L In the 1800s, scientists studied how fat-soluble substances and water-soluble substances interact with cell membranes. Their studies provided
evidence that cell membranes are structured to perform certain functions. What function did these studies suggest that cell membranes
perform?
carry materials throughout the interior of the cell
control the cell's activities
control which substances enter the cell
form proteins from fat-soluble substances

control which substances enter the cell

Explanation:

The fat soluble non-polar  (hydrophobic)and the polar  water soluble( Hydrophilic) simply describe the bi-layer structure of the cell membrane. The phospholipid bilayer is in such a way that the fat soluble hydrophobic tails point inwards towards one another,with the water soluble hydrophobic points outwards.

This arrangement ensure that polar molecules can only pass through the hyrophilic(water loving head),which points outwards while non-polar could only pass through the hydrophobic tails,which form the inner layer.

Thus the inner later only allow fat soluble substances passages,while the outer soluble layer allows only polar substances.

Therefore based on the arrangement of 1800,the cell membranes controls what enters  the cell.

the answer is:

carbon dioxide