Cell Structure, Organelles, and the Cytoskeleton: Study Notes for General Biology

The Cell: Structure and Organelles

Overview of Cell Organelles

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular life. Understanding the role of each organelle is fundamental to cell biology.

• Nucleus: Controls cell activities and houses genetic information (DNA), which is passed to the next generation. It is the site of transcription and regulates gene expression.

• Plasma Membrane: Separates the cell from its external environment and is selectively permeable, allowing certain substances to enter or exit the cell.

• Chloroplasts: Found in plant cells; capture light energy and convert it into chemical energy through photosynthesis. Example: Photosynthesis in Arabidopsis thaliana.

• Cell Wall: Provides additional support, protection, and gives the cell its shape. Contains cellulose and forms a rigid layer outside the plasma membrane in plant cells.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; site of protein synthesis and initial protein folding.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid production, storage of Ca2+, and detoxification.

• Golgi Apparatus: Sorts, modifies, and packages proteins into vesicles for transport within or out of the cell.

• Lysosome: Contains digestive enzymes; digests food, bacteria, and worn-out organelles.

• Mitochondria: Powerhouse of the cell; produces energy (ATP) for growth and development via cellular respiration.

• Vacuole: Stores water, food, enzymes, and other materials; prominent in plant cells.

• Cytoskeleton: Provides structural support, maintains cell shape, and facilitates movement.

• Cilia: Short, hair-like structures for cell locomotion; found in some animal cells.

• Flagella: Long, whip-like structures for cell movement; found in sperm cells and some bacteria.

• Microtubules: Part of the cytoskeleton; separate chromosomes during cell division and help in mitosis.

• Actin Filaments: Part of the cytoskeleton; aid in cellular contractions and movement.

Table: Organelle Functions and Descriptions

Organelle	Main Function	Description
Nucleus	Genetic control	Controls cell activities; contains DNA
Plasma Membrane	Boundary & transport	Separates cell; selectively permeable
Chloroplasts	Photosynthesis	Converts light to chemical energy
Cell Wall	Support & protection	Rigid layer; contains cellulose
Rough ER	Protein synthesis	Studded with ribosomes
Smooth ER	Lipid synthesis	Detoxification; Ca2+ storage
Golgi Apparatus	Protein modification	Sorts and packages proteins
Lysosome	Digestion	Breaks down waste and organelles
Mitochondria	Energy production	Produces ATP
Vacuole	Storage	Stores water, food, enzymes
Cytoskeleton	Structure & movement	Maintains shape; enables movement
Cilia	Locomotion	Short, hair-like; movement
Flagella	Locomotion	Long, whip-like; movement
Microtubules	Cell division	Separate chromosomes
Actin Filaments	Contraction	Aids in movement

Protein Synthesis and Export Pathway

Pathway of Protein Production and Export

Proteins are synthesized and exported from the cell through a coordinated pathway involving several organelles. Each organelle plays a specific role in the process.

1. Nucleus: Contains DNA, which is transcribed into messenger RNA (mRNA).

2. Ribosomes (on Rough ER): mRNA exits the nucleus and is translated into protein by ribosomes attached to the rough ER.

3. Rough ER: Newly synthesized proteins enter the rough ER for folding and initial modifications.

4. Transport Vesicles: Proteins are packaged into vesicles and transported to the Golgi apparatus.

5. Golgi Apparatus: Proteins are further modified, sorted, and packaged for export.

6. Secretory Vesicles: Proteins are enclosed in vesicles that move toward the plasma membrane.

7. Plasma Membrane: Vesicles fuse with the membrane, releasing proteins outside the cell (exocytosis).

Example: Secretion of insulin by pancreatic beta cells follows this pathway.

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: The time required for each step varies, and the amount of protein decreases as it moves through each organelle toward its final destination.

The Cytoskeleton

Components and Functions of the Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement. It consists of three main components, each with distinct roles.

• Microtubules: Hollow tubes made of tubulin; involved in maintaining cell shape, intracellular transport, and separating chromosomes during cell division (mitosis and meiosis).

• Actin Filaments (Microfilaments): Thin filaments composed of actin; responsible for cell movement, muscle contraction, and cytokinesis.

• Intermediate Filaments: Rope-like fibers; provide mechanical strength and help maintain cell integrity.

Consequences of Missing Cytoskeletal Filaments:

• Loss of cell shape and structural integrity

• Impaired cell movement and division

• Defective intracellular transport

Example: Neurons rely on microtubules for axonal transport; muscle cells depend on actin filaments for contraction.

Additional info: Table 4.1 in most textbooks compares the properties and functions of cytoskeletal components.

Table: Cytoskeletal Components and Their Roles

Component	Main Function	Role in Cell
Microtubules	Support, transport, division	Chromosome separation, vesicle movement
Actin Filaments	Movement, contraction	Cell motility, muscle contraction
Intermediate Filaments	Strength, stability	Resist mechanical stress