Question

Movement of phloem sap from a source to a sink

(A) occurs through the apoplast of sieve-tube elements.

(B) depends ultimately on the activity of proton pumps.

(C) depends on tension, or negative pressure potential.

(D) results mainly from diffusion

Short answer

(A) The option “occurs through the apoplast of sieve-tube elements” is false.

(B) The option “depends ultimately on the activity of proton pumps” is true.

(C) The option “depends on tension, or negative pressure potential” is false.

(D) The option “results mainly from diffusion” is false.

Step-by-step solution

Phloem sap 

Phloem sap is composed of minerals, nutrients, sugars, hormones, and water. The phloem sap facilitates the movement of carbohydrates from the source where it is produced and when it reaches the sink where it is stored.

Explanation of option ‘(A)’

The movement of phloem sap cannot occur from the source to the sink directly through the sieve tube elements. These sieve tube elements allow the movement of substances across a concentration gradient and turgor pressure but do not necessarily involve the movement of carbohydrates from source to sink.

Thus, the movement of phloem sap does not occur through the apoplast of sieve elements.

Therefore, the given option is false.

Explanation of option ‘(B)’

The movement of phloem sap can ultimately occur with the help of proton pumps. The proton pumps use energy molecules to produce an electrochemical gradient that pushes the molecules from the source to the sink.

Thus, the movement of phloem sap occurs through proton pumps.

Therefore, the given option is true.

Explanation of option ‘(C)’

The movement of phloem sap does not occur through negative pressures or tension because this type of movement is observed in the xylem. In phloem, the movement is driven by positive hydrostatic pressure. This process causes the movement of the phloem via osmosis.

Thus, the movement of phloem sap does not occur through negative pressures.

Therefore, the given option is false.

Explanation of option ‘(D)’

Proton pumps and perforated sieve tubes drive the movement of the phloem sap. Diffusion does not help in the movement of cell sap. However, diffusion comes into play when the phloem sap has to unload itself at the sap.

Thus, the movement of phloem sap does not occur mainly from diffusion.

Therefore, the given option is false.