24.1 Two Prokaryotic Domains Flashcards
What does it mean for prokaryotes to be paraphyletic?
it excludes some descendants of the last common ancestor of all living organisms, namely eukaryotes
What are the 2 domains of life characterized by prokaryotic cell structure?
Bacteria and Archaea
What makes up for Bacteria and Archaea’s lack in complexity of cell structure?
their dazzling metabolic diversity
these domains underpin the efficient operation of ecosystems on our planet
How were bacteria first distinguished?
because of their small size, they were long dismissed as primitive organisms, distinguished mostly by the eukaryotic features they lack:
- no membrane-bounded nuclei
- no energy-producing organelles
- no sex
Why were the first point of views of bacteria misleading?
bacteria are the diverse and remarkably successful products of nearly 4 billion years of evolution
today, bacterial cells outnumber eukaryotic cells by several orders of magnitude
even in your body, bacteria equal or outnumber human cells
Describe the contents of a bacterial cell.
- DNA is present in a single circular chromosome folded into many loops (in contrast to multiple linear chromosomes characteristic of eukaryotic cells)
- many carry additional DNA in the form of plasmids, small circles of DNA that replicate independently of the cell’s circular chromosome
- have plasma membrane and cell wall
*figure 24.1 textbook
Is plasmid DNA essential for cell survival?
in general, no, but it may contain genes that have adaptive value under specific environmental conditions
Why does transcribed mRNA in bacterial cells immediately translate into proteins by ribosomes?
because no nuclear membrane separates DNA from the surrounding cytoplasm
How are cells processes, such as metabolism, carried out by bacterial cells?
by proteins that float freely in the cytoplasm or are embedded in the plasma membrane
few bacteria (notably the photosynthetic bacteria) contain internal membranes similar to those found in chloroplasts and mitochondria
Where do light-triggered reactions of photosynthetic bacteria take place?
in association with membranes distributed within the cytoplasm
What is peptidoglycan?
a complex polymer of sugars and amino acids
it makes up a cell wall that provides structural support for bacteria
- some bacteria have thick walls made with multiple layers, some have thin walls surrounded by an outer layer of lipids
What was believed to be missing in bacterial cells for many years?
it was believed that bacteria lacked the cytoskeletal framework that organizes cytoplasm in eukaryotic
more recently, careful studies have shown that bacteria do possess an internal scaffolding of proteins that plays an important role in determining the shape, polarity, and other spatial properties of bacterial cells
Describe the size of bacterial cells.
- tiny
- 200-300 nanometers (nm) in diameter
- relatively few are more than 1-2 micrometers long
Why are bacteria so small?
diffusion limits cell size in bacteria
Describe diffusion.
- more molecules move from a region with a higher concentration of the molecule to lower concentration of molecule than move in the opposite direction
- net movement stops only when the two regions achieve equal concentrations of the molecule, but diffusion continues
Give examples of bacterial diffusion.
- photosynthetic bacteria gain the CO2 they need by the diffusion of CO2 from the environment into the cell
- respiring bacteria take in small organic molecules and oxygen by diffusion
Why does diffusion explain why bacterial cells tend to be small?
small cells have more SA in proportion to its V, and the interior parts of a small cell are closer to the surrounding environment than those of a larger cell
as a result, slowly diffusing molecules do not have to travel far to reach every part of a small cell’s interior
What is the surface area of a spherical cell?
the area available for taking up molecules from the environment
it increases as the square of the radius
What is the volume of a spherical cell?
the amount of cytoplasm that is supported by diffusion
it increases as the cube of the radius
What happens as cell size increases?
it becomes harder to supply the cell with the materials needed for growth
for this reason, most bacterial cells are tiny spheres, rods, spirals, or filaments - small enough for molecules to diffuse into the cell’s interior
*figure 24.2 textbook
What is the largest known bacteria?
thiomargarita namibiensis
it lives in oxygen-poor sediments off the coast of southwestrn Africa
its total volume is ~100 million times large than E. coli
*figure 24.1 textbook
How does T. namibiensis “cheat”?
98% of its volume is taken up by a large vacuole, so the metabolically active cytoplasm is restricted to a thin film around the cell’s periphery
thus, the distance through which nutrients move by diffusion is only a few micrometers, as in many other bacteria
