Chapter 6: A Tour of the Cell

Concept 6.2: Prokaryotic vs. Eukaryotic Cells

Cells are classified into two major types: prokaryotic and eukaryotic. Understanding their differences is fundamental to cell biology.

• Prokaryotic cells include Bacteria and Archaea. Their DNA is located in a region called the nucleoid, which is not membrane-bound.

• Eukaryotic cells have most of their DNA enclosed within a nucleus, which is surrounded by a double membrane called the nuclear envelope.

• Both cell types have a plasma membrane, cytoplasm, and ribosomes (sites of protein synthesis).

• Flagella are locomotion organelles found in some bacteria.

Surface area-to-volume ratio is crucial for cell function, as it affects the ability to exchange materials with the environment. As a cell grows, its volume increases faster than its surface area, limiting efficient exchange.

• Microvilli are projections that increase surface area, aiding absorption (e.g., in intestinal cells).

Concept 6.3: The Nucleus, Chromosomes, and Ribosomes

The nucleus is the control center of the eukaryotic cell, housing genetic material and coordinating activities such as growth and reproduction.

• Nuclear envelope: Double membrane that encloses the nucleus, separating it from the cytoplasm.

• Chromatin: Complex of DNA and proteins; condenses to form chromosomes during cell division.

• Nucleolus: Region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Ribosomes: Complexes that synthesize proteins; found free in cytoplasm or bound to the endoplasmic reticulum.

Functions of the nucleus and ribosomes:

• Nucleus stores genetic instructions and coordinates cell activities.

• Ribosomes translate genetic instructions into proteins.

Concept 6.4: The Endomembrane System

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins.

• Nuclear envelope

• Endoplasmic reticulum (ER): Network of membranes; divided into smooth ER (lipid synthesis, detoxification) and rough ER (protein synthesis, has ribosomes attached).

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.

• Lysosomes: Membranous sacs containing hydrolytic enzymes for digestion.

• Vesicles and vacuoles: Storage and transport within cells.

• Plasma membrane: Boundary of the cell, regulates entry and exit of substances.

Key Equations:

• Surface area of a sphere:

• Volume of a sphere:

• Surface area-to-volume ratio:

Concept 6.5: Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells. They are believed to have originated from endosymbiotic events.

• Mitochondria: Sites of cellular respiration; convert energy from sugars, fats, and other fuels into ATP using oxygen.

• Chloroplasts: Sites of photosynthesis in plants and algae; convert solar energy into chemical energy stored in organic molecules.

• Both have double membranes and their own circular DNA, similar to prokaryotes.

• Contain ribosomes and reproduce independently within the cell.

Endosymbiont theory: Suggests that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

Concept 6.6: The Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, organization, and movement within the cell.

• Three main types of cytoskeletal fibers:

  • Microtubules: Hollow tubes; maintain cell shape, facilitate movement (cilia, flagella), chromosome separation during cell division.

  • Microfilaments: Thin, solid rods; involved in cell movement and muscle contraction.

  • Intermediate filaments: Rope-like fibers; provide mechanical support.

• Cilia and flagella are motile structures composed of microtubules; cilia are short and numerous, flagella are longer and fewer.

Additional info:

• Some context and definitions have been expanded for clarity and completeness.

• Tables have been recreated to summarize organelle functions and cytoskeletal fiber types.

• Equations for surface area and volume are provided to support understanding of cell size limitations.