Inorganic ionsEdexcel A-Level Biology B: Subtopic test

1. 1
   A grower raises young tomato plants in hydroponic culture, where the roots sit in a nutrient solution. Plants given a complete solution grow normally. Plants given a solution lacking nitrate ions grow slowly and their older leaves turn pale yellow. Plants given a solution lacking magnesium ions have leaves that turn yellow between the veins.

   (a)

   Which molecule can a plant make only if it has a supply of nitrate ions?

   [1 mark]

   • AAmino acids
   • BCellulose
   • CStarch
   • DGlucose

   (b)

   Which statement best explains the yellowing between the veins of the magnesium-deficient plants?

   [1 mark]

   • AMagnesium is needed to make ATP, so the leaves lack energy to stay green
   • BMagnesium forms the middle lamellae, so the leaf cells separate
   • CMagnesium is needed to make the bases of DNA, so the leaf cells die
   • DMagnesium is needed to make chlorophyll, so less chlorophyll is made

   (c)

   Explain why plants lacking nitrate ions grow slowly.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A strawberry grower compares three nutrient solutions. In solution 1 all essential ions are present and the plants grow well. In solution 2 phosphate ions are missing: the roots grow poorly, the plants take up fewer ions by active transport and the leaves are dull and dark. In solution 3 calcium ions are missing: new tissue at the shoot and root tips is weak and soft, and its cells separate easily when handled.

   (a)

   Which statement describes the role of phosphate ions in a plant?

   [1 mark]

   • AThey form the central ion of chlorophyll
   • BThey bind pectin molecules together in the middle lamellae
   • CThey combine with ADP to make ATP, which transfers energy
   • DThey provide the nitrogen needed for amino acids

   (b)

   What is the most likely cause of the weak, soft tissue and separating cells in solution 3?

   [1 mark]

   • ACellulose microfibrils cannot form without calcium ions
   • BCalcium pectate cannot form, so the middle lamellae between neighbouring cells are weak
   • CChlorophyll cannot form, so the cell walls are not green
   • DAmino acids cannot be made, so the cell walls contain no protein

   (c)

   Explain why plants in solution 2 take up fewer ions by active transport.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Young wheat plants in a sandy field are stunted and have pale yellow leaves. A farmer adds ammonium nitrate to one half of the field and magnesium sulfate to the other half. Three weeks later, plants in the nitrate-treated half have green leaves and have grown well. Plants in the magnesium-treated half show no improvement. Chlorophyll content was low in the plants before treatment and is still low in the magnesium-treated plants, but is high in the nitrate-treated plants.

   (a)

   Deduce which ion was lacking in the soil, and explain why the stunted growth is consistent with this.

   [3 marks]

   (b)

   A student suggests that magnesium deficiency caused the low chlorophyll content. Evaluate whether the farmer's results support this suggestion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is writing a revision summary on why plants need mineral ions. She writes: 'Plants make their own food by photosynthesis, so they only need light, carbon dioxide and water.' Mineral ions such as nitrate, calcium, magnesium and phosphate are absorbed from the soil solution by root hair cells.

   (a)

   Explain why the student's statement is incorrect, by describing the roles of nitrate, calcium, magnesium and phosphate ions in a plant.

   [6 marks]

   (b)

   Explain the effects on a plant of a deficiency of magnesium, calcium or phosphate ions, and justify why a fertiliser should supply all of the essential ions.

   [6 marks]

   Total for question 4: 12 marks

End of questions