3.2 Foundations of Modern Cell Theory

Cell theory forms the foundation of modern biology, explaining how all living things are made of cells. From single-celled bacteria to complex multicellular organisms, cells are the basic units of structure and function. This topic also covers how germ theory grew out of cell theory, connecting the microscopic world to disease.

Foundations of Cell Theory

Core principles of cell theory

Cell theory rests on three main ideas, each built through decades of observation and experiment.

1. All living organisms are composed of one or more cells. Cells are the fundamental structural and functional units of life. Matthias Schleiden studied plant tissues and concluded that all plants are made of cells. Theodor Schwann independently reached the same conclusion for animal tissues. Together, their work in the late 1830s established that cells are universal building blocks across all living things.

2. All cells arise from pre-existing cells. Rudolf Virchow formalized this principle with the phrase "Omnis cellula e cellula" ("every cell from a cell"). Cells reproduce through division (mitosis or meiosis), not through spontaneous generation. Louis Pasteur's famous swan-neck flask experiment provided strong evidence: by using a flask whose curved neck allowed air in but trapped microorganisms, he showed that broth remained sterile unless exposed to outside microbes. This put the idea of spontaneous generation to rest.

3. Cells contain hereditary information. DNA carries the genetic instructions needed to regulate cellular functions and pass traits to daughter cells during division.

Beyond these three pillars, cells also maintain homeostasis, actively regulating their internal environment to keep conditions stable despite changes outside the cell.

Cell structure and function

• All cells are enclosed by a cell membrane (also called the plasma membrane), which controls what enters and exits the cell.
• The cytoplasm is the gel-like material inside the cell where most cellular processes take place.
• Eukaryotic cells contain a membrane-bound nucleus that houses DNA and directs cellular activities. Prokaryotic cells lack this nucleus.
• Various organelles carry out specific tasks: ribosomes synthesize proteins, the endoplasmic reticulum processes and transports molecules, and so on.

Cellular diversity and specialization

Not all cells look or act the same. Through cell differentiation, a single cell type can develop into many specialized forms, each with a distinct structure suited to its role. For example, red blood cells are shaped for carrying oxygen, while neurons are elongated to transmit electrical signals. This specialization is what allows multicellular organisms to carry out complex functions that no single cell could handle alone.

Evidence for endosymbiotic theory

Endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by a larger ancestral cell. Instead of being digested, they formed a mutually beneficial relationship with the host. Over time, they became permanent organelles. Lynn Margulis championed this theory in the 1960s, and multiple lines of evidence support it:

• Own DNA: Both mitochondria and chloroplasts contain their own circular DNA, similar in structure to bacterial chromosomes.
• Independent reproduction: These organelles divide by binary fission, the same process bacteria use to reproduce, rather than being built from scratch by the cell.
• Double membranes: Both organelles have two membranes. The inner membrane likely corresponds to the original prokaryote's membrane, while the outer membrane came from the host cell's engulfing vesicle.
• Bacterial-type ribosomes: Ribosomes inside mitochondria and chloroplasts are 70S, the same size as bacterial ribosomes, rather than the 80S ribosomes found in the eukaryotic cytoplasm.
• Antibiotic sensitivity: Certain antibiotics that target bacteria (such as chloramphenicol) also inhibit protein synthesis in mitochondria and chloroplasts, further linking these organelles to a bacterial ancestor.

Germ Theory

Germ theory states that many diseases are caused by microorganisms invading the body. This idea grew directly out of advances in microscopy and cell theory, and it transformed medicine. Several key scientists contributed to its development.

Scientists' contributions to germ theory

Ignaz Semmelweis (1818–1865) Semmelweis worked in a Vienna hospital maternity ward where puerperal (childbed) fever killed many new mothers. He noticed that doctors who came straight from performing autopsies had much higher infection rates in their patients. He required physicians to wash their hands with a chlorinated lime solution before examining patients, and mortality rates dropped dramatically. He proposed that "decomposing animal matter" on doctors' hands was transmitting the disease. His ideas were largely rejected during his lifetime but were later vindicated.

Louis Pasteur (1822–1895) Pasteur's experiments showed that microorganisms cause fermentation and spoilage, not some mysterious chemical process. He developed pasteurization, a heating technique that kills harmful microbes in beverages like wine and milk. His swan-neck flask experiment disproved spontaneous generation (see above). He also developed vaccines for anthrax and rabies, demonstrating that weakened forms of a pathogen could provide immunity.

Joseph Lister (1827–1912) Lister applied Pasteur's germ theory to surgery. He introduced antiseptic techniques, using carbolic acid (phenol) to clean wounds, surgical instruments, and even the air in operating rooms. Post-operative infection and mortality rates dropped significantly, earning him the title "father of antiseptic surgery."

Robert Koch (1843–1910) Koch developed methods for isolating and growing pure bacterial cultures on solid media like nutrient agar. He identified the specific microbes responsible for anthrax, tuberculosis, and cholera. Most importantly, he established Koch's postulates, a set of criteria for proving that a specific microbe causes a specific disease:

1. The microorganism must be found in all cases of the disease but absent in healthy organisms.
2. The microorganism must be isolated from the diseased host and grown in pure culture.
3. The pure cultured microorganism must cause the same disease when introduced into a healthy, susceptible host.
4. The microorganism must be re-isolated from the experimentally infected host and confirmed to be identical to the original organism.

Koch used these steps to prove that Mycobacterium tuberculosis causes tuberculosis, testing his postulates using guinea pigs as experimental hosts.