Carbohydrates and Polysaccharides

Carbohydrates are polyhydroxy aldehydes or ketones with the general formula $(CH_2O)_n$. We classify them by their functional groups: aldoses contain an aldehyde at $C_1$, while ketoses contain a ketone at $C_2$. In aqueous solutions, these linear chains spontaneously close into rings. This cyclization creates a new chiral center at the anomeric carbon ($C_1$ for glucose). If the hydroxyl group ($-OH$) at $C_1$ points down, it is the **$\alpha$-anomer; if it points up, it is the $\beta$-anomer**. This tiny spatial difference determines whether the resulting polymer is a soft energy source or a rigid structural material.

When two monosaccharides join, they undergo a dehydration synthesis reaction. A hydroxyl group from one sugar reacts with the anomeric carbon of another, releasing a molecule of $H_2O$ and forming a glycosidic bond. The notation for these bonds is critical. For example, an $\alpha(1 \rightarrow 4)$ linkage means the $-OH$ on $C_1$ was in the $\alpha$ position and connected to the $C_4$ of the next sugar. These bonds can create long linear chains or highly branched networks depending on which carbons are involved.

Polysaccharides serve two main purposes: energy storage and structural support. Starch (in plants) and glycogen (in animals) use $\alpha(1 \rightarrow 4)$ linkages, which cause the chains to twist into a helical shape, making them compact and easy for enzymes to break down. Starch consists of amylose (unbranched) and amylopectin (branched via $\alpha(1 \rightarrow 6)$ bonds). Cellulose, however, uses $\beta(1 \rightarrow 4)$ linkages. This causes the chains to remain straight, allowing them to pack tightly and form hydrogen bonds between adjacent chains, creating the immense tensile strength found in plant cell walls.