Protein synthesis and degradation represent substantial costs during plant growth. To obtain a quantitative measure of the rate of protein synthesis and degradation, we supplied 13CO2 to intact Arabidopsis thaliana Col-0 plants and analyzed enrichment in free amino acids and in amino acid residues in protein during a 24 hour pulse and 4 day chase. Whilst many free amino acids labeled slowly and incompletely, alanine showed a rapid rise in enrichment in the pulse and decrease in the chase. Enrichment in free alanine was used to correct enrichment in alanine residues in protein and calculate the rate of protein synthesis. The latter was compared with the relative growth rate to estimate the rate of protein degradation. The relative growth rate was estimated from sequential determination of fresh weight, sequential images of rosette area, and labeling of glucose in the cell wall. In an 8 hour photoperiod, protein synthesis and cell wall synthesis were three-fold faster in the day than at night, protein degradation was slow (3-4% day-1) and flux to growth and degradation resulted in a protein half-life of 3.5 days. In the starchless phosphoglucomutase (pgm) mutant at night, protein synthesis was further decreased and protein degradation increased, whilst cell wall synthesis was totally inhibited, quantitatively accounting for the inhibition of growth in this mutant. We also investigated the rates of protein synthesis and degradation during leaf development, during growth at high temperature, and compared synthesis rates of Rubisco large and small subunits of in the light and dark.