Both plant and animal cells have a mitochondria! Plant cells have a cell wall, a cell membrane and a chloroplast. Animal cells have a lysosome, where as plant cells usually don't have this present.
If the cell membrane were not selectively permeable, then unwanted substances would transfer through instead of being blocked out. Think of it like this: you have a sifter with various sizes of spice in it. You only want the finer grains of spice in your food, so when you shake the sifter, the larger pieces of spice don't fit through the small holes that only allow fine grains. Then you are left with the small spices in your food and the large ones stuck in the sifter.
Hopefully this helps and I wasn't too confusing :)
A single muscle cell is called a myocyte. Inside are sliding filaments called sarcomeres.
