A Tour of the Cell: Structure and Function of Eukaryotic and Prokaryotic Cells

Cell Theory: The Fundamental Units of Life

Overview of Cell Theory

Cell theory is a foundational concept in biology, describing the properties and significance of cells in living organisms.

• All organisms are made of cells: Cells are the basic unit of life.

• The cell is the simplest collection of matter that can be alive: Cells carry out all necessary life functions.

• Multicellular organisms have specialized cells with structures adapted to their functions.

• All cells descend from earlier cells: This reflects the evolutionary continuity from early prokaryotes to modern eukaryotes.

Discovery of the Cell

Historical Contributions

The discovery of cells was made possible by advances in microscopy.

• Robert Hooke (1665): First observed cells in cork using a microscope.

• Anton van Leeuwenhoek (1673): Improved microscopes and observed living cells, including bacteria and protozoa.

Microscopy

Principles and Types

Microscopy is essential for studying cells and their components.

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses. Lenses refract light to magnify images.

• Electron Microscope (EM): Uses beams of electrons for much higher resolution, allowing visualization of subcellular structures.

Key Parameters of Microscopy

• Magnification: Ratio of image size to actual size.

• Resolution: Minimum distance between two distinguishable points.

• Contrast: Visible differences in parts of the sample.

Microscopy Techniques

• Brightfield, phase contrast, differential interference contrast, fluorescence, confocal, super-resolution (LM).

• Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (EM).

Cell Fractionation

Purpose and Method

Cell fractionation separates cellular components for study.

• Homogenization: Breaks cells apart.

• Centrifugation: Separates organelles by size and density.

• Allows correlation of cell structure with function using biochemistry and cytology.

Example: Differential Centrifugation

Speed (g)	Pellet Contents
1,000	Nuclei and cellular debris
20,000	Mitochondria, chloroplasts
80,000	Plasma membranes, fragments, ribosomes

Defining Features of Cells

Common Features (Prokaryotic and Eukaryotic)

• Plasma membrane: Encloses the cell, controls passage of substances.

• Cytosol: Semifluid substance inside the cell.

• Chromosomes: Carry genetic information.

• Ribosomes: Synthesize proteins.

Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (DNA in nucleoid)	Present (membrane-bound)
Organelles	No membrane-bound organelles	Membrane-bound organelles
Size	Generally smaller	Generally larger
Cytoplasm	Bound by plasma membrane	Region between plasma membrane and nucleus

Cell Size and Surface Area-to-Volume Ratio

Limits on Cell Size

Cell size is constrained by metabolic requirements and the surface area-to-volume ratio.

• Surface area increases as the square of cell dimensions.

• Volume increases as the cube of cell dimensions.

• Cells must maintain a high surface area-to-volume ratio for efficient exchange of materials.

Formula

• Surface area of a cube: $6s^2$

• Volume of a cube: $s^3$

• Surface area-to-volume ratio: $\frac{6s^2}{s^3} = \frac{6}{s}$

Panoramic View of the Eukaryotic Cell

Internal Membranes and Organelles

Eukaryotic cells have internal membranes that compartmentalize functions into organelles. Plant and animal cells share most organelles.

• Plasma membrane: Selective barrier, phospholipid bilayer.

• Nucleus: Contains genetic material.

• Endoplasmic reticulum (ER): Rough and smooth regions.

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

• Lysosomes: Digestive organelles.

• Vacuoles: Storage and structural support (especially in plants).

• Mitochondria: ATP production.

• Chloroplasts: Photosynthesis (plants and algae).

• Cytoskeleton: Structural support, motility.

Membranes: Structure and Function

Plasma Membrane

The plasma membrane is a selective barrier composed of a phospholipid bilayer with embedded proteins and carbohydrate side chains.

• Hydrophilic regions: Interact with aqueous environments.

• Hydrophobic regions: Form the core of the membrane, limiting passage of polar molecules.

Major Organelles and Structures

Nucleus

The nucleus is the information center of the cell, containing most of the cell's genes.

• Nuclear envelope: Double membrane separating nucleus from cytoplasm.

• Chromatin: DNA and associated proteins.

• Nucleolus: Site of ribosome synthesis.

Ribosomes

Ribosomes are complexes of ribosomal RNA and protein, responsible for protein synthesis.

• Free ribosomes: Suspended in cytosol, synthesize proteins for use within the cell.

• Bound ribosomes: Attached to ER or nuclear envelope, synthesize proteins for export or membranes.

Endomembrane System

The endomembrane system regulates protein traffic and metabolic functions.

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

Components are connected directly or via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER: Lacks ribosomes, synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Rough ER: Studded with ribosomes, synthesizes glycoproteins, distributes transport vesicles, membrane factory.

Golgi Apparatus

The Golgi apparatus consists of flattened sacs (cisternae) and functions in modifying, sorting, and packaging proteins and macromolecules.

• Cis face: Receiving side.

• Trans face: Shipping side.

Lysosomes

Lysosomes are membranous sacs containing hydrolytic enzymes for digestion of macromolecules.

• Enzymes hydrolyze proteins, fats, polysaccharides, nucleic acids.

• Optimal function in acidic environment.

Summary Table: Major Eukaryotic Organelles

Organelle	Main Function
Nucleus	Genetic information storage
Ribosome	Protein synthesis
ER (Smooth)	Lipid synthesis, detoxification
ER (Rough)	Protein synthesis for export
Golgi apparatus	Protein modification and sorting
Lysosome	Macromolecule digestion
Mitochondrion	ATP production
Chloroplast	Photosynthesis
Vacuole	Storage, structural support

Additional info: These notes cover the main topics and subtopics from Chapter 6 "A Tour of the Cell" in a General Biology course, including cell theory, microscopy, cell fractionation, cell structure, and organelle function.