Chapter 6: A Tour of the Cell – Study Notes

Study Guide - Smart Notes

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This section explores the similarities and differences between prokaryotic and eukaryotic cells, as well as between animal and plant cells.

Prokaryotic Cells: Include Bacteria and Archaea. Their DNA is concentrated in a region called the nucleoid, which is not membrane-bound.
Eukaryotic Cells: Include Protists, Fungi, Animals, and Plants. Their DNA is enclosed within a membrane-bound nucleus.
Key Differences:
- Location of DNA: Prokaryotes (nucleoid, not membrane-bound); Eukaryotes (nucleus, membrane-bound).
- Internal Membranes: Eukaryotes have extensive internal membranes forming organelles; prokaryotes do not.
Cell Size: Surface area-to-volume ratio limits cell size. As a cell grows, its volume increases faster than its surface area, making exchange with the environment less efficient.
Adaptations: Microvilli (surface projections) increase surface area in some cells, such as intestinal cells.

Example: Bacterial cells lack a nucleus and membrane-bound organelles, while animal cells have both.

This section describes the structure and function of the nucleus, chromosomes, and ribosomes in eukaryotic cells.

Nucleus: Contains most of the cell's genetic material. Surrounded by a double membrane called the nuclear envelope, which has nuclear pores for molecular transport.
Chromatin: DNA and proteins that condense to form chromosomes during cell division.
Nucleolus: Region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.
Ribosomes: Complexes of rRNA and protein that carry out protein synthesis. Found free in the cytoplasm or bound to the endoplasmic reticulum (ER).

Example: Ribosomes in the nucleolus assemble into large and small subunits before being exported to the cytoplasm.

The endomembrane system includes organelles that work together in the synthesis, modification, and transport of cellular products.

Main Components:
- Nuclear envelope
- Endoplasmic reticulum (ER)
- Golgi apparatus
- Lysosomes
- Vesicles and vacuoles
- Plasma membrane
Endoplasmic Reticulum (ER):
- Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons, stores calcium ions.
- Rough ER: Studded with ribosomes; synthesizes proteins and membranes, distributes transport vesicles.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.
Lysosomes: Membranous sacs containing hydrolytic enzymes for digesting macromolecules. Acidic internal pH.
Vacuoles: Large vesicles with varied functions, such as storage, waste disposal, and maintaining turgor pressure in plant cells.

Example: The rough ER produces proteins that are sent to the Golgi apparatus for modification and sorting.

Mitochondria and chloroplasts are the energy-converting organelles of eukaryotic cells. They are believed to have originated via endosymbiosis.

Mitochondria: Sites of cellular respiration, using oxygen to generate ATP from sugars, fats, and other fuels.
Chloroplasts: Sites of photosynthesis in plants and algae, converting solar energy to chemical energy stored in sugars.
Endosymbiont Theory: Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.
Structural Features: Both have double membranes, their own circular DNA, and ribosomes. They grow and reproduce independently within the cell.
Adaptations: Highly folded inner membranes (cristae in mitochondria, thylakoids in chloroplasts) increase surface area for energy conversion processes.

Example: Mitochondria are abundant in muscle cells due to high energy demands.

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

Main Types of Cytoskeletal Fibers:
- Microtubules: Hollow tubes; maintain cell shape, guide organelle movement, separate chromosomes during cell division.
- Microfilaments (Actin Filaments): Thin rods; support cell shape, involved in muscle contraction, cell motility, and division.
- Intermediate Filaments: Fibrous proteins; provide mechanical support, anchor organelles.
Functions:
- Maintenance of cell shape
- Mechanical support
- Cell motility (e.g., movement of cilia and flagella)
Cilia and Flagella: Motile appendages built from microtubules. Cilia are short and numerous; flagella are longer and usually singular. Both are used for movement.

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

Feature	Prokaryotic Cells	Eukaryotic Cells
DNA Location	Nucleoid (not membrane-bound)	Nucleus (membrane-bound)
Organelles	Absent (no membrane-bound organelles)	Present (membrane-bound organelles)
Cell Size	Generally smaller (0.1–5 μm)	Generally larger (10–100 μm)
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Organelle	Main Function
Nucleus	Stores genetic information; controls cell activities
Ribosome	Protein synthesis
Endoplasmic Reticulum (ER)	Smooth ER: lipid synthesis, detoxification; Rough ER: protein synthesis
Golgi Apparatus	Modifies, sorts, and packages proteins and lipids
Lysosome	Digestion of macromolecules
Vacuole	Storage, waste disposal, turgor pressure (plants)
Mitochondrion	ATP production via cellular respiration
Chloroplast	Photosynthesis (plants and algae)
Cytoskeleton	Structural support, movement

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology curriculum.