active transport across membranes

• Active transport is the movement of particles against the concentration gradient (from an area of low to high concentration), through a partially permeable membrane with carrier proteins, using ATP (energy).
• In active transport, the substrate binds to receptor sites on the carrier protein, the third phosphate head binds to the carrier cell, and the breaking of ATP into ADP + Pi releases energy and causes the protein to change shape and open on the other side of the membrane.
• The substrate is released and as the Pi is released, the protein retains its original shape.
• Villi are folds in the membrane of the small intestine, they have a large surface area to increase the volume of nutrients absorbed into the blood, a very thin cell membrane to shorten the diffusion pathway, and a good blood supply.
• Each villus has microvilli to further increase the surface area, they are part of one cell.
• The process of co-transport is where sodium ions are removed from epithelial cells in the intestine to the blood via active transport to make a low sodium ion concentration inside the cell.
• The high concentration of sodium ions in the lumen can now bind to the co-transport protein, a glucose molecule now has to bind to the other receptor on the co-transport protein and both molecules move into the cell because they change the shape of the protein.
• The sodium ion is immediately removed via active transport and the glucose molecule is also removed into the blood via facilitated diffusion and the process repeats.
• Endocytosis is things entering the cell.
• Exocytosis is things exiting the cell, it's endocytosis in reverse.
• Most vesicles come from the Golgi apparatus.