复习题(11)(4)

A third function for lysosomes is to handle the products of receptor-mediated endocytosis such as the receptor, ligand and associated membrane. In this case, the early coalescence of vesicles bringing in the receptor and ligand produces an endosome. Then, the introduction of lysosomal enzymes and the lower pH causes release, and degradation of the contents. This can be used for recycling of the receptor and other membrane components.

So lysosomes carry hydrolases that degrade nucleotides, proteins, lipids, phospholipids, and also remove carbohydrate, sulfate, or phosphate groups from molecules. The hydrolases are active at an acid pH which is fortunate because if they leak out of the lysosome, they are not likely to do damage (at pH 7.2) unless the cell has become acidic. A Hydrogen ion ATPase is found in the membrane of lysosomes to acidify the environment.

7 How does the Golgi Complex sort lysosomal enzymes?

The Golgi complex sorts the lysosomal enzyme in the Trans region. It is received from the rough endoplasmic reticulum (RER in this cartoon) in the cis region. There it has a phosphate radical attached to the mannose residue. This mannose-6 phosphate forms a sorting signal that moves through the cisternae to the trans region where it binds to a specific receptor. After it binds to the receptor, it begins to bud and a \It moves away to fuse with a developing lysosome (such as the vacuoles seen in the previous figure). This lysosome contains a hydrogen ion pump on its surface. The pump works to acidify the environment inside the lysosome. This removes the phosphate and dissociates the hydrolase from the receptor. The receptor is then recycled back to the Golgi complex

8 How do you prove membranes are fluid?

Membrane proteins (like lipids) rotate about an axis perpendicular to the plane of the bilayer. However, they may also move laterally. The following experiments show that not only do membrane proteins move, they can rearrange. The first experiment was done on fused cells. One group of cells came from a mouse line and the other from a human cell line. The investigators obtained antibodies to proteins in the membrane and conjugated these antibodies to fluorescein (green) or rhodamine (red) fluorescent compounds. The cells were then fused, forming a \(note the two nuclei). Then, the two sets of antibodies were applied. Initially the cells were labeled half red and half green. However, after 40 min at 37 C, the labels mixed and the cells showed mixing of the two dyes. In the next experiment, the label on the membrane is \label on the protein and results in a clear area (area of no label) in a completely labeled cell.Then, one can watch the recovery in the bleached area as the proteins + label diffuse into the irradiated patch.

9 What is Endoplasmic reticulum and its functions?

Endoplasmic reticulum is a network of tubules, vesicles and sacs that are interconnected. They may serve specialized functions in the cell including protein synthesis, sequestration of calcium, production of steroids, storage and production of glycogen, and insertion of membrane proteins.

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