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Lecture 10

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Biology 1002B
Tom Haffie

Lecture 10: Evolution of Eukaryotes The Paradox  All morphologically complex life is eukaryotic  All eukaryotes share common complex traits:  Nucleus, trafficking, cytoskeleton, sex, phagocytosis, organelles, differentiation into different tissue types  Prokaryotes show no tendency to develop these  So why don’t they if evolution is gradual?  Seems like all of a sudden, eukaryotes got the ability that allowed for multicellular complexity What drove the evolution of Eukaryotes?  Oxygen is the key  The earliest forms of life were anaerobic bacteria  Metabolic process used to get ATP was fermentation  Fueled the ability to keep glycolysis going  Don’t get very much ATP -> break down glucose, but you don’t harness all the free energy that is in a molecule of glucose -> starved for ATP  About 2.2 billion years ago, cyanobacteria developed  Prokaryotic cell structure can do oxygenic photosynthesis  Split water to oxygen; ability is what led to oxygen accumulating in the atmosphere  Cyanobacteria have PS2 and PS1 -> only group of prokaryotic cells that have a photosynthetic structure similar to the chloroplast  Have an oxygen evolving complex  Led to the development of bacteria that can undergo aerobic respiration  Oxidative phosphorylation  Aerobic respiration could only evolve after there was enough oxygen in the atmosphere  Lots of ATP from oxidative phosphorylation compared to glycolysis / fermentation Oxidative Phosphorylation in Bacteria  Bacteria are small compared to eukaryotic cells  Many centres of oxidative phosphorylation in bacteria (blue spots)  Oxidative phosphorylation: electron transport, chemiosmotic ATP synthesis from breakdown of glucose, occurs on plasma membrane  In many bacteria, during electron transport, the protons are pumped outside of the cell and harnessed for ATP synthesis through chemiosmosis when the protons come back into the cell  If you only have centres of phosphorylated oxidation on the membrane, if you try to get bigger, you need more of them because the volume of the cell goes up  More proteins going on in the cell when it’s bigger = more energy needed to support that  Surface area increases as a function of the radius squared, but the volume increases as a function of the radius cubed: as cell gets bigger, the volume increases much faster than the surface area does  Becomes a problem if plasma membrane surface area is where electron transport and oxidative phosphorylation take place  This limits the size of a bacterium; there comes a point where you can’t fill a plasma membrane with enough centres of oxidative phosphorylation to support the ever-increasing volume Eukaryotes have (much) Bigger Genomes  Have more energy  Bacteria have a high plasma membrane surface area to volume ratio  Eukaryotic cells have low plasma membrane surface area to volume ratio  Plasma membrane isn’t the site of ox phos. -> mitochondria is  Each cell has many mitochondria, and Each mitochondria has many ox phos centres  Membrane area internalized in the cell that can be used to make ATP is very high  Can get complex because it can support a larger genome because it has lots of energy  Replicating DNA genome doesn’t take much energy (2%), protein synthesis, however, does (75%) -> constrains the size of a prokaryotic genome because it doesn’t have as much ATP to invest to make novel proteins  A typical bacterium has 13,000 ribosomes, a typical eukaryotic cell has 10,000 times more  Capacity to generate proteins in a eukaryotic is gigantic  Prokaryotic genomes are small because they don’t have the energy to devote to have more proteins being made  Genome size varies tremendously among eukaryotes because they have lots of ATP to duplicate genome, etc. -> not constrained to keep the genome very small Endosymbiosis  Eukaryotes are probably derived from an ancestral bacterium  Endomembrane system must be produced -> defining characteristic of eukaryote  Probably derived by infolding of plasma membrane  Endoplasmic reticulum is connected to the nuclear envelope  Nucleus seems to be an advantage to separating the genomic information from the rest of the cell  Nuclear envelope = ability to tightly regulate transcription and replication of DNA separate from the rest of the cell -> tightly control a nuance transcription in ways that aren’t possible in a bacteria  Specialized transcription factors; tightly regulated transcription & translation separated in space that gives a level of sophi
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