13.1 How Cells Arose There are three possibilities for the appearance of the first living organisms on Earth 1. Extraterrestrial origin Life was transferred to Earth from a distant planet 2. Special creation Life was created by supernatural or divine forces 3. Evolution Life may have evolved from inanimate matter, with selection as the driving force Only the third possibility is scientifically testable

Earth is formed 4.5 billion years ago Fig. 13.1 A clock of biological time

Forming Life’s Building Blocks Life originated ~ 2.5 billion years ago The Earth’s atmosphere then had no oxygen It was rich with hydrogen-rich gases (NH3, CH4) These simple molecules combined to form more complex molecules Lightning provided the energy

Forming Life’s Building Blocks Stanley Miller and Harold Urey reconstructed the oxygen-free atmosphere of early Earth in their lab Cellular building blocks form spontaneously, when the system is subjected to lightning or UV light They concluded that life may have evolved in a “primordial soup” of biological molecules

Forming Life’s Building Blocks Concerns have been raised about the “primordial soup” hypothesis No oxygen => no protective ozone layer Therefore, the UV light would have destroyed the essential ammonia and methane gases Louis Lerman, in 1986, proposed the bubble model Key chemical life-building processes took place within bubbles on the ocean’s surface Inside the bubbles the essential gases would be protected from UV light

Fig. 13.2

The First Cells The first step may have been the formation of tiny bubbles termed microspheres These have cell-like properties Those microspheres better able to incorporate molecules and energy persisted longer than others The first macromolecule produced was RNA It provided a possible early mechanism of inheritance Later, RNA was replaced as hereditary material by the much more stable double-stranded DNA

13.2 The Simplest Organisms The fossil record indicates that prokaryotes appeared 2.5 billion years ago Eukaryotes, on the other hand, appeared only 1.5 billion years ago Today prokaryotes are the simplest and most abundant form of life on earth Prokaryotes occupy a very important place in the web of life on earth

The Structure of a Prokaryote Prokaryotes are small, simply organized, single cells that lack a nucleus Fig. 4.9 Rod Spherical Spiral Prokaryotes include the bacteria and archaea They come in three main shapes Rod-shaped (bacilli) Spherical (cocci) Spirally coiled (spirilla)

The Structure of a Prokaryote The cell membrane of prokaryotes is encased in a cell wall Bacterial cell walls are composed of peptidoglycan Network of polysaccharides linked together by peptide cross-links Archaeal cell walls lack peptidoglycan They are made of proteins, sugars or both

Bacteria can be divided into two groups based on their cell wall architecture Gram-positive Have a thick peptidoglycan layer Lack outer membrane Gram-negative Have a thin peptidoglycan layer Have an outer membrane containing lipopolysaccharide The name refers to a differential stain developed by Hans Christian Gram Gram-positive cells retain the primary crystal violet stain Gram-negative cells don’t and are stained by a safranin (red) counterstain

Fig. 13.3 The structures of bacterial cell walls

Many bacteria have capsules A gelatinous layer found external to the cell wall Many bacteria possess threadlike flagella Long external appendages used for locomotion Some bacteria also possess pili Short external appendages used for attachment In harsh conditions, a few bacteria can form endospores Highly-resistant structures that may germinate into active bacteria when conditions improve

Prokaryotes reproduce by binary fission Fig. 7.1

Some bacteria undergo a process called conjugation The undirectional transfer of plasmid DNA following cell-to-cell contact Fig. 13.4 Both cells contain a complete copy of the plasmid Pilus connecting the two cells together

13.3 Comparing Prokaryotes to Eukaryotes Prokaryotes differ from eukaryotes in many respects They have very little internal organization They are unicellular and much smaller They possess a single chromosome They are far more metabolically diverse

TABLE 13.1

TABLE 13.1

Prokaryotic Metabolism Prokaryotes have evolved many more ways than eukaryotes to acquire carbon and energy Acquisition of Carbon Autotrophs = Use CO2 as their only carbon source Heterotrophs = Use preformed organic compounds as carbon sources Acquisition of Energy Phototrophs = Use light as energy source Chemotrophs = Use chemicals as energy source

