Premed · Premed · General Biology 1

Lecture 5: Cell Theory and Prokaryotic vs. Eukaryotic Cells

General Biology I — Molecular & Cellular

Learning Objectives

By the end of this lecture, students will be able to:

1. State the three tenets of cell theory and describe its historical development
2. Explain why cells are small and the importance of the surface area-to-volume ratio
3. Compare and contrast prokaryotic and eukaryotic cells
4. Describe the key features of bacterial and archaeal cells
5. Identify the major organelles of eukaryotic cells and their functions
6. Explain the endosymbiotic theory for the origin of mitochondria and chloroplasts

Lecture Content

I. Cell Theory

Cell theory stands as one of the foundational unifying ideas in all of biology, comparable in importance to the theory of evolution. It rests on three tenets: first, that all living organisms are composed of one or more cells; second, that the cell is the basic unit of structure and function in living organisms; and third, that all cells arise from pre-existing cells (expressed in Latin as omnis cellula e cellula).

These principles were not established overnight but emerged from centuries of observation and debate. In 1665, Robert Hooke used a compound microscope to examine thin slices of cork and described the tiny compartments he saw as "cells." A decade later, Anton van Leeuwenhoek became the first person to observe living cells, including protists and bacteria, which he called "animalcules." In 1838, the botanist Matthias Schleiden proposed that all plants are composed of cells, and the following year the zoologist Theodor Schwann extended this claim to all animals. The third tenet was articulated by Rudolf Virchow in 1855, and was powerfully supported by Louis Pasteur's elegant swan-neck flask experiments of 1859, which definitively disproved the long-held notion of spontaneous generation.

II. Cell Size and the Surface Area-to-Volume Ratio

Most cells are microscopic, ranging from roughly 1 to 100 micrometers in diameter. This small size is not accidental--it is dictated by a fundamental geometric constraint. As a cell grows larger, its volume increases as the cube of its radius, while its surface area increases only as the square. This means the surface area-to-volume ratio drops with increasing size. Because the plasma membrane (surface area) must support the exchange of nutrients, wastes, and signaling molecules for the entire cell volume, a cell that grows too large will find itself unable to transport materials efficiently across its membrane.

Cells employ several strategies to maintain an adequate surface area-to-volume ratio. The simplest is to remain small. Alternatively, cells may adopt elongated or flattened shapes--neurons extend long, thin axons, and red blood cells are biconcave discs--that increase surface area relative to volume. Some cells create membrane invaginations, such as the microvilli that line the intestinal epithelium, dramatically amplifying the absorptive surface without increasing overall cell size.

<image>A diagram showing three cubes of increasing size (1 cm, 2 cm, and 4 cm per side). For each cube, surface area, volume, and SA:V ratio are calculated and displayed. A graph beside the cubes plots SA:V ratio (y-axis) against cell diameter (x-axis), showing the inverse relationship. An inset shows intestinal epithelial cells with microvilli as an example of increasing surface area.</image>

III. Microscopy

Our understanding of cell biology has been shaped by advances in microscopy. Light microscopy uses visible light and has a resolution limit of approximately 200 nanometers. Various techniques--bright-field, phase-contrast, fluorescence, and confocal microscopy--allow researchers to observe living cells and visualize specific structures using dyes or fluorescent labels. Electron microscopy uses beams of electrons instead of light and achieves resolution down to about 0.2 nanometers, revealing the fine ultrastructure of cells. Transmission electron microscopy (TEM) passes electrons through thin sections of the specimen and reveals internal structures, while scanning electron microscopy (SEM) scans electrons across the specimen surface to produce striking three-dimensional images of cell topology. Electron microscopy requires that specimens be fixed and dehydrated, so it cannot be used to observe living cells. For reference, 1 millimeter equals 10^-3 meters, 1 micrometer equals 10^-6 meters, and 1 nanometer equals 10^-9 meters.

IV. Prokaryotic Cells

The term prokaryote derives from Greek: pro meaning "before" and karyon meaning "nucleus"--prokaryotes lack a membrane-bound nucleus. They are distributed across two domains of life, Bacteria and Archaea, and they are typically small, ranging from 0.1 to 5 micrometers.

Prokaryotic cells lack membrane-bound organelles. Their genetic material resides in a nucleoid region--an irregularly shaped area of the cytoplasm that is not enclosed by a membrane. Most prokaryotes carry a single, circular chromosome, and many also harbor plasmids, small circular DNA molecules that can carry genes for antibiotic resistance, toxin production, or metabolic capabilities. Surrounding the cell is a cell wall that provides shape and physical protection. In bacteria, the cell wall is constructed from peptidoglycan, a polymer of sugars cross-linked by short amino acid chains. Archaea have cell walls made of pseudopeptidoglycan or other polymers, but notably lack true peptidoglycan. The plasma membrane is a phospholipid bilayer, though the lipid chemistry differs between bacteria (ester-linked fatty acids) and archaea (ether-linked branched lipids that confer greater stability in extreme environments).

