Movement of Large Molecules Across Membranes: Exocytosis

00:01 So, that's things coming into the cell.

00:03 Let's talk about moving things out of the cell or exocytosis.

00:07 Clearly, it's not a one-way streak.

00:10 We also need to move things out.

00:11 So, what we're shown here on the left-hand side of the line showing exocytosis is the Golgi complex packaging up a set of proteins in a membrane that are going to be destined for export to the outside world.

00:27 Those are put into vesicles, translocated to the membrane, there, and then, they are released into the outside space.

00:37 What is shown on that panel is a constitutive secretion.

00:42 This happens constantly.

00:45 So, cells like the liver cells will make albumin constantly, and package it up and secrete it into the bloodstream.

00:53 Similarly, plasma cells, mature B cells, will constantly package up and secrete their antibodies, so you can have constitutive secretion.

01:02 On the right-hand side of the line here is actually regulated secretion.

01:06 So, that occurs only after a signal.

01:10 And this is done, for example, in neuronal synapsis, where neuron is gonna signal to the next neuron.

01:16 It's got a packaged collection of material that it will then release only when the appropriate stimulus comes across.

01:24 Same thing happens in the beta islets of the pancreas.

01:27 There, you will pre-package insulin and release it after the appropriate signal.

01:33 So, it's shown at the bottom, it's that we're packaging up in a golgi apparatus some protein.

01:39 It makes a little vesicle, it makes its way to the cell surface, and then when we signal appropriately, it's usually through a membrane receptor interaction that causes a calcium influx, we will get the vesicle to release its contents into the extracellular space, and that's often running. So, we can have constitutive or regulated exocytosis.

02:03 What is this used for? Well, for protein export.

02:06 We've talked about this specific example of antibody from B cells, plasma cells, from albumin, from hepatocytes.

02:13 Various peptide hormones, so again, insulin from the beta islet is a good one.

02:18 That's regulated. Pancreatic enzymes too.

02:21 We only release pancreatic enzymes when there's something to digest.

02:25 So, that is a regulated secretion as well.

02:28 We can have it as presynaptic vesicles for the exocytosis, and cytotoxic T cells have pre-packaged molecules, enzymes, and pore-forming proteins that are going to be directed to a target once they have interacted with it. So, this is exocytosis.