Prokaryotic Metabolism Based on carbon and energy sources, prokaryotes can be divided into four categories 1. Photoautotrophs Use the energy of sunlight to build organic molecules from CO2 Cyanobacteria 2. Chemoautotrophs Obtain energy by oxidizing inorganic substances Nitrifiers oxidize ammonia or nitrite

Prokaryotic Metabolism Based on carbon and energy sources, prokaryotes can be divided into four categories 3. Photoheterotrophs Use light as energy and pre-formed organic molecules as carbon sources Purple nonsulfur bacteria 4. Chemoheterotrophs Use organic molecules as carbon and energy sources Decomposers and most pathogens

13.4 Viruses Infect Organisms Viruses are not living organisms Rather, they are “parasitic” chemicals that can only reproduce within living cells Viruses are very small Range from 17 – 1,000 nm Viruses occur in all organisms In every case, the basic structure is the same Segments of DNA or RNA wrapped in a protein coat called the capsid There is considerable difference, however, in the details

Not found in all viruses Fig. 13.6 The structure of bacterial, plant, and animal viruses Membrane-like layer Not found in all viruses Capsid

The Origin of Viral Diseases Influenza Perhaps the most lethal virus in human history Natural reservoirs ducks and pigs in central Asia AIDS (HIV) First entered humans from chimpanzees in Africa The chimpanzee virus is called simian immunodeficiency virus (SIV) Ebola virus Filamentous virus that attacks connective tissue Natural host of the virus is unknown

The Origin of Viral Diseases Hantavirus Discovered in 1993 Natural host is deer mice SARS Severe acute respiratory syndrome Caused by a coronavirus Natural host is most likely the civet West Nile Virus Mosquito-borne virus Natural host is birds

13.5 The Origin of Eukaryotic Cells The microfossil records suggests that eukaryotic cells appeared 1.7 billion years ago The word eukaryote is derived from the Greek words for “true” and “nucleus” The endoplasmic reticulum and nucleus of eukaryotes may have evolved from infoldings of prokaryotic cell membranes

Endosymbiosis The endosymbiotic theory proposes that engulfed bacteria gave rise to mitochondria and chloroplasts Evidence Organelles are surrounded by two membranes Organelles have circular DNA Organelles have ribosomes that resemble those of prokaryotes Organelles divide by binary fission

Fig. 13.7 The theory of endosymbiosis

13.6 General Biology of Protists Protists are highly variable eukaryotes that share one characteristic: They are not fungi, plants or animals Protists have varied types of cell surfaces Some have cell walls Movement is also accomplished by diverse mechanisms Flagella Cilia Pseudopodia

Many protists form cysts Dormant cells with resistant outer covering Protists employ all forms of nutrition except chemoautotrophy Phototrophs Heterotrophs Phagotrophs (or holozoic feeders) Ingest visible particles of food Osmotrophs (or saprozoic feeders) Ingest food in soluble form

Protists typically reproduce asexually Binary fission  Equal halves Budding  Progeny cell smaller than parent cell Schizogony  Multiple fission Sexual reproduction occurs in times of stress Zygotic meiosis In the Sporozoans Gametic meiosis In ciliates and some flagellates Sporic meiosis In algae

Multicellularity Complex multicellular organisms Individuals are composed of many highly specialized cells that coordinate their activities Three kingdoms exhibit multicellularity Plants Animals Fungi

Multicellularity Two key characteristics distinguish between complex multicellular and simple multicellular organisms Cell specialization Different cells use different genes They therefore develop in different ways Intercell coordination Cells adjust their activity in response to what other cells are doing

Colony moves by beating of flagella of individual cells Multicellularity Colony moves by beating of flagella of individual cells Colonies Permanent association of cells, but little or no integration of cell activities Volvox Unicellular green alga Colony is hollow ball of cells Fig. 13.9

