Movement of Manufactured Food

Movement of Manufactured Food

Plants use sunlight, carbon dioxide and water to manufacture glucose, yielding oxygen as a by-product. Sunlight or radiant light is captured by the green pigment chlorophyll inside of chloroplasts to provide the energy for photosynthesis to occur. Once the food is manufactured in the leaves it needs to be distributed to the entire plant so that the glucose can be used by each cell for respiration and some of the photosynthetic products are then stored for later use.

The glucose is manufactured mainly in the palisade cells where there are more chloroplasts, and then passes into the phloem. Plants usually transport food in the form of the sugar sucrose because it is less reactive than glucose. Sucrose is transported to where it is needed in the the plant via phloem sap, and may be stored in roots, stems or fruit. Transport of food material from leaves to other parts of the plant is called translocation. Understanding the phloem structure is important to understanding how it transports food.

Aphids feeding on phloem sap which is rich in glucose.

How the phloem functions

While the transport of water is usually unidirectional in xylem (upward or lateral), the movement of sugars in the phloem is multi-directional, and occurs by active transport, an energy-dependent process. Sucrose is actively transported against a concentration gradient into sieve-tube elements. The sieve-tube elements have no nuclei but the adjacent companion cells do. Companion cells are closely associated with sieve tubes and carry out all the cellular functions of the sieve tubes.

The cytoplasm of sieve tubes and companion cells is connected through numerous channels called plasmodesmata. These cytoplasmic connections allow the companion cells to regulate the content and activity of the sieve tube cytoplasm. The companion cells also help load the sieve tube with sugar and the other metabolic products that they transport throughout the plant. This lowers the water potential of the sieve-tube element, causing water tomove in by osmosis, creating a pressure that pushes the sap down the tube. The metabolising cells of the plant actively transport sugars out of sieve-tube elements, producing exactly the opposite effect. The diagram below illustrates how the overall process works.

Diagram showing movement in the xylem and phloem vessels. Water movement is upwards in the xylem and lateral into and out of the phloem. Lateral movement also occurs into and out of the companion cells accompanying the phloem vessel.