Chapter 3: The Cell and its Enviornment

Living cells are made up of 85% water

Water is the universal solvent. Almost all substances needed for life can dissolve into it.

Substances dissolved in solvents are called solutes. Solvents and solutes together create a solution. For example, blood anc cell cytoplasm are solutions that contain many solutes, such as carbon dioxide and sugar.

A cell must be able to control the concentration of vital elements both within and surrounding it. For example, oxygen and glucose must be transported into the cell while carbon dioxide and amine groups must be transported out. The passage of molecules into and out of a cell is regulated by the cell membrane or plasma membrane. The plasma membrane is a double layer of phospholipid molecules, called a bilayer, embedded with globular proteins. It’s a semi-permeable barrier that allows certain molecules to pass freely while excluding others. The proteins in it can also regulate molecular passage across the membrane as carriers, channels, receptors, pumps, and enzymes.

Diffusion is a process that occurs whether a cell requires it or not. It may cause the solutes a cell needs to naturally flow in, a process called passive transport, because it requires no energy to occur. Diffusion, facilitated diffusion through a protein channel, and osmosis are all examples of passive diffusion. However, diffusion can also cause solutes a cell needs to flow out. In this case, proteins must fight against passive diffusion and move molecules from an area of low concentration to high concentration. This process requires energy, and is therefore called active transport.

Osmosis is the passive diffusion of water molecules across a membrane. When describing the effect a solution will have on a living cell, three terms of tonicity are used: hypertonic, hypotonic, and isotonic. Hypertonic is when a solution is more concentrated than another, hypotonic is when a solution is less concentrated than another, and isotonic is when they are equal.