What are the difference types of transport across membranes?

Firstly we need to understand the structure of membranes. Cell membranes are made up of phospholipids. These are molecules with a hydrophilic head (it “likes water”) and hydrophobic tail (which “doesn’t like water”). The membrane has two layers of phospholipids, with all the heads on the outside, and all the tails pointing in towards the middle. Dotted throughout the membrane are channel proteins, which are like tunnels through the membrane, and carrier proteins, which also transport substances across the membrane. Simple diffusion is how some molecules, like oxygen and carbon dioxide, pass though the membrane. They go from a higher concentration on one side, to a lower concentration on the other side. Facilitated diffusion is how molecules such as sodium ions pass across the membrane. Facilitated diffusion is still from high concentration to low concentration, but the molecules or ions pass though channel proteins, rather than squeezing between the phospholipid molecules. Active transport is from low concentration to high- we say they are moving up a concentration gradient. This requires the use of ATP to provide energy. Ions are transported from the soil into plant roots by active transport, and from the guy into the cells lining the gut. Osmosis is specifically how water moves across membranes. Water moves from high water potential to low water potential. Water potential is lower if there are more ions dissolved in it, so pure water has a high water potential. The final way substances can be transported is by co-transport. An example of this is in the intestine, where sodium ions are pumped out of an epithelial cell, by active transport, into the blood. This means that the concentration of sodium ions is higher in the intestine lumen than in the epithelial cell. Sodium ions therefore diffuse (facilitated diffusion) into the epithelial cell. The energy from this diffusion is used to co-transport glucose into the epithelial cell.