Phloem and Translocation

Phloem transports organic products of photosynthesis. Phloem contains living conducting cells (sieve tube members). The sieve tubes bear clusters of pores in the walls through which the cytoplasm of adjacent cells is connected. Companion cells, adjacent to the sieve tube members, help to move sugars from regions of intense photosynthetic activity. Companion cells are a type of parenchyma that helps load organic compounds into sieve tubes.

Storage forms of organic molecules (example: starch, fats, proteins) are too large or insoluble for transport throughout the plant body. Cells break them down to smaller molecules for transport. Sucrose is the main carbohydrate transported. Carbohydrates are stored as starches.

Organic molecules travel from photosynthetic sites to organs that need them by translocation. Molecules move through phloem from sources (leaves) to sinks (flowers, fruits, seeds, roots). According to the pressure flow theory, translocation depends on pressure gradients. Solutes move into the phloem from a source , then water follows, pressure builds, and fluids move away to sink areas. At the sink, organic compounds move by active transport into sink cells where they are used.

This animation (Audio - Important) describes the pressure flow theory.

Movement of materials through the plant are interdependent processes.

This animation (Audio - Important) reviews vascular tissues.

REVIEW: The cells that function with the sieve tubes are the

REVIEW: The source region in the pressure flow explanation of phloem transport is most often the

REVIEW: The most common form of sugar transported to the roots is

REVIEW: In phloem, organic compounds flow through _____ .

REVIEW: _____ conducts water and ions; _____ conducts food.

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