A Tour of the Cell: Cytoskeleton, Cell Surfaces, and Connections

Ch 4: A Tour of the Cell

Introduction to the Cell

Cells are the fundamental units of life, forming the basis of all living organisms. Eukaryotic cells contain specialized structures called organelles, each with distinct functions that contribute to the cell's survival and activity.

• Nucleus and Ribosomes: Control center for genetic information and protein synthesis.

• Endomembrane System: Network of membranes involved in synthesis, transport, and processing of cellular materials.

• Energy-Converting Organelles: Mitochondria and chloroplasts convert energy for cellular use.

• Cytoskeleton and Cell Surfaces: Provide structure, movement, and communication between cells.

Cytoskeleton and Cellular Extensions: Move and Move Stuff

The Cytoskeleton

The cytoskeleton is a dynamic network of protein fibers that provides structural support, facilitates transport within the cell, and enables cellular movement and division.

• Functions: Support, intracellular transport, mitosis, and cell movement.

• Hierarchy of Thickness: Microtubules > Intermediate Filaments > Microfilaments.

Microtubules

• Composed of globular proteins called tubulins.

• Can be disassembled and reassembled elsewhere in the cell.

• Form the centrosome (a pair of centrioles in animal cells), which organizes microtubules during cell division.

• Involved in chromosome movement during mitosis and meiosis.

• Form the structural basis for cilia and flagella, enabling cell movement.

Intermediate Filaments

• Made of fibrous proteins (often super-coiled).

• More permanent structures compared to microtubules and microfilaments.

• Reinforce cell shape and anchor organelles (e.g., the nucleus).

• Common in cells subject to mechanical stress, such as skin cells.

Microfilaments

• Composed of globular proteins called actin.

• Form twisted double chains.

• Support the cell's shape and are involved in cell movements.

• Work with myosin in muscle contraction, and are involved in amoeboid movement and white blood cell migration.

Cellular Extensions

Specialized structures that extend from the cell surface, aiding in movement and environmental interaction.

• Cilia: Short, numerous projections that move fluid over the cell surface or propel single cells.

• Flagella: Longer, usually singular extensions that propel cells (e.g., sperm cells).

Structure of Cilia and Flagella: Both have a "9+2" arrangement of microtubules (nine doublets surrounding two central microtubules), cross-linking proteins, and motor proteins (dyneins) that generate movement.

Extracellular Matrix and Intercellular Junctions: Staying Connected

Extracellular Matrix (ECM)

The ECM is a complex network of proteins and carbohydrates outside animal cells that provides structural support, segregates tissues, and regulates intercellular communication.

• Fills space and binds cells together.

• Collagen: A glycoprotein making up about 40% of the body's proteins, providing tensile strength.

• Integrins: Membrane proteins that connect the ECM to the cytoskeleton, transmitting signals between the cell's exterior and interior.

• Other components: Glycoproteins, proteoglycans, and polysaccharides.

• Specialized matrices: Blood plasma, bone, and cartilage matrices.

Intercellular Junctions

Specialized structures that connect adjacent cells, allowing them to communicate, adhere, and form tissues.

• Tight Junctions: Seal neighboring cells together to prevent leakage of extracellular fluid.

• Anchoring Junctions (Desmosomes): Fasten cells together into strong sheets; involve intermediate filaments and are important in tissues subject to stretching (e.g., muscle).

• Gap (Communicating) Junctions: Allow ions and small molecules to pass directly from one cell to another, facilitating communication.

• Plasmodesmata: Channels between plant cells that allow the passage of molecules and communication.

Cell Walls: Providing Support

Cell walls are rigid structures that provide support and protection to certain cells.

• Plant Cell Walls: Composed mainly of cellulose, a polysaccharide that gives strength and rigidity.

• Fungi Cell Walls: Made of chitin, a different polysaccharide.

• Plasmodesmata: Channels in plant cell walls that allow communication and transport between cells.

Table: Eukaryotic Cell Structures and Their Functions

Structure	Main Function(s)
Nucleus	Genetic control: DNA replication, RNA synthesis, assembly of ribosomal subunits
Ribosomes	Protein synthesis
Rough ER	Synthesis of membrane lipids and proteins, secretory proteins, hydrolytic enzymes; formation of transport vesicles
Smooth ER	Lipid synthesis, detoxification, calcium ion storage
Golgi apparatus	Modification, sorting, and shipping of macromolecules; formation of lysosomes and transport vesicles
Lysosomes	Digestion of ingested food, bacteria, and damaged organelles
Peroxisomes	Breakdown of toxic hydrogen peroxide byproducts
Mitochondria	Conversion of chemical energy in food to ATP (cellular respiration)
Chloroplasts (plants)	Conversion of solar energy to chemical energy (photosynthesis)
Cytoskeleton	Maintenance of cell shape, anchorage and movement of organelles, cell movement
Plasma membrane	Selective barrier; regulates passage of substances in and out of cell
Cell wall (plants, fungi, some protists)	Support and protection; binding of cells in tissues
ECM (animals)	Support, regulation, and connection of cells
Cell junctions	Communication and binding of cells in tissues

Additional Info

• The "Twizzlers Pull 'n' Peel" analogy is used to help visualize the structure of intermediate filaments, which are made of multiple protein strands twisted together, similar to the candy's strands.

• The "Ig Nobel Prize" examples highlight the importance of creative research in biology, such as painting cows with zebra stripes to reduce biting fly attacks, illustrating the application of biological principles to real-world problems.

• The phrase "All roads lead to Rome" is used metaphorically to show how various cellular structures (like cytoskeletal elements) converge to support cell function.