Prokaryotic ribosomes are 70S in size (composed of 50S and 30S subunits), smaller than the 80S ribosomes of eukaryotes--a difference exploited by many antibiotics that selectively target bacterial protein synthesis. Many prokaryotes possess flagella for motility and pili (or fimbriae) for attachment to surfaces and for conjugation, a form of horizontal gene transfer. Some species secrete a capsule or slime layer of polysaccharides that provides protection from the host immune system and facilitates adhesion. Prokaryotic cells come in characteristic shapes--cocci (spherical), bacilli (rod-shaped), and spirilla (spiral)--and reproduce rapidly by binary fission, an asexual process.

<image>A labeled diagram of a typical bacterial cell. The cell is rod-shaped (bacillus) and shows: cell wall (peptidoglycan layer), plasma membrane beneath it, cytoplasm, nucleoid region with circular DNA, scattered 70S ribosomes, plasmids (small circular DNA), flagellum at one end, pili on the surface, and an outer capsule. An inset compares the cell wall composition of Gram-positive bacteria (thick peptidoglycan layer) and Gram-negative bacteria (thin peptidoglycan between inner and outer membranes, with lipopolysaccharide on the outer membrane).</image>

V. Eukaryotic Cells

The name eukaryote comes from eu ("true") and karyon ("nucleus"), reflecting the defining feature of these cells: a membrane-bound nucleus. Eukaryotes belong to the domain Eukarya, which includes protists, fungi, plants, and animals. Eukaryotic cells are generally much larger than prokaryotes (10 to 100 micrometers) and are characterized by extensive internal compartmentalization--a system of membrane-bound organelles that segregate incompatible biochemical processes and increase the efficiency of cellular operations. Their genetic material is organized into linear chromosomes housed within a nuclear envelope. Their ribosomes are 80S (composed of 60S and 40S subunits), and an elaborate cytoskeleton of protein filaments provides structural support, enables intracellular transport, and drives cell movement. Eukaryotic cells reproduce by mitosis for growth and repair, and by meiosis for the production of gametes in sexual reproduction.

VI. Comparison of Prokaryotic and Eukaryotic Cells

VII. Plant Cells vs. Animal Cells

Although all eukaryotic cells share fundamental features, plant and animal cells differ in several important respects. Plant cells possess a rigid cell wall of cellulose outside the plasma membrane, providing structural support and preventing the cell from bursting when water enters. They contain chloroplasts, the organelles of photosynthesis, and a large central vacuole that can occupy up to 80% of cell volume and serves in storage, waste disposal, and the maintenance of turgor pressure--the internal hydrostatic pressure that keeps the plant upright. Plasmodesmata are channels that traverse the cell walls between adjacent plant cells, enabling direct communication and transport of small molecules.

Animal cells lack cell walls, chloroplasts, and central vacuoles but possess features absent in plants. Centrioles, found within the centrosome, help organize the mitotic spindle during cell division. Lysosomes are generally more prominent in animal cells and carry out intracellular digestion. Specialized cell-cell junctions--tight junctions, desmosomes, and gap junctions--connect animal cells in ways appropriate to the tissue. Both cell types share the essential eukaryotic toolkit: a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, a plasma membrane, and a cytoskeleton.

VIII. The Endosymbiotic Theory

One of the most elegant explanations in cell biology is the endosymbiotic theory, proposed by Lynn Margulis in 1967. According to this theory, mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by an ancestral eukaryotic cell. Rather than being digested, the engulfed prokaryotes survived and established a mutually beneficial relationship with their host: the aerobic bacterium (ancestor of mitochondria) provided efficient ATP production, and the photosynthetic cyanobacterium (ancestor of chloroplasts) provided the ability to capture light energy.

The evidence supporting this theory is extensive and compelling. Both mitochondria and chloroplasts possess their own circular DNA, reminiscent of bacterial chromosomes. They are bounded by double membranes--the inner membrane is thought to be the original prokaryotic membrane, while the outer membrane derives from the host's engulfing vesicle. They contain 70S ribosomes, the same size as prokaryotic ribosomes rather than the 80S ribosomes of the eukaryotic cytoplasm. They replicate by binary fission, independently of the host cell's division cycle. Their size is comparable to that of bacteria. Phylogenetic analysis of their DNA sequences reveals that mitochondria are most closely related to alpha-proteobacteria and chloroplasts to cyanobacteria. Even certain antibiotics that target bacterial ribosomes--such as chloramphenicol--also inhibit the ribosomes of mitochondria and chloroplasts, further underscoring their prokaryotic heritage.

<image>A diagram illustrating the endosymbiotic theory. Step 1: An ancestral eukaryotic cell (large, with a nucleus) engulfs an aerobic bacterium (small, rod-shaped). Step 2: The bacterium is enclosed in a double membrane and becomes an endosymbiont, eventually becoming a mitochondrion. Step 3: A mitochondrion-containing cell engulfs a photosynthetic cyanobacterium. Step 4: The cyanobacterium becomes a chloroplast, giving rise to the plant/algal lineage. Evidence boxes surround the diagram listing: own circular DNA, 70S ribosomes, double membrane, binary fission, and phylogenetic similarity to bacteria.</image>

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