1. Receptors (protein molecules) recognize which substances can pass through. Thus, the membrane is called selectively permeable.
2. Diffusion is the passing of objects across a semipermeable membrane due to concentration differences. Substances are said to move "down the concentration gradient" during diffusion. This means that the substances move from an area of higher concentration to an area of lower concentration.
Diffusion of water molecules is called osmosis.
3. Hypotonic substances have lower osmotic pressure than other substances.
So, pure water is hypotonic to salt water.
Isotonic means that two substances have the same osmotic pressure. Isotonic substances have reached equilibrium in which water is moving at equal rates in each direction across the membrane.
4. Facilitated diffusion is a form of passive transport across the membrane. It does not require energy. However, it utilizes channel and carrier proteins to allow substances that are too big to fit between the gaps of the phospholipids that make up the cell membrane to pass through it.
5. Passive transport does not require energy whereas active transport does. The energy used is in the form of ATP. Active transport requires energy because it pushes substances against the concentration gradient (from low to high concentrations).
6. See answer number one.
7. Simple diffusion is a form of passive transport that does not require a carrier protein. Facilitated diffusion is a form of passive transport that does require a carrier protein.
8. When the process uses ATP as the form of energy it is called primary active transport. When the process uses an electrochemical gradient the it is classified as secondary.
9. It functions in the active transport of sodium and potassium against their concentration gradients.
10. Phagocytosis is cell eating. Pinocytosis is cell drinking. Both are forms of endocytosis. The ions are pumped to opposite sides of the membrane. This builds an electrical gradient that can be used to drive other transport processes.
11. Cell walls give the cell an extra layer of support and protection. Cell adhesion molecules are proteins found on the outside of a cell membrane that helps the cell stick to other cells (thus the term "adhesion").
12. Tight junctions, gap junctions, adherens