of filaments: actin filaments, intermediate filaments, and microtubules.

Actin Filaments: Composed of actin; involved in cell movement and shape.
Intermediate Filaments: Composed of keratins, lamins, etc.; provide structural support.
Microtubules: Composed of tubulin dimers; involved in chromosome movement and intracellular transport.

Types of cytoskeletal filaments

Actin Filaments and Cell Movement

Actin filaments interact with myosin to produce cell movements, including muscle contraction, cell crawling, cytokinesis, and cytoplasmic streaming.

Actin-Myosin Interaction: Myosin heads bind to actin, hydrolyze ATP, and cause filament sliding.
Cytokinesis: Actin-myosin interactions divide animal cells.
Cytoplasmic Streaming: Moves cytoplasm in plant cells.

Actin filament structure and functions Actin and myosin interaction Cytokinesis and cytoplasmic streaming

Intermediate Filaments

Intermediate filaments anchor the nucleus and other organelles, maintaining cell shape by resisting tension.

Structural Support: Provide mechanical strength.
Anchoring: Secure organelles in place.

Intermediate filament structure and functions Intermediate filaments in cell

Microtubules

Microtubules are hollow tubes that move chromosomes during cell division, assist in cell plate formation, and provide tracks for intracellular transport. They are formed from α- and β-tubulin dimers.

Chromosome Movement: Essential during mitosis and meiosis.
Intracellular Transport: Kinesin and other motor proteins move vesicles along microtubule tracks.
Cell Shape: Resist compression forces.

Microtubule structure and functions Centrosome and microtubule organization

Kinesin and Intracellular Transport

Kinesin is a motor protein that "walks" along microtubule tracks, transporting vesicles and organelles. Each step requires ATP hydrolysis.

ATP Hydrolysis: Provides energy for movement.
Directional Transport: Moves cargo toward the plus end of microtubules.

Kinesin walking along microtubule track Kinesin motor protein

Summary Table: Cytoskeletal Filaments

Filament Type	Structure	Main Functions
Actin Filaments	Two coiled strands (7 nm)	Cell shape, movement, cytokinesis, organelle transport
Intermediate Filaments	Fibers wound into cables (10 nm)	Structural support, anchoring organelles
Microtubules	Hollow tubes (25 nm)	Chromosome movement, intracellular transport, cell shape

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