Nutrition in Plants, practice questions

C, Magnesium is a component of chlorophyll, so its absence prevents new chlorophyll from forming between the veins while the veins stay green.

B, Active transport is needed because mineral ions in the soil are often at a lower concentration than inside the root hair cell.

B, The control set-up, containing the complete nutrient solution, shows how the plant grows normally so that any deficiency symptom in the test set-up can be attributed to the one missing ion.

B, Rhizobium bacteria live inside root nodules, swellings on the roots of legumes, where they fix nitrogen gas from the air.

B, An epiphyte, unlike a parasite, does not draw water or nutrients from its host's living tissue; it uses the host only for physical support.

B, Insectivorous plants photosynthesise normally for carbohydrates; they trap insects mainly to obtain nitrogen that is scarce in the poor soils where they grow.

B, The fine threads of the fungus extend the effective absorptive surface area of the root system, increasing uptake of water and mineral ions such as phosphorus.

C, Iron is not very mobile within the plant, so its deficiency appears first in the youngest leaves near the growing point.

B, The long, thin extension of a root hair cell greatly increases the surface area in contact with the soil.

C, Rhizobium bacteria and the legume both benefit: the bacteria fix nitrogen the plant can use, and the plant supplies the bacteria with carbohydrates.

• (a) Iron
• (b) Iron cannot be moved easily from older to younger parts of the plant, so when it is in short supply the youngest leaves near the growing point, which cannot obtain iron from older tissue, are affected first and turn pale or yellow

• (a) It acts as a control, showing normal growth when every ion is present, so any symptom in the other set-ups can be linked to the specific ion left out
• (b) Interveinal chlorosis: yellowing between the veins while the veins remain green
• (c) Poor root growth and purplish leaves

• Rafflesia: parasitic, because it has no chlorophyll and draws water, minerals and food directly from a living host vine
• Orchid: epiphytic, because it grows on a tree purely for physical support and absorbs its own water and minerals from rain, air and debris
• Pitcher plant: insectivorous, because it photosynthesises normally but also traps and digests insects mainly to obtain extra nitrogen from poor soil

Macronutrients (e.g. nitrogen, phosphorus, potassium) are needed in large amounts; micronutrients are needed in small amounts.

Nitrogen is needed for proteins, magnesium for chlorophyll; a lack of a nutrient causes a specific deficiency symptom.

Water enters root hair cells by osmosis, moving down the water potential gradient.

Mineral ions are absorbed by active transport, which needs energy because the soil is often more dilute than the cell.

Root hair cells are long and thin with a large surface area to speed up absorption.

Some plants are parasitic, epiphytic or insectivorous, gaining nutrients in unusual ways.

A lack of nitrogen causes stunted growth and yellowing of older leaves because nitrogen is needed for protein and chlorophyll; a lack of phosphorus gives poor root growth and purplish leaves; a lack of potassium causes yellow, curling leaf margins; and a lack of magnesium causes interveinal chlorosis because magnesium is a component of chlorophyll.

Rhizobium bacteria live inside swellings called root nodules on the roots of legumes such as peas, beans and groundnuts. The bacteria fix nitrogen gas from the air into nitrate and ammonium compounds the plant can absorb, while the plant supplies the bacteria with carbohydrates, making this a mutualistic relationship rather than parasitism.

Many plant roots form a partnership with soil fungi called mycorrhiza. The fungal threads greatly extend the effective surface area of the root system, absorbing extra water and mineral ions, especially phosphorus, from a larger volume of soil, and in return the fungus receives sugars made by the plant in photosynthesis.

Rafflesia is a parasitic plant with no chlorophyll of its own that draws water and food directly from a host vine; the pitcher plant and Venus flytrap are insectivorous, trapping and digesting insects mainly for nitrogen while still photosynthesising; and many orchids are epiphytes, using a tree only for physical support and absorbing water and minerals from rain and air.

Growing plants in water culture (hydroponics), a nutrient solution with one mineral ion deliberately left out, lets a specific deficiency symptom be observed and linked to that missing ion, which is how the role of each macronutrient was first established experimentally.

Micronutrients such as iron, manganese, zinc and boron are needed only in tiny amounts but are still essential; for example, iron is required to make chlorophyll, and a shortage of it also causes yellowing of leaves, similar in appearance to a magnesium deficiency but affecting young leaves first rather than old ones.

The Venus flytrap has sensitive trigger hairs on its hinged leaves that snap shut when touched twice in quick succession, trapping an insect, while the pitcher plant has a deep, slippery, nectar-lined trap that insects fall into and cannot climb out of; both then secrete digestive enzymes to break down the prey and absorb the released nutrients.