A spectrophotometer measures the number of photons emitted to estimate the intensity of light spectra absorbed and transmitted by a sample. This provides information on the amount of a compound in the sample.
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed.
Trichloroacetic acid (TCA) precipitation of proteins is commonly used to concentrate protein samples or remove contaminants, including salts and detergents, prior to downstream applications such as SDS-PAGE or 2D-gels. TCA precipitation denatures the protein, so it should not be used if the protein must remain in its folded state (e.g., if you want to measure a biochemical activity of the protein).
Determining the protein concentration in your sample is an important step in any laboratory workflow that involves protein extraction and/or analysis. Knowing how much protein you have can help you compare results from one protein to another and from one experiment to the next.