Transport systems

Why needed?

• All synthesis of protein takes place in the cytoplasm
• Functions such as growth, development, repair, that depend on movement of molecules or organelles to another part requires transport
• Receptors, signaling proteins, enzymes for synthesis of neurotransmitter must be moved to sites of use in distant axon terminals or dendrites

What things are transported?

1.  complex molecules- enzymes, structural elements like tubulin proteins
2. organelles- vesicles and mitochondria

Anterograde transport

Movement from  the cell body/center to the periphery of a cell

Speed ranges from 0.5 mm/day to 400 mm/day

Fast transport

• up to 410 mm/day
• specific for membrane bound organelles or materials
• can transport a variety of molecules, including complex proteins, but must be packaged in vesicle for transport

Intermediate transport

• about 200 mm/day

Slow transport

• less than 6 mm/day
• moves mostly soluble proteins not bound in vesicle
• two kinds carry two different kinds of molecules, one carries mostly alpha and beta tubulin, and protein that makes neurofilaments, another transports actin, the calcium regulator calmodulin, and various enzymes  

1. SCa - slow component of slow transport 0.5 to 3 mm/day
2. SCb - fast component of slow transport 5-6 mm/day

Retrograde transport

Movement of materials from periphery back to cell body/center of the cell

• moves only materials packaged in vesicles
• only one rate so far found, about 200 mm/day

Mechanisms of transport

Requirements

Energy
Transport is energy requiring process involving hydrolysis of ATP, if cell poisoned with metabolic inhibitor (stops ATP production), all transport ceases

 Microtubules

• microtubules serve as structural grid for transport
• pharmacological agents that dissociate tubulin stop transport
• transport takes place along microtubules extracted from cell so long as ATP is present

Microfilaments

• microfilaments also serve as tracks  for  movement of cargo by motor proteins

Specialized motor proteins

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