Inside the Cell: Structure, Function, and Dynamics of Cellular Components

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Inside the Cell

Learning Objectives

Describe the structure and function of common components of prokaryotic and eukaryotic cells
Compare and contrast the cellular structure and components of prokaryotic vs. eukaryotic cells
Compare and contrast the cellular structure and components of plant vs. animal cells
Explain protein import into the nucleus, secretion from the cell, targeting to the ER, and sorting in the Golgi apparatus
Compare and contrast the structures and functions of the three cytoskeletal filaments

The Prokaryotic Cell

General Structure

Prokaryotic cells are structurally simpler than eukaryotic cells and lack membrane-bound organelles. Their main components include the plasma membrane, cell wall, cytoplasm, ribosomes, and genetic material.

Plasma membrane: Selectively permeable barrier that encloses the cell contents.
Cell wall: Provides structural support and shape.
Cytoplasm: Gel-like substance where cellular processes occur.
Ribosomes: Sites of protein synthesis.
Genetic material: Typically a single, circular chromosome located in the nucleoid region.
Plasmids: Small, circular DNA molecules carrying extra genes, such as those for antibiotic resistance.

Diagram of a prokaryotic cell showing chromosome, plasmid, ribosomes, cytoplasm, plasma membrane, and cell wall

Organization of Prokaryotic DNA

Prokaryotic DNA is organized as a single, circular chromosome. DNA is highly compacted by supercoiling to fit within the cell. Plasmids provide additional genetic capabilities, such as antibiotic resistance, and are important tools in biotechnology.

Nucleoid region: Area where the chromosome is located, not membrane-bound.
Supercoiling: Mechanism for compacting DNA within the cell.

Images showing E. coli chromosome and supercoiled DNA

Extracellular Structures

Prokaryotic cells possess specialized extracellular structures for movement and attachment.

Flagella: Long, whip-like structures that rotate to propel the cell through liquid environments.
Fimbriae: Short, hair-like projections that enable attachment to surfaces and play a role in establishing infections.

Electron micrograph of a prokaryotic cell with labeled flagella and fimbriae

The Eukaryotic Cell

General Animal Cell Structure

Eukaryotic cells are characterized by the presence of membrane-bound organelles, including a nucleus. Animal cells contain various organelles that compartmentalize cellular functions.

Nucleus: Contains genetic material and is surrounded by a nuclear envelope.
Mitochondria: Sites of ATP production via cellular respiration.
Endoplasmic reticulum (ER): Rough ER is studded with ribosomes and synthesizes proteins; smooth ER synthesizes lipids.
Golgi apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosomes: Contain digestive enzymes for breaking down macromolecules.
Peroxisomes: Involved in oxidation reactions and detoxification.
Cytoskeleton: Provides structural support and facilitates movement.

Diagram of a generalized animal cell with labeled organelles

General Plant Cell Structure

Plant cells share many features with animal cells but also have unique structures.

Cell wall: Rigid outer layer composed of cellulose, providing structural support.
Chloroplasts: Sites of photosynthesis, containing chlorophyll.
Central vacuole: Large organelle for storage and maintaining cell turgor.
Plasmodesmata: Channels between plant cells for communication.
Other organelles similar to those in animal cells (nucleus, mitochondria, ER, Golgi, etc.).

Diagram of a generalized plant cell with labeled organelles

Comparison: Prokaryotic vs. Eukaryotic Cells

Prokaryotes: No nucleus, no membrane-bound organelles, generally smaller, single circular chromosome.
Eukaryotes: Nucleus present, many membrane-bound organelles, generally larger, multiple linear chromosomes.

Example: Escherichia coli (prokaryote) vs. human liver cell (eukaryote).

Comparison: Plant vs. Animal Cells

Plant cells: Have cell walls, chloroplasts, and large central vacuoles; perform photosynthesis.
Animal cells: Lack cell walls and chloroplasts; contain centrioles and lysosomes.

Protein Trafficking and Secretion

The Secretory Pathway

Proteins destined for secretion or for certain organelles follow a specific pathway through the cell, known as the secretory pathway.

Protein synthesis begins on ribosomes in the cytosol.
Proteins with an ER signal sequence are directed to the rough ER, where synthesis is completed and initial modifications occur.
Proteins are packaged into vesicles and transported to the Golgi apparatus for further modification and sorting.
From the Golgi, proteins are sorted and sent to their final destinations: the plasma membrane, lysosomes, or secretion outside the cell.

