Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Translocation is the movement of sucrose () and amino acids in the phloem from a source to a sink.
A 'Source' is the part of the plant where sucrose is produced (e.g., mature leaves during photosynthesis) or released from storage (e.g., germinating seeds or tubers in spring).
A 'Sink' is the part of the plant where sucrose is stored (e.g., roots, tubers, developing seeds) or used for growth (e.g., growing shoot tips, flowers, and fruits).
Phloem tissue consists of sieve tube elements (living cells with no nucleus to allow easy flow) and companion cells (which provide for the active loading of sucrose).
Unlike the transport of in the xylem, which is unidirectional (upwards), translocation in the phloem is bidirectional (upwards or downwards) depending on the needs of the plant.
The plant transports carbohydrates as sucrose () rather than glucose () because sucrose is less reactive and more efficient for transport.
📐Formulae
💡Examples
Problem 1:
Identify the source and sink in a potato plant during the height of the summer growing season.
Solution:
Source: Mature green leaves; Sink: Growing tubers (potatoes underground).
Explanation:
In summer, leaves perform photosynthesis to produce glucose (), which is converted to sucrose () for translocation via the phloem down to the tubers, where it is converted to starch for storage.
Problem 2:
During the early spring, a deciduous tree begins to grow new leaves using energy stored in its roots. Identify the source and sink in this scenario.
Solution:
Source: Roots/Storage organs; Sink: Developing leaf buds.
Explanation:
In early spring, no photosynthesis occurs because there are no leaves. The stored starch in the roots is converted back into sucrose () and moved upwards through the phloem to provide energy for the growth of new buds.
Problem 3:
Explain why aphid stylets (mouthparts) are used by scientists to study translocation.
Solution:
Aphids penetrate the phloem directly to feed on the sap, which contains high concentrations of and amino acids.
Explanation:
Because phloem sap is under high pressure, it flows out through the aphid's stylet even if the aphid's body is removed, allowing researchers to analyze the chemical composition of the transported materials.