Describe how increasing the size of a cell will affect the ability of diffusion to transport materials into and out of a cell.
Diffusion of substances into and out of a cell occurs across the cell surface.
If the size of a cell increases it will be more difficult (all other things being equal) for diffusion to transport materials out of the cell. The reason for this is because of the change in the surface to area ratio of the cell.
Diffusion must occur, as you say, across the cell's surface. Therefore, whatever is going to be transported out of the cell must contact the cell's surface. If the cell gets bigger, there will be less surface area compared to the area within the cell. Therefore, less of the cell's interior will be able to come in contact with the cell's surface. If less of it comes in contact, less of it can cross the cell membrane.
So -- the greater the size of the cell, the lower the surface to area ratio and the harder it is for anything to diffuse out of the cell.
Transport of molecules across the membrane may be active or passive. Both active and passive transport involve selectivity. Some molecules cross the membrane more easily than others. A carrier molecule may also be involved in both systems.
The passage of molecules through the membrane from a high concentration to a low concentration region is called passive transport. Transfer of molecules takes place along the concentration gradient, and no energy is required.
In active transport molecules usually move from regions of low concentration, to regions of high concentration, i.e. against the concentration gradient. It requires energy.