Movement of manufactured food
Plants use carbon dioxide and water to manufacture glucose and oxygen is the waste product. Sunlight and enzymes are necessary 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(manufacture energy).
Sunlight and enzymes
water + carbon dioxide ↔ glucose (carbohydrates) + oxygen
The glucose is manufactured mainly in the palisade cells and then passes into the phloem. Transport of food material from leaves to other parts of the plantis called translocation. This food may be stored in roots, stems or fruit.
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Phloem tissue is made up of two different types of cells which are sieve tubes and companion cells. Sieve tubes are the main conducting cells. These cellslook like a sieve and phloem sap moves from cell to cell though the phloem walls. Unlike cells of the xylem, sieve tubes are alive at functional maturity,but do not have nuclei. Companion cells have nuclei and are closely associated with sieve tubes. Companion cells carry out all the cellular functions of thesieve tubes. The cytoplasm of sieve tubes and companion cells is connected through numerous channels called plasmodesmata. These cytoplasmic connectionsallow the companion cells to regulate the content and activity of the sieve tube cytoplasm. The companion cells also help load the sieve tube with sugarand the other metabolic products that they transport throughout the plant.
Aphids feeding on phloem sap