Eukaryotic Cells and Microorganisms Flashcards
The Evolutionary History and Development of Eukaryotic Cells
Approximately to billion years ago, the first eukaryotic cells emerged. Research suggests that Bacteria, Archaea, and Eukarya all descended from a common progenitor known as the Last Common Ancestor (LCA). This ancestor was likely a large organism with poorly defined boundaries that used RNA as its primary genetic material. Over time, genetic material may have transitioned to DNA, potentially influenced by DNA viruses. The earliest eukaryotes were likely independent, single-celled organisms that eventually formed colonies. Within these colonies, individual cells began to specialize in specific functions. As cells became increasingly dependent on one another and lost the ability to survive in isolation, complex multicellular organisms evolved.
The development of eukaryotic organelles is largely explained by the theory of endosymbiosis. Around billion years ago, pre-eukaryotic cells are believed to have engulfed or been parasitized by bacteria. These bacteria eventually evolved into specialized organelles: mitochondria and chloroplasts. Evidence for this includes the fact that these organelles contain their own circular DNA, possess bacteria-sized ribosomes, and are capable of independent division within the cell.
External and Boundary Structures of the Eukaryotic Cell
Eukaryotic appendages for movement include flagella and cilia. Eukaryotic flagella are distinct from their bacterial counterparts, being approximately times thicker and significantly more complex in structure. They consist of a long, sheathed cylinder containing regularly spaced microtubules and are covered by an extension of the cell membrane. Cilia share a similar internal structure but are smaller, more numerous, and are found only in a specific group of protozoa and certain animal cells.
The glycocalyx is the outermost layer of the cell, composed of polysaccharides. It can appear as a network of fibers, a slime layer, or a protective capsule. This structure is essential for protection, environmental adherence, and signal reception. Below the glycocalyx, some eukaryotes possess a cell wall. In fungi, the cell wall is rigid, providing structural support. It is chemically distinct from bacterial walls, featuring a thick inner layer of polysaccharide fibers made of chitin or cellulose and a thin outer layer of mixed glycans. Notably, protozoa and helminths lack a cell wall.
The cytoplasmic membrane is a typical phospholipid bilayer embedded with proteins. Unlike many prokaryotes, eukaryotic membranes contain various sterols, which provide relative rigidity and stability. This is particularly important for cells that lack the support of a cell wall. The membrane serves as a selectively permeable barrier for the cell.
Internal Structures: The Nucleus and Endomembrane System
The nucleus is the most prominent eukaryotic organelle, housing the genetic material. It is enclosed by the nuclear envelope, a double-membrane structure separated by a narrow space and perforated with nuclear pores. These pores allow the migration of macromolecules between the nucleus and the cytoplasm. Within the nucleoplasm is the nucleolus, the primary site for ribosomal RNA (rRNA) synthesis. The genetic material itself is organized as chromatin, consisting of linear DNA bound to histone proteins.
The endoplasmic reticulum (ER) consists of a series of membrane tunnels used for transport and storage. The rough endoplasmic reticulum (RER) is studded with ribosomes and is involved in transporting materials from the nucleus toward the cell exterior. The smooth endoplasmic reticulum (SER) lacks ribosomes and functions in nutrient processing as well as the synthesis and storage of lipids and other non-protein macromolecules.
The Golgi apparatus acts as the site for protein modification and shipping. It is composed of flattened, disc-shaped sacs called cisternae. The Golgi works in tandem with the ER through a specific transport process: transitional vesicles carrying proteins from the ER are received by the Golgi, where the proteins are chemically modified with polysaccharides or lipids. Finally, condensing vesicles pinch off from the Golgi to be delivered to lysosomes or transported outside the cell membrane via exocytosis.
Nature’s Assembly Line and Intracellular Digestion
The coordinated effort between the nucleus, ER, and Golgi is known as Nature’s Assembly Line. The process begins when a segment of DNA is transcribed into RNA, which exits the nucleus via pores to reach ribosomes on the RER. Proteins synthesized there are deposited into the RER lumen, then transported to the Golgi for modification and packaging into functional vesicles.
Specific vesicles include lysosomes and vacuoles. Lysosomes contain various enzymes for intracellular digestion of food and protection against pathogens, as well as the removal of debris from damaged tissue. Vacuoles are membrane-bound sacs used for storage or digestion. In phagocytic cells, food particles are engulfed into a food vacuole (phagosome), which then merges with a lysosome to form a phagolysosome where digestion occurs.
