PASSIVE TRANSPORT

PASSIVE TRANSPORT

Naturally, molecules move from an area of high concentration to low concentration without the need of energy. This type of movement is passive transport. Molecules with strong electrical charges such as ions cannot simply diffuse across the cell membrane. Irrespective of their size, their charge prevents them from moving freely across the cell membrane. Other molecules such as proteins, starch and sugar are simply too large to diffuse across the membrane. Sometimes, some of these large molecules are transported across the cell membranes by carrier proteins; this does not require energy and as a result is a form of passive transport. There are three types of passive transport;

Simple diffusion: Hydrocarbons, carbon dioxide, and oxygen are hydrophobic substances that can pass easily across the cell membrane by diffusion and travel down the concentration gradient (Figure 8). This type of diffusion relies on the thermal motion energy intrinsic to the molecule in question. It is a form of passive transport because the cell expends no energy in moving the substances.

Figure 8: Diffusion of molecules across a semi-permeable membrane from an area of high concentration to an area of low concentration in order to achieve a balanced concentration.

Osmosis: The movement of water across a selectively permeable membrane is osmosis. A cell has one of three water relationships with the environment around it.

• In an isotonic solution there will be no net movement of water across the plasma membrane. Water crosses the membrane, but at the same rate in both directions.
• In a hypertonic solution the cell will lose water to its surroundings. The hyper - prefix refers to more solutes in the water around the cell, hence, the movement of water to the higher (hyper-) concentration of solutes. In this case the cell loses water to the environment, shrinks, and may die.
• In a hypotonic solution water will enter the cell faster than it leaves. The hypo - prefix refers to fewer solutes in the water around the cell, hence, the movement of water into the cell where the solutes are more heavily concentrated. In this case the cell will expand and may burst, unless protected by a cell wall such as that found in bacteria and plant cells.

Facilitated diffusion: Ions and polar molecules cannot pass easily across the membrane. The process by which ions and hydrophilic substances travel across the cell membrane with the help of transport proteins is called facilitated diffusion (Figure 9). Transport proteins are specific (like enzymes) for the substances they transport. They work in one of two ways:

• They provide a hydrophilic channel through which the molecules in question can pass.
• They bind loosely to the molecules in question and carry them through the membrane.