21 terms

Chapter 15

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Major episodes in the history of life
Earth was formed about 4.6 billion years ago

the first living things
-prokaryotes evolved by about 3.5 billion years ago,
-began oxygen production about 2.7 billion years ago,
-lived alone for more than a billion years, and
-continue in great abundance today
Single celled eukaryotes first evolved
-about 2.1 billion years ago
-multi-cellular eukaryotes first evolved at least 1.2 billion years ago
Rise in animal diversity
All the major phyla of animals evolved by the end of the Cambrian explosion, which
-began about 540 million years ago

Life moves onto land (Plants and fungi)
- first colonized land about 500 million years ago and
- were followed by amphibians that evolved from fish
The origin of life
We may never know for sure how life on Earth began

Today, most biologists think it is possible that life on early Earth evolved from simple cells produced by
-chemical and
-physical processes
A four-stage hypotheses for the origin of life
According to one hypotheses, the first organisms were products of chemical evolution in four stages

-abiotic synthesis of organic monomers
-abiotic synthesis of polymers
-formation of pre-cells
-origin of self-replicating molecules
Stage 1: Abiotic Synthesis of Organic Monomers
An apparatus was built to mimic the early Earth atmosphere and included

-hydrogen gas (H2), methane (CH4), ammonia (NH3), and water vapor (H20)

-sparks that were discharged into the chamber to mimic the prevalent lightning of early Earth, and

-a condenser that cooled the atmosphere, causing water and dissolved compounds to "rain" into the miniature "sea"
Stage 1: Abiotic Syntheses Cont.
After the apparatus had run for a week an abundance of organic molecules essential for life had collected in the "sea", including amino acids, the monomers of proteins

These laboratory experiments
-have been repeated and extended by other scientists, and
-support the idea that organic molecules could have arisen abiotically on early earth
Stage 2: Abiotic Synthesis of Polymers
Researchers have brought about the polymerization of monomers to form polymers, such as proteins and nucleic acids, by dripping solutions of organic monomers onto

-hot sand,
-clay, or
-rock
Stage 3: Formation of Pre-Cells
A key step in the origin of life was the isolation of a collection of abiotically created molecules within a membrane

Laboratory experiments demonstrate that pre-cells could have formed spontaneously from abiotically produced organic compounds. They

-have selectively permeable surface
- can grow by absorbing molecules from their surroundings
Stage 4: Origin of Self-Replicating Molecules
Life is defined partly by the process of inheritance, which is based on self-replicating molecules

One hypothesis is that the first genes were short strands of RNA that replicated themselves

-without the assistance of proteins,
-perhaps using RNA's that can act as enzymes, called ribozymes
-can be called the "RNA world"
Prokaryotes
Prokaryotes lived and evolved all alone on Earth for about 2 billion years

Prokaryotes
-are found wherever there is life,
-have a collective biomass that is at least ten times that of all eukaryotes,
-thrive in habitats too extreme for any eukaryote
-cause about half of all human diseases, and
-are more commonly benign or beneficial
They're everywhere!
Compared to eukaryotes, prokaryotes are
-much more abundant and
-typically much smaller

Prokaryotes living in soil and at the bottom of lakes, rivers, and oceans, help to decompose dead organisms and other organic waste material, returning vital chemical elements to the environment
Prokaryotic Cells
-lack a membrane-enclosed nucleus
-lack other membrane-enclosed organelles
-typically have cell walls exterior to their plasma membranes, but
-display an enormous range of diversity
Prokaryotic Forms
The three most common shapes of prokaryotes are
1. spherical (cocci)
2. rod-shaped (bacilli) and,
3. spiral or curved

All prokaryotes are unicellular

Some species
-exist as groups of two or more cells
Prokaryotic Forms and Reproduction
About half of all prokaryotes are mobile, and many of these travel using one or more flagella

Most prokaryotes can reproduce
-by dividing in half by binary fission and
-at very high rates if conditions are favorable

Some Prokaryotes form endospores, which are
-thick-coated, protective cells
-produced when the prokayote is exposed to unfavorable conditions
Protists
Protists are
-eukaryotes that are not fungi, animals, or plants,
-mostly unicellular, and
-ancestral to all other eukaryotes
-consists of multiple clades (kingdoms) but
The origin of Eukaryotic Cells
Eukaryotic cells evolved by
-the infolding of the plasma membrane of a prokaryotic cell to form the endomembrane system and

-a process know as endosymbiosis
Symbiosis
is a more general association between organisms of two or more species
Endosymbiosis
-refers to one species living inside another host species and
-is the process by which eukaryotes gained mitochondria and chloroplasts
The diversity of protists
The group called protists
-consist of multiple clades but
-remain a convenient term to refer to eukaryotes that are not plants, animals, or fungi
Protists obtain their nutrition in a variety of ways
-Algae are autotrophs, producing their food by photosynthesis

-Other protists are heterotrophs.
-Some Protists eat bacteria or other protists
-Other protists are fungus-like and obtain organic molecules by absorption
-Parasites derive their nutrition from a living host, which is harmed by the interaction