Energy Synthesis, Ribosomes, and the Cytoskeleton
Mitochondria are the primary sites of energy production, extracting chemical energy from nutrients to store it as ATP. They feature a smooth outer membrane and an inner membrane folded into cristae, which house the enzymes for aerobic respiration. Chloroplasts, found in algae and plants, convert sunlight into chemical energy through photosynthesis, producing oxygen as a byproduct. They are larger than mitochondria and contain thylakoids stacked into grana within the stroma matrix.
Ribosomes are distributed throughout the cytoplasm, attached to the RER, or located within mitochondria and chloroplasts. The eukaryotic ribosome is an type, consisting of a large subunit and a small subunit. The cell's structural framework, the cytoskeleton, is composed of three fiber types: actin filaments (thin protein strands for movement), intermediate filaments (rope-like structural support), and microtubules (long, hollow tubes for organelle anchoring and transport).
Biology and Impact of Fungi
Fungi can be unicellular (yeasts) or multicellular (molds/helminths with larval stages). Yeasts are round to oval cells that reproduce asexually through budding, often forming chains called pseudohyphae. Molds consist of long, thread-like cells called hyphae. Septate hyphae are divided by cross walls (septa), while nonseptate hyphae are continuous cells. A mass of hyphae is called a mycelium.
Fungi are heterotrophic, obtaining nutrients through absorption. Most are saprobes, living off dead matter, while a few are parasites. They reproduce primarily through spores, which can be asexual (sporangiospores in a sac or conidiospores which are free) or sexual (formed by the linking of genes from two parents). Fungi have significant human impacts: they are essential decomposers and antibiotic producers, though they also cause community-acquired and opportunistic infections (mycoses), produce toxins like those from Aspergillus flavus, and destroy approximately of fresh produce annually.
Characteristics and Classification of Protozoa
Protozoa are a diverse group of approximately single-celled organisms, most of which are harmless inhabitants of soil and water. Their cytoplasm is divided into a clear outer ectoplasm (movement/feeding) and a granular inner endoplasm (containing organelles). They typically range in size from to , though some reach several millimeters.
Their life cycle involves two main stages: the trophozoite (the active, motile feeding stage) and the cyst (a dormant, resistant stage). Cysts allow protozoa to survive harsh conditions and are crucial for the spread of diseases like those caused by Entamoeba histolytica and Giardia lamblia. Protozoa are classified by their motility: Sarcodina (pseudopods), Ciliophora (cilia), Mastigophora (flagella), and Sporozoa (non-motile). Notable pathogens include Trypanosoma cruzi, which causes Chagas disease and is spread by insect feces, and Plasmodium species, which cause malaria.
Helminthology: Parasitic Worms
Helminths include flatworms (Phylum Platyhelminthes) and roundworms (Phylum Aschelminthes/Nematodes). Flatworms are thin and segmented, further divided into cestodes (tapeworms) and trematodes (flukes). Roundworms are elongated and unsegmented. While multicellular with complex organ systems, pathogenic helminths often have highly developed reproductive tracts and reduced digestive or nervous systems.
The life cycle involves an egg stage, larval stage, and adult stage. The intermediate host is where larval development occurs, while the definitive host is where adulthood and mating take place. A single female helminth can lay between and eggs per day. A common example is the pinworm (Enterobius vermicularis), where eggs are swallowed, hatch in the intestine, and the female migrates to the anus to deposit eggs, causing itching that facilitates further transmission.
Questions & Discussion
Question: Which characteristics provide evidence that mitochondria were once prokaryotic cells?
Answer: Evidence includes their circular chromosomes, prokaryotic-sized ribosomes, the ability to divide independently of the cell, and membranes that can be inhibited by specific antibiotics.
Question: Which organelle is responsible for energy production in the cell?
Answer: The mitochondrion is the primary organelle for energy production.
Question: From which sources can fungi derive nutrients?
Answer: Fungi can obtain nutrients from dead plants and animals, living tissues, and even synthetic materials like rubber and petroleum products.
Question: What is the active, feeding, and motile stage of the protozoan life cycle called?
Answer: This is known as the trophozoite stage.
Question: Where do adulthood and mating of helminths typically occur?
Answer: These processes occur in the definitive host.