Living cells are always surrounded by a watery environment. This may be the freshwater or salt water in which unicellular organisms live; or the intercellular fluid that bathes the body cells of higher animals. Materials flow between the cell and its environment mainly by diffusion and osmosis. Let us see how these processes occur and how changes in its environment can affect the living cell.
Nature of Matter and States of Matter
Diffusion and osmosis are possible because of the nature of matter. We can define matter as anything that has mass and occupies space. It includes all the living and non-living things around us.
Matter is made up of tiny particles which may be molecules or ions. These particles are moving all the time. Matter can exist in three states: as a solid, a liquid and a gas.
In solid, the particles are closely packed with very little space between them. The forces of attraction between the particles are strong so that they cannot move around but vibrate in fixed positions. In a liquid, the particles are not so closely packed together. They can move around but they are still attracted to one another so that their movements are within a given space. In a gas, the particles move very fast. They are widely separated from one another and there is very little attraction between the them. Thus, the basic difference between the three states of matter is the degree of movement of their particles.
Any substance can exist as a solid, a liquid or a gas under the appropriate conditions of temperature and pressure. A familiar example of such a substance is water. A normal pressure (760 mmHg), pure water exists as the solid ice, below 0°C. On heating, the molecules in ice vibrate faster and break away from their fixed positions to move about, that is ice melt to form liquid water. Water exists as a liquid between 0°C and 100°C. On further heating, the molecules in water move about even faster, collide with one another, and eventually escape from the liquid to form water vapor (gas). We can reverse the above process by cooling water vapor.
Cells and Osmosis
A living cell is bound by a plasma membrane. This membrane is selectively permeable and allows water and certain solute molecules and ions pass through it. As a result, the plasma membrane regulates the movement of materials between the cell and its environment. In contrast, a semi-permeable membrane only allows water molecules to pass through it.
Osmosis in animal cells : Animal cells contain mainly cytoplasm and cell organelles. In higher animals, the cells are bathed in intercellular fluid or plasma. The concentration of the solutes in these fluids is important for the well-being and functioning of the cells.
A living cell may find itself in any one of the following situations:
The fluid surrounding the cell is more concentrated than the inside of the cell. In the case, the surrounding fluid is said to be hypertomic to the contents of the cell. There is a net movement of water molecules out of the cell into the surrounding. This is known as exosmosis. It causes the cell to shrink.
The fluid surrounding the cell is less concentrated than the inside of the cell. Here, the surrounding fluid is said to be hypotomic to the contents of the cell. There is a net movement of water molecules from the surrounding fluid into the cell. This is known as endosmosis. It causes the cell to swell, and eventually rupture.
The surrounding fluid and the cell content have the same concentration. Hence, they are said to be isotomic. There is no net movement of water molecules in or out of the cell.
To survive and function well, the living cell and the fluid that bathes it must be isotonic or be able to maintain an osmotic balance. Endosmosis and exosmosis can lead to the eventual death of an animal.
Osmosis in plant cells : plant cells have cell membrane and cell walls. The cell wall is a tough and fairly elastic structure that is freely permeable to all molecules and ions. The cell membrane, however, is selectively permeable.
Unlike an animal, most of the space in a plant cell is occupied by a large central backup that contains cell sap. It has a high concentration and tends to draw in water into the cell from the surroundings by osmosis. When endosmosis occurs, water flows into the vacuole of a plant cell, causing the cell to swell. The cell, however, does not rupture because, although the cell wall stretches to a certain extent, it is tough and does not break. It also prevents the cell membrane from expanding. A high pressure builds up inside the cell and makes it turgid. When exosmosis occurs, water flows out of the vacuole of the plant cell into the surroundings. As a result, the vacuole shrinks and eventually pulls the cytoplasm from the cell wall. This process is known as plasmolysis. Turgidity is important in land plants. It make the plant form and gives support, especially to herbaceous plants. If plant cells are not turgid, the plant will wilt. The plant will easily recover if it supplied with water. However, if water is not supplied, the plant will die when all its cells becomes plasmolysed.