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: Cells are the smallest units of life capable of independent existence.

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

• All cells are related by descent: Modern cells have evolved from ancestral cells through processes of division and genetic inheritance.

Cell Theory

Principles of Cell Theory

Cell theory is a foundational concept in biology describing the properties of cells.

• All living things are composed of cells.

• The cell is the basic unit of structure and function in living organisms.

• All cells arise from pre-existing cells.

Cell Organization and Homeostasis

Cellular Organization

Cells maintain homeostasis by controlling their internal environment and organizing their components.

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

• Specialized regions and organelles: Cells contain distinct structures that perform specific functions.

• Cell size limitations: The surface area to volume ratio restricts cell size, as nutrient and waste exchange must be efficient.

Surface Area to Volume Ratio

The efficiency of cellular processes depends on the ratio of surface area to volume.

• As cells grow, volume increases faster than surface area.

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

Formula:

• For a sphere:

• For a sphere:

Studying Cells: Microscopy and Fractionation

Microscopy

Microscopy is essential for visualizing cells and their components.

• Light Microscopy (LM): Uses visible light to observe cells; 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): Visualizes cell surfaces in 3D.

  • Transmission Electron Microscopy (TEM): Visualizes internal cell structures by passing electrons through thin samples.

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

Cell Fractionation

Cell fractionation separates cellular components for study.

• Homogenization: Cells are broken apart to release organelles.

• Centrifugation: Separates organelles by size and density.

• Purpose: Allows scientists to study the function of individual organelles.

Eukaryotic vs. Prokaryotic Cells

Comparison of Cell Types

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

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 such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

Nucleus and Ribosomes

Nucleus: Information Central

The nucleus stores genetic information and coordinates cellular activities.

• Nuclear envelope: Double membrane separating nucleus from cytoplasm.

• Nuclear pores: Regulate entry and exit of molecules.

• 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.

• Structure: Composed of rRNA and proteins; two subunits (large and small).

• Location: Free in cytosol or bound to endoplasmic reticulum (ER).

• Function: Catalyze peptide bond formation during 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)

• ER Lumen- internal aqueous compartment in ER

• 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: Controls entry and exit of substances

Energy Converting Organelles

Mitochondria

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

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

• Matrix: Internal compartment containing enzymes and mitochondrial DNA

• Function: Aerobic respiration:

• Own DNA: Mitochondria have their own circular DNA and replicate independently

Plastids (Chloroplasts)

Plastids are organelles in plants and algae involved in food production and storage. Chloroplasts perform photosynthesis.

• Double membrane structure

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

• Stroma: Fluid inside the inner membrane

• Function: Photosynthesis:

• Own DNA: Chloroplasts have their own circular DNA

Endosymbiotic Theory

This theory explains the origin of mitochondria and chloroplasts as formerly independent prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, circular DNA, ribosomes similar to bacteria, autonomous replication

• Genetic transfer: Many genes have moved to the host cell nucleus

Cytoskeleton

Components and Functions

The cytoskeleton is a network of protein filaments that provides structural support, enables movement, and organizes cell contents.

• 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

Outside the Cell

Cell Walls and Extracellular Structures

Cells produce external structures for protection, support, and communication.

• Prokaryotes: Cell wall, outer envelope, capsule (glycocalyx)

• Plants: Thick cell walls made of cellulose; primary and secondary walls

• Fungi: Cell walls made of chitin

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

Extracellular Matrix (ECM)

The ECM is a network of proteins and carbohydrates outside animal cells, providing structural support and regulating cell behavior.

• Main components: Collagen (structural protein), fibronectin (glycoprotein), integrins (membrane proteins)

• Functions: Cell recognition, communication, structural reinforcement

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

Key Learning Objectives

• Describe cell theory and its implications.

• Explain the importance of surface area to volume ratio in cell size.

• Compare and contrast prokaryotic and eukaryotic cells.

• Identify major organelles and their functions.

• Understand the structure and function of the cytoskeleton.

• Describe the endosymbiotic theory for mitochondria and chloroplasts.

• Distinguish between plant and animal cells based on organelle content.

• Explain the role of the extracellular matrix in animal cells.