Transport Across Membranes
The membranes of cells are called selectively permeable. This simply means that membranes select or allow only certain molecules to pass through them. Some molecules cross the membrane without the use of cellular energy. This is called passive transport. Other molecules cross the membrane using cellular energy. This is called active transport. All molecules that enter or exit the cell use a variation of either passive or active transport.

Passive Transport
Passive transport occurs due to diffusion. Diffusion is when molecules move from an area of high concentration to an area of low concentration. Diffusion happens in liquids or gases because that atoms that make them up are in constant motion due to heat energy. This random motion of atoms causes them to mix and spread out. When the concentration of a substance varies across a distance, it is said to have a concentration gradient. A substance moves down a concentration gradient when it moves from an area of high concentration to an area of low concentration. Passive transport can only move substances down a concentration gradient. Because of this, passive transport stops when there is an equal amount of the substance on each side of the membrane. (equilibrium)

Passive transport can occur in three general ways. One way is for molecules to diffuse across the lipids layers of the membrane. Only very small molecules or small uncharged molecules can cross the membrane in this manner. Oxygen, carbon dioxide, urea, and alcohol are small enough to diffuse across the lipid layers. Water due to its small size can also diffuse across the lipid layers despite its having an uneven charge. When water diffuses across a membrane it is called osmosis.

The two other means of passive transport involve two types of proteins embedded in the cell membrane. Facilitated diffusion is the name given this process. Facilitated diffusion is similar to simple diffusion due its not requiring cellular energy and transport is again down the concentration gradient. One type of facilitated diffusion uses channel proteins. These proteins are quite specific for the type of molecule they let across the membrane, such as inorganic ions. The size, shape, and charge of the protein must be correct for them to pass through the channel. Some channel proteins have gates for controlling permeability that open in response to an impulse like when nerve cells quickly release Na+ ions (sodium ions).

Carrier proteins are used in the second type of facilitated diffusion. These proteins also only move molecules down a concentration gradient, so no cellular energy is needed. Carrier proteins only transport specific molecules because of how they work. Fist the molecule must have the right shape and charge to bond to the protein. Due to this binding, the protein changes shape and move the molecule across the membrane. The completion of this shift in shape then causes the molecule to be released. With the release of the molecule the carrier protein returns to its original shape. Sugars like glucose, amino acids and nucleotides are facilitated across membranes by carrier proteins.

Active Transport
Some molecules needed by cells cannot passively crossed a membrane. This is because the molecules will have to move up a concentration gradient. Moving molecules from an area of low concentration to an area of high concentration requires cellular energy and is called active transport. Like facilitated diffusion, active transport uses transport proteins that only move a specific kind of molecule. This is because the molecule must first bind to the protein. Then the protein receives a phosphate from an ATP molecule. The energy received through this transfer changes the shape of the protein and moves the substance through the membrane. When the molecule and phosphate are released from the protein, the protein returns to its original shape. Sometimes two molecules are transported across a membrane in opposite directions using the energy from the same ATP molecule. The first molecule is moved when the phosphate attaches to the protein and the second is moved when the phosphate is released. Needed minerals are moved up a concentration gradient into the root cells of plants by active transport.

Sometimes a cell needs to move substances that are too large for transport proteins to handle. Large amounts of a substance can be brought into the cell by endocytosis. In this process, the plasma membrane folds inward to surround the substance in a pouch of membrane. The plasma membrane then pinches off the newly formed package, releasing it into the cytoplasm. Large amounts of a substance can be put out of a cell by exocytosis. In this process, the substance is first enclosed in a membrane package. This package then moves to and joins with the cell's membrane, putting the substance outside the cell. Nerve cells release and muscle cells take in neurotransmitters by exocytosis and endocytosis.

Copy the following items and fill in the blanks.

1. A membrane's ability to only allow certain molecules to pass through it is called .......... .
2. ............... transport is powered by heat energy.
3. ............... is the movement of a substance from an area of high concentration to an area of low concentration.
4. If the concentration of a substance varies from one place to another, it is said to have a ............... .
5. Active transport and all diffusion ............... when the molecules are equally spread out which is called equilibrium.
6. Small, uncharged, molecules can ............... across the lipid layers of a membrane.
7. The diffusion of ............... across a membrane is called osmosis.
8. In ............... , substances move cross a membrane with the help of embedded proteins that don't use cellular energy.
9. Facilitated diffusion only occurs ............... a concentration gradient.
10. ............... only let substances that are the correct size, shape, and charge through and across a membrane.
11. Molecules must have the correct ............... and ............... to bind with a carrier protein.
12. The carrier proteins of facilitated diffusion don't use ............... when they move substances by changing shape.
13. Binding, changing shape, and releasing are the three steps ............... do as they funciton.
14. ............... requires the use of cellular energy to move a substance across a membrane.
15. Moving molecules from an area of low concentration to an area of high concentration requires ............... .
16. In active transport, membrane proteins receive energy when a ............... is attached to them from ATP.
17. Binding, receiving a phosphate, changing shape, and releasing are the steps used by proteins in ............... .
18. ............... and ............... are how cells move large amounts of substances across their membranes.
19. In ............... , the cell's membrane encloses substances outside the cell and then brings them in.
20. In .............. , a membrane package inside the cell joins with the cell's membrane to put substances out of the cell.