Nutrition in Plants, worked answers

The missing ion is nitrogen. Nitrogen is a component of proteins and chlorophyll, so a lack of it slows protein synthesis and growth, causing stunted growth. Because nitrogen is mobile within the plant, it is withdrawn from older leaves and relocated to younger, actively growing parts when supply is limited, which is why the older leaves turn uniformly yellow first.

nitrogenproteinchlorophyllstunted growthmobile nutrient

The concentration of a mineral ion in the soil solution is often lower than its concentration inside the root hair cell, so absorbing more of the ion means moving it against its own concentration gradient. Diffusion can only move a substance down a concentration gradient, so it cannot account for this movement; active transport uses energy released by respiration in the root cells to move the ion against the gradient and into the cell.

active transportconcentration gradientagainst the gradientenergy from respirationroot hair cell

The complete nutrient solution set-up acts as a control, showing how the plant grows when every known mineral ion is available, so any difference seen in the other set-up can be attributed specifically to the missing ion rather than to some other factor. In the magnesium-free set-up, the plant is predicted to develop interveinal chlorosis, yellowing between the veins while the veins remain green, because magnesium cannot be obtained to make new chlorophyll.

control set-upwater culturemagnesiuminterveinal chlorosisnamed variable

An epiphyte uses the tree only for physical support; it absorbs its own water and mineral ions from rainwater, moisture in the air and decaying debris around its roots, so it does not remove any nutrients or water from the host tree's tissues. A parasitic plant such as Rafflesia has no chlorophyll of its own and instead draws water, minerals and food directly from the living tissue of a host vine through specialised absorptive structures, which removes resources the host needs for itself and can weaken or damage it.

epiphytephysical support onlyparasitic plantno chlorophyllhost tissue

Photosynthesis provides the plant with carbohydrates, but it does not supply nitrogen or other mineral ions, which the plant still needs to make proteins and other molecules. Insectivorous plants such as the Venus flytrap and the pitcher plant typically grow in nutrient-poor, often waterlogged soils where these minerals are scarce, so trapping and digesting insects gives them an additional source of nitrogen and other nutrients that their roots cannot easily obtain from the soil.

nitrogenpoor soilinsectivorous plantadditional sourcephotosynthesis still occurs

Legumes have root nodules containing Rhizobium bacteria that fix nitrogen gas from the air into nitrate and ammonium compounds the plant can absorb, in a mutualistic relationship where the plant supplies the bacteria with carbohydrates in return. When the legume crop is later ploughed back into the soil or its roots decompose, this fixed nitrogen is released into the soil, enriching it with a nutrient that the next crop can then use, which is why legumes are grown in crop rotation before a nitrogen-demanding crop.

Rhizobiumroot nodulesnitrogen fixationmutualistic relationshipcrop rotation

The fine threads of the fungus spread through a much larger volume of soil than the plant's own roots could reach alone, greatly increasing the effective surface area available for absorbing water and mineral ions, particularly phosphorus, which the fungus passes on to the plant. In return, the fungus receives sugars made by the plant during photosynthesis, which it cannot produce for itself since it has no chlorophyll, making this a mutualistic relationship in which both organisms benefit.

mycorrhizafungal threadssurface areaphosphorusmutualistic relationship