The cell membrane. It is described as semipermeable
The cell wall in plants and the cell membrane in animals and bacteria.
it is the cell wall ! :)
The cell membrane is reposnible for allowing what substances can enter and leave a cell, this happens by different methods of transport for instance "active transport" in plants. The phospholipid bilayer containing cholesterol, proteins and carbohydrates, also helps in allowing which substances to pass.
The cell part that allows only certain substances to diffuse into and out of the cell is called the semi permeable membrane. This allows good substances to enter the cell.
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Cell membrane
The Cell Membrane protects the cell and regulates what substances enter and leave it. It is like the "gate keeper"
You could be referring to the cell membrane or carrier proteins. Both allow certain substances to enter cells.
Ask a college professor, not me.
The answer is nucleus :)
The Nucleus
I think you are talking about the nucleus, which is the "brain" of the cell. It controls everything that cell does.