In 1665, Robert Hooke, an English scientist observed cork under a microscope and noted that it was composed of small units. He called these units cells. Later it was observed that most living things are composed of cells. This led to cell theory. Cell theory is that living things are composed of cells.
A theory is a hypothesis that scientific investigations have shown to be true most of the time. A law is a hypothesis that scientific investigations have shown to be true all of the time. Cell theory is not cell law. This is because life forms called viruses, viriods and priods are not composed of cells. Other than these limited exceptions, all life is composed of cells. The cell is the basic unit of life.
Many organisms such as bacteria, amoeba, some algae, and yeast are composed of only a single cell. The single celled organisms are described as being "unicellular." However, most organisms are composed of many cells and are described as being "multicellular." All cells live in a fluid environment. The human body, for example, is about 70% water. To live, cells must be able to obtain material from and release substances into a liquid environment. It is the cell membrane which allows materials to come into a cell, as well as allowing substances to be released from a cell. An important property of the cell membrane is that it is semipermeable. Semipermeable means that the membrane only allows certain things to pass into or out of the cell. Generally, only those materials that need to enter the cell are allowed to enter. Generally, only those materials which need to be released from the cell are allowed to leave the cell.
Cell membranes are composed of two layers of molecules called phospholipid. This phospholipid membrane is composed of combinations of atoms called a phosphate group, and lipids (fats) such as cholesterol. This phospholipid bilayer functions to keep water outside and inside of the cell. However, in the space between the layers, there is no water. The outside parts of the bilayer are described as being hydrophilic (water loving) and the inside parts are described as being hydrophobic (water fearing). Since water is the universal solvent, things can dissolve in water outside and inside the membrane and join with the membrane to enter or leave the cell.
Embedded into the membrane are proteins. Some of these proteins are bonded together to form pores through which certain things can pass into and out of the cell. A mosaic is a work of art composed of bits and pieces of things inlaid into some uniform surface. Since the outside and inside of the cell membrane attract water, and because this surface is inlaid with protein, the model of the cell membrane is often described as being "fluid Mosaic Model.
Some substances enter the cell by diffusion. Diffusion is the random movement of particles resulting in their movement from a region of greater concentration to one of lesser concentration. If you have a balloon filled with oxygen and you allow this oxygen to be released in a closed room, the random movements of the oxygen atoms will cause the oxygen to diffuse or spread out and become evenly distributed in the room. Most cell membranes are permeable to oxygen. As blood brings oxygen to cells, through diffusion, oxygen enters the cells. It is also through diffusion that carbon dioxide leaves a cell as a waste product of the sugar which was burned to produce energy. So diffusion is one way that things can enter or leave cells. In diffusion, a substance moves from an area of greater concentration to an area of lesser concentration until equilibrium is reached.
Osmosis is the diffusion of water through a semipermeable membrane. Since water is so important to maintaining life, cells must stay in osmotic balance. Osmotic balance exists when the amount of water which enters the cell is equal to the amount of water which leaves the cell. Through a semipermeable membrane, water will pass from an area of higher concentration to an area of lesser concentration. If you stay in a swimming pool for a long time, you are in an environment in which the outside of your skin is exposed to a higher concentration of water than the inside. Water will pass by osmosis to the inside of your skin. This will make your skin puff up and wrinkle from the water which was drawn into your skin. If salt or sugar is added to the liquid environment of bacteria, the salt or sugar causes there to be a lesser concentration of water outside the bacteria than inside the bacteria. Through osmosis water leaves the bacteria causing dehydration and death. This is why salty or sugary foods seldom grow bacteria.
Osmosis and diffusion allow things to enter cells through passive transport. Passive transport means that the cell does not have to use energy to move these substances into or out of the cell. There is another kind of passive transport called facilitated diffusion. In facilitated diffusion, proteins in the cell membrane form pores which allow certain substances to pass. Without the protein pore, these substances would be too large to pass directly into the cell. The use of protein to aid (facilitate) the passage of particles from a region of greater concentration to a region of lesser concentration is called facilitated diffusion. The proteins involved in facilitated diffusion are called permease. It is through facilitated diffusion that sodium and glucose enter cells.
Whether passive transport occurs by osmosis, simple diffusion or facilitated diffusion, it always involves a movement of particles from a region of greater to lesser concentration. The cell does not have to expend any energy to transport these substances. But sometimes a cell must transport something from a region of lesser concentration to a region of greater concentration. To do this the cell must use energy to create this movement.
When cell energy is used to transport a particle from an area of lesser to greater concentration it is called active transport. Active transport also involves permease molecules. Permease involved in active transport are known as pumps. It is by active transport that cells rid themselves of harmful particles which could poison and kill the cell.
Materials which are too large to enter or leave the cell by passive or active
transport can enter or leave through endocytosis or exocytosis. Endocytosis is
the process by which a cell uses energy to surround and take in large particles.
If a solid particle is taken in through endocytosis, this process is referred to
as being phagocytosis, a word which means "cell eating." It is through
phagocytosis that white blood cells eat bacteria. If a liquid is taken in through
endocytosis, this process is called pinocytosis, a word which means "cell
drinking." A reverse process is used to rid cells of wastes or to expel useful
substances needed elsewhere. This process is called exocytosis (cell exiting).