Biology (Kimball)

Mixtures are heterogeneous forms of matter. Mixtures are composed of variable proportions of molecules and atoms.

The composition of a mixture is variable with each components retaining its characteristic properties. Its components are easily separated. Examples of Mixtures: soil, ocean water and other solutions, air, the cytosol of a cell n contrast, compounds are homogeneous forms of matter. Their constituent elements (atoms and/or ions) are always present in fixed proportions . Properties of compounds include

• The relative proportions of the elements in a compound are fixed.
• The components of a compound do not retain their individual properties. Both sodium and chlorine are poisonous; their compound, table salt (NaCl) is absolutely essential to life.
• It takes large inputs of energy to separate the components of a compound.

Examples of Compounds

• water (H O)
• table salt (NaCl)
• sucrose (table sugar, C H O )

Separating the Components of a Mixture

Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative

• size
• density
• solubility
• electrical charge

Dialysis

Dialysis is the separation of small solute molecules or ions (e.g., glucose, Na , Cl ) from macromolecules (e.g., starch) by virtue of their differing rates of diffusion through a differentially permeable membrane.

Figure : Small-molecule dialysis using dialysis tubing. (CC-SA-BY-3.0; Potcherboy).

As shown in Figure , the cellophane used to construct a bag is perforated with tiny pores that permit ions and small molecules to pass through, but exclude molecules with molecular weights greater than about 12,000. If a cellophane bag is mixed with a mixture of sugar and starch and place it in salt water, the sugar molecules (teal dots) will diffuse out into the water until 2 12 22 11 + – 1.1.1 1.1.1 1.1.2 https://bio.libretexts.org/@go/page/3741 equilibrium is reached; i.e., until their concentrations are equal on both sides of the membrane. Similarly, the salt (red dots) will diffuse into the bag. However, because of their large size, all the starch (big blue disks) will be retained within the tubing.

Chromatography

Chromatography is the term used for several techniques for separating the components of a mixture. The different types of chromatography techniques used are: paper chromatography, exclusion chromatography, and affinity chromatography.

Paper chromatography technique provides an easy way to separate the components of a mixture. A drop of mixture is placed in one corner of a square of absorbent paper.

One edge of the paper is immersed in a solvent. (a)

The solvent migrates up the sheet by capillary attraction.

As it does so, the substances in the drop are carried along at different rates. (b)

Each compound migrates at a rate that reflects the size of its molecule and its solubility in the solvent.

After a second run at right angles to the first (often using a different solvent), the various substances will be spread out at distinct spots across the sheet, forming a chromatogram. (c)

The identity of each spot can be determined by comparing its position with the position occupied by known substances under the same conditions.

In many cases, a fragment of the paper can be cut away from the sheet and chemical analysis run on the tiny amount of substance in it.

Figure 1.1.2: Paper chromatography

Autoradiography

If the mixture contains molecules that have been labeled with a radioactive isotope, these can be located by placing the chromatogram next to a sheet of X-ray film. The location of dark spots on the developed film (because of radiation emitted by the isotope) can be correlated with the position of the substances on the chromatogram.