Chapter 6: A Tour of the Cell – General Biology Study Notes

Chapter 6: A Tour of the Cell

Overview: The Fundamental Units of Life

Cells are the basic structural and functional units of all living organisms. Understanding cell composition, organization, and function is essential for studying biology.

• All organisms are made of cells: This is the foundation of cell theory.

• Cells are the simplest collection of matter that can be alive: They perform all necessary life processes.

• Cell structure is correlated to cellular function: The shape and internal organization of a cell are closely related to its role.

• All cells are related by descent from earlier cells: Cells arise from pre-existing cells.

Cell Theory

Principles of Cell Theory

Cell theory is a cornerstone of biology, describing the properties and significance of cells.

• All living things are composed of cells.

• The cell is the basic unit of life.

• All cells arise from pre-existing cells.

Cell Organization and Homeostasis

Cell Structure and Function

Cells maintain homeostasis through specialized structures and organization.

• Plasma membrane: Surrounds the cell, separating internal contents from the external environment.

• Specialized regions and organelles: Allow compartmentalization of functions.

• Cell size is limited: Due to the surface area to volume ratio, which affects nutrient uptake and waste removal.

Surface Area to Volume Ratio is critical for cell efficiency. As cells grow, volume increases faster than surface area, limiting cell size.

• Small cells have a greater surface area relative to volume, facilitating efficient exchange of materials.

Equation:

• For a sphere: $\text{Surface Area} = 4\pi r^2$

• For a sphere: $\text{Volume} = \frac{4}{3}\pi r^3$

Studying Cells: Microscopy and Fractionation

Microscopy

Microscopy is essential for visualizing cells and their components.

• Light Microscopy (LM): Uses visible light and glass lenses. Can magnify up to 1000x. Suitable for viewing cell shape and some organelles.

• Electron Microscopy (EM): Uses electron beams for much higher resolution (down to 1 nm). Includes:

  • Scanning Electron Microscopy (SEM): Views cell surfaces in 3D.

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

• Fluorescence Microscopy: Uses fluorescent dyes to label specific cell components, allowing detailed visualization.

Key parameters:

• Magnification: Ratio of image size to actual size.

• Resolution: Minimum distance between two distinguishable points.

• Contrast: Difference in brightness between parts of the sample.

Cell Fractionation

Cell fractionation separates cellular components for study.

• Homogenization: Breaking cells open.

• Centrifugation: Separates organelles by size and density.

Allows determination of organelle functions through biochemical analysis.

Eukaryotic vs. Prokaryotic Cells

Comparison of Cell Types

Cells are classified as prokaryotic or eukaryotic based on their structure.

Feature	Prokaryotes	Eukaryotes
Nucleus	No (nucleoid region)	Yes
DNA Location	Cytoplasm	Nucleus
Membrane-bound Organelles	No	Yes
Cell Size	Small (1-2 μm)	Larger (10-20 μm)
Cell Wall Composition	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), none (animals)

Compartments in Eukaryotic Cells

Cell Regions and Organelles

Eukaryotic cells have internal membranes that create compartments for specialized functions.

• Cytoplasm: Region outside the nucleus, containing cytosol and organelles.

• Nucleoplasm: Region within the nuclear envelope.

• Organelles: Membrane-bound structures with specific functions (e.g., nucleus, mitochondria, endoplasmic reticulum).

Nucleus and Ribosomes

Nucleus: Information Central

The nucleus stores genetic information and coordinates cellular activities.

• Nuclear envelope: Double membrane with pores for molecular transport.

• Chromatin: DNA and associated proteins; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes: Protein Factories

Ribosomes synthesize proteins by translating genetic information.

• Free ribosomes: Located in cytosol; produce proteins for use within the cell.

• Bound ribosomes: Attached to endoplasmic reticulum or nuclear envelope; produce proteins for membranes or export.

• Structure: Composed of large and small subunits, made of rRNA and proteins.

