Transport in plants

●        Introduction

●        Plants contain two types of transport vessel:

o    Xylem vessels – transport water and minerals from the roots to the stem and leaves

o    Phloem vessels – transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem.

●        These vessels are arranged throughout the root, stem and leaves in groups called vascular bundles.

●        Xylem vessels 

●        Xylem cells lose their top and bottom walls to form a continuous tube through which water moves through from the roots to the leaves.

●        Adaptations:

o    Cells joined end to end with no cross walls to form a long continuous tube

o    Cells are essentially dead, without cell contents, to allow free passage of water

o    Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant

●        Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration.

●        Transpiration is defined as the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata.

●        Phloem vessels:

●        Phloem is vascular tissue that transports food (sugar dissolved in water) from photosynthetic cells to other parts of the plant for growth or storage.

●        This type of tissue consists of living cells that are separated by end walls with tiny perforations or holes. .

●         Sucrose is taken up actively by phloem, which increases its concentration, and water moves into phloem by osmosis. This flow enables the transport of sucrose to all parts of the plant that need it.

●        The transport of sucrose and amino acids in the phloem, from regions of production to regions of storage or use, is called translocation

●        Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year; however dissolved food is always transported from the source (where it’s made) to sink (where it’s stored or used)