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Unformatted text preview: M. S. Shell 2009 1/11 last modified 10/27/2010 Intracellular transport in eukaryotes Overview Compartmentalization and inner membranes enables eukaryotic cells to be 1000-10000 times larger than prokaryotes to isolate specialized chemical processes in specific parts of the cell to produce packages (vesicles) of chemical components that can be shuttled around the cell actively Membrane-enclosed organelles take up ~50% of the volume of eukaryotic cells: nucleus genomic function endoplasmic reticulum synthesis of lipids; on the border with the cytosol, synthesis of proteins destined for many organelles and the plasma membrane Golgi apparatus modification, sorting, and packaging of proteins and lipids for specific intracellular destination (akin to a mail sort facility) lysosomes degradation endosomes sorting of endocytosed (engulfed) material by the cell peroxisomes oxidation of toxic species mitochondria , chloroplasts energy conversion Cells contain 10 gG 10 g protein molecules that are constantly being synthesized and degraded Proteins are synthesized in the cytosol , but not all proteins remain there and many must be transported to the appropriate compartment For comparison: transport by diffusion Even without active transport requiring free energy transduction, movement of molecules in the cell is rapid by diffusive motion M. S. Shell 2009 2/11 last modified 10/27/2010 Consider a sea of molecules. Pinpoint one molecule and note its starting position at time 0. Due to thermal motion, the particle on average makes a random jump of length g every G units of time. The jump is random in the radial direction. This is called a random walk . Repeat this process for many jumps n and interrogate the final distance of the particle from its starting point We could imagine doing many such experiments. What would be the expected , , as a function of ? 0 0 0 since the process is spherically symmetric. What about ? Consider the case in going from step n to n+1: y y 2 2 2 Here, , , are the random amounts by which we change the length at one step. Notice that we have the constraint g . Therefore 2 2 2 g Now, we average over all possible (random) trajectories for the same starting point: 2 2 2 g...
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