Nutrition in Plants
Plants make their own food by photosynthesis and take up mineral nutrients from the soil. This chapter covers the mineral nutrients a plant needs, how it absorbs water and minerals, and the diversity of plant nutrition.
Photosynthesis itself is explored in depth on the process pages; here the focus is on nutrients and uptake.
Each macronutrient has a specific role and a recognisable deficiency symptom. Too little nitrogen, which builds proteins and chlorophyll, causes stunted growth and yellowing of older leaves; too little phosphorus, needed for roots and energy transfer, gives poor root growth and purplish leaves; too little potassium causes yellow, curling leaf margins; and too little magnesium specifically prevents new chlorophyll from forming, giving yellow leaves with the veins still green (interveinal chlorosis).
Plants also obtain some nutrients through partnerships with other organisms. Rhizobium bacteria living in the root nodules of legumes such as peas and beans fix nitrogen gas from the air into compounds the plant can use, while the plant supplies the bacteria with carbohydrates, a mutualistic relationship exploited in crop rotation.
Many plants also form a mycorrhizal association with soil fungi, which greatly increases the root surface available for absorbing water and mineral ions, especially phosphorus, in exchange for sugars.
This chapter is often assessed by giving a picture or description of a deficiency symptom and asking candidates to name the missing mineral and explain its role, or by describing an unusual plant, such as one that traps insects or grows on a tree, and asking how it obtains its nutrients. Naming the specific ion or the specific relationship, not just 'lack of nutrients', is what earns full marks.
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Content standards in this chapter
Key concepts
- Main inorganic nutrients
- Macronutrients (e.g. nitrogen, phosphorus, potassium) are needed in large amounts; micronutrients are needed in small amounts.
- Role of nutrients
- Nitrogen is needed for proteins, magnesium for chlorophyll; a lack of a nutrient causes a specific deficiency symptom.
- Uptake of water
- Water enters root hair cells by osmosis, moving down the water potential gradient.
- Uptake of mineral salts
- Mineral ions are absorbed by active transport, which needs energy because the soil is often more dilute than the cell.
- Root hair adaptations
- Root hair cells are long and thin with a large surface area to speed up absorption.
- Diversity in plant nutrition
- Some plants are parasitic, epiphytic or insectivorous, gaining nutrients in unusual ways.
- Macronutrient deficiency symptoms
- 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.
- Nitrogen fixation and root nodules
- 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.
- Mycorrhizal association
- 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.
- Named examples of diverse nutrition
- 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.
- Investigating mineral requirements
- 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
- 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.
- How the Venus flytrap and pitcher plant trap insects
- 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.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Matching a mineral deficiency to its symptom.
- Explaining how roots absorb water and mineral salts differently.
- Describing an adaptation of a root hair cell.
- Matching a described deficiency symptom, such as chlorosis or purplish leaves, to the specific mineral ion that is missing.
- Explaining why legumes are grown to improve soil nitrogen content, linking the answer to Rhizobium bacteria in root nodules.
- Distinguishing a parasitic plant, an epiphyte and an insectivorous plant by exactly how and why each obtains extra nutrients.
- Interpreting a water-culture (hydroponics) experiment in which one mineral ion is removed, and predicting the resulting deficiency symptom.
Common mistakes
What students write: Saying minerals enter roots by osmosis.
What earns the mark: Water enters by osmosis, but mineral ions are absorbed by active transport, which needs energy.
What students write: Writing that plants get all nutrients from photosynthesis.
What earns the mark: Photosynthesis makes carbohydrates, but plants still need mineral ions from the soil for proteins, chlorophyll and other molecules.
What students write: Confusing macronutrients and micronutrients.
What earns the mark: Macronutrients are needed in large amounts (e.g. nitrogen); micronutrients in tiny amounts.
What students write: Saying insectivorous plants do not photosynthesise.
What earns the mark: Insectivorous plants still photosynthesise; they catch insects mainly to obtain nitrogen from poor soils.
What students write: Saying magnesium deficiency and nitrogen deficiency produce the same symptom.
What earns the mark: Nitrogen deficiency turns whole older leaves yellow and stunts growth, while magnesium deficiency causes interveinal chlorosis, yellowing between the veins while the veins themselves stay green, because magnesium is specifically needed to make chlorophyll.
What students write: Thinking Rhizobium bacteria are parasites living off the legume's roots.
What earns the mark: Rhizobium and the legume have a mutualistic relationship: the bacteria fix nitrogen the plant can use, and the plant supplies the bacteria with carbohydrates, so both organisms benefit.
What students write: Saying an epiphyte such as an orchid steals nutrients from its host tree.
What earns the mark: An epiphyte uses its host tree only for physical support; it absorbs water and minerals from rainwater, air and decaying debris around it, so the host tree is not harmed the way a true parasite's host is.
What students write: Believing an insectivorous plant gets all of its energy from digesting insects.
What earns the mark: An insectivorous plant such as the pitcher plant still photosynthesises to make its own carbohydrates; it traps insects mainly to obtain extra nitrogen and other minerals from nutrient-poor soil.
What students write: Confusing iron deficiency with magnesium deficiency because both cause yellow leaves.
What earns the mark: Iron is not very mobile in the plant, so an iron deficiency shows first as chlorosis in young leaves near the shoot tip, whereas magnesium is mobile and its deficiency shows first in older leaves.
What students write: Thinking hydroponics has no practical use outside a school laboratory.
What earns the mark: Hydroponics is used commercially to grow vegetables such as lettuce and tomatoes without soil, particularly in areas such as the Cameron Highlands, by supplying all the needed mineral ions directly in a nutrient solution.
Study this chapter
Processes in this chapter
Frequently asked questions
How do roots absorb water and mineral salts?
Why does a plant need mineral nutrients if it makes food by photosynthesis?
Why are root hair cells good at absorbing water?
Why do farmers plant legumes such as peas or beans to improve soil fertility?
What is the difference between a parasitic plant, an epiphyte and an insectivorous plant?
How does interveinal chlorosis help identify a magnesium deficiency?
Why does an iron deficiency turn young leaves yellow while a magnesium deficiency affects older leaves first?
Source:SRC-DSKP-EN, SRC-FORMAT
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