It is common knowledge that plants photosynthesise to make sugars and then store much of it as starch that can later be metabolized to generate energy for the plant, for feed inganimals or microbes.
Besides its use fornutrition, starch is also used in the food for its physicochemical properties, for starching laundry (ask your parents!) and as wallpaper paste. Starchderivatives are also common ingredients in skincare products, and claimed to have wound-healing, antimicrobial, antioxidant and anti-inflammatory, antitumoral, and anti-aging properties [1].
Starch macromolecule are branched chains containing 300 to 1000 glucose units, and it takes bit of energy to string them together. But why should plants invest instarch rather than keeping photosynthate as glucose? Most of the answer lies in the physical properties of solutions. When sugar or any other material dissolves in water, the solution gets an osmotic “pressure”, actually a negative water potential,(designated ψ in physiology). The negative ψ allows plants to suck up more water, preventing wilting and driving growth. Animal cells also have negative ψ, which is why blood cells burst when placed in water.
This may seem counterintuitive, but ψ changes in proportion to the number of dissolved molecules, with little regard to their molecular size. A 5% glucose solution(~2 teaspoons per cup) has a ψ of minus 7.7 atmospheres. For the same amount of glucose polymerised into starch (at 1000 glucose per starch molecule), ψ is a mere–0.0077 atmospheres. All the sugar produced in photosynthesis would create a HUGE negative ψ,causing all sorts of problems; this problem is resolved by converting glucose to starch and storing it till needed in roots, tubers and seeds, etc..
When cereals germinate, the first step is to break down the starch into maltose (a glucose-glucose dimer) and the ψ that develops sucks in more water to drive germination. Stopping barley germination at the first stage is how malt for brewing is produced).
And for skincare? Starch is fine, but sugar would suck your skin dry.
1. Albuquerque, P.B.S., et al.: Skincare application of medicinal plant polysaccharides — A review. Carbohydrate Polymers. 277, 118824 (2022). https://doi.org/10.1016/J.CARBPOL.2021.118824
