Summary
Highlights
Plants have mass transport systems to move sugars (translocation) and water (transpiration). Sugars produced in leaves via photosynthesis need to be transported to other parts of the plant for energy or storage.
Translocation is the process of sugar transport, primarily done by phloem cells arranged into long columns called phloem tubes. These tubes have small pores between cells, allowing for the movement of sap (water and sugar mixture). Phloem can transport substances in both upward and downward directions.
Adjacent to phloem are xylem tubes, made of dead xylem cells with no ends, forming long hollow tubes strengthened by lignin. Their role is to transport water and mineral ions from the roots to the leaves for photosynthesis.
Water movement through a plant is driven by the evaporation of water from the leaves, known as transpiration. This creates a 'transpiration stream' as evaporating water molecules pull others up the plant, maintaining a continuous flow.
Four main factors influence the rate of transpiration: light intensity, temperature, airflow, and humidity. Higher light intensity means more photosynthesis, open stomata, and increased transpiration. Warmer temperatures increase water evaporation. High airflow blows away water molecules, maintaining a steep concentration gradient and increasing transpiration. High humidity, conversely, reduces the concentration gradient, decreasing transpiration.
The inside of a leaf has a higher water concentration than the outside, creating a concentration gradient. Transpiration rate depends on the strength of this gradient; a stronger gradient (e.g., with high airflow) leads to faster diffusion of water out of the leaf.