evolution of simple cells

simple cells evolve

Simple organic molecules such as proteins and nucleic acids eventually became complex organic substances. Sci- entists think that the organic molecules adhered to clay minerals, which provided the structure needed for these substances to organize. The clays, along with their metal cations, catalyzed the chemical reactions that caused the molecules to form polymers. The first RNA fragments could also have come together on ancient clays. E. coli (Escherichia coli) is a primitive prokaryote that may resemble the earliest cells. For an organic molecule to become a cell, it must be able to separate itself from its environment. To enclose the molecule, a lipid membrane grew around the organic material. Eventually the molecules could synthesize their own organic material and replicate themselves. These became the first cells.

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prokaryotes

The earliest cells were prokaryotes (Figure 1.1). Although prokaryotes have a cell membrane, they lack a cell nucleus and other organelles. Without a nucleus, RNA was loose within the cell. Over time the cells became more complex. LUCA was a prokaryote but differed from the first living cells because its genetic code was based on DNA. The oldest fossils are tiny microbe-like objects that are 3.5 billion years old. Evidence for bacteria, the first single-celled life forms, goes back 3.5 billion years (Figure 1.2).

photosynthesis

The earliest life forms did not have the ability to photosynthesize. Without photosynthesis what did the earliest cells eat? Most likely they absorbed the nutrients that floated around in the organic soup that surrounded them. After hundreds of millions of years, these nutrients would have become less abundant. Sometime around 3 billion years ago (about 1.5 billion years after Earth formed!), photosynthesis began. Photo- synthesis allowed organisms to use sunlight and inorganic molecules, such as carbon dioxide and water, to create chemical energy that they could use for food. To photosynthesize, a cell needs chloroplasts (Figure 1.3). A diagram of a bacterium. Chloroplasts are visible in these cells found within a moss.

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importance of photosynthesis

In what two ways did photosynthesis make the planet much more favorable for life? 1. Photosynthesis allowed organisms to create food energy so that they did not need to rely on nutrients floating around in the environment. Photosynthesizing organisms could also become food for other organisms. 2. A byproduct of photosynthesis is oxygen. When photosynthesis evolved, all of a sudden oxygen was present in large amounts in the atmosphere. For organisms used to an anaerobic environment, the gas was toxic, and many organisms died out. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL:

cyanobacteria

What were these organisms that completely changed the progression of life on Earth by changing the atmosphere from anaerobic to aerobic? The oldest known fossils that are from organisms known to photosynthesize are cyanobac- teria. Cyanobacteria were present by 2.8 billion years ago, and some may have been around as far back as 3.5 billion years. Cyanobacteria were the dominant life forms in the Archean. Why would such a primitive life-form have been dominant in the Precambrian? Many cyanobacteria lived in reef-like structures known as stromatolites (Figure These rocks in Glacier National Park, Montana may contain some of the oldest fossil microbes on Earth. Modern cyanobacteria are also called blue-green algae. These organisms may consist of a single or many cells and they are found in many different environments (Figure 1.5). Even now cyanobacteria account for 20% to 30% of photosynthesis on Earth. A large bloom of cyanobacteria is harmful to this lake.

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instructional diagrams

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questions

the earliest cells may have formed when

-->  a) organic molecules adhered to clay minerals.

b) clay minerals developed the ability to replicate.

c) metal cations combined with organic molecules.

d) none of these.

for organic molecules to become a cell they had to

a) separate themselves from their environment with a membrane.

b) synthesize their own organic material.

c) replicate themselves.

-->  d) all of these.

stromatolites are cyanobacteria reefs that dominated the archean.

-->  a) true

b) false

cyanobacteria were the first organisms to photosynthesize but they died out before the end of the precambrian.

a) true

-->  b) false

early cells took nutrients from the air.

a) true

-->  b) false

the first organisms to photosynthesize were

a) plants

-->  b) cyanobacteria

c) coral reef

d) none of the above

the organisms referred to as luca was

a) was a prokaryote with dna as its genetic replicator.

b) the last universal common ancestor.

c) the point at which all evolutionary lineages of all organisms converge.

-->  d) all of these.

prokaryotes consist of

a) organelles

b) a nucleus

-->  c) a cell membrane

d) all of the above

why did photosynthesis made earth more favorable for life?

a) organisms could figure out what to do with all the carbon dioxide in the atmosphere.

-->  b) organisms did not need to rely on nutrients floating by to get food energy.

c) the oxygen it produced allowed all life forms alive at the time to thrive.

d) all of these.

to photosynthesize, a cell needs

a) chloroplasts

b) sunlight

c) nutrients

-->  d) all of these

diagram questions

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