Cells: The Working Units of Life – Structure, Function, and Origin

Cells: The Working Units of Life

5.1 What Features Make Cells the Fundamental Units of Life?

Cells are the basic structural and functional units of all living organisms. The cell theory is a foundational concept in biology, describing the properties and implications of cells in life.

• Cell Theory:

  • All organisms are composed of cells.

  • Cells are the fundamental units of life.

  • All cells come from preexisting cells.

  • Modern cells evolved from a common ancestor.

• Implications of Cell Theory:

  • Functions of all cells are similar.

  • Life is continuous.

  • The origin of life was the origin of cells.

The Scale of Life

Most cells are small, which is essential for their function. Exceptions include large cells such as bird eggs, some algae, and certain bacteria.

• Size Comparison: Cells are much larger than atoms and molecules, but smaller than multicellular structures. The scale ranges from nanometers (nm) for molecules to micrometers (μm) for cells.

• Microscopy: Electron microscopes are used for very small structures; light microscopes for most cells.

Surface Area-to-Volume Ratio

The efficiency of cellular processes depends on the surface area-to-volume ratio. Smaller cells have a higher ratio, which facilitates exchange of materials.

• Definition: The surface area-to-volume ratio is the amount of surface area per unit volume of a cell.

• Formulas:

  • Surface area of a sphere: $A = 4\pi r^2$

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

  • Surface area-to-volume ratio: $\frac{A}{V}$

• Importance:

  • High ratio allows efficient exchange of nutrients and waste.

  • Limits cell size; larger organisms have more cells, not larger cells.

  • Specialized shapes (e.g., villi, neurons) can increase surface area.

Diameter	Surface Area	Volume	Surface Area-to-Volume Ratio
1 μm	3.14 μm²	0.52 μm³	6:1
2 μm	12.56 μm²	4.19 μm³	3:1
3 μm	28.26 μm²	14.18 μm³	2:1

Plasma Membrane

The plasma membrane is the outer boundary of every cell, composed of a phospholipid bilayer with embedded proteins. It acts as a selective barrier for the passage of substances.

• Functions:

  • Controls entry and exit of ions, nutrients, and waste products.

  • Maintains homeostasis.

5.2 What Features Characterize Prokaryotic Cells?

Prokaryotic cells are simpler and lack membrane-bound organelles. They include Bacteria and Archaea.

• Characteristics:

  • Enclosed by a plasma membrane.

  • Genetic material located in the nucleoid region.

  • Cytoplasm contains cytosol (water, ions, small molecules, macromolecules).

  • Most have a rigid cell wall (peptidoglycan in bacteria).

  • Some have a capsule (slimy polysaccharide layer).

  • Flagella for movement (made of flagellin).

  • Pili for attachment and DNA exchange.

5.3 What Features Characterize Eukaryotic Cells?

Eukaryotic cells are larger and more complex, with membrane-bound organelles that compartmentalize functions.

• Organelles:

  • Nucleus: Contains genetic material (chromosomes), surrounded by nuclear envelope.

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

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Contain digestive enzymes for macromolecule breakdown.

  • Mitochondria: Site of cellular respiration, ATP production.

  • Plastids (in plants): Chloroplasts for photosynthesis.

  • Peroxisomes: Break down toxic by-products.

  • Cytoskeleton: Microtubules, microfilaments, intermediate filaments for structure and movement.

• Compartmentalization: Allows specialization and formation of tissues and organs in multicellular organisms.

Feature	Animal Cell	Plant Cell
Chloroplasts	No	Yes
Central Vacuole	No	Yes
Plasmodesmata	No	Yes
Lysosomes	Yes	No
Centrioles	Yes	No
Flagella	Yes	No (except some plant sperm)

5.4 What Are the Roles of Extracellular Structures?

Cells are supported and connected by extracellular structures, which vary between plants and animals.

• Plant Cell Walls:

  • Composed of cellulose fibers embedded in polysaccharides and proteins.

  • Connected by plasmodesmata (channels for communication).

• Animal Extracellular Matrix (ECM):

  • Composed of collagen, proteoglycans, and other proteins.

  • Binds cells together, contributes to tissue properties, regulates movement and signaling.

  • Integrins connect ECM to cytoskeleton.

• Cell Junctions:

  • Tight junctions: Seal cells together.

  • Desmosomes: Anchor cells into strong sheets.

  • Gap junctions: Provide channels for communication.

5.5 How Did Eukaryotic Cells Originate?

The origin of eukaryotic cells involved the development of internal compartments and organelles. Two main theories explain this process.

• Inward Folding Theory: The endomembrane system and nucleus may have originated from inward folds of the plasma membrane in ancestral prokaryotes.

• Endosymbiotic Theory: Mitochondria and plastids (chloroplasts) originated when a cell engulfed another cell, which then lived symbiotically inside the host.

• Evidence for Endosymbiosis:

  • Mitochondria and chloroplasts have double membranes.

  • They contain their own circular DNA, similar to prokaryotes.

  • They have ribosomes similar to those in prokaryotes.

  • Observed cases of eukaryotes ingesting algae that become functional organelles.

Example: Tay-Sachs Disease

Tay-Sachs is a lysosomal storage disease caused by a defective gene producing a nonfunctional enzyme, leading to toxic accumulation of lipids in neurons and early death.

Summary Table: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Cell wall	Yes (most)	Yes (plants, fungi)
Size	Small (1-10 μm)	Larger (10-100 μm)
DNA location	Nucleoid	Nucleus

Additional info: These notes expand on the original content by providing definitions, formulas, and context for key biological concepts, ensuring a self-contained study guide for General Biology students.