Cellular Biology Quiz on Organelles and Processes

The endosymbiotic theory explains how mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotes. Over time, these prokaryotes formed a stable symbiotic relationship with the host cell, eventually evolving into essential organelles. Evidence includes their circular DNA, double membranes, bacterial-like ribosomes, and replication through binary fission. This theory helps explain why these organelles function semi-autonomously compared to other cell structures.

Fungi thrive best in mildly acidic environments. A pH of around 5 allows fungal enzymes and metabolic pathways to function optimally. Acidic conditions also reduce competition from many bacteria that prefer neutral or slightly alkaline pH. This adaptation helps fungi efficiently decompose organic matter in soil, decaying logs, and food surfaces. Understanding fungal pH preference is important in food preservation, agriculture, and microbiology research.

“Mould” is a broad, informal term used for fungi that grow as filamentous mycelia across surfaces without forming visible fruiting bodies like mushrooms. These fungi reproduce by spores and spread quickly across organic material. The term is imprecise because moulds include many unrelated groups of fungi. Yeasts are unicellular, and Actinomycetes are filamentous bacteria, not fungi. The distinction helps classify microbial growth patterns in laboratory and environmental settings.

Yeasts are nonfilamentous, unicellular fungi that grow as individual cells rather than forming long hyphae. They reproduce primarily through budding or fission and have spherical or oval morphology. Their unicellular form allows rapid nutrient uptake and fast reproduction. Yeasts are vital in food science, biotechnology, and human health, used in breadmaking, fermentation, and recombinant protein production. Their simplicity makes them widely used as model organisms for genetics and cell biology research.

The three domains—Bacteria, Archaea, and Eukarya—represent the highest level of biological classification. This system is based on molecular phylogeny, particularly ribosomal RNA analysis. Bacteria and Archaea are prokaryotic but differ significantly in membrane composition and genetics. Eukarya includes all organisms with a nucleus and membrane-bound organelles. This three-domain model reflects evolutionary divergence and clarifies the relationships among all living organisms.

Mitochondria are present only in eukaryotic cells, not prokaryotes. Their presence is a defining feature distinguishing eukaryotes from bacteria and archaea. As the energy-producing centers of the cell, mitochondria perform aerobic respiration and host metabolic pathways essential for ATP generation. Their double membrane and unique DNA provide strong evidence for their prokaryotic origin. Prokaryotes instead contain simpler structures and lack internal compartmentalization, highlighting the major evolutionary leap of eukaryotic cells.

The cell membrane is composed mainly of a phospholipid bilayer, which creates a semi-permeable barrier separating the cell interior from its environment. Each phospholipid has hydrophilic heads and hydrophobic tails, enabling the bilayer to form spontaneously in aqueous conditions. Embedded proteins contribute to transport, signaling, and structural functions. This architecture maintains homeostasis and regulates substance movement in and out of the cell, which is central to cell survival and physiology.

Ribosomes are the molecular machines responsible for translating mRNA into proteins. They consist of rRNA and protein subunits and function in both eukaryotic and prokaryotic cells. In eukaryotes, ribosomes can be free in the cytoplasm or attached to the rough endoplasmic reticulum, allowing synthesis of cytosolic or membrane-bound proteins. Their universal presence highlights their essential role in cellular function. Other organelles like lysosomes or the Golgi handle processing, not protein assembly.

Lysosomes contain hydrolytic enzymes that break down macromolecules, damaged organelles, and foreign particles. Their acidic interior optimizes enzyme activity and protects the rest of the cell from uncontrolled digestion. This recycling function is vital for maintaining cellular health and preventing buildup of waste. Lysosomal defects can lead to storage disorders, highlighting their importance. They differ from mitochondria, which produce energy, and from storage organelles that hold lipids or other molecules.

Osmosis is the passive movement of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration. This process helps maintain cell turgor, hydration, and fluid balance. Unlike active transport, osmosis requires no energy. It is essential in plant water regulation, kidney function, and maintaining internal cellular stability. Understanding osmosis helps explain swelling, shrinking, and overall water dynamics within biological systems.