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the outer edge of the cytoplasm is the cell membrane.
The outer edge means the furthest most point from the center. The outer edge is usually the end of the object before the dropping off point.
The leading edge
Radius is from the center of a circle to the outer edge.
Diameter is from the outer edge to the opposite outer edge, i.e., through the center.
The diameter has twice the length of the radius.
The out edge of a cup is called the 'Rim'.