Question

Phloem sap is mainly made of (a) water and sucrose (b) water and minerals (c) oligosaccharides and hormones (d) sucrose only.

Short answer

The main components of phloem sap are (a) water and sucrose.

Step-by-step solution

Identify Principal Components of Phloem Sap

Phloem sap is a part of the plant's transport system and carries necessary nutrients throughout the plant. To determine its main components, we need to consider the substances that are commonly transported by phloem tissue in plants.

Review Options for Phloem Sap Composition

Look at each given option and verify against known scientific knowledge about phloem sap composition. Phloem is known to transport sugars, amino acids, hormones, and other organic substances manufactured during photosynthesis from the leaves to other parts of the plant.

Select the Correct Option Based on Phloem Function

Since the primary function of the phloem is to distribute soluble organic material (primarily sucrose), which is a product of photosynthesis, the most suitable option from the list would be the one that includes sucrose.

Key concepts

Plant Transport System

The plant transport system is a sophisticated network that enables the distribution of nutrients, water, and other solutes necessary for plant growth and development. Fundamental to this system are two types of vascular tissues: xylem and phloem. The xylem is responsible for the transport of water and dissolved minerals from the roots to the rest of the plant, whereas the phloem carries soluble organic substances.

Phloem sap is indeed a crucial part of this system, with its composition primarily consisting of water and sucrose. It acts like the plant's circulatory system, delivering the food produced in the leaves, through photosynthesis, to tissues that require energy for growth, storage, or other metabolic processes. In addition to sucrose, the phloem sap can include amino acids, plant hormones, and other nutrients, reflecting the complex biochemical needs of the plant.

Sucrose Transport

Sucrose is manufactured in the leaves of plants through the process of photosynthesis. Once produced, it needs to be transported to different parts of the plant where it can be utilized or stored. This is where the process of sucrose transport comes into play, facilitated by the phloem. The movement of sucrose, known as translocation, is driven by a pressure-flow mechanism, which involves loading sucrose into the phloem at the source (typically leaves), and unloading it at the sink (where it is needed, such as roots, fruits, and seeds).

Pressure-Flow Hypothesis

The pressure-flow hypothesis provides insight into how sucrose is actively transported against its concentration gradient in some parts, while at other parts, it moves down the concentration gradient. This is a sophisticated balance of active and passive transport that ensures energy-efficient distribution of food resources within the plant.

Photosynthesis Products

Photosynthesis is the process through which green plants use sunlight to synthesize foods with carbon dioxide and water. The primary products of photosynthesis are glucose and oxygen. The glucose formed is often converted to sucrose, which is more readily transported within the plant, and it's this sucrose that forms the bulk of what is carried through the phloem sap.

Other than sucrose, photosynthesis also produces amino acids and lipids which are critical for plant growth. These organic compounds synthesized during photosynthesis are essential not just for the plant's survival but also form the foundational basis of the food chain, sustaining life on Earth.