Cell Structure and Function: Organelles, Cytoskeleton, and Protein Trafficking

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Inside the Cell

Photosynthesis and Cellular Respiration

Cells carry out essential energy transformations through photosynthesis and cellular respiration, each occurring in specialized organelles.

Photosynthesis Equation:
Organelle: Occurs in the chloroplast of plant cells.
Cellular Respiration Equation:
Organelle: Occurs in the mitochondrion of eukaryotic cells.
Example: Plants use chloroplasts for photosynthesis; animals use mitochondria for respiration.

Functions of Cell Organelles

Each organelle in a eukaryotic cell has a distinct function, contributing to cellular homeostasis and metabolism.

Nucleus: Stores genetic material (DNA); site of transcription.
Nucleolus: Synthesizes ribosomal RNA (rRNA) and assembles ribosome subunits.
Rough Endoplasmic Reticulum (RER): Synthesizes proteins for secretion or membrane insertion; studded with ribosomes.
Smooth Endoplasmic Reticulum (SER): Synthesizes lipids, detoxifies chemicals, stores calcium ions.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
Lysosomes: Digest macromolecules; contain hydrolytic enzymes and maintain low pH.
Peroxisomes: Contain catalase and peroxidase; break down hydrogen peroxide and fatty acids.
Mitochondria: Generate ATP via cellular respiration; contain their own DNA.
Chloroplasts: Conduct photosynthesis; contain their own DNA.

Nuclear Lamina and Nucleolus

The nuclear lamina provides structural support to the nucleus, while the nucleolus is essential for ribosome biogenesis.

Nuclear Lamina: Meshwork of intermediate filaments (lamins); supports nuclear envelope.
Nucleolus: Site of rRNA synthesis and ribosome assembly.

Protein Targeting: Free vs. Bound Ribosomes

Proteins synthesized by free ribosomes and those by ER-bound ribosomes have different cellular destinations.

Free Ribosomes: Make proteins for cytosol, nucleus, mitochondria, chloroplasts, peroxisomes.
Bound Ribosomes (RER): Make proteins for secretion, plasma membrane, lysosomes, ER, Golgi.
Example: Enzymes for glycolysis are made by free ribosomes; insulin is made by RER-bound ribosomes.

Smooth ER vs. Rough ER

The endoplasmic reticulum is divided into two types based on structure and function.

Smooth ER: Lacks ribosomes; lipid synthesis, detoxification, calcium storage.
Rough ER: Has ribosomes; protein synthesis and folding.

Golgi Apparatus: Protein Processing and Transport

The Golgi apparatus modifies proteins and lipids, sorts them, and packages them into vesicles for transport.

Protein Movement: Proteins move between ER and Golgi via vesicles, not freely.
Enzyme Recycling: Golgi recycles enzymes through vesicle trafficking.

Lysosomes: Acidic Environment and Function

Lysosomes maintain a low pH for optimal enzyme activity, achieved by active proton transport.

Low pH: Means high concentration of H+ ions.
Proton Transport: Protons are pumped into lysosomes by ATP-driven proton pumps; they do not freely diffuse.

Peroxisomes: Enzymes and Functions

Peroxisomes contain enzymes that detoxify harmful substances.

Catalase: Converts hydrogen peroxide to water and oxygen.
Peroxidase: Breaks down peroxides.

Organelles Containing DNA

Some organelles have their own DNA, supporting the endosymbiotic theory.

Nucleus: Contains most cellular DNA.
Mitochondria: Contains mitochondrial DNA.
Chloroplasts: Contains chloroplast DNA.

Endosymbiotic Theory

This theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

Evidence: Double membranes, own DNA, ribosomes similar to bacteria.

Nuclear Transport

Macromolecules move in and out of the nucleus via nuclear pore complexes.

Import: Proteins with nuclear localization signals (NLS) are imported.
Export: mRNA, ribosomal subunits, and proteins are exported.
Experiment: Adding NLS to cytoplasmic protein causes its import into nucleus, demonstrating signal-mediated transport.