Multicellularity Aggregates Multicellular organisms Transient collection of cells Cellular slime molds Cells come together during starvation Multicellular organisms Individuals are composed of many interacting cells that regulate the activities of one another True but simple multicellularity has been achieved by three groups of protists Brown, green, and red algae

13.7 Kinds of Protists Protists are the most diverse eukaryotic kingdoms The kingdom Protista is an artificial group Not representative of evolutionary relationships There is little consensus, even among experts, as to how protists should be classified Single, very diverse kingdom vs. Several different kingdoms

There are 15 distinct phyla of protists These are grouped into five general groups based on shared characteristics 1. Presence/absence of flagella/cilia 2. Presence and kinds of pigments 3. The type of mitosis 4. The types of cristae in mitochondria 5. Molecular genetics of the ribosomal “S” subunit 6. Types of inclusions 7. Overall body form 8. Body shell or armor 9. Modes of nutrition and movement

Fig. 13.10 The major protist groups Functional groupings Accepted taxa

TABLE 13.2

TABLE 13.2

13.8 A Fungus Is Not a Plant The study of fungi is called mycology Fungi have traditionally been included in the plant kingdom However, there are significant differences between fungi and plants Fungi are heterotrophs Fungi have filamentous bodies Fungi have nonmotile sperm Fungi have cell walls made up of chitin Fungi have nuclear mitosis

The Body of a Fungus Fungi exist mainly as slender filaments called hyphae (singular, hypha) Hyphae are strings of cells separated by septa (singular, septum) Pores in the septa allow for cytoplasmic streaming between cells Because of cytoplasmic streaming, many nuclei may be connected by shared cytoplasm

Reproductive structures The main body of a fungus is not the familiar mushroom But rather an extensive network of hyphae termed a mycelium (plural, mycelia) Reproductive structures Fig. 13.11 All parts of the fungal body are metabolically active Morel Amanita

How Fungi Reproduce Fungi reproduce both sexually and asexually Sexual reproduction is initiated when two hyphae of different mating types come in contact and fuse The two nuclei do not fuse immediately Heterokaryon Hyphae containing nuclei derived from two genetically different individuals Homokaryon Hyphae containing nuclei derived from two genetically similar individuals

How Fungi Reproduce Fungi have three types of reproductive structures Gametangia Form haploid gametes that fuse to form zygote Sporangia Produce haploid spores that are dispersed Conidiophores Produce asexual spores Fig. 13.12 Puffball spores Spores are a common means of fungal reproduction

How Fungi Obtain Nutrients Fungi obtain nutrients by external digestion They secrete digestive enzymes into their surroundings and absorb the resulting organic molecules Fig. 13.13 The oyster mushroom Some fungi are active predators Immobilizes nematodes then eats them! Others are even more active predators Snare or trap prey

13.9 Kinds of Fungi ~ 74,000 species of fungi have been named so far They are divided into four phyla Zygomycota Ascomycota Basidiomycota Chitrydiomycota Distinguished primarily by their mode of sexual reproduction A fifth group, the imperfect fungi, is artificial It’s a “catch-all” grouping of fungi in which sexual reproduction has not been observed yet!

TABLE 13.3

Ecological Roles of Fungi as Decomposers Fungi, together with bacteria, are the principal decomposers in the biosphere Fungi are virtually the only organisms that can break down lignin Fungi cause animal diseases Fungi are the most harmful pests of living plants

Commercial Uses Many commercial products are dependent on the biochemical activities of fungi Bread Beer Cheese Soy sauce Penicillin Some fungi are used to convert one complex organic molecule into another

Fungal Associations Two kinds of mutualistic associations between fungi and autotrophic organisms are ecologically important 1. Mycorrhizae Symbiotic association between a fungus and the roots of plants Fig. 13.14 Lichens growing on rock 2. Lichens Symbiotic association between a fungus and a green algae or cyanobacterium