endocytosis - envelops food particles 

• substances required for growth are sometimes too large to cross the bilayer
• 3 main types
• phagocytosis - enveloping particulate, organic matter
• pinocytosis - enveloping liquid
• receptor-mediated endocytosis - transfer of specific molecules
  • only food particles that fit attach to the receptor
  • clathrin - protein that coats the inner pit
  • each pit folds inward to form a vesicle
• low-density lipoprotein - brings cholestrol into the cell to be used in the membranes

exocytosis - reverse of endocytosis 

• discharges material from vesicles
• used by plants to send material for cell wall construction through the membrane
• used by animals to secrete hormones, neurotransmitters, enzymes, etc
• protists use contractile vacuoles for exocytosis

active transport - moves substance against the concentration gradient 

• powered by ATP
• uses selective protein channels like facilitated diffusion
• makes cells independent from environmental conditions

sodium-potassium pump - moves sodium and potassium ions across the membrane 

1. 3 sodium ions bind to protein on the cytoplasm side, causing the protein to change shape
2. protein turns ATP into ADP (adenine diphosphate) and a phosphate
3. protein changes shape again, moves the 3 sodium to the exterior
4. 2 potassium ions bond to protein once 3 sodium ions leave
5. protein changes shape again, releasing phosphate group
6. protein goes back to original shape, release potassium into the cell, attract new sodium

• process found in all animal cells

coupled transport - uses energy from 1 molecule's gradient to move another molecule 

• energy released from a molecule moving w/ its gradient is used to help move another molecule against its gradient
• sodium moves back into the cell along its gradient to move glucose into the cell against its gradient
• symport - both molecules moving the same direction through a membrane
• countertransport - molecules move in opposite directions through a membrane; molecules attach to the protein known as an antiport in these situations