Advanced Biology: Human Anatomy help!!?

1) Small molecules (polar or non polar but NEVER IONIZED) enter the cell through the cellular membrane by means of diffusion. Diffusion is movement of matter from a place where matter is plentiful to a place where there is less of it. Kinda like water spilling. The molecules are small enough to pass through the membrane with little or no trouble. Example: water, oxygen, carbon dioxide molecules.

2) Larger molecules like sugars and amino acids enter the cell the same way as the smaller ones do, by diffusion. The only difference is, they are too big to pass through the membrane. The membrane has specialized transport proteins that allow larger molecules to enter. It's kinda like people who can enter a house through doors but cars can't. But if we open the garage doors it'll technically be in there.

The process is called facilitated diffusion.

3) Very large molecules are too big even for the transport proteins. The cell actually swallows them whole. The membrane twists around the molecule and takes it all inside. The process is called endocytosis (or phagocytosis).

4. All ions are unable to pass through the membrane as they are electrically charged. They used specialised proteins called "pumps" to pump the ions in or out of the cell. However, unlike the previous types of transport, they have to spend energy to do this. They spend energy molecules called ATP. The most prominent pump in our bodies' membranes is the Sodium-Potassium pump. The pump is always sending Sodium ions outside the membrane and the Potassium ions inside. As a result, Sodium ions are most concentrated outside the cell.