Chapter 6: A Tour of the Cell

Overview: The Fundamental Units of Life – Cell

Cells are the basic structural and functional units of all living organisms. Understanding their organization and diversity is essential for studying biology.

• Cell Theory: All organisms are made of cells. Cells arise only from pre-existing cells.

• Unicellular vs. Multicellular Organisms: Many organisms consist of a single cell (unicellular), while others are composed of many cells (multicellular).

• Cell Structure and Function: The cell is the simplest collection of matter that can be alive. Cells vary in structure and function, reflecting evolutionary adaptation.

• Levels of Organization: Cells are organized into higher levels such as tissues, organs, and organ systems.

• Cell Diversity: Cells differ in their structure and function, but share basic structural organization.

Concept 6.1: Biologists Use Microscopes and the Tools of Biochemistry to Study Cells

Microscopy and biochemistry are essential for investigating cell structure and function.

• Microscopes: Devices that magnify small objects, allowing visualization of cell structures.

• Light Microscopes (LM): Use visible light to illuminate specimens. Magnification up to ~1000x. Resolution limited by wavelength of light.

• Electron Microscopes (EM): Use electron beams for much higher resolution (up to 2 nm). Two main types:

  • Scanning Electron Microscope (SEM): Produces 3D images of specimen surfaces.

  • Transmission Electron Microscope (TEM): Reveals internal cell structures by passing electrons through thin sections.

• Cell Fractionation: Technique to separate cell components for biochemical analysis. Involves centrifugation to isolate organelles.

Table: Comparison of Light and Electron Microscopes

Concept 6.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Cells are classified as prokaryotic or eukaryotic based on their internal organization and membrane-bound structures.

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

• Eukaryotic Cells: Have a nucleus and numerous membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Plasma Membrane: All cells are surrounded by a selective barrier called the plasma membrane, which regulates passage of substances.

• Cell Size: Most cells are between 1–100 μm in diameter. Surface area-to-volume ratio limits cell size.

• Compartmentalization: Internal membranes create distinct environments for specialized functions.

Table: Prokaryotic vs. Eukaryotic Cells

Concept 6.3: The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

The nucleus contains most of the cell’s genetic material and directs cellular activities. Ribosomes synthesize proteins based on genetic instructions.

• Nucleus: Surrounded by a double membrane called the nuclear envelope. Contains chromosomes made of DNA and proteins (chromatin).

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

• Ribosomes: Complexes of rRNA and protein that carry out protein synthesis. Can be free in the cytosol or bound to the endoplasmic reticulum.

Protein Synthesis Process:

• DNA is transcribed into messenger RNA (mRNA) in the nucleus.

• mRNA exits the nucleus and is translated by ribosomes into polypeptides.

Equation:

Concept 6.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions in the Cell

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

• Endoplasmic Reticulum (ER): Network of membranes; two types:

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons.

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

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules.

• Vacuoles: Storage organelles; plant cells have a large central vacuole.

• Plasma Membrane: Controls entry and exit of substances.

Table: Functions of Endomembrane System Components

Additional info:

• Cell fractionation is a key method in cell biology for isolating organelles and studying their functions.

• Surface area-to-volume ratio is a critical factor limiting cell size and influencing cell shape.

• Internal membranes in eukaryotic cells allow for compartmentalization and specialization of cellular processes.