Bio Lec 15

The source, "Lec15.pdf", discusses the evolution of microbial life, beginning with the historical ideas about the origin of life and progressing through the emergence and characteristics of prokaryotes and protists.

Historically, one prominent idea was "Spontaneous Generation," which posited that life could emerge from inanimate materials. This belief persisted until the mid-1800s, with examples cited such as flies appearing on meat or mice in dirty laundry.

Louis Pasteur, a chemist, did not believe in spontaneous generation. In 1859, he conducted experiments using a "Swan Neck Flask". He boiled meat broth in a flask, heated and bent the neck into an S-shape. This design allowed air to enter but trapped airborne microorganisms in the neck due to gravity. As expected, no microorganisms grew in the broth. These experiments demonstrated the principle of Biogenesis – that life cannot arise from non-life. Pasteur is also associated with "Pasteurisation," a process of heating milk to a high temperature and bottling it to kill microorganisms.

The source notes that there is no evidence that life can arise from non-life (abiogenesis or spontaneous generation) on Earth today. This presents a paradox: if life cannot arise from non-life now, how did the first life arise?. The resolution proposed is that the environment of early Earth was drastically different from current conditions. The early atmosphere contained very little oxygen, which is significant because oxygen can break down complex molecules. Favorable conditions existed for chemical reactions that could generate small organic molecules, or monomers, from inorganic ones. Without an ozone layer (which requires oxygen to form), high levels of UV radiation reached Earth. UV light and lightning could have served as energy sources to catalyze these chemical reactions.

Laboratory experiments, such as the famous Miller-Urey experiment, have attempted to replicate these early Earth conditions. These experiments used flasks and tubes containing an atmosphere of methane, ammonia, hydrogen, and water vapor, representing the early atmosphere, and a flask of water representing the primordial "sea". Electric discharges simulated lightning. The results showed that basic organic molecules were formed and deposited in the water.

Following the formation of monomers, small molecules could have joined to form polymers, possibly as water containing the monomers splashed onto hot rocks or in pools where water condensed. This joining of monomers to form polymers involves dehydration reactions.

Eventually, complex molecules are thought to have become self-replicating, with DNA being the key molecule. It is suggested that the first genes were likely short strands of RNA. Laboratory experiments have shown that small RNA strands can assemble spontaneously. Importantly, RNA molecules can also function as enzymes, catalyzing the synthesis of more RNA molecules; these are called ribozymes. This suggests that a complex protein enzyme was not necessarily required to evolve first, as RNA molecules could serve as both a template for replication and an "enzyme". This period is referred to as the "RNA world".

After the development of self-replicating molecules, pre-cells may have formed. Liposomes, which are hollow spheres made of a membrane of fats, are suggested as possible precursors. These liposomes could have enclosed nucleic acids and proteins, forming a precursor to a cell. Over time, these pre-cells could have become increasingly cell-like, with the formation of the first cells explained by natural chemical and biological processes.

The Earth formed about 4.6 billion years ago, and the first prokaryotes appeared approximately 3.5 billion years ago. This suggests that the development of these pre-cells into the first prokaryotic cells had hundreds of millions of years to occur.

For the next 2 billion years, only prokaryotes existed on Earth. Prokaryotes are highly diverse and inhabit nearly every environment, including hot or cold, saltwater or freshwater, and acidic, neutral, or basic conditions. There are two domains of prokaryotes: Domain Bacteria and Domain Archaea. Most prokaryotes belong to the Bacteria domain. Bacteria are responsible for many diseases but also include beneficial types, such as those that aid digestion in guts and decomposers that recycle dead organic material. The Domain Archaea was once considered the most primitive prokaryotic cells. Archaea are known as "Extremophiles" because they thrive in extreme habitats, similar to the harsh conditions thought to have existed on early Earth. Examples of Archaea habitats include the Great Salt Lake, deep-sea thermal vents, and anaerobic environments.

Prokaryotic structure is characterized by different shapes:

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Cocci (spherical). Clusters are called staphylococci, and chains are called streptococci.

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Bacillus (rod-shaped), examples include Anthrax and E. coli.

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Spirochetes (spiral-shaped), examples include the bacteria causing Lyme disease and Syphilis.

Prokaryotes lack a true nucleus and membrane-bound organelles. Most reproduce rapidly through binary fission, where DNA is replicated and the cell splits in two.

