Taxonomy used to have two groups of vertebrates and invertebrates. But invertebrates are far more diverse when it comes to the phylogenetic tree. Vertebrates all share same characteristic which is a back bone made of vertebrae, but invertebrates don’t share a common anatomical structure that all have; they are radically different in structure so they can’t be grouped in a single group. You have Arthropoda, which all have jointed limbs and segmented bodies and molt their cuticles. There’s Cnidaria, which most have tentacles and nematocysts and are radially symmetrical. And Mollusca, which have a muscular foot and a part of their body called a mantle. All these groups are invertebrates, but there’s nothing they share in common WITHOUT referring to the fact that they don’t have vertebrae. So basically that’s just an example why they can’t be grouped into one category; they all have different characteristics and are placed in different areas of the animal kingdom’s phylogenetic tree. Hope this helps
Some molecules can move down their concentration gradients by crossing to the lipid portion of the membrane directly,while others must pass through membrane protein s in a process called Facilitated Diffusion
