Origin of Life Prokaryotic Cells Introduction to Biology

Origin of Cellular Life The Earth formed about 4.6 billion years ago. o For about 500 million years, the Earth was continually bombarded by chunks of rock and ice in the solar system. The early atmosphere of Earth contained: o Water vapor H 2 O o Nitrogen N 2 o Carbon dioxide CO 2 o Methane CH 4 o Ammonia NH 3

Origin of Cellular Life How did life arise from such a harsh environment? Two scientists designed a model of what conditions were like on Earth at this time. o This is called the Miller-Urey Apparatus

Miller-Urey Apparatus This apparatus simulated three important conditions on Earth: – The high amount of lightning – Heat and gases released by volcanic activity – Water vapor present in the atmosphere.

Results of Miller-Urey Apparatus Simple compounds including water (H 2 O), methane (CH 4 ), ammonia (NH 3 ), and hydrogen (H 2 ) were used to simulate the atmosphere. After 2 weeks, 10-15% of the carbon had been used to form sugars, amino acids, and parts of nucleic acids. o These simple organic compounds could have produced the proteins, lipids, and carbohydrates that make up life today.

The First Cells The first life forms on Earth were likely single-celled prokaryotic organisms. Prokaryotic organisms are single-celled organisms that do not have a nucleus. o Their DNA or RNA is usually floating freely inside the cell. Prokaryotic cells also do not have any membrane bound organelles.

Most prokaryotes are microscopic, but what they lack in size they make up for in numbers. There are more in a handful of fertile soil than the number of people who ever lived. Prokaryotes thrive almost everywhere, including places too acidic, too salty, too cold, or too hot for most other organisms They have an astonishing genetic diversity

Prokaryotic Energy Sources Prokaryotes are able to get the energy they need for life from four different sources: o Photoautotrophy o Chemoautotrophy o Photoheterotrophy o Chemoheterotrophy “Photo” means light. “Chemo” means inorganic (non-living) chemicals. “Auto” means self. “Hetero” means different.

Relationships to Oxygen Prokaryotic metabolism varies with respect to oxygen: o Obligate aerobes require oxygen o Facultative anaerobes can survive with or without oxygen o Obligate anaerobes are poisoned by oxygen

Archaea Archaea are prokaryotic organisms that are very similar to bacteria in size and structure. Bacteria and archaea likely evolved separately from the original life forms on Earth. Key differences between bacteria and archaea: o Archaea can survive in extremely hot, cold, and salty environments. o Archaea are not affected by many antibiotics. o Archaea are obligate anaerobes ; they can only survive in oxygen- free environments.

Archaea can be classified by the type of environment they live in: o Extreme thermophiles thrive in very hot environments o Extreme halophiles live in high saline environments o Acidophiles live in environments with a very low pH (high amounts of acid).

Reproduction Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours. o Binary fission is asexual reproduction and produces exact clones of the original bacteria. Many prokaryotes form endospores, which can remain viable in harsh conditions for centuries

Reproduction Prokaryotes are able to reproduce so quickly that under the right conditions they grow exponentially. Exponential growth is when the population increases at a faster and faster rate. This only occurs if all the needs of the bacteria culture are met (food, space, etc).

Petri Dishes Bacteria are cultured in petri dishes, which contain agar, a food source. Petri dishes are close to perfect conditions for bacteria to grow, so they reproduce exponentially.

Prokaryotes like bacteria are able to evolve much more quickly than multicellular organisms. o They reproduce quickly. o Their genome is small, so a single mutation can change a bacteria drastically. o Prokaryotes are able to exchange plasmids with each other through pili.

Bacterial Shapes Most prokaryotes are unicellular, although some species form colonies Prokaryotic cells have a variety of shapes The three most common of which are: o Coccus – Spherical o Bacillus – Rod-shaped o Spirilla – Spiral-shaped Video: Tubeworms Video: Tubeworms

LE 27-2 Spherical (cocci) Rod-shaped (bacilli) Spiral 5 µm2 µm1 µm

A typical rod-shaped bacterium A thin section through the bacterium Bacillus coagulans (TEM) 0.5 µm Pili Nucleoid Ribosomes Plasma membrane Cell wall Capsule Flagella Bacterial chromosome

Most prokaryotes have a ring of DNA that is not surrounded by a membrane. o No nucleus. o DNA is kept within a nucleoid region instead. Some species of bacteria also have smaller rings of DNA called plasmids. o These can be exchanged between invididuals. DNA provides instructions to ribosomes, which produce any proteins the bacteria need.

