What is the driving force for photoassimilate movement from the source to the sink?
The driving force for photoassimilate movement from the source to the sink is mainly the concentration gradient. Photoassimilates, such as sugars and other organic molecules, are produced in the source tissues (usually leaves) through photosynthesis. These photoassimilates then move towards growing or storage tissues, which are known as sinks (such as roots, buds, fruits, or storage organs).
The movement of photoassimilates is facilitated by the concentration gradient between the source and the sink. High concentrations of photoassimilates in the source tissue create a pressure potential gradient that drives the movement towards lower concentration areas in the sink tissue. This gradient allows for the movement of photoassimilates through phloem tissues, which are specialized plant vascular tissues responsible for long-distance transport.
Additionally, the process of active transport, facilitated by energy-dependent carriers, also plays a role in the movement of photoassimilates from source to sink. This active transport process helps maintain the concentration gradient and supports the movement of photoassimilates to areas of lower concentration, contributing to their overall movement and distribution within the plant.
The driving force for photoassimilate movement from the source to the sink is primarily osmotic pressure. This process is known as translocation and it occurs in the phloem tissue of plants. During photosynthesis, the source, which is usually the leaves, produces sugars (photoassimilates) through the conversion of light energy. These photoassimilates, such as glucose and sucrose, are then transported through the phloem to the sink, which is typically a growing or storage organ like fruits, flowers, or roots.
The movement of photoassimilates is driven by a concentration gradient between the source and the sink. Glucose and sucrose molecules are actively transported into the phloem sieve tubes in the source tissues, increasing their concentration in the phloem. This creates a higher osmotic potential in the phloem sieve tubes compared to the surrounding tissues. As a result, water from the surrounding tissues enters the phloem by osmosis, creating a positive pressure known as turgor pressure.
This turgor pressure in the phloem sieve tubes pushes the photoassimilates down towards the sink. At the sink, the photoassimilates are actively unloaded from the phloem to be used for growth or storage. This removal of photoassimilates reduces their concentration in the phloem, consequently decreasing the osmotic potential and turgor pressure. This helps maintain the concentration gradient between the source and the sink, allowing the continuous flow of photoassimilates from the source to the sink.