Protein Import into Mitochondria and Chloroplasts

Proteins destined for mitochondria and chloroplasts are encoded both in organelle DNA and nuclear DNA.

Organelle-encoded: Some proteins are made inside the organelle.
Nuclear-encoded: Most proteins are made in cytosol and imported via translocases.

Vesicle Trafficking: Budding and Fusion

Vesicles transport materials between organelles by budding from one membrane and fusing with another.

Endocytosis: Uptake of materials into cell via vesicles.
Exocytosis: Release of materials from cell via vesicles.

Endomembrane System: Protein Pathway

Proteins synthesized in the RER follow a specific pathway through the endomembrane system.

Pathway: RER → Golgi → vesicles → plasma membrane or lysosome.

Protein Synthesis in the RER

Proteins destined for secretion or membranes are synthesized in the RER with the help of signal recognition particles (SRP) and translocons.

SRP: Recognizes signal sequence, pauses translation, directs ribosome to ER.
Translocon: Protein channel in ER membrane; allows polypeptide entry.
Folding Enzymes: Chaperones (e.g., BiP) assist folding in ER lumen.

Transmembrane Protein Folding

Transmembrane proteins fold in the RER, with amphipathic amino acids orienting according to membrane environment.

Amphipathic: Both hydrophobic and hydrophilic regions; hydrophobic regions embed in membrane.

Glycosylation: Types and Locations

Glycosylation is the addition of carbohydrate groups to proteins, affecting their function and localization.

N-linked Glycosylation: Occurs on asparagine residues in ER.
O-linked Glycosylation: Occurs on serine/threonine residues in Golgi.
ER vs. Golgi: Glycosylation patterns differ; ER adds core oligosaccharides, Golgi modifies them.
Destination: Glycosylated portion faces extracellular space when protein reaches plasma membrane.

Lysosome Function and Cycle

Lysosomes degrade macromolecules, recycle components, and maintain cellular health.

Cycle: Formation, fusion with vesicles, digestion, recycling.

Cytoskeleton: Filament Types and Functions

The cytoskeleton provides structural support, facilitates movement, and organizes cell contents.

Actin Filaments (Microfilaments): Structure: two intertwined strands of actin; function: cell shape, movement.
Intermediate Filaments: Structure: rope-like; function: mechanical strength, nuclear lamina.
Microtubules: Structure: hollow tubes of tubulin; function: cell shape, transport, mitosis.

Filament	Structure	Function
Actin	Double helix of G-actin	Cell movement, shape
Intermediate	Rope-like, lamins	Strength, nuclear support
Microtubules	Hollow tubes of tubulin	Transport, mitosis

Actin: G-actin and F-actin

Actin exists as monomeric G-actin and polymeric F-actin, forming dynamic filaments.

G-actin: Globular actin monomer.
F-actin: Filamentous actin polymer.

Actin Polymerization and Treadmilling

Actin filaments grow and shrink through polymerization and depolymerization, enabling cell movement.

Treadmilling: Addition at plus end, loss at minus end; maintains filament length.

Cell Movement with Actin

Cells move by extending actin-rich structures (lamellipodia, filopodia) and contracting via actin-myosin interactions.

Actin-Myosin Cycle: Myosin binds actin, uses ATP to generate force.
Regulation: Actin-binding proteins can prevent myosin binding.

Intermediate Filaments: Function

Intermediate filaments provide mechanical strength and support, especially in the nuclear lamina.

Microtubules: Structure and Function

Microtubules are dynamic polymers of tubulin, essential for intracellular transport and cell division.

Centrosomes: Microtubule organizing centers; contain centrioles.
Motor Proteins: Dynein (moves toward minus end), Kinesin (moves toward plus end).

Motor Protein	Direction	Function
Dynein	Toward minus end	Retrograde transport
Kinesin	Toward plus end	Anterograde transport

Additional info: Academic context was added to clarify protein targeting, glycosylation, cytoskeleton dynamics, and nuclear transport experiments.