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The key points covered of this video include:

1. The Phloem
2. Phloem Tissue
3. Active Loading

The Phloem

Plants need to transport water from their roots upwards and sugars and other assimilates both upwards and downwards. Assimilate is a general term for an organic molecule such as a carbohydrate or amino acid that can be used by an organism. Plants have two transport systems grouped into tissue called vascular bundles. The phloem transports sugars and assimilates. These molecules are produced by the leaves and required by all the plants tissues.

Phloem Tissue

Phloem tissue consists of two types of cell: 1. Sieve-tube elements are lined up end-to-end to allow the flow of sap. Sieve tube elements have little cytoplasm and no nucleus to maximise space for sap but they are still alive. 2. Companion cells control transport in the sieve tube elements. Companion cells contain many mitochondria because the phloem requires active transport to move sap in translocation. Translocation is the transport of assimilates through a plant. This electron-micrograph shows a cross-section of phloem tissue in a vascular bundle.

Active Loading

The process of loading sucrose into the sieve tube elements is called active loading: Step 1: H+ ions are pumped out from the companion cells to the surrounding leaf tissue creating a diffusion gradient of H+. Step 2: H+ ions diffuse back into the companion cells through cotransporter proteins bringing sucrose with them. Step 3: High concentrations of sugar in companion cells cause sucrose to diffuse in to the sieve tube elements.

Summary

The phloem transports sap containing water and assimilates up and down the plant
Phloem tissue consists of closely packed sieve tube elements and companion cells
Sieve tube elements contain the sap that is transported
Companion cells actively load sucrose and other assimilates into the sieve tube elements