Inside the Eukaryotic Cell: Structure and Function of Cell Components

Chapter 7: Inside The Cell Part I

Overview of Eukaryotic Cell Structure

Eukaryotic cells possess a variety of internal structures, each contributing to the cell's overall function and specialization. The collaboration of these organelles and systems underlies the properties of life.

• Key Question: What are the parts of the cell?

• Main Topics: Eukaryotic cell structures, how organelles fit into a whole, nuclear transport, endomembrane system, dynamic cytoskeleton.

Eukaryotic Cell Structure: A Parts List

Mitochondria

Mitochondria are the primary suppliers of ATP, the energy currency of the cell. Their unique structure and dynamic behavior are essential for cellular metabolism.

• Membranes: Two membranes surround mitochondria.

  • Outer membrane: Defines the organelle's surface.

  • Inner membrane: Folded into sac-like cristae, increasing surface area for ATP production.

• Mitochondrial matrix: The solution enclosed within the inner membrane, containing enzymes for the citric acid cycle.

• Dynamic morphology: Mitochondria undergo fusion and fission, forming networks or remaining as individual organelles.

• Genetic autonomy: Mitochondria have their own mitochondrial DNA (mtDNA) and ribosomes, allowing them to grow and divide independently of the cell cycle.

Example: Muscle cells contain many mitochondria to meet high energy demands.

Chloroplasts

Chloroplasts are found in plant and algal cells and are the site of photosynthesis, converting light energy into chemical energy.

• Membranes: Three membranes surround chloroplasts.

  • Innermost membrane: Contains thylakoids, which are flattened sacs.

  • Thylakoids: Arranged in stacks called grana.

  • Stroma: The fluid surrounding thylakoids, containing enzymes for sugar production.

• Genetic autonomy: Like mitochondria, chloroplasts have their own DNA and ribosomes, and can grow and divide independently.

Example: Leaf cells are packed with chloroplasts to maximize photosynthesis.

Endosymbiosis Theory

The endosymbiosis theory proposes that mitochondria and chloroplasts originated as free-living bacteria engulfed by ancestral eukaryotic cells, leading to a mutually beneficial relationship.

• Evidence: Presence of DNA and ribosomes, double membranes, and independent division.

Cytoskeleton

The cytoskeleton is an extensive network of protein fibers that provides structural support, maintains cell shape, and facilitates intracellular transport.

• Functions:

  • Gives cells shape and stability

  • Transports materials within the cell

  • Organizes organelles and cellular structures

Cell Wall and Extracellular Matrix

Plant, algal, and fungal cells possess a cell wall outside the plasma membrane, providing structural support and protection. Animal cells lack a cell wall but are supported by the extracellular matrix (ECM), a network of secreted proteins and polysaccharides.

• Cell wall: Rigid, durable outer layer in plants and algae.

• ECM: Provides support and regulates cell behavior in animals.

Cell Systems: Nuclear Transport

Nucleus and Nuclear Envelope

The nucleus is the information center of eukaryotic cells, housing genetic material and coordinating gene expression.

• Nuclear envelope: Double membrane separating nucleus from cytoplasm, perforated by nuclear pore complexes (NPCs).

• Nuclear pores: Regulate selective transport of molecules (e.g., nucleoside triphosphates, proteins for DNA replication and transcription).

• Nuclear localization signals: "Zip codes" that direct proteins to the nucleus.

Cell Systems II: The Endomembrane System

Manufacturing, Shipping, and Recycling

The endomembrane system is responsible for the synthesis, modification, transport, and recycling of cellular materials.

• Components: Rough ER, Smooth ER, Golgi apparatus, lysosomes, transport vesicles.

• Protein targeting: Proteins have specific molecular "zip codes" for delivery to correct compartments (e.g., acid hydrolases to lysosomes).

Secretory Pathway

Proteins destined for secretion or specific organelles follow a pathway starting in the rough ER and ending at the plasma membrane or lysosomes.

• Pulsed labeling experiments: Demonstrate movement of proteins from ER to Golgi to secretory vesicles.

• Signal hypothesis: Proteins bound for the endomembrane system have an ER signal sequence (20 amino acids) that directs them to the rough ER.

• Glycosylation: Addition of carbohydrate groups to proteins in the ER, forming glycoproteins.

• Vesicle transport: Proteins are packaged into vesicles that bud from the ER and fuse with the Golgi apparatus.

• Golgi apparatus: Dynamic organelle where proteins are further modified and sorted for delivery.

• Sorting signals: Proteins exiting the Golgi carry molecular tags (e.g., mannose-6-phosphate for lysosomal targeting).

• Exocytosis: Transport vesicles fuse with the plasma membrane to secrete contents outside the cell.

Lysosome Recycling Pathways

Lysosomes digest large molecules and recycle cellular components through multiple pathways, ensuring efficient resource use.

Cell Systems III: The Dynamic Cytoskeleton

Cytoskeletal Elements

The cytoskeleton consists of three major types of protein filaments, each with distinct structure and function.

• Actin filaments (microfilaments): Smallest, composed of actin subunits, abundant in animal cells, involved in cell shape and movement.

• Intermediate filaments: Diverse family (e.g., keratins, lamins), provide mechanical strength and nuclear stability.

• Microtubules: Largest, hollow tubes made of α- and β-tubulin dimers, originate from microtubule organizing centers (MTOCs), essential for chromosome movement and intracellular transport.

Microtubule-Based Transport

Microtubules serve as tracks for vesicle transport, powered by motor proteins such as kinesin.

• Kinesin: Motor protein that "walks" along microtubules, converting ATP into mechanical energy to move vesicles.

Flagella and Cilia

Flagella and cilia are cell surface projections that enable movement. Their structure and mechanism differ between prokaryotes and eukaryotes.

• Prokaryotic flagella: Composed of flagellin, rotate like a propeller.

• Eukaryotic flagella and cilia: Composed of microtubules in a 9+2 arrangement (axoneme), move by whipping motion powered by dynein motor proteins.

Example: Sperm cells use flagella for motility; respiratory tract cells use cilia to move mucus.

Summary Table: Eukaryotic Cell Components

Organelle	Main Function	Key Features
Mitochondria	ATP production	Double membrane, cristae, own DNA/ribosomes
Chloroplasts	Photosynthesis	Three membranes, thylakoids/grana, own DNA/ribosomes
Nucleus	Genetic information storage	Double membrane, nuclear pores
Endoplasmic Reticulum (ER)	Protein/lipid synthesis	Rough (ribosomes), Smooth (no ribosomes)
Golgi Apparatus	Protein modification/sorting	Stacked cisternae, dynamic structure
Lysosome	Digestion/recycling	Acidic enzymes, single membrane
Cytoskeleton	Structural support, movement	Actin filaments, intermediate filaments, microtubules
Cell Wall/ECM	Structural support	Rigid wall (plants), ECM (animals)

Key Equations and Concepts

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Endosymbiosis: $\text{Eukaryotic cell} + \text{bacterium} \rightarrow \text{organelle}$

Additional info: Some details inferred from standard biology textbooks to ensure completeness and clarity.