Eukaryotic Cell Overview PDF

Overview of the Eukaryotic Cell First primitive eukaryotes were likely single-celled and independent Over time cells aggregated, forming colonies Cells within colonies became specialized to perform a specific function Complex multicellular organisms evolved as individual cells lost the abili...

Overview of the Eukaryotic Cell First primitive eukaryotes were likely single-celled and independent Over time cells aggregated, forming colonies Cells within colonies became specialized to perform a specific function Complex multicellular organisms evolved as individual cells lost the ability to survive separately from the intact colony External Structures Appendages for Moving: Flagella and Cilia Eukaryotic flagella: Different from bacterial flagella About ten times thicker Structurally more complex Covered by an extension of the cell membrane Long, sheathed cylinder containing regularly spaced microtubules Eukaryotic cilia: Similar to flagella in structure, but are smaller and more numerous Found only in a single group of protozoa and certain animal cells Microtubules in Flagella Regularly-spaced microtubules extend along the entire length of the flagellum Nine pairs of microtubules surround a single pair Known as the 9 + 2 arrangement Also present in cilia Aaron J. Bell/Science Source Flagella Cross-Section The Glycocalyx An outermost layer that comes into direct contact with the environment Composed of polysaccharides Appears as: A network of fibers  A slime layer A capsule Contributes to protection, adherence, and signal reception Boundary Structures: The Cell Wall Protozoa and helminths do not have cell walls Cell walls of fungi: Rigid and provide structural support and shape Different in chemical composition from bacterial and archaeal cell walls Thick inner layer of polysaccharide fibers composed of chitin or cellulose Thin outer layer of mixed glycans Boundary Structures: The Cell Membrane Typical bilayer of phospholipids in which protein molecules are embedded Contain sterols of various kinds: Relative rigidity gives stability to the membrane Important in cells that do not have a cell wall Cytoplasmic membranes of eukaryotes have a similar function as those in bacteria and archaea, serving as selectively permeable barriers Internal Structures: The Nucleus The most prominent organelle of eukaryotic cells Separated from the cell cytoplasm by an external boundary called the nuclear envelope: Composed of two parallel membranes (lipid-bilayers) separated by a narrow space Perforated with small, regularly spaced pores, formed at sites where the membranes unite Macromolecules migrate through the pores to the cytoplasm and vice versa The Nucleus 1 Nucleolus: Found in the nucleoplasm Site for ribosomal RNA synthesis Collection area for ribosomal subunits Chromatin: Made of linear DNA and histone proteins  Genetic material of the cell Internal Structures: Endoplasmic Reticulum A series of membrane tunnels used in transport and storage Rough endoplasmic reticulum (RER): Allows transport materials from the nucleus to the cytoplasm and to the cell’s exterior Ribosomes attached to its membrane surface hence its ‘rough’ appearance Smooth endoplasmic reticulum (SER): Closed tubular network without ribosomes Functions in nutrient processing and in synthesis and storage of nonprotein macromolecules such as lipids Internal Structures: Golgi Apparatus The site in the cell in which proteins are modified and then sent to their final destinations Consists of several flattened, disc-shaped sacs called cisternae Always closely associated with the endoplasmic reticulum: Transitional vesicles from the endoplasmic reticulum are picked up at the face of the Golgi apparatus Proteins are modified within the cisternae by the addition of polysaccharides and lipids Condensing vesicles pinch off of the Golgi apparatus and are then conveyed to lysosomes or transported outside the cell Nature’s Assembly Line Nucleus, endoplasmic reticulum, and Golgi apparatus: A segment of DNA containing the instructions for producing a protein is copied into RNA, and this RNA transcript is passed out through the nuclear pores directly to the ribosomes on the endoplasmic reticulum Specific proteins on the RER are deposited in the lumen and transported to the Golgi apparatus Proteins in the Golgi apparatus are chemically modified and packaged into vesicles to be used by the cell Vesicles Lysosomes: Bud off the Golgi apparatus as a vesicle Contain a variety of enzymes involved in the intracellular digestion of food particles and protection against invading microorganisms Participate in the removal of cell debris in damaged tissue Vacuoles: Membrane-bound sacs containing fluids or solid particles to be digested, excreted, or stored Found in phagocytic cells in response to food and other substances that have been engulfed Contents of a food vacuole are digested through a merger of a vacuole with a lysosome Mitochondria Generate energy for the cell Composed of a smooth, continuous outer membrane with an inner folded membrane Folds on the inner membrane are called cristae: Hold the enzymes and electron carriers of aerobic respiration Extracts chemical energy contained in nutrient molecules and stores it in the form of high-energy molecules, or ATP Mitochondria