AP BIOLOGY

Detailed explanation-1: -Function of companion cell is to load sugar and amino acids into sieve elements. These cells use transmembrane proteins to take up these molecules by active transport.

Detailed explanation-2: -Companion cells provide sieve tube members with proteins necessary for signaling and ATP in order to help them transfer molecules between different parts of the plant. It is the companion cells that helps transport carbohydrates from outside the cells into the sieve tube elements.

Detailed explanation-3: -Companion cells move sugar and amino acids into and out of the sieve elements. In ‘source’ tissue like leaf companion cells use transmembrane proteins to require up sugar and amino acids by transport. Loading is a setup of a water potential gradient that facilitates movement of the sugar.

Detailed explanation-4: -This active transport of sugar into the companion cells occurs via a proton-sucrose symporter; the companion cells use an ATP-powered proton pump to create an electrochemical gradient outside of the cell.

Detailed explanation-5: -Sieve tubes are responsible for the transport of food throughout the plant body while companion cells are responsible for regulating the activity of the sieve element.