<Li> Energy flow occurs within cells . </Li> <Li> Heredity information (DNA) is passed on from cell to cell . </Li> <Li> All cells have the same basic chemical composition . </Li> <P> The cell was first discovered by Robert Hooke in 1665 using a microscope . The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s . In this theory the internal contents of cells were called protoplasm and described as a jelly - like substance, sometimes called living jelly . At about the same time, colloidal chemistry began its development, and the concepts of bound water emerged . A colloid being something between a solution and a suspension, where Brownian motion is sufficient to prevent sedimentation . The idea of a semipermeable membrane, a barrier that is permeable to solvent but impermeable to solute molecules was developed at about the same time . The term osmosis originated in 1827 and its importance to physiological phenomena realized, but it wasn't until 1877, when the botanist Pfeffer proposed the membrane theory of cell physiology . In this view, the cell was seen to be enclosed by a thin surface, the plasma membrane, and cell water and solutes such as a potassium ion existed in a physical state like that of a dilute solution . In 1889 Hamburger used hemolysis of erythrocytes to determine the permeability of various solutes . By measuring the time required for the cells to swell past their elastic limit, the rate at which solutes entered the cells could be estimated by the accompanying change in cell volume . He also found that there was an apparent nonsolvent volume of about 50% in red blood cells and later showed that this includes water of hydration in addition to the protein and other nonsolvent components of the cells . </P>

Who said that all living things are made of cells