Prokaryotes have significant ecological impacts. They can cause Disease by producing toxins (exotoxins or endotoxins). A tiny amount of botulinum exotoxin can be lethal, and Salmonella poisoning is caused by an endotoxin in the bacterial cell wall. Prokaryotes are crucial for Chemical Recycling. Cyanobacteria are important for restoring oxygen to the atmosphere and for nitrogen fixation, converting atmospheric nitrogen into a form usable by plants. Some symbiotic bacteria live with plant roots (like legumes) and fix nitrogen, making it available to the plant. Many bacteria are decomposers, breaking down organic materials. Prokaryotes are also used in Bioremediation to clean up the environment. Decomposers can break down sewage, and some bacteria are used to treat oil spills by breaking down petroleum products.

Protists are described as simple, primitive eukaryotes. They are considered the ancestors of all other eukaryotes, including plants, animals, and fungi. Protists evolved about 1.7 billion years ago. Membrane-bound organelles in eukaryotes may have formed from in-foldings of cell membranes. The Endosymbiont Theory proposes that chloroplasts and mitochondria evolved when one prokaryotic cell engulfed a smaller cell, and the two came to live together symbiotically. Evidence for this theory includes the fact that chloroplasts and mitochondria are similar in size to small bacteria and contain their own DNA, RNA, and ribosomes separate from the rest of the cell.

The diversity of Protists is vast. All protists are eukaryotic. Most are unicellular, possessing complex cells as they function as complete organisms. Some protists are colonial or multicellular, and while most are small, some, like giant kelps, can be very large. There are four major groups: Protozoans, Slime Molds, Algae, and Seaweeds.

Most protists are aquatic, living as planktonic (free-floating) or sessile (attached) forms. Planktonic protists are vital parts of aquatic food webs. Photosynthetic plankton and bacteria contribute significantly to global photosynthesis. Protists can be free-living, live symbiotically, or be parasitic. Terrestrial protists require damp places for survival and water for sexual reproduction (to transport gametes).

The source details the four groups of Protozoans:

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Flagellates: Move using one or more flagella. Most are free-living, but some are parasitic, like Giardia (causing diarrhea in humans) and Trypanosoma (causing African sleeping sickness).

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Amoebas: Lack flagella or cilia and move using pseudopodia (temporary cell extensions). The source mentions the "brain-eating amoeba" as an example.

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Apicomplexans: All are parasitic. This group includes Plasmodium, the organism responsible for malaria, which is transmitted by the Anopheles mosquito. Malaria symptoms are flu-like and can progress to severe conditions, causing significant mortality worldwide.

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Ciliates: Use cilia for movement and feeding. A familiar example is Paramecium, which lives in freshwater.

Slime Molds are described as fungus-like protists that are heterotrophs. They are not closely related to true fungi. Plasmodial slime molds are found in moist areas like leaf litter and logs in forests and exist as a single cell with many nuclei.

Algae are plant-like protists that are photosynthetic. They live in wet or moist habitats and lack a cuticle. The source discusses several groups of algae, including unicellular and multicellular forms. The groups covered are Dinoflagellates, Diatoms, Green Algae, and Seaweed.

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Dinoflagellates: Mostly unicellular with cellulose plates covering their cells. They have two flagella and move with a spinning motion. Most are photosynthetic, though some ingest other microorganisms. Many are endosymbionts with marine invertebrates, called zooxanthellae, providing carbohydrates to their hosts like corals and being responsible for almost all primary production in coral reefs. Dinoflagellates are also responsible for "Red Tides" and produce neurotoxins harmful to fish and humans.

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Diatoms: Mostly unicellular, typically reproducing asexually. They have a two-part shell made of silica (glass-like) and are classified by their shell pattern. They exist as planktonic and sessile forms in freshwater and marine habitats and are important in primary production. Massive accumulations of diatom shells form diatomaceous earth, which is mined for various uses (insulation, filtering, abrasives). The White Cliffs of Dover are noted as a large accumulation of diatoms.

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Green Algae: Photosynthetic, found in aquatic and moist terrestrial habitats. They have various growth forms (unicellular, colonial, coenocytic, multicellular) and are often motile at some point in their life. A green algae is believed to be the ancestor of modern plants. Shared characteristics with plants include pigments (chlorophyll a & b, carotenoids), storage products (mainly starch), and cell walls with cellulose. They are important in food webs and oxygenate water. Green algae reproduce both sexually (forming gametes in gametangia) and asexually (cell division, fragmentation, spores). Volvox is given as an example of a colonial green algae.

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Seaweeds: Large, multicellular marine algae. They are classified into three main groups based on pigments: green, red, and brown algae. Seaweeds are harvested for food, such as kombu (brown algae) in Japanese and Korean soups and nori (red algae) used for sushi wraps. A gel-like substance from seaweed is used as a thickener in puddings and ice cream.

Finally, the source mentions that a green algae ancestor, specifically a Charophycean, is believed to have given rise to true land plants.