Question

Explain what types compounds get transported in the phloem and why Explain the concept of sources and sinks in terms of phloem translocation; know examples of source and sink organs for sugars and nitrogenous compounds Explain how a leaf can be a sink and a source for photosynthate (sugars) at different times

Answer 1

1. Phloem is a conducting tissue in the vascular plants. This tissue transports organic materials like sugars, from the sites of their synthesis to the sites of their use or storage. This transport plays an important role in the plant life as if the plant produces excess organic matter( carbohydrates) they are transported for storage for the time when plant cannot produce it or to the sites of seed production, etc.

2.

SOURCE : Source are the tissues where the phloem tissue gets loaded with the material it carries (organic molecules) and sink are the tissues wher it is unloaded.

The green parts of the plant(leaves) which synthesis the organic molecules like glucose and then sugar from it are the source and the tissues where these material are used (roots, cannot prepare food as they lack chlorophyll and are not exposed to light) act as sink like the storage organs(fruits).

Source of nitrogen are the roots which absorb nitrates front the soil and sink of nitrogen are the tissues which store amino acids like the protein rich seeds.

3. Leaves can act as both source and sink for photosynthate as :

When photosynthesis occurs in plants during day times and during summers leaves are the sources of dugars because they synthesis sugars which sre transported to other tissues by phloem, where as leaves can act as sink when there is no photosynthesis as during night where respiration is going but no photosynthesis.