are unique organelles Divide independently of the cell Contain circular strands of DNA Have bacteria-sized 70S ribosomes These characteristics led scientists to hypothesize that mitochondria evolved from bacterial cells following an interaction between bacteria and an archaea-like cell Chloroplasts Found in algae and plant cells Capable of converting energy from sunlight into chemical energy through photosynthesis Produce oxygen gas as a by-product of photosynthesis Resemble mitochondria but are larger, contain special pigments, and are more varied in shape Ribosomes Distributed throughout the cell: Scattered freely in the cytoplasm and cytoskeleton Attached to the rough endoplasmic reticulum Appear inside mitochondria and chloroplasts Multiple ribosomes are often found arranged in short chains called polyribosomes (polysomes) Size and structure: Large and small subunits of ribonucleoprotein  Eukaryotic ribosome is 80S, a combination of 60S and 40S subunits The Cytoskeleton Functions: Anchoring organelles Moving RNA and vesicles Permitting shape changes and movement Three main types of cytoskeletal elements: Actin filaments: long, thin protein strands Intermediate filaments: ropelike structures Microtubules: long, hollow tubes Fungal Cells 2 basic cell forms: Yeasts: Round to oval shape Asexual reproduction, budding Hyphae: Long, threadlike cells found in the bodies of filamentous fungi Pseudohyphae are chains of yeast cells Some fungal cells are considered dimorphic and can take either form, depending on growth conditions Fungi and Human Disease 1 Nearly 300 species of fungi can cause human disease Three types of fungal disease in humans: Community-acquired infections caused by environmental pathogens Hospital-associated infections caused by fungal pathogens in clinical settings Opportunistic infections caused by low-virulence species infecting already-weakened individuals Fungi and Human Disease 2 Harmless spores can cause opportunistic infections in AIDS patients Fungal cell walls give off chemical substances that can trigger allergies Toxins produced by poisonous mushrooms can induce neurological disturbances and even death Aspergillus flavus synthesizes a poison called aflatoxin, potentially lethal to animals who eat contaminated grain Agricultural Impact of Fungi A number of species are pathogenic to corn and grain: Reduces crop production Can cause disease in domestic animals consuming contaminated feed crops Fungi rot fresh produce during shipping and storage: 40% of yearly fruit crop is consumed by fungi Benefits of Fungi Play an essential role in decomposing organic matter and returning minerals to the soil Form stable associations with plant roots and increase their ability to absorb water and nutrients Fungi have been engineered to produce large quantities of antibiotics, alcohol, organic acids, and vitamins Some fungi are eaten or used to provide flavoring to food Fungal Nutrition All fungi are heterotrophs: they acquire nutrients from a wide variety of organic substrates Saprobes: fungi that acquire nutrients from the remnants of dead plants and animals in soil or aquatic habitats Parasites: fungi that grow on the bodies of living animals or plants, although very few require a living host Fungi penetrate the substrate and secrete enzymes that reduces it to small molecules that can be absorbed Fungi are often found in nutritionally poor or adverse environments, and those with high salt or sugar content They can digest a wide array of organic substrates such as rubber, petroleum products, wood, and hair. Morphology of Fungi Cells of most microscopic fungi grow in loose associations or colonies Colonies of yeasts are much like bacteria: they have a soft, uniform texture and appearance Colonies of filamentous fungi are noted for the striking cottony, hairy, or velvety texture Mycelium: the woven, intertwining mass of hyphae that makes up the body or colony of a mold Septa: the nature of the septa varies from solid partitions with no communication between the compartments to partial walls with small pores that allow the flow of organelles and nutrients between adjacent compartments:  Nonseptate hyphae consist of one, long, continuous cell Vegetative hyphae are responsible for the visible mass of growth that appears on a substrate Reproductive, or fertile, hyphae produce spores Reproductive Strategies and Spore Formation Most can propagate by the outward growth of existing hyphae or by fragmentation Spores: Primary reproductive mode of fungi Can be dispersed through the environment by air, water, and living things Will germinate upon finding a favorable substrate and produce a new fungus colony in a short time Asexual Spore Formation Sporangiospores: formed by successive cleavages within a saclike head called a sporangium, which is attached to a stalk, the sporangiophore Conidiospores or conidia: free spores not enclosed by a spore-bearing sac Sexual Spore Formation Mixing of DNA from two parent fungi creates offspring with combinations of genes different from that of the parents Variations lead to potentially advantageous adaptations Sexual spores vary from simple fusion of fertile hyphae of two different strains, or as a complex union of male and female structures The Protozoa Name comes from the Greek for “primitive