FACT
You will learn more
about the transport
processes in plants
in:Support and
transport systems in
plants.
Figure 5.42: Leaf structure.Vascular tissueis made up of the xylem and phloem vessels you have learned about earlier
in this chapter. Xylem as you have learned, transports water and minerals to the leaf. Phloem
transports dissolved sucrose made in the leaf out of its site of synthesis to the rest of the leaf.Transport of substances into and out of the leaf DUMMY
The leaf is devised to transport water, sugars, carbon dioxide and oxygen across its surface.
Each of these involves separate processes and cells which we will discuss below.Movement of oxygen and carbon dioxideStomata are the site of gaseous exchange in the leaf. Carbon dioxide enters the leaf and
oxygen leaves. Waste gases from photosynthesis and respiration leave through the stomata as
well. Oxygen diffuses out of the leaf because the concentration of oxygen is greater outside
the leaf. Similarly, carbon dioxide diffuses into the leaf due to its higher concentration
outside the leaf.Movement of water into leafWater is constantly being lost by the leaf through transpiration processes. This results in the
cells in the mesophyll having a lower water concentration that the vascular bundles. Water
thus moves down a concentration gradient from the xylem vessel into the living cells of the
mesophyll layer and to the surface of the mesophyll cell walls. This causes water to move
up from the stem by means of transpirational pull. The movement of water is maintained
because water molecules constantly evaporate into leaf inter-cellular air space out of the
stomatal pore and into the atmosphere.Movement of sugarsSugars are manufactured in the leaves and transported to the rest of the plant. Chloroplasts
found in the palisade layer harvest the energy from the sun to make glucose. Glucose is
subsequently stored in the form of starch. Starch is transported through the phloem vessel
present in the vascular tissue in the leaf.152 5.6. The leaf as an organ