LE 27-8 Chromosome 1 µm

Internal Structures Prokaryotic cells do not have membrane- bound organelles. o Ex: Chloroplast, mitochondria However, some prokaryotes have special membranes that perform some of the same functions. o Thylakoid membranes enable photosynthesis o Respiratory membranes enable the use of oxygen to break down nutrients into ATP.

LE 27-7 Thylakoid membranes Respiratory membrane Photosynthetic prokaryote Aerobic prokaryote 0.2 µm 1 µm

Cell-Surface Structures An important feature of nearly all prokaryotic cells is their cell wall. The cell wall… o Maintains cell shape o Provides physical protection o Prevents the cell from bursting in a hypotonic environment

The cell wall of many prokaryotes is covered by a capsule, a layer of polysaccharides that protect them from being caught by white blood cells. Capsule

Some prokaryotes have fimbriae and pili, which allow them to stick to their growing surface or attach to other prokaryotes to exchange genes.. Fimbriae

Motility Most motile bacteria propel themselves by flagella. In the environment, many bacteria exhibit the ability to move toward or away from certain stimuli Video: Prokaryotic Flagella (Salmonella typhimurium) Video: Prokaryotic Flagella (Salmonella typhimurium)

LE 27-6 Flagellum Filament Cell wall Hook Basal apparatus Plasma membrane 50 nm

Importance of Prokaryotes Prokaryotes are so important to the biosphere that if they were to disappear, most other life would not be able to survive.

Symbiotic Relationships Symbiotic relationships are when two organisms live close together. In mutualism, both symbiotic organisms benefit o Example: Bacteria that live in deep-sea fish, producing bioluminescence. In commensalism, one organism benefits while neither harming nor helping the other. o Example: Most of the bacteria on human skin

Nitrogen Metabolism Prokaryotes also need nitrogen to build amino acids and proteins. One common source of nitrogen is called nitrogen fixation, where prokaryotes convert atmospheric nitrogen to ammonia. Some plants, called legumes, have nodules in their roots that contain bacteria to help them fix nitrogen.

Chemical Recycling Prokaryotes help recycle elements needed for life between living and nonliving parts of the ecosystem. Heterotrophic prokaryotes function as decomposers, breaking down corpses, dead vegetation, and waste products into smaller molecules that can be used by other living things.

Bioluminescence Some bacteria produce light as a result of their internal chemical reactions. Other animals have formed mutualistic relationships with these bacteria to take advantage of this bioluminescence.

Harmful Bacteria In parasitism, one organism, called a parasite, benefits at the expense of the host o Example: The bacteria that causes strep throat. Not all prokaryotes are harmful, but some are human pathogens – they cause disease.

Pathogenic Prokaryotes Prokaryotes cause about half of all human diseases Lyme disease is one example.

Pathogenic prokaryotes typically cause disease by releasing exotoxins or endotoxins Exotoxins are released externally by the bacteria, often as waste products. o Example: Botulism, tetanus, anthrax Endotoxins are released only when bacteria die and their cell walls break down o Example: Meningitis

Antibiotics Antibiotics are anti-bacterial chemicals that originally came from mold. Each antibiotic works in different ways. o Penicillin disrupts the bacteria’s ability to produce a cell wall, causing it to burst due to an influx of water into its cytoplasm.

Antibiotic Resistance Bacteria can mutate and evolve quickly, due to their small size and fast reproduction rate. Sometimes, a mutation will result in their ability to resist the action of antibiotics. o Over time, this mutation can spread throughout an entire colony, creating a strain of antibiotic-resistant bacteria. Resistant bacteria will not be affected by the antibiotics in the same way.