Study Notes: The Cell (BIO 121/ Campbell, Chapter 6)

The Cell

Introduction

The cell is the fundamental unit of life, exhibiting a wide range of structural and functional diversity. Understanding the differences between cell types, their organelles, and the tools used to study them is essential in general biology.

Structural Organization of Prokaryotic and Eukaryotic Cells

Key Differences

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. DNA is located in a region called the nucleoid. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Plants, Animals, Fungi, and Protists.

• Cell Size: Prokaryotic cells are generally smaller (0.1–5 μm) than eukaryotic cells (10–100 μm).

• Advantages of Small Cell Size:

  • Higher surface area-to-volume ratio, facilitating efficient exchange of materials with the environment.

  • Faster cellular processes due to shorter distances for diffusion.

Microscopy in Biology

Types of Microscopy

• Light Microscopy: Uses visible light to observe living or stained cells. Limited to about 1000x magnification and 200 nm resolution.

• Electron Microscopy: Uses electron beams for much higher resolution (up to 0.1 nm). Includes:

  • Transmission Electron Microscopy (TEM): Views internal cell structures.

  • Scanning Electron Microscopy (SEM): Provides detailed images of cell surfaces.

• Advantages and Disadvantages:

  • Light microscopes allow observation of living cells but have lower resolution.

  • Electron microscopes provide higher resolution but require fixed (dead) specimens.

Eukaryotic Cell Structure

Plasma Membrane vs. Cell Membrane, Cytoplasm, and Cytosol

• Plasma Membrane: The outer boundary of the cell, composed of a phospholipid bilayer with embedded proteins. Regulates entry and exit of substances.

• Cell Membrane: Another term for the plasma membrane.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing organelles and cytosol.

• Cytosol: The fluid portion of the cytoplasm where metabolic reactions occur.

Major Organelles and Their Functions

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and detoxifies toxins.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Site of cellular respiration and ATP production.

• Lysosomes: Contain digestive enzymes to break down macromolecules and old organelles.

• Transport Vesicles: Move materials between organelles and to/from the cell membrane.

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

Cytoskeleton

Structure and Functions

• Microfilaments (Actin Filaments): Thin fibers involved in cell movement and shape changes.

• Intermediate Filaments: Provide mechanical support and maintain cell shape.

• Microtubules: Hollow tubes that maintain cell shape, facilitate intracellular transport, and are involved in cell division.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells, providing structural and biochemical support.

• Motility Structures: Cilia and flagella are composed of microtubules and enable cell movement.

• Centrioles: Organize microtubules during cell division in animal cells.

Table: Comparison of Cytoskeletal Elements

Element	Structure	Main Function	Example
Microfilaments	Actin protein, thin fibers	Cell movement, shape	Muscle contraction
Intermediate Filaments	Various proteins, rope-like	Structural support	Keratin in skin
Microtubules	Tubulin protein, hollow tubes	Cell division, transport	Mitotic spindle

Plant vs. Animal Cells

Structural Differences

• Cell Wall: Present in plant cells, provides rigidity and protection. Composed mainly of cellulose.

• Central Vacuole: Large, membrane-bound organelle in plant cells that stores water, nutrients, and waste products. Maintains turgor pressure.

• Chloroplasts: Present in plant cells for photosynthesis (not mentioned in the original, but relevant).

• Lysosomes: Common in animal cells; in plant cells, similar functions are performed by the central vacuole.

Table: Comparison of Plant and Animal Cells

Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Central Vacuole	Large, single	Small or absent
Lysosomes	Rare (central vacuole serves similar function)	Common
Chloroplasts	Present	Absent

Example Question

• Which organelle in a plant cell serves the function of a lysosome? Answer: The central vacuole.

Additional info: Chloroplasts were added for completeness, as they are a key distinguishing feature of plant cells.