Form 5 · Common mistakes

Nutrition in Plants, common mistakes

The mistakes SPM students make on Nutrition in Plants, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying minerals enter roots by osmosis.	Osmosis only explains the movement of water down a water potential gradient; it does not explain the movement of dissolved ions.	Water enters by osmosis, but mineral ions are absorbed by active transport, which needs energy from respiration.
Writing that plants get all of their nutrients from photosynthesis.	Photosynthesis converts carbon dioxide and water into carbohydrates only; it cannot supply nitrogen, phosphorus or any other mineral element.	Photosynthesis makes carbohydrates, but plants still need mineral ions from the soil for proteins, chlorophyll and other molecules.
Confusing macronutrients with micronutrients.	The distinction is based on the quantity needed, not on how important the nutrient is.	Macronutrients such as nitrogen are needed in large amounts; micronutrients such as iron are needed only in small amounts, though both are essential.
Saying insectivorous plants do not photosynthesise.	Trapping insects supplies extra nitrogen, not an alternative energy source in place of photosynthesis.	Insectivorous plants still photosynthesise normally; they catch insects mainly to obtain nitrogen from nutrient-poor soils.
Saying magnesium deficiency and nitrogen deficiency produce the same symptom.	Both cause yellowing, but the pattern of yellowing on the leaf is different because the two nutrients move within the plant in different ways.	Nitrogen deficiency turns whole older leaves yellow, while magnesium deficiency causes interveinal chlorosis, yellowing between veins that stay green.
Thinking Rhizobium bacteria are parasites living off a legume's roots.	A parasite harms its host and gives nothing in return, but Rhizobium supplies the plant with a nutrient it needs.	Rhizobium and the legume have a mutualistic relationship: the bacteria fix nitrogen the plant can use, and the plant supplies the bacteria with carbohydrates.
Saying an epiphyte such as an orchid takes nutrients from its host tree.	An epiphyte's roots are attached to the bark for anchorage only and do not penetrate the host's living tissue.	An epiphyte uses its host tree only for physical support and absorbs water and minerals from rainwater, air and surrounding debris.
Believing an insectivorous plant gets all its energy from digesting insects.	Digested insects supply nitrogen and other minerals, not the carbohydrates the plant uses as an energy source.	An insectivorous plant such as the pitcher plant still makes its own carbohydrates by photosynthesis; digested insects mainly supply extra nitrogen.
Confusing iron deficiency with magnesium deficiency because both cause yellow leaves.	The two symptoms are distinguished by which leaves are affected first, based on how mobile each element is within the plant.	Iron is not very mobile, so its deficiency shows first in young leaves near the shoot tip; magnesium is mobile, so its deficiency shows first in older leaves.
Thinking hydroponics has no practical use outside a school laboratory.	The same principle of supplying mineral ions in solution instead of through soil is used at a commercial scale.	Hydroponics is used commercially to grow crops such as lettuce and tomatoes without soil, by supplying all needed mineral ions directly in a nutrient solution.
Describing a water-culture experiment without including a complete-solution control set-up.	Without a control, a candidate cannot show that the deficiency symptom observed was caused specifically by the one ion left out.	A valid water-culture investigation includes a control set-up with the complete nutrient solution alongside the set-up missing one named ion.
Assuming all root hair adaptations only relate to increasing surface area.	Surface area is one adaptation, but a thin cell wall and a very large number of root hairs also contribute independently to efficient absorption.	Root hair cells are adapted by having a large surface area, a thin wall that shortens the diffusion distance, and by occurring in very large numbers over a root system.

How to avoid these mistakes

Learn the main macronutrients, their roles and one deficiency symptom each.
Compare water uptake (osmosis) and mineral uptake (active transport).
Note the special cases of parasitic and insectivorous plants.
Build a table of macronutrient, one role and one deficiency symptom for nitrogen, phosphorus, potassium and magnesium.
Learn the three special-nutrition strategies, parasitic, epiphytic and insectivorous, with one named example and how each obtains nutrients.
Revise nitrogen fixation by root-nodule bacteria and the mycorrhizal association with fungi as two examples of mutualism that help a plant obtain minerals.
Learn one micronutrient (such as iron) alongside a macronutrient, and compare how each deficiency shows up on young versus old leaves.

Frequently asked questions

How do roots absorb water and mineral salts?

Water is absorbed into root hair cells by osmosis, moving from the more dilute soil solution into the more concentrated cell sap. Mineral salts are absorbed as ions by active transport, which uses energy from respiration because the ions often move from the more dilute soil into the more concentrated cell against the concentration gradient.

Why does a plant need mineral nutrients if it makes food by photosynthesis?

Photosynthesis only makes carbohydrates from carbon dioxide and water. To make other molecules the plant needs mineral ions from the soil, for example nitrogen to make proteins and magnesium to make chlorophyll. A shortage of any of these causes a specific deficiency symptom such as yellow leaves.

Why are root hair cells good at absorbing water?

A root hair cell has a long, thin extension that greatly increases the surface area in contact with the soil, so more water and ions can be absorbed at once. Its thin wall gives a short distance for water to enter, and there are very many root hairs, which together make absorption efficient.