Diagram of the secretory pathway: ER to Golgi to plasma membrane

Targeting Proteins to the ER

Proteins destined for the endomembrane system or secretion possess an ER signal sequence at their N-terminus. This sequence is recognized by the signal recognition particle (SRP), which directs the ribosome to the ER membrane.

Signal peptide: Short amino acid sequence that targets the protein to the ER.

Transport In and Out of the Nucleus

Transport of proteins and RNA between the nucleus and cytoplasm is regulated by the nuclear pore complex (NPC). Proteins destined for the nucleus contain a nuclear localization signal (NLS) that is recognized by import proteins.

Nuclear pore complex: Large protein assembly that spans the nuclear envelope and controls molecular traffic.
Nuclear localization signal (NLS): Amino acid sequence that directs proteins into the nucleus.

Diagram of the nuclear envelope and nuclear pore complex

Protein Processing in the ER and Golgi Apparatus

In the ER, proteins are folded (often with the help of chaperones) and tagged via glycosylation (addition of carbohydrate groups). The tag determines the protein's final destination. Folded and tagged proteins are sent to the Golgi apparatus for further modification and sorting.

Glycosylation: Addition of carbohydrate groups to proteins, important for sorting and function.
Golgi apparatus: Modifies, sorts, and packages proteins for delivery to their final destinations.

Sorting and Final Destinations from the Golgi

To cell membrane: Default pathway for proteins without a specific carbohydrate tag.
To lysosome: Proteins tagged with mannose-6-phosphate are directed to lysosomes (e.g., acid hydrolase enzymes).
To other organelles: Other carbohydrate tags direct proteins to specific organelles.

Pathways to the Lysosome

Three main pathways deliver material to lysosomes for degradation. Acid hydrolase enzymes, sent from the Golgi, complete lysosome formation and enable breakdown of macromolecules.

The Cytoskeleton

Overview of Cytoskeletal Filaments

The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and organizes cellular components. There are three main types of cytoskeletal filaments: actin filaments, intermediate filaments, and microtubules.

Filament	Structure	Subunits	Functions
Actin filaments (microfilaments)	Two coiled strands, 7 nm diameter	Actin	Maintain cell shape by resisting tension (pull) Move cells via muscle contraction or cell crawling Divide animal cells in two Move organelles and cytoplasm in plants, fungi, and animals
Intermediate filaments	Fibers wound into thicker cables, 10 nm diameter	Keratin, lamins, or others	Maintain cell shape by resisting tension (pull) Anchor nucleus and some other organelles
Microtubules	Hollow tube, 25 nm diameter	α- and β-tubulin dimers	Maintain cell shape by resisting compression (push) Move cells via flagella or cilia Move chromosomes during cell division Assist formation of cell plate during plant cell division Provide tracks for intracellular transport

Summary table of cytoskeletal filaments: actin, intermediate filaments, microtubules

Actin Filaments and Myosin: Movement

Actin filaments interact with the motor protein myosin to produce movement. This interaction is essential for muscle contraction, cell crawling, and cytokinesis (cell division). ATP hydrolysis provides the energy for myosin to "walk" along actin filaments, causing the filaments to slide past each other.

Muscle contraction: Actin and myosin interactions shorten muscle fibers.
Cell crawling: Enables movement of cells such as amoebas and immune cells.
Cytokinesis: Division of the cytoplasm during cell division.

Diagram showing actin and myosin interaction for movement

Microtubules and Motor Proteins: Intracellular Transport

Microtubules serve as tracks for the movement of vesicles and organelles within the cell. Motor proteins such as kinesin (moves toward the plus end) and dynein (moves toward the minus end) "walk" along microtubules, powered by ATP hydrolysis. Microtubules are also essential for the movement of cilia and flagella.

Kinesin: Moves cargo toward the cell periphery (plus end of microtubules).
Dynein: Moves cargo toward the cell center (minus end of microtubules).
Flagella and cilia: Structures for cell movement, composed of microtubules arranged in a characteristic pattern.

Diagram of kinesin walking along a microtubule track

Summary: Cytoskeletal Filament Functions

Actin filaments: Cell shape, movement, division.
Intermediate filaments: Structural support, anchoring organelles.
Microtubules: Intracellular transport, cell division, movement via cilia/flagella.

Additional info: The cytoskeleton is highly dynamic, allowing cells to rapidly reorganize their internal structure in response to environmental signals or developmental cues.