Water and oxygen can move across the membrane of the alveoli in the lungs. How would their paths through the alveoli cell membrane differ? Also, in the motor neuron, substances have to move across the membrane to ensure the correct functioning of the nervous system. How is the movement of Na+/K+ (charged ions) different from the movement of neurotransmitters (large protein molecules) across the cell membrane?

In the lungs, both oxygen and water are transported passively across membranes. In the case of oxygen, the concentration of oxygen in the air is greater than the concentration of oxygen in the deoxygenated bloodstream, so oxygen will move from the air into the bloodstream via passive diffusion. Once in the bloodstream, oxygen can bind to hemoglobin in red blood cells, thereby keeping the concentration of oxygen in the blood low and promoting further diffusion from the inhaled air. In the course of metabolizing oxygen in your body, your cells produce carbon dioxide and water. During breathing, the water (and carbon dioxide) move from a high-concentration region (the bloodstream) into a low-concentration region (the alveolus) and are exhaled.
In neurons, ions are moved across the cell membrane through the opening channels. Some channels are voltage gated and open when the membrane reaches a depolarized state (voltage gated Na channel), whereas others are ligand gated and open with the binding of neurotransmitter (GABA-R). Neurotransmitters are released from neurons via vesicle mediated exocytosis. This involves fusion of prepackaged membrane-bound vesicles with the plasma membrane and release of neurotransmitters into the post-synapse.