Floridean starch

Floridean starch is a type of storage glucan found in glaucophytes and in red algae (also known as rhodophytes), in which it is usually the primary sink for fixed carbon from photosynthesis. It is found in grains or granules in the cell's cytoplasm and is composed of an α-linked glucose polymer with a degree of branching intermediate between amylopectin and glycogen, though more similar to the former. The polymers that make up floridean starch are sometimes referred to as "semi-amylopectin".

Floridean starch consists of a polymer of glucose molecules connected primarily by α(1,4) linkages, with occasional branch points using α(1,6) linkages. It differs from other common α-linked glucose polymers in the frequency and position of the branches, which gives rise to different physical properties. The structure of floridean starch polymers is most similar to amylopectin and is sometimes described as "semi-amylopectin". Floridean starch is often described in contrast to starch (a mixture of amylopectin and amylose) and glycogen:

Bacteria: ADP-glucose

Amylose: Almost entirely linear

Historically, floridean starch has been described as lacking amylose. However, amylose has been identified as a component of floridean starch granules in some cases, particularly in unicellular red algae.

Features such as UDP-glucose building blocks and cytosolic storage differentiate the Archaeplastida into two groups: the rhodophytes and glaucophytes, which use floridean starch, and the green algae and plants (Chloroplastida), which use amylopectin and amylose. There is strong phylogenomic evidence that the Archaeplastida are monophyletic and originate from a single primary endosymbiosis event involving a heterotrophic eukaryote and a photosynthetic cyanobacterium.

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