3.1 Spontaneous Generation
- The theory of spontaneous generation states that life arose from nonliving matter. It was a long-held belief dating back to Aristotle and the ancient Greeks.
- Experimentation by Francesco Redi in the 17th century presented the first significant evidence refuting spontaneous generation by showing that flies must have access to meat for maggots to develop on the meat. Prominent scientists designed experiments and argued both in support of (John Needham) and against (Lazzaro Spallanzani) spontaneous generation.
- Louis Pasteur is credited with conclusively disproving the theory of spontaneous generation with his famous swan-neck flask experiment. He subsequently proposed that “life only comes from life.”
3.2 Foundations of Modern Cell Theory
- Although cells were first observed in the 1660s by Robert Hooke, cell theory was not well accepted for another 200 years. The work of scientists such as Schleiden, Schwann, Remak, and Virchow contributed to its acceptance.
- Endosymbiotic theory states that mitochondria and chloroplasts, organelles found in many types of organisms, have their origins in bacteria. Significant structural and genetic information support this theory.
- The miasma theory of disease was widely accepted until the 19th century, when it was replaced by the germ theory of disease thanks to the work of Semmelweis, Snow, Pasteur, Lister, and Koch, and others.
3.3 Unique Characteristics of Prokaryotic Cells
- Prokaryotic cells differ from eukaryotic cells in that their genetic material is contained in a nucleoid rather than a membrane-bound nucleus. In addition, prokaryotic cells generally lack membrane-bound organelles.
- Prokaryotic cells of the same species typically share a similar cell morphology and cellular arrangement.
- Most prokaryotic cells have a cell wall that helps the organism maintain cellular morphology and protects it against changes in osmotic pressure.
- Outside of the nucleoid, prokaryotic cells may contain extrachromosomal DNA in plasmids.
- Prokaryotic ribosomes that are found in the cytoplasm have a size of 70S.
- Some prokaryotic cells have inclusions that store nutrients or chemicals for other uses.
- Some prokaryotic cells are able to form endospores through sporulation to survive in a dormant state when conditions are unfavorable. Endospores can germinate, transforming back into vegetative cells when conditions improve.
- In prokaryotic cells, the cell envelope includes a plasma membrane and usually a cell wall.
- Bacterial membranes are composed of phospholipids with integral or peripheral proteins. The fatty acid components of these phospholipids are ester-linked and are often used to identify specific types of bacteria. The proteins serve a variety of functions, including transport, cell-to-cell communication, and sensing environmental conditions. Archaeal membranes are distinct in that they are composed of fatty acids that are ether-linked to phospholipids.
- Some molecules can move across the bacterial membrane by simple diffusion, but most large molecules must be actively transported through membrane structures using cellular energy.
- Prokaryotic cell walls may be composed of peptidoglycan (bacteria) or pseudopeptidoglycan (archaea).
- Gram-positive bacterial cells are characterized by a thick peptidoglycan layer, whereas gram-negative bacterial cells are characterized by a thin peptidoglycan layer surrounded by an outer membrane.
- Some prokaryotic cells produce glycocalyx coatings, such as capsules and slime layers, that aid in attachment to surfaces and/or evasion of the host immune system.
- Some prokaryotic cells have fimbriae or pili, filamentous appendages that aid in attachment to surfaces. Pili are also used in the transfer of genetic material between cells.
- Some prokaryotic cells use one or more flagella to move through water. Peritrichous bacteria, which have numerous flagella, use runs and tumbles to move purposefully in the direction of a chemical attractant.
3.4 Unique Characteristics of Eukaryotic Cells
- Eukaryotic cells are defined by the presence of a nucleus containing the DNA genome and bound by a nuclear membrane (or nuclear envelope) composed of two lipid bilayers that regulate transport of materials into and out of the nucleus through nuclear pores.
- Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the cytoskeleton, the cell membrane, and/or the cell wall
- The nucleolus, located in the nucleus of eukaryotic cells, is the site of ribosomal synthesis and the first stages of ribosome assembly.
- Eukaryotic cells contain 80S ribosomes in the rough endoplasmic reticulum (membrane bound-ribosomes) and cytoplasm (free ribosomes). They contain 70s ribosomes in mitochondria and chloroplasts.
- Eukaryotic cells have evolved an endomembrane system, containing membrane-bound organelles involved in transport. These include vesicles, the endoplasmic reticulum, and the Golgi apparatus.
- The smooth endoplasmic reticulum plays a role in lipid biosynthesis, carbohydrate metabolism, and detoxification of toxic compounds. The rough endoplasmic reticulum contains membrane-bound 80S ribosomes that synthesize proteins destined for the cell membrane
- The Golgi apparatus processes proteins and lipids, typically through the addition of sugar molecules, producing glycoproteins or glycolipids, components of the plasma membrane that are used in cell-to-cell communication.
- Lysosomes contain digestive enzymes that break down small particles ingested by endocytosis, large particles or cells ingested by phagocytosis, and damaged intracellular components.
- The cytoskeleton, composed of microfilaments, intermediate filaments, and microtubules, provides structural support in eukaryotic cells and serves as a network for transport of intracellular materials.
- Centrosomes are microtubule-organizing centers important in the formation of the mitotic spindle in mitosis.
- Mitochondria are the site of cellular respiration. They have two membranes: an outer membrane and an inner membrane with cristae. The mitochondrial matrix, within the inner membrane, contains the mitochondrial DNA, 70S ribosomes, and metabolic enzymes.
- The plasma membrane of eukaryotic cells is structurally similar to that found in prokaryotic cells, and membrane components move according to the fluid mosaic model. However, eukaryotic membranes contain sterols, which alter membrane fluidity, as well as glycoproteins and glycolipids, which help the cell recognize other cells and infectious particles.
- In addition to active transport and passive transport, eukaryotic cell membranes can take material into the cell via endocytosis, or expel matter from the cell via exocytosis.
- Cells of fungi, algae, plants, and some protists have a cell wall, whereas cells of animals and some protozoans have a sticky extracellular matrix that provides structural support and mediates cellular signaling.
- Eukaryotic flagella are structurally distinct from prokaryotic flagella but serve a similar purpose (locomotion). Cilia are structurally similar to eukaryotic flagella, but shorter; they may be used for locomotion, feeding, or movement of extracellular particles.