Problem 143 - Objective Biology for NEET

Question

Sink and source in sucrose transport may be reversed depending on (a) Plant's need (b) Season (c) Both (a) and (b) (d) None of these

Step-by-Step Solution

Answer

The correct answer is (c) Both (a) and (b).

Step 1: Understand the role of 'sink' and 'source' in plant sucrose transport

In plant sucrose transport, the 'source' refers to where sucrose is produced, typically in the leaves through photosynthesis. The 'sink' refers to where sucrose is needed or stored, this could be roots, stems, fruits, or seeds of the plant. Under normal conditions, sucrose transport occurs from the 'source' to the 'sink'.

Step 2: Determine why 'sink' and 'source' may be reversed

The flow of sucrose from 'source' to 'sink' could be reversed depending on several factors. During a particular growth phase or season, certain parts of the plant may need more energy, switching roles from source to sink. Similarly, during dormancy or winter season when leaves may not produce enough food, stored food in roots or stems (sinks) may become the new sources.

Step 3: Choose the correct answer

With the understanding that plant's needs or seasons could impact the reversal of 'sink' and 'source' roles in sucrose transport, it is clear that the correct answer is (c) Both (a) and (b).

Key Concepts

The Dynamics of Sink and Source

Plants have an efficient system for transporting sucrose, which involves the roles of 'sink' and 'source'. The term 'source' refers to parts of a plant where sucrose is produced, primarily in the leaves via the process of photosynthesis. In contrast, the 'sink' represents areas in need of sucrose for growth or storage, such as roots, stems, seeds, and fruits.

The system is dynamic, meaning that the roles of sink and source can change throughout a plant's life cycle. During periods of rapid growth, such as spring or when a plant is forming fruit, those parts become major sinks. Conversely, if a plant part accumulates excess sucrose, it can act as a temporary source. The adaptability of roles facilitates efficient energy use and storage, depending on the plant's current needs as well as environmental factors.

Understanding Photosynthesis

Photosynthesis is a fundamental process where plants convert sunlight into chemical energy stored as sucrose. This occurs mainly in the leaves, which act as the primary source for sucrose during periods of active photosynthesis. The equation for photosynthesis is:\[6CO_2 + 6H_2O + light\ energy \Rightarrow C_6H_{12}O_6 + 6O_2\]

In this process, carbon dioxide and water are converted into glucose (a type of sugar) and oxygen, with the help of sunlight absorbed by chlorophyll. The glucose is then transformed into sucrose and transported internally to various sink areas where the plant needs it most. During different times of the day or season, the rate of photosynthesis can vary, altering the plant's sucrose source dynamics.

Transitions During Plant Growth Phases

Plants experience different growth phases, each with unique sucrose needs. These phases include germination, vegetative growth, flowering, and fruiting.

- **Germination**: During this early phase, seeds (acting as sources) supply stored energy to emerging seedlings. - **Vegetative Growth**: At this stage, leaves become powerful sources as they photosynthesize to support expansion and root growth. - **Flowering and Fruiting**: Flowers and developing fruits turn into strong sinks, needing significant sucrose for development. This phase often shifts the balance of sucrose away from leaves to help form fruit.

The plant continually assesses where energy is most needed in its development and adjusts the transportation of sucrose accordingly.

Impact of Seasonal Changes on Sucrose Transport

Seasonal changes impact plant biology significantly, especially in terms of sucrose transport. For instance, during the growing season, plants have active leaves as sources, supporting the growth of buds, fruits, and other organs acting as sinks. However, in colder months or when a plant enters a dormant phase, the metabolism slows down, decreasing the leaves' efficacy as sources.

During the winter, if a plant has stored enough energy in its roots or other parts, these sinks can assume the role of sources to ensure survival until the return of the growing season. This seasonal shift in the source-sink relationship assists plants in adapting to climate variations, ensuring efficient use of stored nutrients in adverse conditions.