Transport in Plants

The maintenance of the pressure gradient in sieve tubes is crucial for the process of translocation of nutrients, primarily sugars, within the phloem of vascular plants. Among the given options, the correct answer is:

Option D
Companion cells

Companion cells play a vital role in the maintenance of the pressure gradient in sieve tubes. These cells are closely associated with sieve tube elements to which they are connected by numerous plasmodesmata. Companion cells assist in maintaining the pressure gradient through the following mechanisms:

1. Loading and Unloading of Sugars: Companion cells actively transport sugars into and out of the sieve tubes. This activity creates a concentration gradient, leading to osmosis which in turn generates turgor pressure within the sieve tubes. This pressure drives the flow of sap from source tissues (where sugars are produced) to sink tissues (where sugars are utilized or stored).

2. Metabolic Support: Companion cells provide metabolic support to the sieve tube elements, which lack a nucleus and other organelles necessary for their own metabolism, thereby maintaining the functional integrity of the sieve tubes.

Therefore, companion cells are integral in managing the pressure gradient required for efficient phloem transport, making Option D the correct choice.

The correct option is :

Option D : (A), (B), (D) only

Explanation :

The mass flow hypothesis, also known as the pressure-flow hypothesis, is a model used to explain the movement of sap through the phloem in plants.

(A) Mass flow is faster than diffusion - True. Mass flow is faster because it is driven by pressure differences, while diffusion relies on concentration gradients which usually results in slower movement.

(B) Mass flow is the result of pressure difference between the end points - True. According to the mass flow hypothesis, the movement of sap is driven by a pressure gradient between the source (where sugars are produced by photosynthesis) and the sink (where sugars are used).

(C) Different substances involved in mass flow move at different paces - False. In mass flow, all the components of the sap, including sugars, hormones, and minerals, move together at the same pace due to the pressure gradient.

(D) Mass flow can result through either a positive or a negative hydrostatic pressure gradient - True. The flow of sap in phloem can be both upward and downward, depending on where the source and sink are located, meaning it can follow both positive (from high to low pressure) and negative (from low to high pressure, which requires energy) pressure gradients.

Symport is a type of transport mechanism that occurs in a cell's membrane. In symport, two different molecules or ions are transported across the membrane in the same direction, but they are co-transported by the same carrier protein. So, the correct option is :

Option C : in same direction with the help of the same carrier.

The other three options describe different types of transport:

Option A : describes a situation where two different carriers at a common site are involved, which does not represent symport.

Option B : describes a situation where two different carriers at different sites in the same cell are involved, which also does not represent symport.

Option D : describes a situation where two molecules move in the opposite direction. This is a characteristic of antiport, not symport.

The correct answer is :

Option B :

Statement I is incorrect but Statement II is correct.

Explanation :

Statement I is incorrect. The process of translocation in plants is bidirectional in phloem but unidirectional in xylem. Xylem primarily transports water and nutrients from the roots to the rest of the plant (upward direction), whereas phloem translocates sugars and other metabolic products from sources (like leaves) to sinks (areas of storage or growth, such as roots or fruits), which can be in either upward or downward direction.

Statement II is correct. Phosphorus, sulfur, nitrogen, and potassium are among the most readily mobilized elements in plants, especially during periods of nutritional stress. These nutrients are essential for plant growth and development and are actively translocated to areas where they are most needed.

Movement and accumulation of ions across a membrane against their concentration gradient can be explained by active transport. It uses energy to transport molecules from lower concentration to a higher concentration.

Statement I is correct as measurements reveal that the forces generated by transpiration can create pressures sufficient to lift a xylem sized column of water up to 130 meters high.

Statement II is also correct. Transpiration has a cooling effect on plant tissues. As water evaporates from the surfaces of leaves, it removes heat, reducing the temperature. This process, known as evaporative cooling, can lower the temperature of the leaf surface by 10 to 15 degrees.

Lenticels are lens shaped opening permitting exchange of gases between the outer atmosphere and internal tissue of the stem.

Bark that is formed early in the season is called early or soft bark. Towards the end of the season late or hard bark is formed.

Bark is non-technical term that refer to all tissues exterior to vascular cambium.

Bark refers to a number of tissue types, viz periderm and secondary phloem.

Phellogen is couple of layers thick

Therefore, only statement A and D are correct.

The correct answer is :

Option D : A – II, B – IV, C – I, D – III

Explanation :

(A) Cohesion - (II) Mutual attraction among water molecules. Cohesion is the property that makes water molecules attracted to each other, causing them to stick together.

(B) Adhesion - (IV) Attraction towards polar surfaces. Adhesion is the property of different substances to bond with each other. In the context of water, it refers to water's ability to "stick" to polar surfaces.

(C) Surface tension - (I) More attraction in liquid phase. Surface tension is the property that allows the surface of a liquid to resist external force, due to the cohesive nature of its molecules. It's why small insects can walk on water without breaking the surface.

(D) Guttation - (III) Water loss in liquid phase. Guttation is the process in plants where water is lost from the edges of the leaves in the form of liquid droplets, typically when soil moisture levels are high.

The ascent of xylem sap in plants is mainly due to negative hydrostatic pressure i.e. transpirational pull, which is accomplished by three physical properties of water i.e., cohesion, adhesion and surface tension.

The symplastic system is system of interconnected protoplasts. Neighbouring cell are connected through cytoplasmic strands that extend through plasmodesmata. The water travels through cell cytoplasm and plasmodesmata, hence the movement is relatively slower. Symplastic movement is aided by cytoplasmic streaming.

If some solute is dissolved in pure water, he solution has lower free water and the concentration of water decreases, reducing it's water potential. The magnitude of this lowering due to dissolution of a solute is called solute potential.