Cell adhesion is fundamental for multicellular organisms. Firstly, it ensures that cells remain anchored to one another, maintaining the structural integrity of tissues. For instance, the heart relies on proper cell adhesion to ensure it contracts cohesively. Secondly, cell adhesion aids in cellular communication, ensuring cells can transmit signals efficiently across tissues. It's also crucial during development, as cells need to migrate, differentiate, and organise into specific patterns to form organs and systems. Furthermore, cell adhesion plays roles in wound healing, immune responses, and can even inhibit cancer cells from spreading. In essence, without cell adhesion, multicellular life as we know it wouldn't be possible.

When gated ion channels malfunction, it can severely disrupt neuronal signalling, leading to various neurological disorders. For example, mutations in the genes encoding for sodium or potassium channels might cause these channels to remain open for longer durations or not open at all. This can disrupt the generation and propagation of action potentials. Conditions such as epilepsy, migraines, or certain ataxias can result from these malfunctions. Moreover, defects in ligand-gated channels can affect synaptic transmission, potentially causing conditions like myasthenia gravis. Thus, the proper functioning of gated ion channels is crucial for normal neural activity and communication.

Neurons maintain their ion gradients through the combined action of ion channels and active transporters. After an action potential, the distribution of ions across the neuronal membrane changes. The sodium-potassium pump (a type of active transporter) plays a pivotal role in restoring and maintaining these gradients. This pump actively transports three sodium ions out of the neuron and two potassium ions into the neuron against their respective gradients. This action, powered by ATP, ensures that the concentrations of these ions are reset after each action potential, allowing the neuron to be ready for subsequent signalling events. The constant operation of these pumps ensures that the neuron maintains its gradients even after multiple action potentials.

Vesicles ensure substance specificity primarily through receptor-mediated endocytosis. In this process, the cell membrane contains specific receptors that bind to particular molecules. When these molecules are present in the extracellular environment, they bind to their respective receptors, causing the membrane to invaginate around them. This invagination eventually pinches off to form a vesicle containing the specific molecules. Since the vesicle formation is triggered by the binding of specific molecules to their receptors, this ensures that only certain substances are internalised, making receptor-mediated endocytosis a highly selective process.

The lipid composition of the membrane greatly influences its function. For instance, membranes with a higher concentration of unsaturated fatty acids remain more fluid at lower temperatures due to the 'kinks' in the fatty acid tails. This fluid nature is essential for processes like protein diffusion, vesicle formation, and cell signalling. In contrast, membranes with more saturated fatty acids tend to be less fluid and more rigid, potentially hindering some cellular processes. Moreover, cholesterol interspersed within the bilayer modulates fluidity, preventing fatty acid chains from packing too closely in higher temperatures and maintaining some fluidity in colder temperatures, thereby ensuring the membrane's functionality across different conditions.