BIOL Chapter 6: A Tour of the Cell

Introduction to Cells

• Discovery: Cells were discovered in 1665 by Robert Hooke.

• Cell Theory:

  1. All organisms are composed of cells.

  2. Cells are the smallest living things.

  3. Cells arise only from pre-existing cells.

• Size Considerations:

  • Most cells are relatively small due to their reliance on diffusion.

  • Smaller cells have advantages over larger ones in nutrient uptake and waste removal.

Limitations of Cell Size

• As a cell increases in size, its volume grows faster than its surface area.

• Neurons overcome size limitations by being long and skinny for better nutrient and waste transport.

Comparing Prokaryotic and Eukaryotic Cells

• Basic Features of All Cells:

  • Plasma membrane

  • Semifluid substance called cytosol

  • Chromosomes (carry genes)

  • Ribosomes (make proteins)

Prokaryotic Cells

• Simplest organisms, lack a membrane-bound nucleus.

• DNA present in nucleoid region.

• Have a cell wall outside the plasma membrane for protection and shape.

• Do contain ribosomes, which are not membrane-bound.

• Two domains: Archaea and Bacteria.

Eukaryotic Cells

• Possess a membrane-bound nucleus, more complex than prokaryotic cells.

• Characterized by compartmentalization through membrane-bound organelles and the endomembrane system.

• Contain a cytoskeleton.

Check for Understanding Questions

• Major cause of cell size limits includes:

  • C. Small cells have sufficient surface area for efficient functioning.

Cellular Structures and Their Functions

Endoplasmic Reticulum (ER)

• Types of ER:

  • Rough ER: Contains ribosomes, synthesizes proteins for secretion, lysosomes, or plasma membrane.

  • Smooth ER: Lacks ribosomes, involved in lipid synthesis, calcium storage, and detoxification.

Golgi Apparatus

• Flattened stacks of membranes for packaging and distributing molecules.

• Contains cis and trans faces; vesicles transport molecules to their destinations.

Lysosomes

• Membrane-bound digestive vesicles arising from Golgi apparatus.

• Contain enzymes for macromolecule breakdown and destroying engulfed material through phagocytosis.

Vacuoles

• Membrane-bound storage structures in plants, serve various functions.

• Types: Central vacuole (plants), contractile (some protists), and storage vacuoles.

Mitochondria

• Found in all eukaryotic cells, involved in ATP production through oxidative metabolism.

• Structure: Outer membrane, intermembrane space, inner membrane with cristae, and matrix.

• Mitochondria contain their own DNA.

Chloroplasts

• Present in plant and some eukaryotic cells, contain chlorophyll for photosynthesis.

• Surrounded by two membranes with thylakoids inside and contain their own DNA.

Endosymbiosis Theory

• Eukaryotic organelles originated by symbiosis between free-living cells.

• Prokaryotes were engulfed by ancestral eukaryotes leading to mitochondria and chloroplasts.

Cytoskeleton and Cell Motility

Cytoskeleton

• Composed of protein fibers, supports cell shape, and positions organelles.

• Three types of fibers:

  • Microtubules (thickest)

  • Microfilaments (actin filaments, thinnest)

  • Intermediate filaments (medium thickness).

Microtubules

• Composed of tubulin polymers, involved in maintaining cell shape, motility, and chromosome movement.

• Form cilia and flagella (9 + 2 arrangement).

Microfilaments

• Comprised of actin, aid in maintaining cell shape, and contribute to muscle contraction and cell motility.

Intermediate Filaments

• Composed of keratin proteins, maintain cell shape and anchor organelles.

Extracellular Components

Cell Walls of Plants

• Cell wall distinguishes plant cells from animal cells, providing protection, shape maintenance, and water regulation.

• Composed of cellulose fibers embedded in polysaccharides and proteins.

Extracellular Matrix (ECM)

• Found in animal cells, provides a protective layer, with collagen as a notable component.

• Integrins link ECM to the cytoskeleton.

Intercellular Junctions

• Facilitate adhesion, interaction, and communication between cells through physical contact.

• Types of junctions include tight junctions, desmosomes, and gap junctions.