How are smaller molecules and ions transported across the membrane? a Diffusion b ATP c Transport proteins d Endocytosis and exocytosis

## Step 1
The question is asking about the mechanisms by which smaller molecules and ions across the cell membrane. The cell membrane is a selectively permeable barrier that regulates the movement of substances in and out of the cell.

## Step 2
The options provided are:
a) Diffusion
b) ATP
c) Transport proteins
d) Endocytosis and exocytosis

## Step 3
Diffusion is a process where molecules move from an area of high concentration to an area of low concentration. While this process does involve the movement of molecules across the membrane, it does not specifically address the transport of smaller molecules and ions.

## Step 4
ATP (Adenosine Triphosphate) is the primary energy currency of the cell. It provides the energy needed for many cellular processes, including active transport. However, ATP itself is not mechanism.

## Step 5
Transport proteins are integral membrane proteins movement of ions and small molecules across the cell membrane. They can either facilitate passive transport (like facilitated diffusion) or active transport, which requires energy.

## Step 6
Endocytosis and exocytosis are processes by which the cell engulfs material from the outside environment (endocytosis) or expels material from the inside (exocytosis). These processes are more about the movement of large molecules or particles rather than smaller molecules and ions.

## Step 7
Given these explanations, the most accurate answer is "c) Transport proteins". These proteins facilitate the movement of smaller molecules and ions across the cell membrane, either through passive or active transport mechanisms.