Topic 11: Cells and Cellular Transport Flashcards
Where did cells come from
Where did cells come from? How did atoms give rise to cells billions of years ago, when the earth was hot
and the oxygen in the atmosphere was a tiny fraction of what it is today? Before cells, the first steps towards
life were chemical reactions that formed the precursors to the macromolecules found in cells. Simple one
celled organisms similar to bacteria (prokaryotes) appeared ~600,000 million years later, and after another
billion years eukaryotic organisms appeared, using oxygen and CO2 to synthesize ATP. Multicellular
organisms followed and today we have a diversity of prokaryotic and eukaryotic life that is adapted to the
current conditions on earth.
Where did macromolecules come from
The condiCons of the early earth did not support life, but were capable of supporCng the chemical reacCons needed for synthesizing
macromolecules. An important concept of evoluCon is that organisms change their environment as they adapt to it. Slowly, and step by step,
macromolecules organized in ways that made them resilient to the environment, and slowly changed the environment, allowing yet more complex
macromolecules to be synthesized.
RNA before DNA
And before there was DNA, there was RNA. Importantly, RNA that could auto-catalyze its duplication- acting as a nucleic acid and as a enzyme. This
was a huge step, as accurate and efficient replication of the information in RNA (and DNA) is essential for life.
Evolution of multicellular organisms
Budding yeast is a unicellular eukaryote. Scientists used “directed evolution” in a controlled laboratory experiment to evolve multicellular yeast.
First they selected for cell size, using gravity to separate small cells from larger cells. As a result of random mutations, cells that adhere to each
other (like volvox) remained associated and again could be selected for using gravity as groups of cells have more mass than individual cells.
Within these colonies of cells, some become specialized for reproduction- by dying early they release cells that seed new colonies. Mutations
continue to accumulate at random in the population, and in the presence of a strong selective event (bottleneck), adaptations that benefit
collective living select for some multicellular systems while others perish. This controlled experiment demonstrates that the evolutionary steps to
multicellular organisms that were predicted from Volvox can occur. This also demonstrates that multicellular organisms take advantage of
specialized cell types that have specific functions that benefit the organism.
Why are cells small
Cells roughly approximate a sphere. Recall that the surface area of a sphere is 4πr2, while the formula for its volume is 4πr3/3. As the radius of a
cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius. Therefore, as a cell increases
in size, its surface area-to- volume ratio decreases. This is not infinitely scalable as surface area is key to the exchange of energy with the
environment. For this reason, unicellular organisms are small. And multicellular organisms are composed of small cells.
Cells are tiny ecosystems
As we discussed, cells evolved over billions of years. Every cell is a tiny ecosystem that is adapted for its environment and lifestyle (e.g. solo or
multicellular). And the building materials of cells reflect this.
Nucleus
The interior membranes of cells form compartments (why is this the case). The nucleus is a membrane compartment that holds the DNA (called
chromatin in eukaryotes), and the nucleolus- a specialized region of DNA where the RNA of ribosomes is synthesized. The nucleoplasm is the
aqueous environment within the nucleus. it is not the same as the cytoplasm as it supports different chemical reactions. For example, chemical
reactions needed for transcription occur in the nucleoplasm, while chemical reactions needed for translation occur in the cytoplasm.
Golgi Body
The nucleus, rough endoplasmic reticulum and Golgi are connected into a protein synthesis and transport system. mRNA from the
nucleus is translated by ribosomes on the rough ER, and the polypeptide that is synthesized enters the ER and are modified. Vesicles bud from the
ER and fuse to the Golgi, where proteins are further modified. A transport vesicle delivers the protein to the plasma membrane, where
hydrophobic residues are removed, releasing the protein to the environment. This is how insulin is processed.
Actin microfilaments
Remember that enzymes are energized by the release of P from ATP. This is also true for actin monomers, which form polymers when ATP is bound
in the active site. The polymer is unstable when ATP is converted to ADP.
So grow and shrink using ATP
Microtubules
What’s made from them
Microtubules grow an shrink in the same way, but in this case the energy currency is GTP, not ATP. Microtubules have a larger diameter than
microfilaments. For this reason a microtubule that is 1 micron (1 x 10-6 meter) in length is stiffer than a microfilament 1 micron in length. As you
will see later, microtubules are specialized to transform chemical energy into mechanical energy during the process of mitosis, and the force that is
applied is modulated by the change in The nucleus, rough endoplasmic reticulum and Golgi are connected into a protein synthesis and transport
system length of the microtubule.
Cilia and Flagella
Studying living things
Cells examined with electron microscopy are dead. If they were not dead before, they are dead after being bombarded with electrons! In fact,
biologists struggled to find ways of studying live cells. And in particular they wanted to study the molecules that do all the important stuff in cells.
The solution came when three scientists used jellyfish GFP to label proteins with a light emitting “tag” that could be detected in living cells. They
won the Nobel prize in Chemistry in 2007 for this breakthrough.
Microscopic color pallet
By introducing mutations into the jellyfish GFP, scientists created a color palette ranging from blue to yellow. So you can, if you have enough
cameras (I have two), follow the travels of two different molecules within a living cell.
Using FPs from bioluminescent organisms like jellyfish and corals etc, we now have FPs that span
the visible range of light and can be used in any cell type.
Analysis of the population says
70 mins to divide a cell
Needs to be population, not just one cell
