Question

4. ATP is not directly involved in the functioning of a cotransporter

Answer 1

The Davson-Danielli sandwich model of the membrane has been replaced by the fluid mosaic model, in which amphipathic (containing both hydrophobic and hydrophilic sections) proteins are embedded in the phospholipid bilayer. Proteins with related functions often cluster in patches.

How do phospholipids and proteins move about within the membrane?

They move laterally (side to side). The unsaturated hydrocarbon tails of some phospholipids keep membranes fluid at lower temperatures, while cholesterol helps membranes resist changes in fluidity caused by temperature changes. 

Differences in membrane lipid composition, as well as the ability to change lipid composition, are evolutionary adaptations that ensure membrane fluidity.