Lysosome

00:01 Lysosomes and peroxisomes.

00:04 Peroxisomes, I'm not going to mention too much about except to say that they are involved in the oxidative reduction or oxidative processes, bringing about the oxidation of fatty acids and hydrogen peroxide.

00:24 Lysosomes themselves, as you see in the image, they're single membrane-bound organelles.

00:32 Now, there's a little bit of controversy as to their origins.

00:36 And I'm going to describe what has been the traditional role and function and structure of the lysosome in this slide and then modify that story a little bit in the next slide.

00:49 Lysosomes consist of enzymes that are involved in digesting cell components or large molecules.

01:00 Traditionally, they were described as being bound by a membrane and produced in the Golgi complex, and that membrane was to protect the exterior part of the lysosomes or the rest of the cell from these very strong, hydrolytic, destructive enzymes.

01:22 And when these lysosomes come across or are targeting either damaged organelles or aged organelles, the cells are forever getting rid of surplus organelles so lysosomes break them down as you see here.

01:40 Lysosomes also break down large molecules that the cell ingests, and another function of lysosomes is to break down the cell itself during necrosis.

01:56 When the lysosome you see in this image leaves the Golgi complex full of its enzymes, it's used to be called or at least in some of the older traditional histology teaching, it used to be called a primary lysosome.

02:14 When it fused with a structure that it was going to degrade, it was called a secondary lysosome.

02:25 And then finally, when it has completely broken down the products that it's targeted and it's just lying as residual components in the cell either to stay in the cell or gotten rid of by the cell, then they are termed tertiary lysosomes.

02:46 That was the traditional understanding of what lysosomes were.

02:51 Now, there's a little change in what the thinking is and I just want to describe that briefly in this particular slide.

03:01 The theory currently I think that we tend to accept is that lysosomes are produced by the Golgi but they're not necessarily packaged or activated in the way previously thought.

03:16 They're probably made by the Golgi apparatus as inactivated hydrolytic enzymes and also membrane components and they target ingested endosomes that carry either material that the cell has ingested or they will target old organelles, etc., like it was described in the slide before. So rather than being membrane-bound, they're lying loose in the cytoplasm.

03:49 If you look on the right-hand side, you can see a process called endocytosis.

03:55 Endocytosis is when the cell membrane clusters around products on the cell surface and ingest some to form this early endosome. Now, an early endosome is a vesicle within the cell that has taken in those external molecules from outside the cell, it starts to recycle some of the cell membrane components back to the cell surface, but then as it's finished doing that, it then proceeds to go deeper into the cell and when it actually is targeted by the lysosome enzymes and they're activated, and other membrane components made by the Golgi complex modify the endosome membrane so that these hydrolytic enzymes cannot destruct the cell itself, then it's termed a late endosome, or more or less like the old terminology of a secondary lysosome.

05:03 And then as you see on the top part of the image, the digestion process continues until the cell either keeps that digested material as a residual body or gets rid of it outside the cell.

05:18 So the only real difference really is whether or not the lysosome itself is a vesicle membrane- bound component of these enzymes or whether the enzymes are now thought to be just free within the cytoplasm in an inactivated form, but then when they're targeting ingested endosomes, then they become activated and seal that membrane so that the rest of the cell isn't destructed by the hydrolytic enzymes inside.

05:48 And again, if you look carefully in this slide, you'll see these large circular structures at the top, they're lysosomes.