Nutrition in Plants Ex

1. The figure below shows an apparatus used for water culture experiments.Several plants are grown like this, in identical conditions. Some, the controls, are given a culture solution containing all the minerals needed by plants. Others have a solution from which one mineral is missing. By comparing with the control plants, the effects of a lack of each mineral can be seen.

(a) Explain why the glass vessel is blackened. [3]
(b) Explain the need for the glass tubing. [2]
(c) If the element nitrogen is absent from the culture solution. How will this affect the plant and why? [2]

ANSWERS
1(a) To reduce amount of sunlight entering to prevent algal growth;
Which may grow around the roots and hinder the plant’s growth;
By competing for nutrients with the roots / blocking and reducing surface area of roots that can take up minerals;

(b) Glass tube to supply oxygen; which the roots need for respiration;

(c) Absence of nitrogen (in nitrates) cause poor growth/fewer leaves/small and pale green leaves leading to death;
As nitrogen is essential for amino acid and protein synthesis;
[Reminder: Plants use nitrates for a nitrogen source, not nitrogen gas!]

2.The figure below shows a picture of a leaf attached to a plant throughout the experiment.

6 discs were cut out from regions X, Y and Z of the leaf at 10 a.m., 4 p.m. and 10 p.m.. The plant may have received some treatment before the discs were cut and likewise the discs too, before their total dry weight is measured. The steps are show below:
Time / Treatment to plant before removing leaf discs/ Treatment of leaf discs/ Dry weight of leaf discs (mg)
10 a.m./ exposed to sunlight/ killed in steam/ 186
4 p.m./ exposed to sunlight/ none/ 209
10 p.m./ kept in the dark for 6 hours/ none/ 198

(a)(i) What is the biological process that causes the change in dry weight of the leaf discs between 10 a.m. and 4 p.m.? Support your answer with reasons. [2]

(ii) What is the biological process that causes the change in dry weight of the leaf discs between 4 p.m. and 10 p.m.? Support your answer with reasons. [2]

(b) Why is dry weight measured instead of fresh weight? [2]

ANSWERS
2(a)(i) Photosynthesis [0.5]
Dry weight increases from 186 to 209 mg, as the leaf discs cut at 4 pm were exposed to sunlight for 6 hrs more than those cut at 10 am [0.5]
Cells in the dics cut at 4 pm would have carried out photosynthesis for a longer period, and would have produced more glucose which would be stored as starch, leading to an increase in dry weight;

(a)(ii) Respiration [0.5]
Stored starch is being converted to glucose for respiration as photosynthesis has stopped in the absence of light/rate of respiration exceeds rate of photosynthesis;
Breakdown of starch and glucose would lead to a decrease in weight from 209 to 198 mg [0.5]

(b) Dry weight is more accurate because it is not affected by the water content of the leaf discs;
Water content is easily affected by different environmental conditions/amount of water absorbed by the plant;