animals” About 12,000 species of single-celled creatures Most are harmless, free-living inhabitants of water and soil A few species are pathogens responsible for hundreds of millions of infections each year Protozoan Form and Function 1 Single cells containing all of the major eukaryotic organelles Cytoplasm divided into two parts: Ectoplasm: clear outer layer involved in locomotion, feeding, and protection Endoplasm: granular inner region housing the nucleus, mitochondria, and food and contractile vacuoles Protozoan Form and Function 2 Some organelles act as a primitive nervous system to coordinate movement Can move through fluids by means of pseudopods (“false feet”) Cell membrane regulates food, wastes, and secretions Cell shape can remain constant (as in most ciliates), or change constantly (as in amoebas) Size of most protozoans range from 3 to 300 μm: Giant amoebas and ciliates range from 3 to 4 mm Nutritional and Habitat Range Heterotrophic, require food in a complex organic form Free-living species scavenge dead plant or animal debris or graze on bacteria and algae Some have special feeding structures, such as oral grooves Some absorb food directly through the cell membrane Pathogenic species may live on the fluids of their host, such as plasma and digestive juices, or actively feed on tissues Main limiting factor is availability of moisture Predominant habitats are fresh and marine water, soil, plants, and animals Can survive in extremes of temperature and pH Life Cycles Trophozoite: the motile feeding stage requiring ample food and moisture to stay active Cyst: Dormant, resting stage when conditions in the environment become unfavorable Resistant to heat, drying, and chemicals Can be dispersed by air currents Important factor in the spread of disease Life Cycles and Transmission Some protozoan groups exist only in the trophozoite phase Many alternate between the trophozoite and cyst stage, depending on the habitat Trichomonas vaginalis, a common STD, does not form cysts and must be transmitted by intimate contact Entamoeba histolytica and Giardia lamblia form cysts and are readily transmitted in contaminated water and foods Reproduction All protozoa reproduce by relatively simple, asexual mitotic cell division or multiple fission Sexual reproduction also occurs in most protozoa: Ciliates participate in conjugation, in which two cells fuse and exchange micronuclei This results in new and different genetic combinations that can be advantageous in evolution The Helminths Include tapeworms, flukes, and roundworms Adult specimens are usually large enough to be seen with the naked eye Not all flatworms and roundworms are parasites; many live free in soil and water Disease-causing helminths spend part of their lives in the gastrointestinal tract Flatworms and Roundworms Flatworms (phylum Platyhelminthes): Very thin, often segmented body plan Divided into cestodes (tapeworms) and trematodes (flukes) Roundworms (phylum Aschelminthes): Also called nematodes Elongated, cylindrical, unsegmented body Helminths’ microscopic eggs and larvae are transmitted and can cause disease similar to microbes General Worm Morphology Multicellular animals that are equipped to some degree with organs and organ systems In pathogenic helminths, the most developed organ is the reproductive tract The digestive, excretory, nervous, and muscular systems are more rudimentary Life Cycles and Reproduction Complete life cycle includes the fertilized egg, larval, and adult stages Adults derive nutrients and reproduce sexually in a host’s body Nematodes: sexes are separate and different in appearance Trematodes: sexes can be separate or hermaphroditic (have both male and female sex organs) Cestodes: generally hermaphroditic Helminth life cycle: Must transmit an infective form (egg or larva) to the body of another host The host in which the larva develops is known as the intermediate (secondary) host Adulthood and mating occur in the definitive (final) host Transport host is an intermediate that experiences no parasitic development Sources for human infection are contaminated food, soil, and water or infected animals Egg Laying Fertilized eggs: Released to the environment Provided with a protective shell and extra food to aid their development into larvae Vulnerable to heat, cold, drying, and predators Certain helminths can lay from 200,000 to 25 million eggs a day to assure successful completion of their life cycle A Helminth Cycle: The Pinworm Example: Enterobius vermicularis: Pinworm Common infestation of the large intestine Range from 2 to 12 mm long with a tapered, curved cylindrical shape Cause enterobiasis (rarely serious) Life cycle: Microscopic eggs are swallowed: picked up from another infected person or objects they have touched Eggs hatch in the intestine Larvae mature into adults within 1 month Male and female worms mate Female migrates to the anus to deposit eggs Intense itching ensues Scratching spreads the eggs Distribution and Importance of Parasitic Worms About 50 species of helminths cause disease in humans Distributed in all areas of the world  Higher incidence in tropical areas Yearly estimate of cases is in the billions and are not confined to developing countries Conservative estimate of 50 million helminth infections in North America alone