Monosaccharides and disaccharides are carried in phloem sap, whereas polysaccharides are not part of phloem sap. Define the terms disaccharide and polysaccharide.
Sieve tubes are made up of phloem sieve tube elements. Fig. 4.1 shows a diagram of one sieve tube element and its companion cell. Some structural features of the sieve tube element are missing from the diagram. Complete Fig. 4.1 by drawing and labelling the structural features of the sieve tube element that are missing from the diagram.
A pressure gradient in a sieve tube leads to mass flow of phloem sap from source to sink. At the source, the water potential of the phloem sap decreases and the hydrostatic pressure of the phloem sap increases. Describe the events that bring about each of these changes in the phloem sap.