Equation:

• Protein synthesis: $\text{mRNA} \rightarrow \text{Protein}$ (translation)

Endomembrane System

Components and Functions

The endomembrane system coordinates the synthesis, modification, and transport of cellular materials.

• Nuclear envelope

• Endoplasmic reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis, detoxification)

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids

• Lysosomes: Digestive enzymes for breakdown of macromolecules

• Vacuoles: Storage and maintenance of cell turgor (especially in plants)

• Plasma membrane: Selective barrier for transport

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

Golgi Apparatus

• Cis face: Receives vesicles from ER.

• Medial region: Further modification of proteins and lipids.

• Trans face: Dispatches vesicles to final destinations.

Lysosomes and Vacuoles

• Lysosomes: Contain hydrolytic enzymes for digestion.

• Vacuoles: Large storage organelles in plants; maintain turgor pressure.

Energy Converting Organelles

Mitochondria

Mitochondria are the sites of aerobic respiration, converting chemical energy in food to ATP.

• Double membrane: Outer and highly folded inner membrane (cristae).

• Matrix: Internal compartment containing enzymes and mitochondrial DNA.

• ATP production: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Chloroplasts (Plastids)

Chloroplasts capture light energy for photosynthesis in plants and algae.

• Double membrane

• Stroma: Internal fluid

• Thylakoids: Flattened sacs containing chlorophyll, grouped in stacks called grana

• Photosynthesis equation: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Endosymbiotic Theory

This theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Both have double membranes, their own DNA, and ribosomes.

• They reproduce independently within the cell.

Cytoskeleton

Components and Functions

The cytoskeleton provides structural support, facilitates cell movement, and organizes organelles.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, organelle movement, and cell division.

• Microfilaments: Solid rods of actin; involved in muscle contraction, cell movement, and division.

• Intermediate filaments: Fibrous proteins; provide mechanical strength and maintain cell shape.

Type	Structure	Main Functions	Examples
Microtubules	Hollow tubes (25 nm diameter)	Cell shape, transport, division	Centrioles, cilia, flagella
Microfilaments	Solid rods (7 nm diameter)	Cell movement, muscle contraction	Actin filaments
Intermediate Filaments	Fibrous proteins (8–12 nm diameter)	Structural stability	Keratin, nuclear lamina

Outside the Cell

Cell Walls and Extracellular Matrix

Structures outside the plasma membrane provide protection and support.

• Prokaryotes: Cell wall (peptidoglycan), capsule (glycocalyx)

• Plants: Cell wall (cellulose), primary and secondary walls

• Fungi: Cell wall (chitin)

• Animals: No cell wall; secrete glycocalyx and extracellular matrix (ECM)

Extracellular Matrix (ECM) in animals:

• Collagen: Main structural protein

• Fibronectin: Glycoprotein connecting ECM to cell surface

• Integrins: Membrane proteins that transmit signals between ECM and cytoplasm

ECM influences cell behavior, communication, and gene expression.

Summary Table: Shared and Unique Organelles in Plant and Animal Cells

Organelle	Plant Cells	Animal Cells
Nucleus	Yes	Yes
Mitochondria	Yes	Yes
Chloroplasts	Yes	No
Cell Wall	Yes (cellulose)	No
Central Vacuole	Yes	No
Lysosomes	Rare	Yes

Key Terms and Definitions

• Organelle: Specialized subunit within a cell with a specific function.

• Homeostasis: Maintenance of stable internal conditions.

• Phospholipid bilayer: Fundamental structure of cell membranes.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Chromatin: DNA-protein complex in the nucleus.

• Endosymbiotic theory: Theory explaining the origin of mitochondria and chloroplasts.

Example Applications

• Microscopy: Diagnosing diseases by observing cell morphology.

• Cell fractionation: Isolating mitochondria to study energy metabolism.

• Endosymbiotic theory: Tracing evolutionary relationships using organelle DNA.

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