Blood plasma also includes many dissolved proteins. These are usually negatively charged because their carboxylic acid groups are ionized at neutral pH. Small molecules include dissolved gases like oxygen and carbon dioxide, amino acids and glucose. Since these reservoirs are separated by membranes, these concentration differences imply that the membranes involved can maintain a higher concentration of a particular solute on one side than the other. This is done by active transport , a process which requires energy to move an ion or other small molecule from a region of low concentration to a region of hight concentration. We've seen an example in the proton gradient produced by respiration and used by oxidative phosphorylation.
Blood cells are either white or red.
White blood cells are involved in the immune response. Hemoglobin in erythrocytes is the molecule responsible for this. The iron in the center of the heme group of each of the four protein chains which make up hemoglobin complexes with an oxygen molecule. This oxygen is replaced in turn by oxygen released by hemoglobin. Remember that CO 2 is produced by the citric acid cycle.